Bipedalism in early humans would have enabled them to travel efficiently over long distances, giving them an advantage over quadrupedal apes in moving across barren open terrain between groves of trees. In addition, the earliest humans continued to have the advantage from their ape ancestry of being able to escape into the trees to avoid predators. The benefits of both Bipedalism and agility in the trees may explain the unique anatomy of Australopiths. Their long, powerful arms and curved fingers probably made them good climbers, while their pelvis and lower limb structure were reshaped for upright walking people belong to the genus Homo, which first evolved at least 2.3 million to 2.5 million years ago. The earliest members of this genus differed from the Australopiths in at least one important respect-they had larger brains than did their predecessors.
The evolution of the modern human genus can be divided roughly into three periods: during an early stage, an intermediate period and late. Species of early Homo resembled gracile Australopiths in many ways. Some early Homo species lived until possibly 1.6 million years ago. The period of the middle Homo began perhaps between two million and 1.8 million years ago, overlapping with the end of early Homo. Species of Middle Homo evolved an anatomy much more similar to that of modern humans but had comparatively small brains. The transition from Intermittent to late Homo probably occurred sometime around 200,000 years ago. Species of late Homo evolved large and complex brains and eventually language. Culture also became an increasingly important part of human life during the most recent period of evolution.
The origin of the genus Homo has long intrigued paleoanthropologists and prompted much debate. One of several known species of Australopiths, or one not yet discovered, could have caused the first species of Homo. Scientists also do not know exactly what factors favoured the evolution of a larger and more complex brain-the defining physical trait of modern humans.
Louis Leakey originally argued that the origin of Homo related directly to the development of Toolmaking,-specifically, the making of stone tools. Toolmaking requires certain mental skills and fine hand manipulation that may exist only in members of our own genus. The name Homo habilis (meaning ‘repairer') refer directly to the making and use of tools
However, several species of Australopiths lived just when early Homo, making it unclear which species produced the earliest stone tools. Recent studies of Australopiths hand bones have suggested that at least a robust species, Australopithecus robustus, could have made tools. In addition, during the 1960's and 1970's researchers first observed that some nonhuman primates, such as chimpanzees, make and use tools, suggesting that Australopiths and the apes that preceded them probably also made some kinds of tools.
Scientists began to notice a high degree of variability in body size as they discovered more early Homo fossils. This could have indicated that H. habilis had a large amount of sexual dimorphism. For instance, the Olduvai female skeleton was dwarfed in comparison with other fossils,-exemplified by a sizable early Homo cranium from East Turkana in northern Kenya. However, the differences in size exceeded those expected between males and females of the same species, and this finding later helped convince scientists that another species of early Homo had lived in eastern Africa.
This second species of early Homo was given the name Homo rudolfensis, after Lake Rudolf (now Lake Turkana). The best-known fossils of H. rudolfensis come from the area surrounding this lake and date from about 1.9 million years ago. Paleoanthropologists have not determined the entire time range during which H. rudolfensis may have lived.
This species had a larger face and body than did H. habilis. The cranial capacity of H. rudolfensis averaged about 750 cu cm (46 cu. in.). Scientists need more evidence to know whether the brain of H. rudolfensis in relation to its body size was larger than that proportion in H. habilis. A larger brain-to-body-size, and ratio can indicate increased mental abilities. H. rudolfensis also had large teeth, approaching the size of those in robust Australopiths. The discovery of even a partial fossil skeleton would reveal whether this larger form of early Homo had apelike or more modern body proportions. Scientists have found several modern-looking thighbones that date from between two million and 1.8 million years ago and may belong to H. rudolfensis. These bones suggest a body size of 1.5 m. (5 ft.) and 52 kg. (114 lb.).
By about 1.9 million years ago, the period of middle Homo had begun in Africa. Until recently, paleoanthropologists recognized one species in this period, Homo erectus. Many now recognize three species of middle Homo: Homo. ergaster, Homo. erectus, and Homo. heidelbergensis. However, some still think Homo ergaster is an early African form of H. erectus, or that Homo heidelbergensis is a late form of the Homo erectus.
The skulls and teeth of early African populations of Middle Homo differed subtly from those of later H. erectus populations from China and the island of Java in Indonesia. H. ergaster makes a better candidate for an ancestor of the modern human line because Asian H. erectus has some specialized features not seen in some later humans, including our own species. H. heidelbergensis has similarities to both
H. erectus and the later species. The H. neanderthalensis, even if it may have been a transitional species between middle Homo and the line to which modern humans belong.
Homo ergaster probably first evolved in Africa around two million years ago. This species had a rounded cranium with a brain size of between 700 and 850 cu. cm. (49 to fifty-two cu. in), a prominent brow ridge, small teeth, and many other features that it shared with the later H. erectus. Many paleoanthropologists consider H. ergaster a good candidate for an ancestor of modern humans because it had several modern skull features, including thin cranial bones. Most H. ergaster fossils come from the time range of 1.8 million to 1.5 million years ago.
The most important fossil of this species yet found is a nearly complete skeleton of a young male from West Turkana, Kenya, which dates from about 1.55 million years ago. Scientists determined the sex of the skeleton from the shape of its pelvis. They also determined from patterns of tooth eruption and bone growth that the boy had died when he was between nine and twelve years old. The oldest humanlike fossils outside Africa have also been classified as H. ergaster, dated around 1.75 million year's old. These finds, from the Dmanisi site in the southern Caucasus Mountains of Georgia, consist of several crania, jaws, and other fossilized bones. Some of these are strikingly like East African H. ergaster, but others are smaller or larger than H. ergaster, suggesting a high degree of variation within a single population
H. ergaster, H. rudolfensis, and H. habilis, in addition to possibly two robust Australopiths, all might have coexisted in Africa around 1.9 million years ago. This finding goes against a traditional paleoanthropological view that human evolution consisted of a single line that evolved progressively over time-an Australopiths species followed by early Homo, then Middle Homo, and finally H. sapiens. It appears that periods of species diversity and extinction have been common during human evolution, and that modern H. sapiens has the rare distinction of being the only living human species today.
Although H. ergaster appears to have coexisted with several other human species, they probably did not interbreed. Mating rarely succeeds between two species with significant skeletal differences, such as H. ergaster and H. habilis. Many paleoanthropologists now believe that H. ergaster descended from an earlier population of Homo-perhaps one of the two known species of early Homo-and that the modern human line descended from the H. ergaster.
Paleoanthropologists now know that humans first evolved in Africa and lived only on that continent for a few million years. The earliest human species known to have spread in large numbers beyond the African continent was first discovered in Southeast Asia. In 1891 Dutch physician Eugene Dubois found the cranium of an early human on the Indonesian island of Java. He named this early human Pithecanthropus erectus, or ‘erect ape-man.' Today paleoanthropologists call this species Homo erectus.
H. erectus appears to have evolved in Africa from earlier populations of H. ergaster, and then spread to Asia sometime between 1.8 million and 1.5 million years ago. The youngest known fossils of this species, from the Solo River in Java, may date from about 50,000 years ago (although that dating is controversial). So, H. erectus was a very successful species,-both widespread, having lived in Africa and much of Asia, and long-lived, having survived for possibly more than 1.5 million years.
H. erectus had a low and rounded braincase that was elongated form front to back, a prominent brow ridge, and adult cranial capacity of 800 to 1,250 cu. cm. (50 to eighty cu. in.), an average twice that of the Australopiths. Its bones, including the cranium, were thicker than those of earlier species. Prominent muscle markings and thick, reinforced areas on the bones of H. erectus indicate that its body could withstand powerful movements and stresses. Although it had much smaller teeth than did the Australopiths, it had a heavy and strong jaw.
In the 1920's and 1930's German anatomist and physical anthropologist Franz Weidenreich excavated the most famous collections of H. erectus fossils from a cave at the site of Zhoukoudian (Chou-k ou-tien), China, near Beijing (Peking). Scientists dubbed these fossil humans Sinanthropus pekinensis, or Peking Man, but others later reclassified them as H. erectus. The Zhoukoudian cave yielded the fragmentary remains of more than 30 individuals, ranging from about 500,000 to 250,000 years old. These fossils were lost near the outbreak of World War II, but Weidenreich had made excellent casts of his finds. Further studies at the cave site have yielded more H. erectus remains.
Other important fossil sites for this species in China include Lantian, Yuanmou, Yunxian, and Hexian. Researchers have also recovered many tools made by H. erectus in China at sites such as Nihewan and Bose, and other sites of similar age (at least one million to 250,000 years old).
Ever since the discovery of H. erectus, scientists have debated whether this species was a direct ancestor of later humans, including H. sapiens. The last populations of H. erectus-such as those from the Solo River in Java-may have lived as recently as 50,000 years ago, while did populations of H. sapiens. Modern humans could not have evolved from these late populations of the H. erectus, a much more primitive type of human. However, earlier East Asian populations could have produced The
Homo sapiens.
Many paleoanthropologists believe that early humans migrated into Europe by 800,000 years ago, and that these populations were not Homo erectus. Most scientists refer to these early migrants into Europe,-who predated both Neanderthals and H. sapiens in the region, as
H. heidelbergensis. The species name comes from a 500,000-year-old jaw found near Heidelberg, Germany
Scientists have found few human fossils in Africa for the period between 1.2 million and 600,000 years ago, during which H. heidelbergensis or its ancestors first migrated into Europe. Populations of H. ergaster (or possibly H. erectus) appear to have lived until at least 800,000 years ago in Africa, and possibly until 500,000 years ago in northern Africa. When these populations disappeared, other massive-boned and larger-brained humans-possibly H. heidelbergensis-appear to have replaced them. Scientists have found fossils of these stockier humans at sites in Bodo Ethiopia, Saldanha (also known as Elandsfontein), South Africa, Ndutu, Tanzania, Kabwe, and Zimbabwe.
Scientists have come up with at least three different interpretations of these African fossils. Some scientists place the fossils in the species H. heidelbergensis and think that this species led to both the Neanderthals (in Europe) and H. sapiens (in Africa). Others think that the European and African fossils belong to two distinct species, and that the African populations that, in this view, was not
H. heidelbergensis but a separate species produced Homo sapiens. Yet other scientists advocate a long-head view that H. erectus similarly, Homo sapiens belong to a single evolving lineage, and that the African fossils belong in the category of archaic H. sapiens (archaic meaning not fully anatomically modern).
The fossil evidence does not clearly favour any of these three interpretations over another. Several fossils from Asia, Africa, and Europe have features that are intermediate between early H. ergaster and H. sapiens. This kind of variation makes it hard to decide how to identify distinct species and to determine which group of fossils represents the most likely ancestor of later humans.
Scientists once thought that advances in stone tools could have enabled early humans such as Homo erectus to move into Asia and Europe, perhaps by helping them to obtain new kinds of food, such as the meat of large mammals. If African human populations had developed tools that allowed them to hunt large game effectively, they would have had a good source of food wherever they went. In this view, humans first migrated into Eurasia based on a unique cultural adaptation.
By 1.5 million years ago, early humans had begun to make new kinds of tools, which scientists call Acheulean. Common Acheulean tools included large hand axes and cleavers. While these new tools might have helped early humans to hunt, the first known Acheulean tools in Africa date from later than the earliest known human presence in Asia. Also, most East Asian sites more than 200,000 years old contains only simply shaped cobble and flake tools. In contrast, Acheulean tools were more finely crafted, larger, and more symmetrical. Thus, the earliest settlers of Eurasia did not have a true Acheulean technology, and advances in Toolmaking alone cannot explain the spread out of Africa.
Another possibility is that the early spreads of humans to Eurasia were not unique, but part of a wider migration of meat-eating animals, such as lions and hyenas. The human migration out of Africa occurred during the early part of the Pleistocene Epoch, between 1.8 million and 780,000 years ago. Many African carnivores spread to Eurasia during the early Pleistocene, and humans could have moved along with them. In this view, H. erectus was one of many meat-eating species to expand into Eurasia from Africa, rather than a uniquely adapted species. Relying on meat as a primary food source might have allowed many meat-eating species, including humans, to move through many different environments without having to learn about unfamiliar and potentially poisonous plants quickly.
However, the migration of humans to eastern Asia may have occurred gradually and through lower latitudes and environments similar to those of Africa. If East African populations of H. erectus moved at only 1.6 km. (1 mi.) every twenty years, they could have reached Southeast Asia in 150,000 years. Over this amount of time, humans could have learned about and begun relying on edible plant foods. Thus, eating meat may not have played a crucial role in the first human migrations to new continents. Careful comparison of animal fossils, stone tools, and early human fossils from Africa, Asia, and Europe will help scientists better to determine what factors motivated and allowed humans to venture out of Africa for the first time.
The origin of our own species, Homo sapiens, is one of the most hotly debated topics in Paleoanthropology. This debate centres on whether or not modern humans have a direct relationship to H. erectus or to the Neanderthals, and to a great extent is acknowledged of the more modern group of humans who evolved within the past 250,000 years. Paleoanthropologists commonly use the term anatomically modern Homo sapiens to distinguish people of today from these similar predecessors.
Traditionally, paleoanthropologists classified as Homo sapiens any fossil human younger than 500,000 years old with a braincase larger than that of H. erectus. Thus, many scientists who believe that modern humans descend from a single line dating back to H. erectus use the name archaic Homo sapiens to refer to a variety of fossil humans that predate anatomically modern H. sapiens. The designate with archaic denote a set of physical features typical of Neanderthals and other species of late Homo before modern Homo sapiens. These features include a combination of a robust skeleton, a large but low braincase (positioned in a measure behind, rather than over, the face), and a lower jaw lacking a prominent chin. In this sense, Neanderthals are sometimes classified as a subspecies of archaic H. sapiens and H. Sapiens neanderthalensis. Other scientists think that the variation in archaic fossils falls into clearly identifiable sets of traits, and that any type of human fossil exhibiting a unique set of traits should have a new species name. According to this view, the Neanderthals belong to their own species, H. neanderthalensis.
The Neanderthals lived in areas ranging from western Europe through central Asia from about 200,000 to about 28,000 years ago. The name Neanderthal (sometimes spelled Neanderthal) comes from fossils found in 1856 in the Feldhofer Cave of the Neander Valley in Germany (tal,-a modern form of that-means ‘valley' in German). Scientists realized several years later that prior discoveries, at Engis, Belgium, in 1829 and at Forbes Quarry, Gibraltar, in 1848,-also represented Neanderthal. These two earlier discoveries were the first early human fossils ever found.
In the past. Scientists claimed that Neanderthal differed greatly from modern humans. However, the basis for this claim came from a faulty reconstruction of a Neanderthal skeleton that showed it with bent knees and a slouching gait. This reconstruction gave the common but mistaken impression that Neanderthals were dim-witted brutes who lived a crude lifestyle. On the contrary, Neanderthals, like the species that preceded them, walked fully upright without a slouch or bent knees. In addition, their cranial capacity was quite large at about 1,500 cu. cm. (about ninety cu. in.), larger on average than that of modern humans. (The difference probably relates to the greater muscle mass of Neanderthals as compared with modern humans, which usually correlates with a larger brain size.)
Compared with earlier humans, Neanderthals had a high degree of cultural sophistication. They appear to have acted symbolic rituals, such as the burial of they're dead. Neanderthal fossils,-including some complete skeletons is quite common compared with those of earlier forms of Homo, in part because of the Neanderthal practice of intentional burial. Neanderthals also produced sophisticated types of stone tools known as Mousterian, which involved creating blanks (rough forms) from which several types of tools could be made. Along with many physical similarities, Neanderthals differed from modern humans in several ways. The typical Neanderthal skull had a low forehead, a large nasal area (suggesting a large nose), a forward-projecting nasal and cheek region, a prominent brow ridge with a bony arch over each eye, a non-projecting chin, and obvious space behind the third molar (in front of the upward turn of the lower jaw).
Neanderthals, in addition, had a distinctively heavily built and large-boned skeleton than do modern humans. Other Neanderthal skeletal features included a bowing of the limb bones in some individuals, broad scapulae (shoulder blades), hip joints turned outward, a long and thin pubic bone, short lower leg and arm bones relative to the upper bones, and large surfaces on the joints of the toes and limb bones. Together, these traits made a powerful, compact body of short stature males averaged 1.7 m. (5 ft. 5 in.) tall and 84 kg. (185 lb.), and females averaged 1.5 m. (5 ft.) tall and 80 kg. (176 lb.).
The short, stocky build of Neanderthals conserved heat and helped them withstand extremely cold conditions that prevailed in temperate regions beginning about 70,000 years ago. The last known Neanderthal fossils come from western Europe and date from approximately 36,000 years ago.
Just when Neanderthal populations grew in number in Europe and parts of Asia, other populations of nearly modern humans arose in Africa and Asia. Scientists also commonly refer to these fossils, which are distinct from but similar to those of Neanderthals, as archaic. Fossils from the Chinese sites of Dali, Maba, and Xujiayao display the long, low cranium and large face typical of archaic humans, yet they also have features similar to those of modern people in the region. At the cave site of Jebel Irhoud, Morocco, scientists have found fossils with the long skull typical of archaic humans but also the modern traits of a moderately higher forehead and flatter mid face. Fossils of humans from East African sites older than 100,000 years, such as Ngaloba in Tanzania and Eliye Springs in Kenya-also seem to show a mixture of archaic and modern traits.
The oldest known fossils that possess skeletal features typical of modern humans date from between 130,000 and 90,000 years ago. Several key features distinguish the skulls of modern humans from those of archaic species. These features include a much smaller brow ridge, if any; a globe-shaped braincase; and a flat or only projecting face of reduced size, located under the front of the braincase. Among all mammals, only humans have a face positioned directly beneath the frontal lobe (forward-most area) of the brain. As a result, modern humans tend to have a higher forehead than did Neanderthals and other archaic humans. The cranial capacity of modern humans ranges from about 1,000 to 2,000 cu. cm. (60 to 120 cu. in.), with the average being about 1,350 cu. cm. (80 cu. in.).
Scientists have found both fragmentary and nearly complete cranial fossils of early anatomically modern Homo sapiens from the sites of Singha, Sudan; Omo, Ethiopia; Klasies River Mouth, South Africa, and Skh~l Cave unbounded of Israel. Based on these fossils, many scientists conclude that modern H. sapiens had evolved in Africa by 130,000 years ago and started spreading to diverse parts of the world beginning on a route through the Near East sometime before 90,000 years ago.
Paleoanthropologists are engaged in an ongoing debate about where modern humans evolved and how they spread around the world. Differences in opinion rest on the question of whether the evolution of modern humans took place in a small region of Africa or over a broad area of Africa and Eurasia. By extension, opinions differ as to whether modern human populations from Africa displaced all existing populations of earlier humans, eventually resulting in their extinction.
Those who think of modern humans originating only in Africa and then spreading around the world support as their thesis the out of Africa hypothesis. Those who think modern humans evolved over a large region of Eurasia and Africa support the so-called multi-regional hypothesis. The African origins of Humanity where Richard Leakey's work at Omo-Kibish gave scientists a fresh start in their study of Homo sapiens' origins. In fact, his finds gave them two beginnings. First, they led a few researchers in the 1970s to conclude that the Kibish man was a far more likely ancestor for the Cro-Magnons, a race of early Europeans who thrived about 25,000 years ago, than their immediate predecessors in Europe, the heavyset Neanderthals. Then in the 1980s, a new reconstruction and study of the Kibish man revealed an even more startling possibility-that, and he was a far better candidate as the forbear, not just for the Cro-Magnons but for every one of us in the wake of an ignited awareness for life today, not just Europeans but all the other peoples of the world, from the Eskimos of Greenland to the Twa people of Africa, and from Australian aborigines to Native Americans. In other words, the Kibish man acted as pathfinder for a new genesis for the human species.
In the past few years, many paleontologists, anthropologists, and geneticists have come to agree that this ancient resident of the riverbanks of Ethiopia and all his Kibish kin-both far and near-could are even among our ancestors. However, it has also become clear that the evolutionary pathway of these fledgling modern humans was not an easy one. At one stage, according to genetic data, our species became as endangered as the mountain gorilla is today, its population reduced to only about 10,000 adults. Restricted to one region of Africa, but tempered in the flames of near extinction, this population went on to make a remarkable comeback. It then spread across Africa until-nearly 100,000 years ago-it had colonized much of the continent's savannas and woodlands. We see the imprint of this spread in biological studies that have revealed that races within Africa are genetically the most disparate on the planet, indicating that modern humans have existed there in larger numbers for a longer time than anywhere else.
We can also observe intriguing clues about our African origins in other less obvious but equally exciting arenas. One example comes from Congo-Kinshasa. This huge tropical African country has never assumed much importance in the field of Paleoanthropology, the branch of anthropology concerned with the investigation of ancient humans. Unlike the countries to the east, Ethiopia, Kenya, and Tanzania, Congo-Kinshasa has provided few exciting fossil sites-until recently.
In the neglected western branch of the African Rift Valley, that giant geological slash that has played such a pivotal role in human evolution, the Semliki River runs northward between two large lakes, and its waters eventually from the source of the Nile. Along its banks, sediments are being exposed that were laid down 90,000 years ago, just as Homo sapiens was making its mark across Africa.
At the town of Katanda an archaeological treasure trove: thousands of artifacts, mostly stone tools, and a few bone implements that quite astonished the archaeologists, led by the husband-and-wife team of John Yellen, of the National Science Foundation, Washington, and Alison Brooks, of George Washington University. Among the wonders they have uncovered are sophisticated bone harpoons and knives. Previously it was thought that the Cro-Magnons were the first humans to develop such delicate carving skills. Yet this very much older grouped of Homo sapiens, living in the heartland of Africa, displayed the same extraordinary skills as craft's workers. It was as if, said one observer, a prototype Pontiac car had been found in the attic of Leonardo da. Vinci.
There were other surprises for researchers, however. Apart from the finely carved implements, they found fish bones, including some from two-metre-long catfish. It seems the Katanda people were efficiently and repeatedly catching catfish during their spawning season, indicating that systematic fishing is quite an ancient human skill and not some recently acquired expertise, as many archaeologists had previously thought. In addition, the team found evidence that a Katanda site had at least two separate but similar clusters of stones and debris that looked like the residue of two distinct neighbouring groupings, signs of the possible impact of the nuclear family on society, a phenomenon that now defines the fabric of our lives.
Clearly, our African forbears were sophisticated people. Bands of them, armed with new proficiencies, like those men and women who had flourished on the banks of the Semliki, began an exodus from their African homeland. Slowly they trickled northward, and into the Levant, the region bordering the eastern Mediterranean. Then, by 80,000 years ago, small groups began spreading across the globe, via the Middle East, planting the seeds of modern humanity in Asia and later in Europe and Australia.
Today men and women conduct themselves in highly complex ways: some are uncovering the strange, indeterminate nature of matter, with its building blocks of quarks and leptons; some are probing the first few seconds of the origins of the universe fifteen billion years ago; while others are trying to develop artificial brains capable of staggering feats of calculation. Yet the intellectual tools that allow us to investigate the deepest secrets of our world are the ones that were forged during our fight for survival, in a very different set of circumstances from those that prevail today. How on earth could an animal that struggled for survival like any other creature, whose time was absorbed in a constant search for meat, nuts, and tubers, and who had to maintain constant vigilance against predators, develop the mental hardwiring needed by a nuclear physicist or an astronomer? This is a vexing issue that takes us to the very heart of our African exodus, to the journey that brought us from precarious survival on a single continent to global control.
If we can ever hope to understand the special attributes that delineate a modern human being we have to attempt to solve such puzzles. How was the Kibish man different from his Neanderthal cousins in Europe, and what evolutionary pressures led the Katanda people to develop in such crucially different ways-ironically in the heart of a continent that has for far too long been stigmatized as backward?
Nonetheless, it remains bewildering, but French researchers announced at a press conference on May 22, 1996, the discovery of a new fossil hominid species in central Chad, estimated to have lived between three million and 3.5 million years ago. The fossilized remains of a lower jaw and seven teeth were found in 1995 near Koro Toro, in the desert about 2500 km (about 1500 mi) east of the Great Rift Valley in Africa, the site of many major hominid fossil finds. The leader of the French team that discovered the fossils at Bahr-el-Ghazal, Chad-paleontologist Michel Brunet of the University of Poitiers-named the species Australopithecus bahrelghazali (from the Arabic name of the nearby River of the Gazelles). The research team published its findings in the May 20 bulletin of the French Academy of Sciences. In a letter to the journal Nature published November 16, 1995, the researchers initially classified the fossil as an example of Australopithecus afarensis, the 3.4-million-year-old species that walked upright in eastern Africa. In the letter, Brunet said that more detailed comparisons with other fossils were necessary before he could determine that the jaw came from another species, and he noted that geographic separation can produce differences among animals of the same species. After the letter was published, Brunet travelled to museums to compare the jaw with other hominid bones. The fossil combines both primitive and modern hominid features. The jaw includes the right and left premolars, both canines, and the right lateral incisor. Brunet said the strong canine teeth and the shape of the incisor resemble human teeth more than ape teeth. The chin area is more vertical than the backward-sloping chin of A. afarensis, and it lacks the strong reinforcement for chewing power found among other early hominids. However, the premolars retain primitive characteristics, such as three roots, and modern humans have only one root. Scientists said they needed more fossil material before they can place the species on the evolutionary tree. Brunet cited the find as the first evidence of hominid occupation of areas outside the Great Rift Valley and South Africa, where anthropologists have concentrated their search for hominid fossils. Other experts noted that the eroding volcanic soils in the Great Rift Valley are simply better for preserving and exposing fossils than the soils in most other regions in Africa. Although many digs have occurred in the Great Rift Valley, most scientists believe that hominids existed throughout Africa.
Researchers have conducted many genetic studies and carefully assessed fossils to determine which of these hypotheses agrees more with scientific evidence. The results of this research do not entirely confirm or reject either one. Therefore, some scientists think a compromise between the two hypotheses is the best explanation. The debate between these views has implications for how scientists understand the concept of race in humans. The question raised is whether the physical differences among modern humans evolved deep in the past or most recently, according to the out of Africa hypothesis, also known as the replacement hypothesis, early populations of modern humans from Africa migrated to other regions and entirely replaced existing populations of archaic humans. The replaced populations would have included the Neanderthals and any surviving groups of Homo erectus. Supporters of this view note that many modern human skeletal traits evolved recently,-within the past 200,000 years or so suggesting a single, common origin. In addition, the anatomical similarities shared by all modern human populations far outweigh those shared by premodern and modern humans within particular geographic regions. Furthermore, biological research indicated that most new species of organisms, including mammals, arose from small, geographically isolated populations.
According to the multi-regional hypothesis, also known as the continuity hypothesis, the evolution of modern humans began when Homo erectus spread throughout much of Eurasia around one million years ago. Regional populations retained some unique anatomical features for hundreds of thousands of years, but they also mated with populations from neighbouring regions, exchanging heritable traits with each other. This exchange of heritable traits is known as gene flow.
Through gene flow, populations of H. erectus passed on a variety of increasingly modern characteristics, such as increases in brain size, across their geographic range. Gradually this would have resulted in the evolution of more modern looking humans throughout Africa and Eurasia. The substantial differences among our citizenries today, are, then, a sortal result from hundreds of thousands of years of regional evolution. This is the concept of continuity. For instance, modern East Asian populations have some skull features that scientists also see in H. erectus fossils from that region.
Some critics of the multi-regional hypothesis claim that it wrongly advocates a scientific belief in race and could be used to encourage racism. Supporters of the theory point out, however, that their position does not imply that modern races evolved in isolation from each other, or that racial differences justify racism. Instead, the theory holds that gene flow linked different populations together. These links allowed progressively more modern features, no matter where they arose, to spread from region to region and eventually become universal among humans.
Scientists have weighed the out of Africa and multi-regional hypotheses against both genetic and fossil evidence. The results do not unanimously support either one, but weigh more heavily in favour of the out of Africa hypothesis.
Geneticists have studied difference in the DNA (deoxyribonucleic acid) of different populations of humans. DNA is the molecule that contains our heritable genetic code. Differences in human DNA result from mutations in DNA structure. Mutations may result from exposure to external elements such as solar radiation or certain chemical compounds, while others occur naturally at random.
Geneticists have calculated rates at which mutations can be expected to occur over time. Dividing the total number of genetic differences between two populations by an expected rate of mutation provides an estimate of the time when the two shared a common ancestor. Many estimates of evolutionary ancestry rely on studies of the DNA in cell structures called mitochondria. This DNA is called mtDNA (mitochondrial DNA). Unlike DNA from the nucleus of a cell, which codes for most of the traits an organism inherits from both parents, mtDNA inheritance passes only from a mother to her offspring. MtDNA also accumulates mutations about ten times faster than does DNA in the cell nucleus (the location of most DNA). The structure of mtDNA changes so quickly that scientists can easily measure the differences between one human population and another. Two closely related populations should have only minor differences in their mtDNA. Conversely, two very distantly related populations should have large differences in their mtDNA
MtDNA research into modern human origins has produced two major findings. First, the entire amount of variation in mtDNA across human populations is small in comparison with that of other animal species. This means that all human mtDNA originated from a single since which ancestral lineage-specifically, a single female-of late has been mutating ever. Most estimates of the mutation rate of mtDNA suggest that this female ancestor lived about 200,000 years ago. In addition, the mtDNA of African populations varies more than that of peoples in other continents. This suggests that the mtDNA of African populations sustained of change for a longer time than in populations of any other region. That all living people inherited their mtDNA from one woman in Africa, who is sometimes called the Mitochondrial Eve. Some geneticists and anthropologists have concluded from this evidence that modern humans originated in a small population in Africa and spread from there.
MtDNA studies have weaknesses, however, including the following four. First, the estimated rate of mtDNA mutation varies from study to study, and some estimates put the date of origin closer to 850,000 years ago, the time of Homo erectus. Second, mtDNA makes up a small part of the total genetic material that humans inherit. The rest of our genetic material-about 400,000 times more than the mtDNA,-came from many individuals living at the time of the African Eve, conceivably from many different regions. Third, the time at which modern mtDNA began to diversify does not necessarily coincide with the origin of modern human biological traits and cultural abilities. Fourth, the smaller amount of modern mtDNA diversity but Africa could result from times when European and Asian populations declined in numbers, perhaps due to climate changes.
Despite these criticisms, many geneticists continue to favour the out of Africa hypothesis of modern human origins. Studies of nuclear DNA also suggest an African origin for a variety of genes. Furthermore, in a remarkable series of studies in the late 1990's, scientists recovered mtDNA from the first Neanderthal fossil found in Germany and two other Neanderthal fossils. In each case, the mtDNA does not closely match that of modern humans. This finding suggests that at least some Neanderthal populations had diverged from the line to modern humans by 500,000 to 600,000 years ago. This also suggests that Neanderthals represent a separate species from modern H. sapiens. In another study, however, mtDNA extracted from a 62,000 -year-old Australian H. sapiens fossil was found to differ significantly from modern human mtDNA, suggesting a much wider range of mtDNA variation within H. sapiens than was previously believed. According to the Australian researchers, this finding lends support to the multi-regional hypothesis because it shows that different populations of H. sapiens, possibly including Neanderthals, could have evolved independently in different parts of the world.
As with genetic research, fossil evidence also does not entirely support or refute either of the competing hypotheses of modern human origins. However, many scientists see the balance of evidence favouring an African origin of modern H. sapiens within the past 200,000 years. The oldest known modern-looking skulls come from Africa and date from perhaps 130,000 years ago. The next oldest comes from the Near East, where they date from about 90,000 years ago. Fossils of modern humans in Europe do not exist in advancing their precedence, in as much as lacking generative qualities that extend no further than 40,000 years ago. In addition, the first modern humans in Europe-often called Cro-Magnon people -had elongated lower leg bones, as did African populations adapted too warm, tropical climates. This suggests that populations from warmer regions replaced those in colder European regions, such as the Neanderthals.
Fossils also show that populations of modern humans lived when and in the same regions as did populations of Neanderthals and Homo erectus, but that each retained its distinctive physical features. The different groups overlapped in the Near East and Southeast Asia for between about 30,000 and 50,000 years. The maintenance of physical differences for this amount of time implies that archaically and modern humans could either not or generally did not interbreed. To some scientists, this also means that the Neanderthals belong to a separate species, H. neanderthalensis, and that migratory populations of modern humans entirely replaced archaic humans in both Europe and eastern Asia.
On the other hand, fossils of archaic and modern humans in some regions show continuity in certain physical characteristics. These similarities may indicate multi-regional evolution. For example, both archaic and modern skulls of eastern Asia have flatter cheek and nasal areas than do skulls from other regions. By contrast, the same parts of the face project forward in the skulls of both archaic and modern humans of Europe. If these traits were influenced primarily by genetic inheritance rather than environmental factors, archaic humans may have produced modern humans in some regions or at least interbred with migrant modern-looking humans.
Each of the competing major hypotheses of modern human origins has its strengths and weaknesses. Genetic evidence appears to support the out of Africa hypothesis. In the western half of Eurasia and in Africa, this hypothesis also seems the better explanation, particularly as for the apparent replacement of Neanderthals by modern populations. Also, the multi-regional hypothesis appears to explain some of the regional continuity found in East Asian populations.
Therefore, many paleoanthropologists advocate a theory of modern human origins that combine elements of the out of Africa and the multi-regional hypotheses. Humans with modern features may have initiatively emerged in Africa or come together there as a result of gene flow with populations from other regions. These African populations may then have replaced archaic humans in certain regions, such as western Europe and the Near East. Still, elsewhere,-especially in East Asia-gene flow may have occurred among local populations of archaic and modern humans, resulting in distinct and enduring regional characteristics.
All three of these views,-the two competing positions and the compromiser acknowledge the strong biological unity of all people. In the multi-regional hypothesis, this unity results from hundreds of thousands of years of continued gene flow among all human populations. According to the out of Africa hypothesis, on the other hand, similarities among all living human populations result from a recent common origin. The compromise position accepts both as reasonable and compatible explanations of modern human origins.
The story of human evolution is as much about the development of cultural behaviour as it is about changes in physical appearance. The term culture, in anthropology, traditionally refers to all human creations and activities governed by social customs and rules. It includes elements such as technology, language, and art. Human cultural behaviour depends on the social transfer of information from one generation to the next, which it depends on a sophisticated system of communication, such as language.
The term culture has often been used to distinguish the behaviour of humans from that of other animals. However, some nonhuman animals also appear to have forms of learned cultural behaviours. For instance, different groups of chimpanzees use different techniques to capture termites for food using sticks. Also, in some regions chimps use stones or pieces of wood for cracking open nuts. Chimps in other regions do not practice this behaviour, although their forests have similar nut trees and materials for making tools. These regional differences resemble traditions that people pass from generation to generation. Traditions are a fundamental aspect of culture, and paleoanthropologists assume that the earliest humans also had some types of traditions.
However, modern humans differ from other animals, and probably many early human species. In that, they actively teach each other and are able to pass on an accumulative amounts of resulting knowledge. People also have a uniquely long period of learning before adulthood, and the physical and mental capacity for language. Language of all forms spoken, signed, and written,-provides a medium for communicating vast amounts of information, much more than any other animal could probably transmit through gestures and vocalizations.
Scientists have traced the evolution of human cultural behaviour through the study of archaeological artifacts, such as tools, and related evidence, such as the charred remains of cooked food. Artifacts show that throughout much of human evolution, culture has developed slowly. During the Palaeolithic, or early Stone Age, basic techniques for making stone tools changed very little for periods of well more than a million years.
Human fossils also provide information about how culture has evolved and what effects it has had on human life. For example, over the past 30,000 years, the basic anatomy of humans has undergone only one prominent change: The bones of the average human skeleton have become much smaller and thinner. Innovations in the making and usage of tools and their obtaining food:  results of cultural evolution may have led to more efficient and less physically taxing lifestyles, and thus caused changes in the skeleton.
Culture has played a prominent role in the evolution of Homo sapiens. Within the last 60,000 years, people have migrated to settle most unoccupied regions of the world, such as small island chains and the continents of Australia and the Americas. These migrations depended on developments in transportation, hunting and fishing tools, shelter, and clothing. Within the past 30,000 years, cultural evolution has sped up dramatically. This change shows up in the archaeological record as a rapid expansion of stone tool types and Toolmaking techniques, and in works of art and indications of evolving religion, such as burials. By 10,000 years ago, people first began to harvest and cultivate grains and to domesticate animals-a fundamental change in the ecological relationship between human beings and other life on Earth. The development of agriculture gave people larger quantities and more stable supplies of food, which set the stage for the rise of the first civilizations. Today, culture and particularly technology dominates human life.
Paleoanthropologists and archaeologists have studied many topics in the evolution of human cultural behaviour. These have included the evolution of (1) social life; (2) subsistence (the acquisition and production of food); (3) the making and using of tools; (4) environmental adaptation; (5) symbolic thought and its expression through language, art, and religion; and (6) the development of agriculture and the rise of civilizations.
Most primate species, including the African apes, live in social groups of varying size and complexity. Within their groups, individuals often have multifaceted roles, based on age, sex, status, social skills, and personality. The discovery in 1975 at Hadar, Ethiopia, of a group of several Australopithecus afarensis individuals who died together 3.2 million years ago appears to confirm that early humans lived in social groups. Scientists have referred to this collection of fossils as The First Family.
One of the first physicals changes in the evolution of humans from apes-a decrease in the size of male canine teeth-also, indicating a change in social relations. Male apes sometimes use their large canines to threaten (or sometimes fight with) other males of their species, usually over access to females, territory, or food. The evolution of small canines in Australopiths implies that males had either developed other methods of threatening each other or become more cooperative. In addition, both male and female Australopiths had small canines, indicating a reduction of sexual dimorphism from that in apes. Yet, although sexual dimorphism in canine size decreased in Australopiths, males were still much larger than females. Thus, male Australopiths might have competed aggressively with each other based on sheer size and strength, and the social life of humans may not have differed much from that of apes until later times.
Scientists believe that several of the most important changes from apelike to characteristically human social life occurred in species of the genus Homo, whose members show even less sexual dimorphism. These changes, which may have occurred at different times, included, (1) prolonged maturation of infants, including an extended period during which they required intensive care from their parents; (2) special bonds of sharing and exclusive mating between particular males and females, called pair-bonding; and (3) the focus of social activity at a home base, a safe refuge in a special location known to family or group members.
Humans, who have a large brain, has a prolonged period of infant development and childhood because the brain takes a long time too mature. Since the Australopiths brain was not much larger than that of a chimp, some scientists think that the earliest humans had a more apelike rate of growth, which is far more rapid than that of modern humans. This view is supported by studies of Australopiths fossils looking at tooth development-a good indicator of overall body development.
In addition, the human brain becomes very large as it develops, so a woman must give birth at an early stage of development in order for the infant's head to fit through her birth canal. Thus, human babies require a long period of care to reach a stage of development at which they depend less on their parents. In contrast with a modern female, a female Australopiths could give birth to a baby at an advanced stage of development because its brain would not be too large to pass through the birth canal. The need to give birth early-and therefore to provide more infant care,-may have evolved around the time of the middle Homo's species Homo's ergaster. This species had a brain significantly larger than that of the Australopiths, but a narrow birth canal.
Pair-bonding, usually of a short duration, occurs in a variety of primate species. Some scientists speculate that prolonged bonds developed in humans along with increased sharing of food. Among primates, humans have a distinct type of food-sharing behaviour. People will delay eating food until they have returned with it to the location of other members of their social group. This type of food sharing may have arisen at the same time as the need for intensive infant care, probably by the time of H. ergaster. By devoting himself to a particular female and sharing food with her, a male could increase the chances of survival for his own offspring.
Humans have lived as foragers for millions of years. Foragers obtain food when and where it is available over a broad territory. Modern-day foragers (also known as hunter-gatherers)-such as the San people in the Kalahari Desert of southern Africa,-also set up central campsites, or home bases, and divide work duties between men and women. Women gather readily available plant and animal foods, while men take on the often less successful task of hunting. Female and male family members and relatives bring together their food to share at their home base. The modern form of the home base-that also serves as a haven for raising children and caring for the sick and elderly-may have first developed with middle Homo after about 1.7 million years ago. However, the first evidence of hearths and shelters common to all modern home bases,-comes from only after 500,000 years ago. Thus, a modern form of social life may not have developed until late in human evolution.
Human subsistence refers to the types of food humans eat, the technology used in and methods of obtaining or producing food, and the ways in which social groups or societies organize them for getting, making, and distributing food. For millions of years, humans probably fed on-the-go, much as other primates do. The lifestyle associated with this feeding strategy is generally organized around small, family-based social groups that take advantage of different food sources at different times of year.
The early human diet probably resembled that of closely related primate species. The great apes eat mostly plant foods. Many primates also eat easily obtained animal foods such as insects and bird eggs. Among the few primates that hunt, chimpanzees will prey on monkeys and even small gazelles. The first humans probably also had a diet based mostly on plant foods. In addition, they undoubtedly ate some animal foods and might have done some hunting. Human subsistence began to diverge from that of other primates with the production and use of the first stone tools. With this development, the meat and marrow (the inner, fat-rich tissue of bones) of large mammals became a part of the human diet. Thus, with the advent of stone tools, the diet of early humans became distinguished in an important way from that of apes.
Scientists have found broken and butchered fossil bones of antelopes, zebras, and other comparably sized animals at the oldest archaeological sites, which go of a date from some 2.5 million years ago. With the evolution of late Homo, humans began to hunt even the largest animals on Earth, including mastodons and mammoths, members of the elephant family. Agriculture and the  of animals arose only in the recent past, with H. sapiens.
Paleoanthropologists have debated whether early members of the modern human genus were aggressive hunters, peaceful plant gatherers, or opportunistic scavengers. Many scientists once thought that predation and the eating of meat had strong effects on early human evolution. This hunting hypothesis suggested that early humans in Africa survived particularly arid periods by aggressively hunting animals with primitive stone or bone tools. Supporters of this hypothesis thought that hunting and competition with carnivores powerfully influenced the evolution of human social organization and behaviour; Toolmaking; anatomy, such as the unique structure of the human hand; and intelligence.
Beginning in the 1960's, studies of apes cast doubt on the hunting hypothesis. Researchers discovered that chimpanzees cooperate in hunts of at least small animals, such as monkeys. Hunting did not, therefore, entirely distinguish early humans from apes, and therefore hunting alone may not have determined the path of early human evolution. Some scientists instead argued in favour of the importance of food-sharing in early human life. According to a food-sharing hypothesis, cooperation and sharing within family groups-instead of aggressive hunting-strongly influenced the path of human evolution.
Scientists once thought that archaeological sites as much as two million years old provided evidence to support the food-sharing hypothesis. Some of the oldest archaeological sites were places where humans brought food and stone tools together. Scientists thought that these sites represented home bases, with many social features of modern hunter-gatherers campsites, including the sharing of food between pair-bonded males and females.
Critique of the food-sharing hypothesis resulted from more careful study of animal bones from the early archaeological sites. Microscopic analysis of these bones revealed the marks of human tools and carnivore teeth, indicating that both humans and potential predators,-such as, hyenas, cats, and jackals-were active at these sites. This evidence suggested that what scientists had thought were home bases where early humans shared food were in fact food-processing sites that humans abandoned to predators. Thus, evidence did not clearly support the idea of food-sharing among early humans.
The new research also suggested a different view of early human subsistence-that early humans scavenged meat and bone marrow from dead animals and did little hunting. According to this scavenging hypothesis, early humans opportunistically took parts of animal carcasses left by predators, and then used stone tools to remove marrow from the bones.
Observations that many animals, such as antelope, often die off in the dry season make the scavenging hypothesis quite plausible. Early Toolmaker would have had plenty of opportunity to scavenge animal fat and meat during dry times of the year. However, other archaeological studies,-and a better appreciation of the importance of hunting among chimpanzees suggests that the scavenging hypothesis be too narrow. Many scientists now believe that early humans both scavenged and hunted. Evidence of carnivore tooth marks on bones cut by early human Toolmaker suggests that the humans scavenged at least the larger of the animals they ate. They also ate a variety of plant foods. Some disagreement remains, however, about how much early humans relied on hunting, especially the hunting of smaller animals.
Scientists debate when humans first began hunting on a regular basis. For instance, elephant fossils were made-known to be found existent with tools made by Middle Homo once led researchers to the idea that members of this species were hunters of big game. However, the simple association of animal bones and tools at the same site does not necessarily mean that early humans had killed the animals or eaten their meat. Animals may die in many ways, and natural forces can accidentally place fossils next to tools. Recent excavations at Olorgesailie, Kenya, show that H. erectus cut meat from elephant carcasses but do not reveal whether these humans were regular or specialized hunters
Humans who lived outside Africa,-especially in colder temperate climates almost needed to eat more meat than their African counterparts. Humans in temperate Eurasia would have had to learn about which plants they could safely eat, and the number of available plant foods would drop significantly during the winter. Still, although scientists have found very few fossils of edible or eaten plants at early human sites, early inhabitants of Europe and Asia probably did eat plant foods besides meat.
Sites that provide the clearest evidence of early hunting include Boxgrove, England, where about 500,000 years ago people trapped several large game animals between a watering hole and the side of a cliff and then slaughtered them. At Schningen, Germany, a site about 400,000 years old, scientists have found wooden spears with sharp ends that were well designed for throwing and probably used in hunting large animals.
Neanderthals and other archaic humans seem to have eaten whatever animals were available at a particular time and place. So, for example, in European Neanderthal sites, the number of bones of reindeer (a cold-weather animal) and red deer (a warm-weather animal) changed depending on what the climate had been like. Neanderthals probably also combined hunting and scavenging to obtain animal protein and fat.
For at least the past 100,000 years, various human groups have eaten foods from the ocean or coast, such as shellfish and some sea mammals and birds. Others began fishing in interior rivers and lakes. Between probably 90,000 and 80,000 years ago people in Katanda, in what is now the Democratic Republic of the Congo, caught large catfish using a set of barbed bone points, the oldest known specialized fishing implements. The oldest stone tips for arrows or spears date from about 50,000 to 40,000 years ago. These technological advances, probably first developed by early modern humans, indicate an expansion in the kinds of foods humans could obtain. Beginning 40,000 years ago humans began making even more significant advances in hunting dangerous animals and large herds, and in exploiting ocean resources. People cooperated in large hunting expeditions in which they killed many reindeer, bison, horses, and other animals of the expansive grasslands that existed at that time. In some regions, people became specialists in hunting certain kinds of animals. The familiarity these people had with the animals they hunted appears in sketches and paintings on cave walls, dating from as much as 32,000 years ago. Hunters also used the bones, ivory, and antlers of their prey to create art and beautiful tools. In some areas, such as the central plains of North America that once teemed with a now-extinct type of large bison (Bison occidentalis), hunting may have contributed to the extinction of entire species.
The making and use of tools alone probably did not distinguish early humans from their ape predecessors. Instead, humans made the important breakthrough of using one tool to make another. Specifically, they developed the technique of precisely hitting one stone against another, known as knapping. Stone Toolmaking characterized the period that on give occasion to have to do with the Stone Age, which began at least 2.5 million years ago in Africa and lasted until the development of metal tools within the last 7,000 years (at different times in different parts of the world). Although early humans may have made stone tools before 2.5 million years ago, Toolmaker may not have remained long enough in one spot to leave clusters of tools that an archaeologist would notice today.
The earliest simple form of stone Toolmaking involved breaking and shaping an angular rock by hitting it with a palm-sized round rock known as a hammerstone. Scientists refer to tools made in this way as Oldowan, after Olduvai Gorge in Tanzania, a site from which many such tools have come. The Oldowan tradition lasted for about one million years. Oldowan tools include large stones with a chopping edge, and small, sharp flakes that could be used to scrape and slice. Sometimes Oldowan Toolmaker used anvil stones (flat rocks found or placed on the ground) on which hard fruits or nuts could be broken open. Chimpanzees are known to do this today.
Humans have always adapted to their environments by adjusting their behaviour. For instance, early Australopiths moved both in the trees and on the ground, which probably helped them survive environmental fluctuations between wooded and more open habitats. Early Homo adapted by making stone tools and transporting their food over long distances, thereby increasing the variety and quantities of different foods they could eat. An expanded and flexible diet would have helped these Toolmaker survive unexpected changes in their environment and food supply
When populations of H. erectus moved into the temperate regions of Eurasia, but they faced unseasoned challenges to survival. During the colder seasons they had to either move away or seek shelter, such as in caves. Some of the earliest definitive evidence of cave dwellers dates from around 800,000 years ago at the site of Atapuerca in northern Spain. This site may have been home too early H. heidelbergensis populations. H. erectus also used caves for shelter.
Eventually, early humans learned to control fire and to use it to create warmth, cook food, and protect themselves from other animals. The oldest known fire hearths date from between 450,000 and 300,000 years ago, at sites such as Bilzingsleben, Germany; Verteszöllös, Hungary; and Zhoukoudian (Chou-k ou-tien), China. African sites as old as 1.6 million to 1.2 million years contain burned bones and reddened sediments, but many scientists find such evidence too ambiguous to prove that humans controlled fire. Early populations in Europe and Asia may also have worn animal hides for warmth during glacial periods. The oldest known bone needles, which indicate the development of sewing and tailored clothing, date from about 30,000 to 26,000 years ago.
Behaviour relates directly to the development of the human brain, and particularly the cerebral cortex, the part of the brain that allows abstract thought, beliefs, and expression through language. Humans communicate through the use of symbols-ways of referring to things, ideas, and feelings that communicate meaning from one individual to another but that need not have any direct connection to what they identify. For instance, a word, or utterance is only one type of symbolization, in that of doing or not as the usually related directional thing or, perhaps, as an ideal symbol represents; it is nonrepresentational English-speaking people use the word lion to describe a lion, not because a dangerous feline looks like the letters  I i-o-n, but because these letters together have a meaning created and understood by people.
People can also paint abstract pictures or play pieces of music that evoke emotions or ideas, even though emotions and ideas have no form or sound. In addition, people can conceive of and believe in supernatural beings and powers-abstract concepts that symbolize real-world events such as the creation of Earth and the universe, the weather, and the healing of the sick. Thus, symbolic thought lies at the heart of three hallmarks of modern human culture: language, art, and religion.
In language, people creatively join words together in an endless variety of sentences, hopefully graduating to phrases and perhaps, the paragraphs and lastly with the grandiosities fulfilled in writing a book. Each set-category has a distinct meaning as accorded to its set-classification by mental rules, or grammar. Language provides the ability to communicate complex concepts. It also allows people to exchange information about both past and future events, about objects that are not present, and about complex philosophical or technical concepts
Language gives people many adaptive advantages, including the ability to plan, to communicate the location of food or dangers to other members of a social group, and to tell stories that unify a group, such as mythologies and histories. However, words, sentences, and languages cannot be preserved like bones or tools, so the evolution of language is one of the most difficult topics to investigate through scientific study.
It appears that modern humans have an inborn instinct for language. Under normal conditions not developing language is almost impossible for a person, and people everywhere go through the same stages of increasing language skill at about the same ages. While people appear to have inborn genetic information for developing language, they learn specific languages based on the cultures from which they come and the experiences they have in life.
The ability of humans to have language depends on the complex structure of the modern brain, which has many interconnected, specific areas dedicated to the development and control of language. The complexity of the brain structures necessary for language suggests that it probably took a long time to evolve. While paleoanthropologists would like to know when these important parts of the brain evolved, endocasts (inside impressions) of early human skulls do not provide enough detail to show this.
Some scientists think that even the early Australopiths had some ability to understand and use symbols. Support for this view comes from studies with chimpanzees. A few chimps and other apes have been taught to use picture symbols or American Sign Language for simple communication. Nevertheless, it appears that language-as well as art and religious ritual-became vital aspects of human life only during the past 100,000 years, primarily within our own species.
Humans also express symbolic thought through many forms of art, including painting, sculpture, and music. The oldest known object of possible symbolic and artistic value dates from about 250,000 years ago and comes from the site of Berekhat Ram, Israel. Scientists have interpreted this object, a figure carved into a small piece of volcanic rock, as a representation of the outline of a female body. Only a few other possible art objects are known from between 200,000 and 50,000 years ago. These items, from western Europe and usually attributed to Neanderthals, include two simple pendants-a tooth and a bone with bored holes and several grooved or polished fragments of tooth and bone.
Sites dating from at least 400,000 years ago contain fragments of red and black pigment. Humans might have used these pigments to decorate bodies or perishable items, such as wooden tools or clothing of animal hides, but this evidence would not have survived to today. Solid evidence of the sophisticated use of pigments for symbolic purposes-such as in religious rituals,-comes only from after 40,000 years ago. From early in this period, researchers have found carefully made types of crayons used in painting and evidence that humans burned pigments to create a range of colours.
People began to create and use advanced types of symbolic objects between about 50,000 and 30,000 years ago. Much of this art appears to have been used in rituals-possibly ceremonies to ask spirit beings for a successful hunt. The archaeological record shows a tremendous blossoming of art between 30,000 and 15,000 years ago. During this period people adorned themselves with intricate jewellery of ivory, bone, and stone. They carved beautiful figurines representing animals and human forms. Many carvings, sculptures, and paintings depict stylized images of the female body. Some scientists think such female figurines represent fertility.
Early wall paintings made sophisticated use of texture and colour. The area of what is now. Southern France contains many famous sites of such paintings. These include the caves of Chauvet, which contain art more than 30,000 years old, and Lascaux, in which paintings date from as much as 18,000 years ago. In some cases, artists painted on walls that can be reached only with special effort, such as by crawling. The act of getting to these paintings gives them a sense of mystery and ritual, as it must have to the people who originally viewed them, and archaeologists refer to some of the most extraordinary painted chambers as sanctuaries. Yet no one knows for sure what meanings these early paintings and engravings had for the people who made them.
Graves from Europe and western Asia indicate that the Neanderthals were the first humans to bury their dead. Some sites contain very shallow graves, which group or family members may have dug simply to remove corpses from sight. In other cases it appears that groups may have observed rituals of grieving for the dead or communicating with spirits. Some researchers have claimed that grave goods, such as meaty animal bones or flowers, had been placed with buried bodies, suggesting that some Neanderthal groups might have believed in an afterlife. In a large proportion of Neanderthal burials, the corpse had its legs and arms drawn in close to its chest, which could indicate a ritual burial position.
Other researchers have challenged these interpretations, however. They suggest that perhaps the Neanderthals had practically rather than religious reasons for positioning dead bodies. For instance, a body manipulated into a fetal position would need only a small hole for burial, making the job of digging a grave easier. In addition, the animal bones and flower pollen near corpses could have been deposited by accident or without religious intention.
Many scientists once thought that fossilized bones of cave bears (a now-extinct species of large bear) found in Neanderthal caves indicated that these people had what has been referred to as a cave bear cult, in which they worshipped the bears as powerful spirits. However, after careful study researchers concluded that the cave bears probably died while hibernating and that Neanderthals did not collect their bones or worship them. Considering current evidence, the case for religion among Neanderthal prevails upon disputatiousness.
One of the most important developments in human cultural behaviours occurred when people began to domesticate (control the breeding of) plants and animals.  and the advent of agriculture led to the development of dozens of staple crops (foods that forms the basis of an entire diet) in temperate and tropical regions around the world. Almost the entire population of the world today depends on just four of these major crops: wheat, rice, corn, and potatoes.
The growth of farming and animal herding initiated one of the most remarkable changes ever in the relationship between humans and the natural environment. The change first began just 10,000 years ago in the Near East and has accelerated very rapidly since then. It also occurred independently in other places, including areas of Mexico, China, and South America. Since the first of plants and animals, many species over large areas of the planet have come under human control. The overall number of plant and animal species has decreased, while the populations of a few species needed to support large human populations have grown immensely. In areas dominated by people, interactions between plants and animals usually fall under the control of a single species-Homo sapiens.
By the time of the initial transition to plant and animal, the cold, glacial landscapes of 18,000 years ago had long since given way to warmer and wetter environments. At first, people adapted to these changes by using a wider range of natural resources. Later they began to focus on a few of the most abundant and hardy types of plants and animals. The plant's people began to use in large quantities included cereal grains, such as wheat in western Asia; wild varieties of rice in eastern Asia; and maize, of which corn is one variety, in what is now Mexico. Some of the animals people first began to herd included wild goats in western Asia, wild ancestors of chickens in eastern Asia, and llamas in South America.
By carefully collecting plants and controlling wild herd animals, people encouraged the development of species with characteristics favourable for growing, herding, and eating. This process of selecting certain species and controlling their breeding eventually created new species of plants, such as oats, barley, and potatoes, eatable animals, including cattle, sheep, and pigs. From these domesticated plant and animal species, people obtained important products, such as flour, milk, and wool.
By harvesting and herding domesticated species, people could store large quantities of plant foods, such as seeds and tubers, and have a ready supply of meat and milk. These readily available supplies gave people an abounding overindulgence-designate with a term food security. In contrast, the foraging lifestyle of earlier human populations never provided them with a significant store of food. With increased food supplies, agricultural peoples could settle into villages and have more children. The new reliance on agriculture and change to settled village life also had some negative effects. As the average diet became more dependent on large quantities of one or a few staple crops, people became more susceptible to diseases brought on by a lack of certain nutrients. A settled lifestyle also increased contact between people and between people and their refuse and waste matter, both of which acted to increase the incidence and transmission of disease.
People responded to the increasing population density-and a resulting overuse of farming and grazing lands-in several ways. Some people moved to settle entirely new regions. Others devised ways of producing food in larger quantities and more quickly. The simplest way was to expand onto new fields for planting and new pastures to support growing herds of livestock. Many populations also developed systems of irrigation and fertilization that allowed them to reuse crop-land and to produce greater amounts of food on existing fields.
The rise of civilizations-the large and complex types of societies in which most people still live today-developed along with surplus food production. People of high status eventually used food surpluses as a way to pay for labour and to create alliances among groups, often against other groups. In this way, large villages could grow into city-states (urban centres that governed them) and eventually empires covering vast territories. With surplus food production, many people could work exclusively in political, religious, or military positions, or in artistic and various skilled vocations. Command of food surpluses also enabled rulers to control labourers, such as in slavery. All civilizations developed based on such hierarchical divisions of status and vocation.
The earliest civilization arose more than 7,000 years ago in Sumer in what is now Iraq. Sumer grew powerful and prosperous by 5,000 years ago, when it centred on the city-state of Ur. The region containing Sumer, known as Mesopotamia, was the same area in which people had first domesticated animals and plants. Other centres of early civilizations include the Nile Valley of Northeast Africa, the Indus. Valley of South Asia, the Yellow River Valley of East Asia, the Oaxaca and Mexico valleys and the Yucatán region of Central America, and the Andean region of South America, China and Inca Empire
All early civilizations had some common features. Some of these included a bureaucratic political body, the military, a body of religious leadership, large urban centres, monumental buildings and other works of architecture, networks of trade, and food surpluses created through extensive systems of farming. Many early civilizations also had systems of writing, numbers and mathematics, and astronomy (with calendars); road systems; a formalized body of law; and facilities for education and the punishment of crimes. With the rise of civilizations, human evolution entered a phase vastly different from all before which came. Before this time, humans had lived in small, family-centred groups essentially exposed to and controlled by forces of nature. Several thousand years after the rise of the first civilizations, most people now live in societies of millions of unrelated people, all separated from the natural environment by houses, buildings, automobiles, and numerous other inventions and technologies. Culture will continue to evolve quickly and in unforeseen directions, and these changes will, in turn, influence the physical evolution of Homo sapiens and any other human species to come.
During the fist two billion years of evolution, bacteria were the sole inhabitants of the earth, and the emergence of a more complex form is associated with networking and symbiosis. During these two billion years, prokaryote, or organisms composed of cells with no nucleus (namely bacteria), transformed he earth's surface and atmosphere. It was the interaction of these simple organisms that resulted in te complex processes of fermentation, photosynthesis, oxygen breathing, and the removal of nitrogen gas from the air. Such processes would not have evolved, however, if these organisms were atomized in the Darwinian sense or if the force of interaction between parts existed only outside the parts.
In the life of bacteria, bits of genetic material within organisms are routinely and rapidly transferred to other organisms. At any given time, an individual bacteria have the use of accessory gene, often from very different strains, which execute unprepared functions are not carried through by its own DNA. Some of this genetic material can be incorporated into the DNA of the bacterium and some may be passed on to other bacteria. What this picture indicates, as Margulis and Sagan put it, is that "all the worlds' bacteria have access to a single gene pool and hence to the adaptive mechanisms of the entire bacterial kingdom."
Since the whole of this gene pool operates in some sense within the parts, the speed of recombination is much greater than that allowed by mutation alone, or by random changes inside parts that alter interaction between parts. The existence of the whole within parts explains why bacteria can accommodate change on a worldwide cale in a few years. If the only mechanism at work were mutation inside organisms, millions of years would require for bacteria to adapt to a global change in the conditions for survival. "By constantly and rapidly adapting to environmental conditions," wrote Margukis and Sagan, "the organisms of the microcosm support the entire biota, their global exchange network ultimately affecting every living plant and animal."
The discovery of symbiotic alliance between organisms that become permanent is other aspect of the modern understanding of evolution that appears to challenge Darwin's view of universal struggle between atomized individual organisms. For example, the mitochondria fond outside the nucleus of modern cells allows the cell to utilize oxygen and to exist in an oxygen-rich environment. Although mitochondria enacts upon integral and essential functions in the life of the cell, they have their own genes composed of DNA, reproduced by simple division, and did so at time different from the rest of the cells.
The most reasonable explanation for this extraordinary alliance between mitochondria and the rest of the cell that oxygen-breathing bacteria in primeval seas combined with the organisms. These ancestors of modern mitochondria provided waste disposal and oxygen-derived energy in exchange for food and shelter and evolved via symbiosis more complex forms of oxygen-breathing life, since the whole of these organisms was lager than the sum of their symbiotic pats, this allowed for life functions that could not be carried to completion by the mere collection of pasts. The existence of the whole within the parts coordinates metabolic functions and overall organization
Awaiting upon the unformidable future, of which the future has framed its proposed modern understanding of the relationship between mind and world within the larger content of the history of mathematical physics, the origin and extensions of the classical view of the functional preliminaries in association with scientific knowledge, and the various ways that physics has attempted to prevent previous challenges to the efficacy of classical epistemology. There is no basis in contemporary physics or biology for believing in the stark Cartesian division between mind and world that some have moderately described as ‘the disease of the Western mind.' The dialectic orchestrations will serve as background for understanding a new relationship between parts and wholes in physics, with a similar view of that relationship that has emerged in the so-called ‘new-biology' and in recent studies of the evolution of a scientific understanding to a more conceptualized representation of ideas, and includes its ally ‘content'.
Recent studies on the manner in which the brains of our ancestors evolved the capacity to acquire and use complex language systems also present us with a new view of the relationship between parts and wholes in the evolution of human consciousness. These studies suggest that cognitive narrations cannot fully explain the experience of consciousness about the physical substrates of consciousness, or that the whole that corresponds with any moment of conscious awareness is an emergent phenomenon that a stable and cohering cognizance cannot fully explain as to the sum of its constituent parts. This also suggests that the pre-adaptive change in the hominid brain that enhanced the capacity to use symbolic communication over a period of 2.5 million years cannot be fully explained as to the usual dynamics of Darwinian evolution.
Recent studies on the manner in which the brains of our ancestors evolved the capacity to acquire and use complex language systems also present us with a new view of the relationship between parts and wholes in the evolution of human consciousness. These studies suggest that the experience of consciousness cannot be fully explained through the physical substrates of consciousness, or that the whole that corresponds with any moment of conscious awareness is an emergent phenomenon that cannot be fully explained as to the sum of its constituent parts. This also suggests that the pre-adaptive change in the hominid brain that enhanced the capacity to use symbolic communication over a period of 2.5 million years cannot be fully explained as for the usual dynamics of Darwinian evolution
Part and wholes in Darwinian theory cannot reveal the actual character of living organisms because that organism exists only in relation to the whole of biological life. What Darwin did not anticipate, however, is that the whole that is a living organism appears to exist in some sense within the parts, and that more complex life forms evolved in precesses in which synergy and cooperation between parts (organisms) result in new wholes (more complex of parts) withe emergent properties that do not exist in the collection of parts. More remarkable, this new understanding of the relationship between part and whole in biology seems very analogous to the disclosed by the discovery of non-locality in physics. We should stress, however, that this view of the relationship between parts and wholes in biologic reality is most orthodox and may occasion some controversy in the community of biological scientists.
Since Darwin's understanding of the relations between part and whole was essentially classical and mechanistic, the new understanding of this relationship is occasioning some revising of his theory of evolution. Darwin made his theory public for the first time in a paper derived to the Linnean Society in 1858. The paper began, ‘All nature hidden, extorted by its adhering agenda embedded to the primitivity of its shaken hostilities as once founded imbedded within the organisms of one another, or with other congestive appetites that gives to the characterology by some externalized nature. In the Origins of Species, Darwin speaks more specifically about the charter of this war: "There must be in every case a struggle for existence one individual either with another of the same species, or with the individual with another of the same species, and still, with the individuals of distinct species, or with physical condition of life." All these assumptions are apparent in Darwin's definition of natural selection: If under chancing conditions of life organic brings present individual differences in almost every part of their structure, and that all construing metabolisms cannot dispute this: If there be, owing to their geometrical rate of an increase, a severe struggle for life to some age, season, or year, and this certainty can then, be considered the infinite complexity of the relating of all organic being to each other and to their conditions of life causing an infinite diversity in structure, constitution, habits, to be advantageous as  those that it would be most extraordinary fact if no variations had ever occurred usefully to each being' own welfare. Nevertheless, in the variations useful any organic being ever d occurred, absurdly individuals thus characterized will have the best chance of being preserved in the struggle for life, and from the strong principle of inheritances, that often have a tendency to produce offsprings similarly characterized. Thus  the principle of preservation, of resembling the survival of the fittest-is called Natural Selection.
Based on the assumption that the study of variation in domestic animals and plants, ‘afforded the best and safest clue' to understanding evolution. Unforeseeably, the humans who domesticated animals were the first to fall victim to the newly evolved germs, but those humans then evolved substantial resistance to the new diseases. When such partly immune people came into contact with others who had no previous exposure to the germ, epidemics resulted in which up to 99 percent of the previously unexposed population was killed. Germs thus acquired ultimately from domestic animals played decisive roles in the European conquests of Native Americans, Australians, South Africans, and Pacific islanders.
Yet as before, the same pattern repeated itself elsewhere in the world, whenever peoples lacking native wild mammal species suitable for finally had the opportunity to acquire by Native Americans in both North and South America, within a generation of the escape of horses from Europe settlements. For example, by the 19th century North America's Great Plain Indians were famous as expert horse-mounted warriors and bison hunters, but they did not even obtain horses until the late 17th century. Sheep acquired from Spaniards similarly transformed Navajo Indian society and led to, among other things, the weaving of the beautiful woolen blankets for which the Navajo have become renowned. Within a decade of Tasmania's settlement by Europeans with dogs, Aboriginal Tasmanian's who had never before seen dogs, began to breed them in large numbers for use in hunting. Thus, among the thousands of culturally diverse native peoples of Australia. The America, and Africa, no universal cultural taboo stood in the way of animal.
Surely, if some local wild mammal species of those continents had been domesticable, some Australian, American, and African peoples would have domesticated them and gained great advantage from them, just as they benefited from the Eurasian domestic animals that they immediately adopted when those became available. For instance, consider all the peoples of sub-Saharan Africa living within the range of wild zebras and buffalo. Why wasn't there at least on African hunter-gatherer tribe that domesticated those zebras and buffalo and that thereby gained sway over other Africans, without having to await the arrival of Eurasian horses and cattle? All these facts show that the explanation for the lack of native mammal  outside Eurasia lay with the locally available wild mammals themselves, nor with the local people.
To the point, evidence for the same interpretation comes from pets. Keeping wild animals as pets, and taming them. Constitute an initial stage in. However, pets have been reported from virtually all traditional human societies on all continents. The variety of wild animals thus tamed is far grater than the variety eventually domesticated, and includes some species that we would scarcely have imagined as pets.
Given our proximity to the animals we love, we must be getting constantly bombarded by their microbes. Those invaders get winnowed by natural selection, and only a few of them succeed in establishing themselves as human diseases.
The first stage is illustrated by dozens of diseases that we now and then pick up directly from our pets and domestic animals. They include cat-scratch fever from our cats, leptospirosis from our dogs, psittacosis from our chickens and parrots, and brucellosis from our cattle. We're similarly liable to pick up diseases from wild animals, such as the tularaemia that hunters can get from skinning wild rabbits. All those microbes are still at an early stage in their evolution into specialized human pathogens. They still don't get transmitted directly from one person to another, and even their transfer to us from animals remain uncommon.
In the second stage a former animal pathogen evolves to the point where it does get transmitted directly between people and causes epidemics. However, the epidemic dies out for any of several reasons, such for being cured by modern medicine, or being stopped when everybody around has already been infected and either becomes immune or dies. For example, a previously unknown fever termed O'nyong-nyong fever appeared in East Africa in 1959 and proceeded to infect several million Africans. It probably arose from a virus of monkeys and was transmitted humans by mosquitoes. The fact that patients recovered quickly and became immune too further attack helped the new disease die out quickly. Closer to home for Americans, Fort Gragg fever was the name applied to a new leptospiral disease that broke out in the United States an the summer of 1942 and soon disappeared.
A third stage in the evolution of our major diseases is represented by former animal pathogens that did establish themselves in humans, whom have not (not yet?) died out, and that may or may not still become major killers of humanity. The future remains very uncertain for Lassa fever, caused by a virus derived probably from rodents. Lassa fevers were first observed in 1969 in Nigeria, were it causes a fatal illness so contagious that Nigerian hospitals have been closed down if even a single case appears. Better established is Lyme disease, caused by a spirochete that we get from the bite of ticks carried by mice and deer. Although the first known human cases in the United States appeared only as recently as 1962, Lyme disease is already reaching epidemic proportions in many parts of our country. the future of AIDS, derived from monkey viruses and first documented in humans around 1959, is even secure (from the virus's perspective).
The final stage of this evolution is represented by the major, long-established epidemic diseases confined to humans. These diseases must have been the evolutionary survivors of far more pathogens that tried to make the jump to us from animal, and mostly failed.
In short, diseases represent evolution in progress, and microbes adapt by natural selection to new hosts and vectors. Nonetheless, compared with cows' bodies, ours offer different immune defences, lice, faeces, and chemistries. In that new environment, a microbe must evolve new ways to live and to propagate itself. In several instructive cases doctors or veterinarians have been able to observe microbes evolving those new ways.
Darwin concluded that nature could by crossbreeding and selection of traits, provide new species. His explanation of the mechanism in nature that results in a new specie took the form of a syllogism: (1) the principle of geometric  increases indicated that more individuals in each species will have produced than can survive, (2) the struggle for existence occurs as one organism competes with another, (3) in this struggle for existence, slight variations, if they prove advantageous will accumulate to produce new species, in analogy with the animal breeder's artificial selection of traits Darwin termed the elimination of the disadvantaged and the promotion of the advantaged natural selection.
In Darwin's view, the struggle for existence occurs ‘between' an atomized individual organism and of the atomized individual organisms in the same species: ‘between' and ‘atomized' individual organisms of new species with that of a different species, or ‘between' an atomized individual organism and the physical conditions of life the whole as Darwin conceived it is the collection of all atomized individual organisms, or parts. The struggle for survival occurs ‘between' or ‘outside' the parts. Since Darwin's viewing this struggle as the only limiting condition in which the accountable rate of an increase in organises, he assumed that rate will be geometrical when the force of a struggle between parts is weak and that the rate will decline with the force becomes stronger.
Natural selection occurred, said Darwin, when variations applicatively form; as each being accountable for through his own welfare,' or useful to the welfare of an atomized individual organism, provides a survival advantage and the organism produces ‘offspring similarly characterized.' Since the force that makes this selection operates ‘outside' the totality of parts. For example, the ‘infinite complexities of relations of all organic beings to each other and to their condition of liveliness' refers to dealing relations between parts, and the ‘infinite diversity in structure, constitute habit' refers to remaining traits within the atomized part. It seems clear in our view that the atomized individual organism in Darwin's biological machine reassembles classical atoms and that the force that drives the interactions of the atomized parts, the ‘struggle for life' resembles Newton's force of universal gravity. Although Darwin parted company with classical determinism in the claim that changes, of mutations, within organisms occurred randomly, his  view of the relationship between parts and wholes essentially mechanistic.
Darwinism belief in the theory of ‘evolution' by natural selection took form in its original formality from the observation of Malthus, although belonging principally to the history of science, as these encountering beliefs are met straight on into a  philosophically influenced Malthus's Essay on Population (1798) in undermining the Enlightenment belief in unlimited possibilities of human progress and perfection. The Origin of Species was principally successful in marshalling the evidence for evolution, than providing a convincing mechanism for genetic change; Darwin himself remained open to the search for additional in its mechanisms, while also remaining convinced that naturae section was at the heart of it. It was only with the later discovery of him ‘gene' as the unit of inheritance hast the synthesis known as ‘neo-Darwinism' became the orthodox theory of evolution in life science.
Human Evolution, is pressively the process through which a lengthy period of change is admissively given by people who have originated from apelike ancestors. Scientific evidence shows that the physical and behavioural traits shared by all people evolved over a period of at least six million years.
One of the earliest defining human traits, Bipedalism -walking on two legs as the primary form of locomotion-undergoes an evolution of more than four million years ago. Other important human characteristics-such as a large and complex brain, the ability to make and use tools, and the capacity for language-developed more recently. Many advanced traits,-including complex symbolic expression, such as art, and elaborate cultural diversity emerged mainly during the past 100,000 years.
Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the so-called great apes (large apes) of Africa-chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas,-share a common ancestor that lived sometime between eight million and six million years ago. The earliest humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between six million and two million years ago come entirely from Africa.
Early humans first migrated out of Africa into Asia probably between two million and 1.7 million years ago. They entered Europe so-so later, generally within the past one million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.
The scientific study of human evolution is called Paleoanthropology. Paleoanthropology is a sub-field of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behaviour. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, Paleoanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, as well as the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world came to be. Yet many people have come to reconcile such beliefs with the scientific evidence.
All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring,-that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, and two-part scientific name. In this system, modern humans are classified as Homo sapiens.
The mechanism for evolutionary change resides in genes-the basic units of heredity. Genes affect how the body and behaviour of an organism develop during its life. The information contained within genetical change is a latent process known as mutation. The way particular genes are expressive articulated as they affect the body or behaviour of an organism-can also change. Over time, genetic change can alter a species's overall way of life, such as what it eats, how it grows, and where it can live.
Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population-a group of organisms of the same species that share a particular local habitat. Many factors can favour new adaptations, but changes in the environment often play a role. Ancestral human species adapted to new environments as their genes changed, altering their anatomy (physical body structure), physiology (bodily functions, such as digestion), and behaviour. Over long periods, evolution dramatically transformed humans and their ways of life.
Geneticists estimate that the human line began to diverge from that of the African apes between eight million and five million years ago (paleontologists have dated the earliest human fossils, too, at least, six million years ago). This figure comes from comparing differences in the genetic makeup of humans and apes, and then calculating how long it probably took for those differences to develop. Using similar techniques and comparing the genetic variations among human populations around the world, scientists have calculated that all people may share common genetic ancestors that lived sometime between 290,000 and 130,000 years ago.
Humans belong to the scientific order named Primates, a group of more than 230 species of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. Modern humans, early humans, and other species of primates all have many similarities as well as some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits, as well as the significance of each step in human evolution.
All primates, including humans, share at least part of a set of common characteristics that distinguish them from other mammals. Many of these characteristics evolved as adaptations for life in the trees, the environment in which earlier primates evolved. These include more reliance on sight than smell; overlapping fields of vision, allowing stereoscopic (three-dimensional) sight; limbs and hands adapted for clinging on, leaping from, and swinging on tree trunks and branches; the ability to grasp and manipulate small objects (using fingers with nails instead of claws); large brains in relation to body size; and complex social lives.
The scientific classification of primates reflects evolutionary relationships between individual species and groups of species. Strepsirhines (meaning ‘turned-nosed') primates-of that the living representatives include lemurs, lorises, and other groups of species all commonly known as prosimians-evolved earliest and are the most primitive forms of primates. The earliest monkeys and apes evolved from ancestral haplorhine (meaning ‘simple-nosed') primates, of which the most primitive living representative is the tarsier. Humans evolved from ape ancestors.
Tarsiers have traditionally been grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinct traits, and group the three together. Monkeys, apes, and humans-who share many traits not found in other primates-together make up the suborder Anthropoidea. Apes and humans together make up the super-family bestowed upon Hominoidea, a grouping that emphasizes the close relationship among the species of these two groups.
Strepsirhines are the most primitive types of living primates. The last common ancestors of Strepsirhines and other mammals-creatures similar to tree shrews and classified as Plesiadapiformes-evolved at least sixty-five million years ago. The earliest primates evolved by about fifty-five million years ago, and fossil species similar to lemurs evolved during the Eocene Epoch (about fifty-five million to thirty-eight million years ago). Strepsirhines share all of the basic characteristics of primates, although their brains are not particularly large or complex and they have a more elaborate and sensitive olfactory system (sense of smell) than do other primates  are the only living representatives of a primitive group of primates that ultimately led to monkeys, apes, and humans. Fossil species called omomyids, with some traits similar to those of tarsiers, evolved near the beginning of the Eocene, followed by early tarsier-like primates. While the omomyids and tarsiers are separate evolutionary branches (and there are no living omomyids), they both share features having to do with a reduction in the olfactory system, a trait shared by all haplorhine primates, including humans.
The anthropoid primates are divided into New World (South America, Central America, and the Caribbean Islands) and Old World (Africa and Asia) groups. New World monkeys-such as marmosets, capuchins, and spider monkeys-belong to the infra-order of platyrrhine (broad-nosed) anthropoids. Old World monkeys and apes belong to the infra-order of catarrhine (downward-nosed) anthropoids. Since humans and apes together make up the hominoids, humans are also catarrhine anthropoids.
The first catarrhine primates evolved between fifty million and thirty-three million years ago. Most primate fossils from this period have been found in a region of northern Egypt known as Al fay y~? m (or the Fayum). A primate group known as Propliopithecus, one lineage of which is sometimes called Aegyptopithecus, had primitive catarrhine features-that is, it had many of the basic features that Old World monkeys, apes, and humans share today. Scientists believe, therefore, that Propliopithecus resembles the common ancestor of all later Old World monkeys and apes. Thus, Propliopithecus may also be considered an ancestor or a close relative of an ancestor of humans evolved during the Miocene Epoch (24 million to five million years ago). Among the oldest known hominoids is a group of primates known by its genus name, Proconsul. Species of Proconsul had features that suggest a close link to the common ancestor of apes and humans-for example, the lack of a tail. The species Proconsul heseloni lived in the trees of dense forests in eastern Africa about twenty million years ago. An agile climber, it had the flexible backbone and narrow chest characteristic of monkeys, but also a wide range of movement in the hip and thumb, traits characteristic of apes and humans.
Early in their evolution, the large apes underwent several radiations-periods when new and diverse species branched off from common ancestors. Following Proconsul, the ape genus Afropithecus evolved about eighteen million years ago in Arabia and Africa and diversified into several species. Soon afterward, three other ape genera evolved-Griphopithecus of western Asia about 16.5 million years ago, the earliest ape to have spread from Africa; Kenyapithecus of Africa about fifteen million years ago; moreover, Dryopithecus of Europe exceeds twelve million years ago. Scientists have not yet determined which of these groups of apes may have given rise to the common ancestor of modern African apes and humans.
Scientists do not all agree about the appropriate classification of hominoids. They group the living hominoids into either two or three families: Hylobatidae, Hominidae, and sometimes Pongidae. Hylobatidae consists of the small or so-called lesser apes of Southeast Asia, commonly known as gibbons and siamangs. The Hominidae (hominids) includes humans and, according to some scientists, the great apes. For those who categorize its properties of being only human among the Hominidae, are as yet, unconditionally positioned as out of place, and contained to the great apes, including the orangutans of Southeast Asia, from which belong to the family Pongidae.
In the past only humans were considered to belong to the family Hominidae, and the term hominid referred only to species of humans. Today, however, genetic studies support placing all of the great apes and humans together in this family and the placing of African apes-chimpanzees and gorillas-together with humans at an even lower level, or subfamily.
According to this reasoning, the evolutionary branch of Asian apes leading to orangutans, which separated from the other hominid branches by about thirteen million years ago, belongs to the subfamily Ponginae. The ancestral and living representatives of the African ape and human branches together belong to the subfamily Homininae (sometimes called Hominines). Lastly, the line of early and modern humans belongs to the tribe (classificatory level above genus) Hominini, or hominins.
This order of classification corresponds with the genetic relationships between ape and human species. It groups humans and the African apes together at the same level in which scientists group together, for example, all types of foxes, all buffalo, or all flying squirrels. Within each of these groups, the species are very closely related. However, in the classification of apes and humans the similarities between the name's hominoid, hominid, hominine, and hominin can be confusing. In this article the term early human refers to all species of the human family tree since the divergence from a common ancestor with the African apes. Popular writing often still uses the term hominid to mean the same thing.
About 98.5 percent of the genes in people and chimpanzees are identical, making chimps the closest living biological relatives of humans. This does not mean that humans evolved from chimpanzees, but it does indicate that both species evolved from a common ape ancestor. Orangutans, the great apes of Southeast Asia, differ much more from humans genetically, indicating a more distant evolutionary relationship.
Modern humans have a number of physical characteristics reflective of an ape ancestry. For instance, people have shoulders with a wide range of movement and fingers capable of strong grasping. In apes, these characteristics are highly developed as adaptations for brachiation-swinging form branch to branch in trees. Although humans do not brachiate, the general anatomy from that earlier adaptation remains. Both people and apes also have larger brains and greater cognitive abilities than do most other mammals.
Human social life, too, shares similarities with that of African apes and other primates-such as baboons and rhesus monkeys-that live in large and complex social groups. Group behaviour among chimpanzees, in particular, strongly resembles that of humans. For instance, chimps form long-lasting attachments with each other; participate in social bonding activities, such as grooming, feeding, and hunting; and form strategic coalitions with each other in order to increase their status and power. Early humans also probably had this kind of elaborate social life.
However, modern humans fundamentally differ from apes in many significant ways. For example, as intelligent as apes are, people's brains are much larger and more complex, and people have a unique intellectual capacity and elaborate forms of culture and communication. In addition, only people habitually walk upright, can precisely manipulate very small objects, and have a throat structure that makes speech possible.
By around six million years ago in Africa, an apelike species had evolved with two important traits that distinguished it from apes: (1) small canine, or eye, teeth (teeth next to the four incisors, or front teeth) and (2) Bipedalism, that is walking on two legs as the primary form of locomotion. Scientists refer to these earliest human species as australopithecines, or Australopiths for short. The earliest Australopiths species known today belong to three genera: Sahelanthropus, Orrorin, and Ardipithecus. Other species belong to the genus Australopithecus and, by some classifications, Paranthropus. The name australopithecine translates literally as ‘southern ape,' in reference to South Africa, where the first known Australopiths fossils were found.
The Great Rift Valley, a region in eastern Africa in which past movements in Earth's crust have exposed ancient deposits of fossils, has become famous for its Australopiths finds. Countries in which scientists have found Australopiths fossils include Ethiopia, Tanzania, Kenya, South Africa, and Chad. Thus, Australopiths ranged widely over the African continent.
Fossils from several different early Australopiths species that lived between four million and two million years ago clearly show a variety of adaptations that marks the transition from ape too human. The very early period of this transition, before four million years ago, remains poorly documented in the fossil record, but those fossils that do exist show the most primitive combinations of ape and human features.
Fossils reveal much about the physical build and activities of early Australopiths, but not everything about outward physical features such as the colour and texture of skin and hair, or about certain behaviours, such as methods of obtaining food or patterns of social interaction. For these reasons, scientists study the living great apes-specifically the African apes, particularly to familiarize a-bettering understanding of how early the Australopiths might have looked and behaved to his transition from ape too human might have occurred. For example, Australopiths probably resembled the great apes in characteristics such as the shape of the face and the amount of hair on the body. Australopiths also had brains roughly equal in size to those of the great apes, so they probably had apelike mental abilities. Their social life probably resembled that of chimpanzees.
Most of the distinctly human physical qualities in Australopiths related to their bipedal stance. Before Australopiths, no mammal had ever evolved an anatomy for habitual upright walking. Australopiths also had small canine teeth, as compared with long canines found in almost all other catarrhine primates.
Other characteristics of Australopiths reflected their ape ancestry. They had a low cranium behind a projecting face, and a brain size of 390 to 550 cu. cm. (24 to thirty-four cu. in.)-in the range of an ape's brain. The body weight of Australopiths, as estimated from their bones, ranged from twenty-seven to 49 kg. (60 to 108 lb.), and they stood 1.1 to 1.5 m. (3.5 to 5 ft.) tall. Their weight and height compare closely to those of chimpanzees (chimp height measured standing). Some Australopiths species had a large degree of sexual dimorphism-males were much larger than females-a trait also found in gorillas, orangutans, and another primates.
Australopiths also had curved fingers and long thumbs with a wide range of movement. In comparison, the fingers of apes are longer, more powerful, and more curved, making them extremely well adapted for hanging and swinging from branches. Apes also have very short thumbs, which limits their ability to manipulate small objects. Paleoanthropologists speculate as to whether the long and dexterous thumbs of Australopiths allowed them to use tools more efficiently than do apes.
The anatomy of Australopiths shows a number of adaptations for Bipedalism, in both the upper and lower body. Adaptations in the lower body included the following: The Australopiths ilium, or pelvic bone, which rises above the hip joint, was much shorter and broader than it is in apes. This shape enabled the hip muscles to steady the body during each step. The Australopiths pelvis also had a bowl-like shape, which supported the internal organs in an upright stance. The upper legs angled inward from the hip joints, which positioned the knees better to support the body during upright walking. The legs of apes, on the other hand, are positioned almost straight down from the hip, so that when an ape walks upright for a short distance, its body sways from side to side. Australopiths also had short and fewer flexible toes than do apes. The toes worked as rigid levers for pushing off the ground during each bipedal step.
Other adaptations occurred above the pelvis. The Australopiths spine had a S-shaped curve, which shortened the overall length of the torso and gave it rigidity and balance when standing. By contrast, apes have a straight spine. The Australopiths skull also had an important adaptation related to Bipedalism. The opening at the bottom of the skull through which the spinal cord attaches to the brain, called the foramen magnum, was positioned more forward than it is in apes. This position set the head in balance over the upright spine.
Australopiths clearly walked upright on the ground, but paleoanthropologists debate whether the earliest humans also spent a significant amount of time in the trees. Certain physical features indicate that they spent at least some of their time climbing in trees. Such features included they're curved and elongated fingers and elongated arms. However, their fingers, unlike those of apes, may not have been long enough to allow them to brachiate through the treetops. Study of fossil wrist bones suggests that early Australopiths had the ability to lock their wrists, preventing backward bending at the wrist when the body weight was placed on the knuckles of the hand. This could mean that the earliest bipeds had an ancestor that walked on its knuckles, as African apes do
Compared with apes, humans have very small canine teeth. Apes-particularly males-have thick, projecting, sharp canines that they use for displays of aggression and as weapons to defend themselves. The oldest known bipeds, who lived at least six million years ago, still had large canines by human standards, though not as large as in apes. By four million years ago Australopiths had developed the human characteristic of having smaller, flatter canines. Canine reduction might have related to an increase in social cooperation between humans and an accompanying decrease in the need for males to make aggressive displays.
The Australopiths can be divided into an early group of species, known as gracile Australopiths, which arose before three million years ago; and a later group, known as robust Australopiths, which evolved after three million years ago. The gracile Australopiths of that several species evolved between 4.5 million and three million years ago-generally had smaller teeth and jaws. The later-evolving robusts had larger faces with large jaws and molars (cheek teeth). These traits indicate powerful and prolonged chewing of food, and analyses of wear on the chewing surface of robust Australopiths molar teeth support this idea. Some fossils of early Australopiths have features resembling those of the later species, suggesting that the robusts evolved from one or more gracile ancestors.
Paleoanthropologists recognize at least eight species of early Australopiths. These include the three earliest established species, which belong to the genera Sahelanthropus, Orrorin, and Ardipithecus, a species of the genus Kenyanthropus, and four species of the genus Australopithecus.
The oldest known Australopiths species is Sahelanthropus tchadensis. Fossils of this species were first discovered in 2001 in northern Chad, Central Africa, by a research team led by French paleontologist Michel Brunet. The researchers estimated the fossils to be between seven million and six million years old. One of the fossils is a fracture, yet nearly completes cranium that shows a combination of apelike and humanlike features. Apelike features include small brain size, an elongated brain case, and areas of bone where strong neck muscles would have attached. Humanlike features  made up of small, flat canine teeth, a short middle part of the face, and a massive brow ridge (a bony, protruding ridge above the eyes) similar to that of later human fossils. The opening where the spinal cord attaches to the brain is tucked under the brain case, which suggests that the head was balanced on an upright body. It is not certain that Sahelanthropus walked bipedally, however, because bones from the rest of its skeleton have yet to be discovered. Nonetheless, its age and humanlike characteristics suggest that the human and African ape lineages had divided from one another by at least six million years ago.
In addition to reigniting debate about human origins, the discovery of Sahelanthropus in Chad significantly expanded the known geographic range of the earliest humans. The Great Rift Valley and South Africa, from which almost all other discoveries of early human fossils came, are apparently not the only regions of the continent that preserve the oldest clues of human evolution.
Orrorin tugenensis lived about six million years ago. This species was discovered in 2000 by a research team led by French paleontologist Brigitte Senut and French geologist Martin Pickford in the Tugen Hills region of central Kenya. The researchers found more than a dozen early human fossils dating between 6.2 million and six million years old. Among the finds were two thighbones that possess a groove indicative of an upright stance and bipedal walking. Although the finds are still being studied, the researchers consider these thighbones to be the oldest evidence of habitual two-legged walking. Fossilized bones from other parts of the skeleton show apelike features, including long, curved finger bones useful for strong grasping and movement through trees, and apelike canine and premolar teeth. Because of this distinctive combination of ape and human traits, the researchers gave a new genus and species name to these fossils, Orrorin tugenensis, which in the local language means ‘original man in the Tugen region.' The age of these fossils suggests that the divergence of humans from our common ancestor with chimpanzees occurred before six million years ago.
In 1994 an Ethiopian member of a research team led by American paleoanthropologists Tim White discovered human fossils estimated to be about 4.4 million year's old. White and his colleagues gave their discovery the name Ardipithecus ramidus. Ramid means ‘root' in the Afar language of Ethiopia and refers to the closeness of this new species to the roots of humanity. At the time of this discovery, the genus Australopithecus was scientifically well established. White devised the genus name Ardipithecus to distinguish this new species from other Australopiths because its fossils had a very ancient combination of apelike and humanlike traits. More recent finds indicate that this species may have lived as early as 5.8 million to 5.2 million years ago.
The teeth of Ardipithecus ramidus had a thin outer layer of enamel-a trait also seen in the African apes but not in other Australopiths species or older fossil apes. This trait suggests a close relationship with an ancestor of the African apes. In addition, the skeleton shows strong similarities to that of a chimpanzee but has slightly reduced canine teeth and adaptations for Bipedalism.
In 1965 a research team from Harvard University discovered a single arm bone of an early human at the site of Kanapoi in northern Kenya. The researchers estimated this bone to be four million years old, but could not identify the species to which it belonged or return at the time to look for related fossils. It was not until 1994 that a research team, led by British-born Kenyan paleoanthropologists Meave Leakey, found numerous teeth and fragments of bone at the site that could be linked to the previously discovered fossil. Leakey and her colleagues determined that the fossils were those of a species very primitives from those of the Australopiths, which was given the name Australopithecus Anamensis. Researchers have since found other A. Anamensis fossils at nearby sites, dating between about 4.2 million and 3.9 million years old. The skull of this species appears apelike, while its enlarged tibia (lower leg bone) indicates that it supported its full body weight on one leg at a time, as in regular bipedal walking
Australopithecus Anamensis was quite similar to another, much better-known species, A. afarensis, a gracile Australopiths that thrived in eastern Africa between about 3.9 million and three million years ago. The most celebrated fossil of this species, known as Lucy, is a partial skeleton of a female discovered by American paleoanthropologists Donald Johanson in 1974 at Hadar, Ethiopia. Lucy lived 3.2 million years ago. Scientists have identified several hundred fossils of A. afarensis from Hadar, including a collection representing at least thirteen individuals of both sexes and various ages, all from a single site.
Researchers working in northern Tanzania have also found fossilized bones of A. afarensis at Laetoli. This site, dated at 3.6 million years old, is best known for its spectacular trails of bipedal human footprints. Preserved in hardened volcanic ash, these footprints were discovered in 1978 by a research team led by British paleoanthropologists Mary Leakey. They provide irrefutable evidence that Australopiths regularly walked bipedally.
Paleoanthropologists have debated interpretations of the characteristics of A. afarensis and its place in the human family tree. One controversy centres on the Laetoli footprints, which some scientists believe show that the foot anatomy and gait of A. afarensis did not exactly match those of the modern humans. This observation may suggest that early Australopiths did not live primarily on the ground or at least spent a significant amount of time in the trees. The skeleton of Lucy also suggests that A. afarensis had longer, more powerful arms than most later human species, suggesting that this species was adept at climbing trees.
A third controversy arises from the claim that A. afarensis was the common ancestor of both later Australopiths and the modern human genus, Homo. While this idea remains a strong possibility, the similarity between this and another Australopiths species-one from southern Africa, named Australopithecus africanus-makes it difficult to decide which of the two species gave rise to the genus Homo.
Australopithecus africanus thrived in the Transvaal region of what is now South Africa between about 3.3 million and 2.5 million years ago. Australian-born anatomist Raymond Dart discovered this species-the first known Australopiths,-in 1924 at Taung, South Africa. The specimen that of a young child, came to be known as the Taung Child. For decades after this discovery, almost no one in the scientific community believed Dart's claim that the skull came from an ancestral human. In the late 1930's teams led by Scottish-born South African paleontologist Robert Broom unearthed many more A. africanus skulls and other bones from the Transvaal site of Sterkfontein.
A. africanus generally had a more globular braincase and less primitive-looking face and teeth than did A. afarensis. Thus, some scientists consider the southern species of early Australopiths to be a likely ancestor of the genus Homo. According to other scientists, however, certain heavily built facial and cranial features of A. africanus from Sterkfontein identify it as an ancestor of the robust Australopiths that lived later in the same region. In 1998 a research team led by South African paleoanthropologists Ronald Clarke discovered an almost complete early Australopiths skeleton at Sterkfontein. This important find may resolve some of the questions about where A. africanus fits in the story of human evolution
Working in the Lake Turkana's region of northern Kenya, a research team led by paleontologist Meave Leakey uncovered in 1999 a cranium and other bone remains of an early human that showed a mixture of features unseen in previous discoveries of early human fossils. The remains were estimated to be 3.5 million years old, and the cranium's small brain and earhole was similar to those of the earliest humans. Its cheekbone, however, joined the rest of the face in a forward position, and the region beneath the nose opening was flat. These are traits found in later human fossils from around two million years ago, typically those classified in the genus Homo. Noting this unusual combination of traits, researchers named a new genus and species, Kenyanthropus platyops, or ‘flat-faced humans from Kenya.' Before this discovery, it seemed that only a single early human species, Australopithecus afarensis, lived in East Africa between four million and three million years ago. Yet Kenyanthropus suggests that a diversity of species, including a more humanlike lineage then A. afarensis, lived in this time, just as in most other eras in human prehistory.
The human fossil record is poorly known between three million and two million years ago, from which estimates make recent results in finding from the site of Bouri, Ethiopia, particularly important. From 1996 to 1998, a research team led by Ethiopian paleontologist Berhane Asfaw and American paleontologist Tim White found the skull and other skeletal remains of an early human specimen about 2.5 million years old. The researchers named it Australopithecus garhi; the word garhi means ‘surprise' in the Afar language. The specimen is unique in having large incisors and molars in combination with an elongated forearm and thighbone. Its powerful arm bones suggest a tree-living ancestry, but its longer legs show the ability to walk upright on the ground. Fossils of A. garhi are associated with some of the oldest known stone tools, along with animal bones that were cut and cracked with tools. It is possible, then, that this species was among the first to make the transition to stone Toolmaking and to eating meat and bone marrow from large animals
By 2.7 million years ago the later, robust Australopiths had evolved. These species had what scientists refer to as megadont cheek teeth-wide molars and premolars coated with thick enamel. Their incisors, by contrast, were small. The robusts also had an expanded, flattened, and more vertical face than did gracile Australopiths. This face shape helped to absorb the stresses of strong chewing. On the top of the head, robust Australopiths had a sagittal crest (ridge of bone along the top of the skull from front to back) to which thick jaw muscles attached. The zygomatic arches (which extend back from the cheek bones to the ears), curved out wide from the side of the face and cranium, forming very large openings for the massive chewing muscles to pass through near their attachment to the lower jaw. Together, these traits say that the robust Australopiths chewed their food powerfully and for long periods.
Other ancient animal species that specialized in eating plants, such as some types of wild pigs, had similar adaptations in their facial, dental, and cranial anatomy. Thus, scientists think that the robust Australopiths had a diet consisting partly of tough, fibrous plant foods, such as seed pods and underground tubers. Analyses of microscopic wear on the teeth of some robust Australopiths specimens appear to support the idea of a vegetarian diet, although chemical studies of fossils suggest that the southern robust species may also have eaten meat.
Scientists originally used the word robust to refer to the late Australopiths out of the belief that they had much larger bodies than did the early, gracile Australopiths. However, further research has revealed that the robust Australopiths stood about the same height and weighed roughly the same amount as Australopithecus afarensis and A. africanus.
The earliest known robust species, Australopithecus aethiopicus, lived in eastern Africa by 2.7 million years ago. In 1985 at West Turkana, Kenya, American paleoanthropologists Alan Walker discovered a 2.5-million-year- old fossil skull that helped to define this species. It became known as the ‘black skull' because of the colour it had absorbed from minerals in the ground. The skull had a tall sagittal crest toward the back of its cranium and a face that projected far outward from the forehead. A. aethiopicus shared some primitive features with A. afarensis-that is, features that originated in the earlier East African Australopiths. This may suggest that A. aethiopicus evolved from A. afarensis.
Australopithecus boisei, the other well-known East African robust Australopiths, lived over a long period of time, between about 2.3 million and 1.2 million years ago. In 1959 Mary Leakey discovered the original fossil of this species-a nearly complete skull-at the site of Olduvai Gorge in Tanzania. Kenyan-born paleoanthropologists Louis Leakey, husband of Mary, originally named the new species Zinjanthropus boisei (Zinjanthropus translates as ‘East African man'). This skull-dating from 1.8 million years ago-has the most specialized features of all the robust species. It has a massive, wide and dished-in face capable of withstanding extreme chewing forces, and molars four times the size of those in modern humans. Since the discovery of Zinjanthropus, now recognized as an Australopiths, scientists have found great numbers of A. boisei fossils in Tanzania, Kenya, and Ethiopia.
The southern robust species, called Australopithecus robustus, lived between about 1.8 million and 1.3 million years ago in the Transvaal, the same region that was home to A. africanus. In 1938 Robert Broom, who had found many A. africanus fossils, bought a fossil jaw and molar that looked distinctly different from those in A. africanus. After finding the site of Kromdraai, from which the fossil had come, Broom collected many more bones and teeth that together convinced him to name a new species, which he called Paranthropus robustus (Paranthropus meaning ‘beside man'). Later scientists dated this skull at about 1.5 million years old. In the late 1940's and 1950 Broom discovered many more fossils of this species at the Transvaal site of Swartkrans.
Paleoanthropologists believe that the eastern robust species, A. aethiopicus and A. boisei, may have evolved from an early Australopiths of the same region, perhaps A. afarensis. According to this view, A. africanus gave rise only to the southern species A. robustus. Scientists refer to such a case characteristics in different places or at different times-as parallel evolution. If parallel evolution occurred in Australopiths, the robust species would make up two separate branches of the human family tree.
The last robust Australopiths died out about 1.2 million years ago. At about this time, climate patterns around the world entered a period of fluctuation, and these changes may have reduced the food supply on which robusts depended. Interaction with larger-brained members of the genus Homo, such as Homo erectus, may also have contributed to the decline of late Australopiths, although no compelling evidence exists of such direct contact. Competition with several other species of plant-eating monkeys and pigs, which thrived in Africa at the time, may have been an even more important factor. Nevertheless, the reason that the robust Australopiths became extinct after flourishing for such a long time is not yet known for sure.
Scientists have several ideas about why Australopiths first split off from the apes, initiating the course of human evolution. Virtually all hypotheses suggest that environmental change was an important factor, specifically in influencing the evolution of Bipedalism. Some well-established ideas about why humans first evolved include (1) the savanna hypothesis, (2) the woodland-mosaic hypothesis, and (3) the variability hypothesis.
The global climate cooled and became drier between eight million and five million years ago, near the end of the Miocene Epoch. According to the savanna hypothesis, this climate change broke up and reduced the area of African forests. As the forests shrunk, an ape population in eastern Africa became separated from other populations of apes in the more heavily forested areas of western Africa. The eastern population had to adapt to its drier environment, which contained larger areas of grassy savanna.
The expansion of dry terrain favoured the evolution of terrestrial living, and made it more difficult to survive by living in trees. Terrestrial apes might have formed large social groups in order to improve their ability to find and collect food and to fend off predators-activities that also may have required the ability to communicate well. The challenges of savanna life might also have promoted the rise of tool use, for purposes such as scavenging meat from the kills of predators. These important evolutionary changes would have depended on increased mental abilities and, therefore, may have correlated with the development of larger brains in early humans.
Critics of the savanna hypothesis argue against it on several grounds, but particularly for two reasons. First, discoveries by a French scientific team of Australopiths fossils in Chad, in Central Africa, suggest that the environments of East Africa may not have been fully separated from those farther west. Recent research suggests that open savannas were not prominent in Africa until sometime after two million years ago.
Criticism of the savanna hypothesis has spawned alternative ideas about early human evolution. The woodland-mosaic hypothesis proposes that the early Australopiths evolved in patchily wooded areas-a mosaic of woodland and grassland-that offered opportunities for feeding both on the ground and in the trees, and that ground feeding favoured Bipedalism.
The variability hypothesis suggests that early Australopiths experienced many changes in environment and ended up living in a range of habitats, including forests, open-canopy woodlands, and savannas. In response, their populations became adapted to a variety of surroundings. Scientists have found that this range of habitats existed at the time when the early Australopiths evolved. So the development of new anatomical characteristics,-particularly Bipedalism-combined with an ability to climb trees, may have given early humans the versatility to live in a variety of habitats.
Bipedalism in early humans would have enabled them to travel efficiently over long distances, giving them an advantage over quadrupedal apes in moving across barren open terrain between groves of trees. In addition, the earliest humans continued to have the advantage from their ape ancestry of being able to escape into the trees to avoid predators. The benefits of both Bipedalism and agility in the trees may explain the unique anatomy of Australopiths. Their long, powerful arms and curved fingers probably made them good climbers, while their pelvis and lower limb structure were reshaped for upright walking people belong to the genus Homo, which first evolved at least 2.3 million to 2.5 million years ago. The earliest members of this genus differed from the Australopiths in at least one important respect-they had larger brains than did their predecessors.
The evolution of the modern human genus can be divided roughly into three periods: early, middle, and late. Species of early Homo resembled gracile Australopiths in many ways. Some early Homo species lived until possibly 1.6 million years ago. The period of middle Homo began perhaps between two million and 1.8 million years ago, overlapping with the end of early Homo. Species of middle Homo evolved an anatomy much more similar to that of modern humans but had comparatively small brains. The transition from middle to late Homo probably occurred sometime around 200,000 years ago. Species of late Homo evolved large and complex brains and eventually language. Culture also became an increasingly important part of human life during the most recent period of evolution.
The origin of the genus Homo has long intrigued paleoanthropologists and prompted much debate. One of several known species of Australopiths, or one not yet discovered, could have given rise to the first species of Homo. Scientists also do not know exactly what factors favoured the evolution of a larger and more complex brain-the defining physical trait of modern humans.
Louis Leakey originally argued that the origin of Homo related directly to the development of Toolmaking -specifically, the making of stone tools. Toolmaking requires certain mental skills and fine hand manipulation that may exist only in members of our own genus. Literally, the name Homo habilis (meaning ‘handy man') refer directly to the making and use of tools.
However, several species of Australopiths lived at the same time as early Homo, making it unclear which species produced the earliest stone tools. Recent studies of Australopiths hand bones have suggested that at least one of the robust species, Australopithecus robustus, could have made tools. In addition, during the 1960's and 1970's researchers first observed that some nonhuman primates, such as chimpanzees, make and use tools, suggesting that Australopiths and the apes that preceded them probably also made some kinds of tools.
According to some scientists, however, early Homo probably did make the first stone tools. The ability to cut and pound foods would have been most useful to these smaller-toothed humans, whereas the robust Australopiths could chew even very tough foods. Furthermore, early humans continued to make stone tools similar to the oldest known kinds for a time long after the gracile Australopiths died out. Some scientists think that a period of environmental cooling and drying in Africa set the stage for the evolution of Homo. According to this idea, many types of animals suited to the challenges of a drier environment originated during the period between about 2.8 million and 2.4 million years ago, including the first species of Homo.
A Toolmaking human might have had an advantage in obtaining alternative food sources as vegetation became sparse in increasingly dry environments. The new foods might have included underground roots and tubers, as well as meat obtained through scavenging or hunting. However, some scientists disagree with this idea, arguing that the period during which Homo evolved fluctuated between drier and wetter conditions, rather than just becoming dry. In this case, the making and use of stone tools and an expansion of the diet in early Homo-as well as an increase in brain size-may all have been adaptations to unpredictable and fluctuating environments. In either case, more scientific documentation is necessary to support strongly or refute the idea that early Homo arose as part of a larger trend of rapid species extinction and the evolution of many new species during a period of environmental change.
Paleoanthropologists generally recognize two species of early Homo-Homo habilis and H. rudolfensis (although other species may also have existed). The record is unclear because most of the early fossils that scientists have identified as species of Homo,-rather than robust Australopiths who lived at the same time occur as isolated fragments. In many places, only teeth, jawbones, and pieces of skull-without any other skeletal remains-suggest that new species of smaller-toothed humans had evolved as early as 2.5 million years ago. Scientists cannot always tell whether these fossils belong to late-surviving gracile Australopiths or early representatives of Homo. The two groups resemble each other because Homo likely descended directly from a species of gracile Australopiths.
In the early 1960's, at Olduvai Gorge, Tanzania, Louis Leakey, British primate researcher John Napier, and South African paleoanthropologists Philip Tobias discovered a group of early human fossils that showed a cranial capacity from 590 to 690 cu. cm. (36 to forty-two cu. in.). Based on this brain size, which was completely above the range of that in known Australopiths, the scientists argued that a new genus, Homo, and a new species, Homo habilis, should be recognized. Other scientists questioned whether this amount of brain enlargement was sufficient for defining a new genus, and even whether H. habilis were different from Australopithecus africanus, as the teeth of the two species look similar. However, scientists now widely accept both the genus and species names designated by the Olduvai team.
H. habilis lived in eastern and possibly southern Africa between about 1.9 million and 1.6 million years ago, and maybe as early as 2.4 million years ago. Although the fossils of this species moderately resemble those of Australopiths, H. habilis had smaller and narrower molar teeth, premolar teeth, and jaws than did its predecessors and contemporary robust Australopiths.
A fragmented skeleton of a female from Olduvai shows that she stood only about one m. (3.3 ft.) tall, and the ratio of the length of her arms to her legs was greater than that in the Australopiths Lucy. At least in the case of this individual, therefore, H. habilis had very apelike body proportions. However, H. habilis had more modern-looking feet and hands capable of producing tools. Some of the earliest stone tools from Olduvai have been found with H. habilis fossils, suggesting that this species made and used the tools at this site.
Scientists began to notice a high degree of variability in body size as they discovered more early Homo fossils. This could have suggested that H. habilis had a large amount of sexual dimorphism. For instance, the Olduvai female skeleton was dwarfed in comparison with other fossils-exemplified by a sizable early Homo cranium from East Turkana in northern Kenya. However, the differences in size exceeded those expected between males and females of the same species, and this finding later helped convince scientists that another species of early Homo had lived in eastern Africa.
This second species of early Homo was given the name Homo rudolfensis, after Lake Rudolf (now Lake Turkana). The best-known fossils of H. rudolfensis come from the area surrounding this lake and date from about 1.9 million years ago. Paleoanthropologists have not determined the entire time range during which H. rudolfensis may have lived.
This species had a larger face and body than did
H. habilis. The cranial capacity of H. rudolfensis averaged about 750 cu. cm. (46 cu. in.). Scientists need more evidence to know whether the brain of H. rudolfensis in relation to its body size was larger than that proportion in H. habilis. A larger brain-to-body-size ratio can suggest increased mental abilities. H. rudolfensis also had large teeth, approaching the size of those in robust Australopiths. The discovery of even a partial fossil skeleton would reveal whether this larger form of early Homo had apelike or more modern body proportions. Scientists have found several modern-looking thighbones that date from between two million and 1.8 million years ago and may belong to H. rudolfensis. These bones suggest a body size of 1.5 m. (5 ft.) and 52 kg. (114 lb.).
The skulls and teeth of early African populations of middle Homo differed subtly from those of later H. erectus populations from China and the island of Java in Indonesia. H. ergaster makes a better candidate for an ancestor of the modern human line because Asian H. erectus has some specialized features not seen in some later humans, including our own species. H. heidelbergensis has similarities to both H. erectus and the later species H. neanderthalensis, although it may have been a transitional species between middle Homo and the line to which modern humans belong.
Homo ergaster probably first evolved in Africa around two million years ago. This species had a rounded cranium with a brain size of between 700 and 850 cu. cm. (49 to fifty-two cu. in.), a prominent brow ridge, small teeth, and many other features that it shared with the later H. erectus. Many paleoanthropologists consider H. ergaster a good candidate for an ancestor of modern humans because it had several modern skull features, including a thin cranial bones. Most H. ergaster fossils come from the time range of 1.8 million to 1.5 million years ago.
The most important fossil of this species yet found is a nearly complete skeleton of a young male from West Turkana, Kenya, which dates from about 1.55 million years ago. Scientists determined the sex of the skeleton from the shape of its pelvis. They also found out from patterns of tooth eruption and bone growth that the boy had died when he was between nine and twelve years old.
The Turkana boy, as the skeleton is known, had elongated leg bones and arm, leg, and trunk proportions of which essentially match those of a modern humans, in sharp contrast with the apelike proportions H. habilis and Australopithecus afarensis. He appears to have been quite tall and slender. Scientists estimate that, had he grown into adulthood, the boy would have reached a height of 1.8 m. (6 ft.) and a weight of 68 kg (150 lb.). The anatomy of the Turkana boy shows that H. ergaster was particularly well adapted for walking and perhaps for running long distances in a hot environment (a tall and slender body dissipates heat well) but not for any significant amount of tree climbing.
The oldest humanlike fossils outside of Africa have also been classified as H. ergaster, dated around 1.75 million year's old. These finds, from the Dmanisi site in the southern Caucasus Mountains of Georgia, consist of several crania, jaws, and other fossilized bones. Some of these are strikingly like East African H. ergaster, but others are smaller or larger than H. ergaster, suggesting a high degree of variation within a single population.
H. ergaster, H. rudolfensis, and H. habilis, in addition to possibly two robust Australopiths, all might have coexisted in Africa around 1.9 million years ago. This finding goes against a traditional paleoanthropological view that human evolution consisted of a single line that evolved progressively over time,- an Australopiths species followed by early Homo, then middle Homo, and finally H. sapiens. It appears that periods of species diversity and extinction have been common during human evolution, and that modern H. sapiens has the rare distinction of being the only living human species today.
Although H. ergaster appears to have coexisted with several other human species, they probably did not interbreed. Mating rarely succeeds between two species with significant skeletal differences, such as H. ergaster and H. habilis. Many paleoanthropologists now believe that H. ergaster descended from an earlier population of Homo-perhaps one of the two known species of early Homo-and that the modern human line descended from H. ergaster.
Paleoanthropologists now know that humans first evolved in Africa and lived only on that continent for a few million years. The earliest human species known to have spread in large numbers beyond the African continent was first discovered in Southeast Asia. In 1891 Dutch physician Eugene Dubois found the cranium of an early human on the Indonesian island of Java. He named this early human Pithecanthropus erectus, or ‘erect ape-man.'Today paleoanthropologists refer to this species as Homo erectus.
H. erectus appears to have evolved in Africa from earlier populations of H. ergaster, and then spread to Asia sometime between 1.8 million and 1.5 million years ago. The youngest known fossils of this species, from the Solo River in Java, may date from about 50,000 years ago (although that dating is controversial). So H. erectus was a very successful widespread species-as both having lived in Africa and much of Asia, and long-lived, having survived for possibly more than 1.5 million years.
Homo erectus had a low and rounded braincase that was elongated to example the peripheral frontage to measurements extending inward to the back, a prominent brow ridge, and adult cranial capacity of 800 to 1,250 cu. cm. (50 to 80 cu. in.), an average twice that of the Australopiths. Its bones, including the cranium, were thicker than those of earlier species. Prominent muscle markings and thick, reinforced areas on the bones of H. erectus indicate that its body could withstand powerful movements and stresses. Although it had much smaller teeth than did the Australopiths, it had a heavy and strong jaw.
In the 1920's and 1930's German anatomist and physical anthropologist Franz Weidenreich excavated the most famous collections of H. erectus fossils from a cave at the site of Zhoukoudian (Chou-k'ou-tien), China, near Beijing (Peking). Scientists dubbed these fossil humans Sinanthropus pekinensis, or Peking Man, but others later reclassified them as H. erectus. The Zhoukoudian cave yielded the fragmentary remains of more than thirty individuals, ranging from about 500,000 to 250,000 years old. These fossils were lost near the outbreak of World War II, but Weidenreich had made excellent casts of his finds. Further studies at the cave site have yielded more H. erectus remains.
Other important fossil sites for this species in China include Lantian, Yuanmou, Yunxian, and Hexian. Researchers have also recovered many tools made by H. erectus in China at sites such as Nihewan and Bose, and other sites of similar age (at least one million to 250,000 years old).
Ever since the discovery of Homo erectus, scientists have debated whether this species was a direct ancestor of later humans, including H. sapiens. The last populations of H. erectus-such as those from the Solo River in Java,-may have lived as recently as 50,000 years ago, at the same time as did populations of H. sapiens. Modern humans could not have evolved from these late populations of H. erectus, a much more primitive type of human. However, earlier East Asian populations could have given rise to H. sapiens.
Many paleoanthropologists believe that early humans migrated into Europe by 800,000 years ago, and that these populations were not Homo erectus. A growing number of scientists refer to these early migrants into Europe-who predated both Neanderthals and H. sapiens in the region,-as H. heidelbergensis. The species name comes from a 500,000-year-old jaw found near Heidelberg, Germany.
Scientists have found few human fossils in Africa for the period between 1.2 million and 600,000 years ago, during which
H. heidelbergensis or its ancestors first migrated into Europe. Populations of H. ergaster (or possibly H. erectus) appear to have lived until at least 800,000 years ago in Africa, and possibly until 500,000 years ago in northern Africa. When these populations disappeared, other massive-boned and larger-brained humans,-possibly H. heidelbergensis appears to have replaced them. Scientists have found fossils of these stockier humans at sites in Bodo, Ethiopia; Saldanha (also known as Elandsfontein), South Africa; Ndutu, Tanzania; and Kabwe, Zimbabwe.
Scientists have come up with at least three different interpretations of these African fossils. Some scientists place the fossils in the species H. heidelbergensis and think that this species gave rise to both the Neanderthals (in Europe) and H. sapiens (in Africa). Others think that the European and African fossils belong to two distinct species, and that the African populations that, in this view, was not H. heidelbergensis but a separate species gave rise to H. sapiens. Yet other scientists advocate a long-head view that H. erectus and H. sapiens belong to a single evolving lineage, and that the African fossils belong in the category of archaic H. sapiens (archaic meaning not fully anatomically modern).
The fossil evidence does not clearly favour any of these three interpretations over another. A growing number of fossils from Asia, Africa, and Europe have features that are intermediate between early H. ergaster and H. sapiens. This kind of variation makes it hard to decide how to identify distinct species and to find out which group of fossils represents the most likely ancestor of later humans.
Humans evolved in Africa.  Lived took of their stand for only as long as four million years or more, so scientists wonder what finally triggered the first human migration out of Africa (a movement that coincided with the spread of early human populations throughout the African continent). The answer to this question depends, in part, on knowing exactly when that first migration occurred. Some studies claim that site in Asia and Europe contain crude stone tools and fossilized fragments of humanlike teeth that date from more than 1.8 million years ago. Although these claims remain unconfirmed, small populations of humans may have entered Asia before 1.8 million years ago, followed by a more substantial spread between 1.6 million and one million years ago. Early humans reached northeastern Asia by around 1.4 million years ago, inhabiting a region close to the perpetually dry deserts of northern China. The first major habitation of central and western Europe, on the other hand, does not appear to have occurred until between one million and 500,000 years ago.
Scientists once thought that advances in stone tools could have enabled early humans such as Homo erectus to move into Asia and Europe, perhaps by helping them to obtain new kinds of food, such as the meat of large mammals. If African human populations had developed tools that allowed them to hunt large game effectively, they would have had a good source of food wherever they went. In this view, humans first migrated into Eurasia based on a unique cultural adaptation.
By 1.5 million years ago, early humans had begun to make new kinds of tools, which scientists call Acheulean. Common Acheulean tools included large hand axes and cleavers. While these new tools might have helped early humans to hunt, the first known Acheulean tools in Africa date from later than the earliest known human presence in Asia. Also, most East Asian sites more than 200,000 years old contains only simply shaped cobble and flake tools. In contrast, Acheulean tools were more finely crafted, larger, and more symmetrical. Thus, the earliest settlers of Eurasia did not have a true Acheulean technology, and advances in Toolmaking alone cannot explain the spread out of Africa.
Another possibility is that the early spread of humans to Eurasia was not unique, but parts of a wider migration of meat -eating animals, such as lions and hyenas. The human migration out of Africa occurred during the early part of the Pleistocene Epoch, between 1.8 million and 780,000 years ago. Many African carnivores spread to Eurasia during the early Pleistocene, and humans could have moved along with them. In this view, H. erectus seems one of many meat-eating species to expand into Eurasia from Africa, rather than a uniquely adapted species. Relying on meat as a primary food source might have allowed many meat-eating species, including humans, to move through many different environments without having to learn about unfamiliar and potentially poisonous plants quickly.
However, the migration of humans to eastern Asia may have occurred gradually and through lower latitudes and environments similar to those of Africa. If East African populations of H. erectus moved at only 1.6 km. (1 mi.) every twenty years, they could have reached Southeast Asia in 150,000 years. Over this amount of time, humans could have learned about and begun relying on edible plant foods. Thus, eating meat may not have played a crucial role in the first human migrations to new continents. Careful comparison of animal fossils, stone tools, and early human fossils from Africa, Asia, and Europe will help scientists better to find what factors motivated and allowed humans to venture out of Africa for the first time.
The origin of our own species, Homo sapiens, is one of the most hotly debated topics in Paleoanthropology. This debate centres on whether or not modern humans have a direct relationship to H. erectus or to the Neanderthals, and to a great extent is acknowledged of the more modern group of humans who evolved within the past 250,000 years. Paleoanthropologists commonly use the term anatomically modern Homo sapiens to distinguish people of today from these similar predecessors.
Traditionally, paleoanthropologists classified as Homo sapiens any fossil human younger than 500,000 years old with a braincase larger than that of H. erectus. Thus, many scientists who believe that modern humans descend from a single line dating back to H. erectus use the name archaic Homo sapiens to refer to a wide variety of fossil humans that predate anatomically modern H. sapiens. The archaic term denotes a set of physical features typical of Neanderthals and other species of late Homo before modern Homo sapiens. These features include a combination of a robust skeleton, a large but low braincase (positioned in a measure behind, rather than over, the face), and a lower jaw lacking a prominent chin. In this sense, Neanderthals are sometimes classified as a subspecies of archaic H. sapiens-H. neanderthalensis. Other scientists think that the variation in archaic fossils falls into clearly identifiable sets of traits, and that any type of human fossil exhibiting a unique set of traits should have a new species name. According to this view, the Neanderthals belong to their own species, H. neanderthalensis.
In the past, scientists claimed that Neanderthals differed greatly from modern humans. However, the basis for this claim came from a faulty reconstruction of a Neanderthal skeleton that showed it with bent knees and a slouching gait. This reconstruction gave the common but mistaken impression that Neanderthals were dim-witted brutes who lived a crude lifestyle. On the contrary, Neanderthals, like the species that preceded them, walked fully upright without a slouch or bent knees. In addition, their cranial capacity was quite large at about 1,500 cu. cm. (about ninety cu. in.), larger on average than that of modern humans. (The difference probably relates to the greater muscle mass of Neanderthals as compared with modern humans, which usually correlates with a larger brain size.).
Compared with earlier humans, Neanderthals had a high degree of cultural sophistication. They appear to have encountered some informality, as perhaps something as primitive in construction showing symbolic rituals, such as the burial of they're dead. Neanderthal fossils-including a number of fairly complete skeletons,-are quite common compared with those of earlier forms of Homo, in part because of the Neanderthal practice of intentional burial. Neanderthals also produced sophisticated types of stone tools known as Mousterian, which involved creating blanks (rough forms) from which several types of tools could be made.
Along with many physical similarities, Neanderthals differed from modern humans in several ways. The typical Neanderthal skull had a low forehead, a large nasal area (suggesting a large nose), a forward-projecting nasal and cheek region, a prominent brow ridge with a bony arch over each eye, a non-projecting chin, and obvious space behind the third molar (in front of the upward turn of the lower jaw).
Neanderthals were heavily built and had prominently-boned skeleton body structures than do modern humans. Other Neanderthal skeletal features included a bowing of the limb bones in some individuals, broad scapulae (shoulder blades), hip joints turned outward, a long and thin pubic bone, short lower leg and arm bones on the upper bones, and large surfaces on the joints of the toes and limb bones. Together, these traits made a powerful, compact body of short stature of males averaged 1.7 m. (5 ft. 5 in.) tall and 84 kg. (185 lb.), and females averaged 1.5 m. (5 ft.) tall and 80 kg. (176 lb.). The short, stocky build of Neanderthals conserved heat and helped them withstand extremely cold conditions that prevailed in temperate regions beginning about 70,000 years ago. The last known Neanderthal fossils come from western Europe and date from approximately 36,000 years ago.
At the same time as Neanderthal populations grew in number in Europe and parts of Asia, other populations of nearly modern humans arose in Africa and Asia. Scientists also commonly refer to these fossils, which are distinct from but similar to those of Neanderthals, as archaic. Fossils from the Chinese sites of Dali, Maba, and Xujiayao display the long, low cranium and large face typical of archaic humans, yet they also have features similar to those of modern people in the region. At the cave site of Jebel Irhoud, Morocco, scientists have found fossils with the long skull typical of archaic humans but also the modern traits of a higher forehead and flatter mid face. Fossils of humans from East African sites older than 100,000 years,  such as Ngaloba in Tanzania and Eliye Springs in Kenya,-also seem to show a mixture of archaic and modern traits.
The oldest known fossils that possess skeletal features typical of modern humans date from between 130,000 and 90,000 years ago. Several key features distinguish the skulls of modern humans from those of archaic species. These features include a much smaller brow ridge, if any; a globe-shaped braincase; and a flat or only projecting face of reduced size, located under the front of the braincase. Among all mammals, only humans have a face positioned directly beneath the frontal lobe (forward-most area) of the brain. As a result, modern humans tend to have a higher forehead than did Neanderthals and other archaic humans. The cranial capacity of modern humans ranges from about 1,000 to 2,000 cu. cm. (60 to 120 cu. in.), with the average being about 1,350 cu. cm. (80 cu. in.).
Scientists have found both fragmentary and nearly complete cranial fossils of early anatomically modern Homo sapiens from the sites of Singha, Sudan; Omo, Ethiopia; Klasies River Mouth, South Africa; and Skh -Cave, Israel. Based on these fossils, many scientists conclude that modern H. sapiens had evolved in Africa by 130,000 years ago and started spreading to diverse parts of the world beginning on a route through the Near East sometime before 90,000 years ago.
Paleoanthropologists are engaged in an ongoing debate about where modern humans evolved and how they spread around the world. Differences in opinion rest on the question of whether the evolution of modern humans took place in a small region of Africa or over a broad area of Africa and Eurasia. By extension, opinions differ as to whether modern human populations from Africa displaced all existing populations of earlier humans, eventually resulting in their extinction.
Those, who think modern humans originated exclusively in Africa, and then spread around the world support what is known as the out of Africa hypothesis. Those who think modern humans evolved over a large region of Eurasia and Africa support the so-called multi-regional hypothesis.
Researchers have conducted many genetic studies and carefully assessed fossils to figure out which of these hypotheses agrees more with scientific evidence. The results of this research do not entirely confirm or reject either one. Therefore, some scientists think a compromise between the two hypotheses is the best explanation. The debate between these views has implications for how scientists understand the concept of race in humans. The dubious question that raises is a distributed contribution  guised in curiously of itself is to whether the physical differences among modern humans evolved deep in the past or recent, in which is accorded to the out of Africa hypothesis.  It is also known as the replacement hypothesis, by which early populations of modern humans out from Africa migrated to other regions and entirely replaced existing populations of archaic humans. The replaced populations would have included the Neanderthals and any surviving groups of Homo erectus. Supporters of this view note that many modern human skeletal traits evolved most recently-within the past 200,000 years or so,-suggesting a single, common origin. Additionally, the anatomical similarities shared by all modern human populations far outweigh those shared by premodern and modern humans within particular geographic regions. Furthermore, biological research suggested that most new species of organisms, including mammals, arose from small, geographically isolated populations.
According to the multi-regional hypothesis, also known as the continuity hypothesis, the evolution of modern humans began when Homo erectus spread throughout much of Eurasia around one million years ago. Regional populations retained some unique anatomical features for hundreds of thousands of years, but they also mated with populations from neighbouring regions, exchanging heritable traits with each other. This exchange of heritable traits is known as gene flow.
Through gene flow, populations of H. erectus passed on a variety of increasingly modern characteristics, such as increases in brain size, across their geographic range. Gradually this would have resulted in the evolution of more modern looking humans throughout Africa and Eurasia. The physical differences among people on this day, then, would result from hundreds of thousands of years of regional evolution. This is the concept of continuity. For instance, modern East Asian populations have some skull features that scientists also see in H. erectus fossils from that region.
Noticeably critics of the multi-regional hypothesis claim that it wrongly advocates a scientific belief in race and could be used to encourage racism. Supporters of the theory point out, however, that their position does not imply that modern races evolved in isolation from each other, or that racial differences justify racism. Instead, the theory holds that gene flow linked different populations together. These links allowed progressively more modern features, no matter where they arose, to spread from region to region and eventually become universal among humans.
Scientists have weighed the out of Africa and multi-regional hypotheses against both genetic and fossil evidence. The results do not unanimously support either one, but weigh more heavily in favour of the out of Africa hypothesis.
Geneticists have studied the amount of difference in the DNA (deoxyribonucleic acid) of different populations of humans. DNA is the molecule that contains our heritable genetic code. Differences in human DNA result from mutations in DNA structure. Mutations may result from exposure to external elements such as solar radiation or certain chemical compounds, while others occur naturally at random.
Geneticists have calculated rates at which mutations can be expected to occur over time. Dividing the total number of genetic differences between two populations by an expected rate of mutation provides an estimate of the time when the two gave cause to be joined of a common ancestor. Many estimates of evolutionary ancestry rely on studies of the DNA in cell structures called mitochondria. This DNA is referred to as mtDNA (mitochondrial DNA). Unlike DNA from the nucleus of a cell, which codes for most of the traits an organism inherits from both parents, mtDNA inheritance passes only from a mother to her offspring. MtDNA also accumulates mutations about ten times faster than does DNA in the cell nucleus (the location of most DNA). The structure of mtDNA changes so quickly that scientists can easily measure the differences between one human population and another. Two closely related populations should have only minor differences in their mtDNA. Conversely, two very distantly related populations should have large differences in their mtDNA.
MtDNA research into modern human origins has produced two major findings. First, the entire amount of variation in mtDNA across human populations is small in comparison with that of other animal species. This significance, in that all human mtDNA originated from a single since which ancestral lineage-specifically, a single female-of late has been mutating ever. Most estimates of the mutation rate of mtDNA suggest that this female ancestor lived about 200,000 years ago. In addition, the mtDNA of African populations varies more than that of peoples in other continents. This suggests that the mtDNA of African populations have proven in identifying their place of a value on a longer time than it has in populations over any other region. In that all living people inherited their mtDNA from one woman in Africa, who is sometimes called the Mitochondrial Eve, in addition geneticists and anthropologists have concluded from this evidence that modern humans originated in a small population in Africa and spread out from there.
MtDNA studies have weaknesses, however, including the following four. First, the estimated rate of mtDNA mutation varies from study to study, and some estimates put the date of origin closer to 850,000 years ago, the time of Homo erectus. Second, mtDNA makes up a small part of the total genetic material that humans inherit. The rest of our genetic material-about 400,000 times more than the amount of mtDNA,-came from many individuals living at the time of the African Eve, conceivably from many different regions.  This intermittent interval of which time modern mtDNA began to diversify does not necessarily coincide with the origin of modern human biological traits and cultural abilities. Fourth, the smaller amount of modern mtDNA diversity outside of Africa could result from times when European and Asian populations declined in numbers, perhaps due to climate changes.
Regardless of these criticisms, many geneticists continue to favour the out of Africa hypothesis of modern human origins. Studies of nuclear DNA also suggest an African origin for a variety of genes. Furthermore, in a remarkable series of studies in the late 1990s, scientists recovered mtDNA from the first Neanderthal fossil found in Germany and two other Neanderthal fossils. In each case, the mtDNA does not closely match that of modern humans. This finding suggests that at least some Neanderthal populations had diverged from the line to modern humans by 500,000 to 600,000 years ago, and the depriving of an augmented potential of possible  occurrence is apprehensibly actualized, and which can be known as having an existence as categorized in virtue been no attributed thing but some substantiation by a form of something exacted to have happened. Also to suggest that Neanderthals represent a separate species from modern H. sapiens. In another study, however, mtDNA extracted from a 62,000-year-old Australian H. sapiens fossil was found to differ significantly from modern human mtDNA, suggesting a much wider range of mtDNA variation within H. sapiens than was previously believed. According to the Australian researchers, this finding lends support to the multi-regional hypothesis because it shows that different populations of H. sapiens, possibly including Neanderthals, could have evolved independently in different parts of the world.
As with genetic research, fossil evidence also does not entirely support or refute either of the competing hypotheses of modern human origins. However, many scientists see the balance of evidence favouring an African origin of modern H. sapiens within the past 200,000 years. The oldest known modern-looking skulls come from Africa and date from perhaps 130,000 years ago. The next oldest comes from the Near East, where they date from about 90,000 years ago. Fossils of modern humans in Europe do not exist between years from before 40,000 years ago. In addition, the first modern humans in Europe-often referred to as Cro-Magnon people had elongated lower leg bones, as did African populations that were adapted too warm, tropical climates. This suggests that populations from warmer regions replaced those in colder European regions, such as the Neanderthals.
Fossils also show that populations of modern humans lived at the same time and in the same regions as did populations of Neanderthals and Homo erectus, but that each retained its distinctive physical features. The different groups overlapped in the Near East and Southeast Asia for between about 30,000 and 50,000 years. The maintenance of physical differences for this amount of time implies that archaically and modern humans could either not or generally did not interbreed. To some scientists, this also means that the Neanderthals belong to a separate species, H. neanderthalensis, and that migratory populations of modern humans entirely replaced archaic humans in both Europe and eastern Asia.
On the other hand, fossils of archaic and modern humans in some regions show continuity in certain physical characteristics. These similarities may indicate multi-regional evolution. For example, both archaic and modern skulls of eastern Asia have flatter cheek and nasal areas than do skulls from other regions. By contrast, the same parts of the face project forward in the skulls of both archaic and modern humans of Europe. Assuming that these traits were influenced primarily by genetic inheritance rather than environmental factors, archaic humans may have given rise to modern humans in some regions or at least interbred with migrant modern-looking humans.
Each of the competing major hypotheses of modern human origins has its strengths and weaknesses. Genetic evidence appears to support the out of Africa hypothesis. In the western half of Eurasia and in Africa, this hypothesis also seems the better explanation, particularly in regard to the apparent replacement of Neanderthals by modern populations. At the same time, the multi-regional hypothesis appears to explain some of the regional continuity found in East Asian populations.
Therefore, many paleoanthropologists advocate a theory of modern human origins that combines elements of the out of Africa and the changing regional hypotheses. Humans with modern features may have first come forth in Africa or come together there as a result of gene flow with populations from other regions. These African populations may then have replaced archaic humans in certain regions, such as western Europe and the Near East. Nevertheless, elsewhere,-especially in East Asia-gene flow may have occurred among local populations of archaic and modern humans, resulting in distinct and enduring regional characteristics.
All three of these views-the two competing positions and the compromise; acknowledge the strong biological unity of all people. In the multi-regional hypothesis, this unity results from hundreds of thousands of years of continued gene flow among all human populations. According to the out of Africa hypothesis, on the other hand, similarities among all living human populations result from a recent common origin. The compromise position accepts both of these as reasonable and compatible explanations of modern human origins.
The story of human evolution is as much about the development of cultural behaviour as it is about changes in physical appearance. The term culture, in anthropology, traditionally refers to all human creations and activities governed by social customs and rules. It includes elements such as technology, language, and art. Human cultural behaviour depends on the social transfer of information from one generation to the next, which it depends on a sophisticated system of communication, such as language.
The term culture has often been used to distinguish the behaviour of humans from that of other animals. However, some nonhuman animals also appear to have forms of learned cultural behaviours. For instance, different groups of chimpanzees use different techniques to capture termites for food using sticks. Also, in some regions chimps use stones or pieces of wood for cracking open nuts. Chimps in other regions do not practice this behaviour, although their forests have similar nut trees and materials for making tools. These regional differences resemble traditions that people pass from generation to generation. Traditions are a fundamental aspect of culture, and paleoanthropologists assume that the earliest humans also had some types of traditions.
Nonetheless, modern humans differ from other animals, and probably many earlier human species, in that they actively teach each other and can pass on and accumulate unusually large amounts of knowledge. People also have a uniquely long period of learning before adulthood, and the physical and mental capacity for language. Language of all forms, spoken, signed, and written in provides a medium for communicating vast amounts of information, much more than any other animal appears to be able to transmit through gestures and vocalizations.
Scientists have traced the evolution of human cultural behaviour through the study of archaeological artifacts, such as tools, and related evidence, such as the charred remains of cooked food. Artifacts show that throughout much of human evolution, culture has developed slowly. During the Palaeolithic, or early Stone Age, basic techniques for making stone tools changed very little for periods of well more than a million years.
Human fossils also provide information about how culture has evolved and what effects it has had on human life. For example, over the past 30,000 years, the basic anatomy of humans has undergone only one prominent change: The bones of the average human skeleton have become much smaller and thinner. Innovations in the making and use of tools and in obtaining food.- results of cultural evolution may have led to more efficient and less physically taxing lifestyles, and thus caused changes in the skeleton.
Paleoanthropologists and archaeologists have studied many topics in the evolution of human cultural behaviour. These have included the evolution of (1) social life; (2) subsistence (the acquisition and production of food); (3) the making and using of tools; (4) environmental adaptation; (5) symbolic thought and its expression through language, art, and religion; and (6) the development of agriculture and the rise of civilizations.
One of the first physical changes in the evolution of humans from apes-a decrease in the size of male canine teeth
- also indicates a change in social relations. Male apes sometimes use their large canines to threaten (or sometimes fight with) other males of their species, usually over access to females, territory, or food. The evolution of small canines in Australopiths implies that males had either developed other methods of threatening each other or become more cooperative. In addition, both male and female Australopiths had small canines, indicating a reduction of sexual dimorphism from that in apes. Yet, although sexual dimorphism in canine size decreased in Australopiths, males were still much larger than females. Thus, male Australopiths might have competed aggressively with each other based on sheer size and strength, and the social life of humans may not have differed much from that of apes until later times.
Scientists believe that several of the most important changes from apelike to characteristically human social life occurred in species of the genus Homo, whose members show even less sexual dimorphism. These changes, which may have occurred at different times, included (1) prolonged maturation of infants, including an extended period during which they required intensive care from their parents; (2) special bonds of sharing and exclusive mating between particular males and females, called pair-bonding; and (3) the focus of social activity at a home base, a safe refuge in a special location known to family or group members.
Humans, who have a large brain, have a prolonged periods of infant development and childhood because the brain takes a long time too mature. Since the Australopiths brain was not much larger than that of a chimp, some scientists think that the earliest humans had a more apelike rate of growth, which is far more rapid than that of modern humans. This view is supported by studies of Australopiths fossils looking at tooth development-a good indicator of overall body development.
In addition, the human brain becomes very large as it develops, so a woman must give birth to a baby at an early stage of development in order for the infant's head to fit through her birth canal. Thus, human babies require a long period of care to reach a stage of development at which they depend less on their parents. In contrast with a modern female, a female Australopiths could give birth to a baby at an advanced stage of development because its brain would not be too large to pass through the birth canal. The need to give birth early,-and therefore, to provide more infant care-may have evolved around the time of the middle Homo species Homo ergaster. This species had a brain significantly larger than that of the Australopiths, but a narrow birth canal.
Pair-bonding, usually of a short duration, occurs in a variety of primate species. Some scientists speculate that prolonged bonds developed in humans along with increased sharing of food. Among primates, humans have a distinct type of food-sharing behaviour. People will delay eating food until they have returned with it to the location of other members of their social group. This type of food sharing may have arisen at the same time as the need for intensive infant care, probably by the time of H. ergaster. By devoting himself to a particular female and sharing food with her, a male could increase the chances of survival for his own offspring.
Humans have lived as foragers for millions of years. Foragers obtain food when and where it is available over a broad territory. Modern-day foragers (also known as hunter-gatherers) such as, the San people in the Kalahari Desert of southern Africa who also set up central campsites, or home bases, and divide work duties between men and women. Women gather readily available plant and animal foods, while men take on the often less successful task of hunting. For most of the time since the ancestors of modern humans diverged from the ancestors of the living great apes, around seven million years ago, all humans on Earth f ed themselves exclusively by hunting wild animals and gathered wild planets, as the Blackfeet still did in thee 19th century. It was only within the last 11,000 years that some peoples turned to what is termed food production: that is, domesticating wild animals and planets and eating the resulting livestock and crops. Today, most people on Earth consume food that they produced themselves or that someone else produced for them. Some current rates of change, within the next decade the few remaining bands of hunter-gatherers will abandon their ways, disintegrate, or die out, thereby ending our million of the years of commitment to the hunter-gatherers lifestyle. Those few peoples who remained hunter-gatherers into the 20th century escaped replacement by food producers because they ere confined to areas not fit for food production, especially deserts and Arctic regions. Within the present decade, even they will have been seduced by the attractions of civilization, settled down under pressure from bureaucrats or missionaries, or succumbed to germs.
Nevertheless, female and male family members and relatives bring together their food to share at their home base. The modern form of the home base,-that also serves as a haven for raising children and caring for the sick and elderly-may have first developed with middle Homo after about 1.7 million years ago. However, the first evidence of hearths and shelters, -common to all modern home bases-comes from only after 500,000 years ago. Thus, a modern form of social life may not have developed until late in human evolution.
Human subsistence refers to the types of food humans eat, the technology used in and methods of obtaining or producing food, and the ways in which social groups or societies organize them for getting, making, and distributing food. For millions of years, humans probably fed on-the-go, much as other primates do. The lifestyle associated with this feeding strategy is generally organized around small, family-based social groups that take advantage of different food sources at different times of year.
The early human diet probably resembled that of closely related primate species. The great apes eat mostly plant foods. Many primates also eat easily obtained animal foods such as insects and bird eggs. Among the few primates that hunt, chimpanzees will prey on monkeys and even small gazelles. The first humans probably also had a diet based mostly on plant foods. In addition, they undoubtedly ate some animal foods and might have done some hunting. Human subsistence began to diverge from that of other primates with the production and use of the first stone tools. With this development, the meat and marrow (the inner, fat-rich tissue of bones) of large mammals became a part of the human diet. Thus, with the advent of stone tools, the diet of early humans became distinguished in an important way from that of apes.
Scientists have found broken and butchered fossil bones of antelopes, zebras, and other comparably sized animals at the oldest archaeological sites, which go on a date from about 2.5 million years ago. With the evolution of late Homo, humans began to hunt even the largest animals on Earth, including mastodons and mammoths, members of the elephant family. Agriculture and the  of animals arose only in the recent past, with H. sapiens.
Paleoanthropologists have debated whether early members of the modern human genus were aggressive hunters, peaceful plant gatherers, or opportunistic scavengers. Many scientists once thought that predation and the eating of meat had strong effects on early human evolution. This hunting hypothesis suggested that early humans in Africa survived particularly arid periods by aggressively hunting animals with primitive stone or bone tools. Supporters of this hypothesis thought that hunting and competition with carnivores powerfully influenced the evolution of human social organization and behaviour; Toolmaking; anatomy, such as the unique structure of the human hand; and intelligence.
Beginning in the 1960s, studies of apes cast doubt on the hunting hypothesis. Researchers discovered that chimpanzees cooperate in hunts of at least small animals, such as monkeys. Hunting did not, therefore, entirely distinguish early humans from apes, and therefore hunting alone may not have determined the path of early human evolution. Some scientists instead argued in favour of the importance of food-sharing in early human life. According to a food-sharing hypothesis, cooperation and sharing within family groups,- instead of aggressive hunting-strongly influenced the path of human evolution.
Scientists once thought that archaeological sites as much as two million years old provided evidence to support the food-sharing hypothesis. Some of the oldest archaeological sites were places where humans brought food and stone tools together. Scientists thought that these sites represented home bases, with many social features of modern hunter-gatherers campsites, including the sharing of food between pair-bonded males and females.
A critique of the food-sharing hypothesis resulted from more careful study of animal bones from the early archaeological sites. Microscopic analysis of these bones revealed the marks of human tools and carnivore teeth, showing that both humans and potential predators, such as hyenas, cats, and jackals were active at these sites. This evidence suggested that what scientists had thought were home bases where early humans shared food were in fact food-processing sites that humans abandoned to predators. Thus, evidence did not clearly support the idea of food-sharing among early humans.
The new research also suggested a different view of early human subsistence that early humans scavenged meat and bone marrow from dead animals and did little hunting. According to this scavenging hypothesis, early humans opportunistically took parts of animal carcasses left by predators, and then used stone tools to remove marrow from the bones.
Observations that many animals, such as antelope, often die off in the dry season make the scavenging hypothesis quite plausible. Early Toolmaker would have had plenty of opportunity to scavenge animal fat and meat during dry times of the year. However, other archaeological studies-and a better appreciation of the importance of hunting among chimpanzees-suggest that the scavenging hypothesis is too narrow. Many scientists now believe that early humans both scavenged and hunted. Evidence of carnivore tooth marks on bones cut by early human Toolmaker suggests that the humans scavenged at least the larger of the animals they ate. They also ate a variety of plant foods. Some disagreement remains, however, as to how much early humans relied on hunting, especially the hunting of smaller animals.
Scientists debate when humans first began hunting on a regular basis. For instance, elephant fossils found with tools made by middle Homo once led researchers to the idea that members of this species were hunters of big game. However, the simple association of animal bones and tools at the same site does not necessarily mean that early humans had killed the animals or eaten their meat. Animals may die in many ways, and natural forces can accidentally place fossils next to tools. Recent excavations at Olorgesailie, Kenya, show that H. erectus cut meat from elephant carcasses but give rise of not revealing to whether these humans were regular or specialized hunters.
Humans who lived outside of Africa,-especially in colder temperate climates,-almost necessitated eating more meat than their African counterparts. Humans in temperate Eurasia would have had to learn about which plants they could safely eat, and the number of available plant foods would drop significantly during the winter. Still, although scientists have found very few fossils of edible or eaten plants at early human sites, early inhabitants of Europe and Asia probably did eat plant foods in addition to meat.
Sites that provide the clearest evidence of early hunting include Boxgrove, England, where about 500,000 years ago people trapped a great number of large game animals between a watering hole and the side of a cliff and then slaughtered them. At Schningen, Germany, a site about 400,000 years old, scientists have found wooden spears with sharp ends that were well designed for throwing and probably used in hunting large animals.
Neanderthals and other archaic humans seem to have eaten whatever animals were available at a particular time and place. So, for example, in European Neanderthal sites, the number of bones of reindeer (a cold-weather animal) and red deer (a warm-weather animal) changed depending on what the climate had been like. Neanderthals probably also combined hunting and scavenging to obtain animal protein and fat.
For at least the past 100,000 years, various human groups have eaten foods from the ocean or coast, such as shellfish and some sea mammals and birds. Others began fishing in interior rivers and lakes. Between probably 90,000 and 80,000 years ago people in Katanda, in what is now the Democratic Republic of the Congo, caught large catfish using a set of barbed bone points, the oldest known specialized fishing implements. The oldest stone tips for arrows or spears date from about 50,000 to 40,000 years ago. These technological advances, probably first developed by early modern humans, indicate an expansion in the kinds of foods humans could obtain.
Beginning 40,000 years ago humans began making even more significant advances in hunting dangerous animals and large herds, and in exploiting ocean resources. People cooperated in large hunting expeditions in which they killed great numbers of reindeer, bison, horses, and other animals of the expansive grasslands that existed at that time. In some regions, people became specialists in hunting certain kinds of animals. The familiarity these people had with the animals they hunted appears in sketches and paintings on cave walls, dating from as much as 32,000 years ago. Hunters also used the bones, ivory, and antlers of their prey to create art and beautiful tools. In some areas, such as the central plains of North America that once teemed with a now-extinct type of large bison (Bison occidentalis), hunting may have contributed to the extinction of entire species.
The making and use of tools alone probably did not distinguish early humans from their ape predecessors. Instead, humans made the important breakthrough of using one tool to make another. Specifically, they developed the technique of precisely hitting one stone against another, known as knapping. Stone Toolmaking characterized the period sometimes referred to as the Stone Age, which began at least 2.5 million years ago in Africa and lasted until the development of metal tools within the last 7,000 years (at different times in different parts of the world). Although early humans may have made stone tools before 2.5 million years ago, Toolmaker may not have remained long enough in one spot to leave clusters of tools that an archaeologist would notice today.
The earliest simple form of stone Toolmaking involved breaking and shaping an angular rock by hitting it with a palm-sized round rock known as a hammerstone. Scientists refer to tools made in this way as Oldowan, after Olduvai Gorge in Tanzania, a site from which many such tools have come. The Oldowan tradition lasted for about one million years. Oldowan tools include large stones with a chopping edge, and small, sharp flakes that could be used to scrape and slice. Sometimes Oldowan Toolmaker used anvil stones (flat rocks found or placed on the ground) on which hard fruits or nuts could be broken open. Chimpanzees are known to do this today.
Scientists once thought that Oldowan Toolmaker intentionally produced several different types of tools. It now appears that differences in the shapes of larger tools were some byproducts of detaching flakes from a variety of natural rock shapes. Learning the skill of Oldowan Toolmaking assiduously required observation, but not necessarily instruction or language. Thus, Oldowan tools were simple, and their makers used them for such purposes as cutting up animal carcasses, breaking bones to obtain marrow, cleaning hides, and sharpening sticks for digging up edible roots and tubers.
Oldowan Toolmaker sought out the best stones for making tools and carried them to food-processing sites. At these sites, the Toolmaker would butcher carcasses and eat the meat and marrow, thus avoiding any predators that might return to a kill. This behaviour of bringing food and tools together contrasts with an eat-as-you-go strategy of feeding commonly seen in other primates.
The Acheulean Toolmaking traditions, which began sometime between 1.7 million and 1.5 million years ago, consisted of increasingly symmetrical tools, most of which scientists refer as to hand-axes and cleavers. Acheulean Toolmaker, such as Homo erectus, also worked with much larger pieces of stone than did Oldowan Toolmaker. The symmetry and size of later Acheulean tools show increased planning and design-and thus probably increased intelligence-on the part of the Toolmaker. The Acheulean tradition continued for more than 1.35 million years.
The next significant advances in stone Toolmaking were made by at least 200,000 years ago. One of these methods of Toolmaking, known as the prepared core technique (and Levallois in Europe), involved carefully and exactingly knocking off small flakes around one surface of a stone and then striking it from the side to produce a preformed tool blank, which could then be worked further. Within the past 40,000 years, modern humans developed the most advanced stone Toolmaking techniques. The so-called prismatic-blade core Toolmaking technique involved removing the top from a stone, leaving a flat platform, and then breaking off multiple blades down the sides of the stone. Each blade had a triangular cross-section, giving it excellent strength. Using these blades as blanks, people made exquisitely shaped spearheads, knives, and numerous other kinds of tools. The most advanced stone tools also exhibit distinct and consistent regional differences in style, indicating a high degree of cultural diversity.
Early humans experienced dramatic shifts in their environments over time. Fossilized plant pollen and animal bones, along with the chemistry of soils and sediments, reveal much about the environmental conditions to which humans had to adapt.
By eight million years ago, the continents of the world, which move over very long periods, had come to the positions they now occupy. However, the crust of the Earth has continued to move since that time. These movements have dramatically altered landscapes around the world. Important geological changes that affected the course of human evolution include those in southern Asia that formed the Himalayan mountain chain and the Tibetan Plateau, and those in eastern Africa that formed the Great Rift Valley. The formation of major mountain ranges and valleys led to changes in wind and rainfall patterns. In many areas dry seasons became more pronounced, and in Africa conditions became generally cooler and drier.
By five million years ago, the amount of fluctuation in global climate had increased. Temperature fluctuations became quite pronounced during the Pliocene Epoch (five million to 1.6 million years ago). During this time the world entered a period of intense cooling called an ice age, which began from place to place of 2.8 million years ago. Ice ages cycle through colder phases known as glacial (times when glaciers form) and warmer phases known as interglacial (during which glaciers melt). During the Pliocene, glacial and interglacial each lasted about 40,000 years each. The Pleistocene Epoch (1.6 million to 10,000 years ago), in contrast, had much larger and longer ice age fluctuations. For instance, beginning about 700,000 years ago, these fluctuations repeated roughly every 100,000 years.
Between five million and two million years ago, a mixture of forests, woodlands, and grassy habitats covered most of Africa. Eastern Africa entered a significant drying period around 1.7 million years ago, and after one million years ago large parts of the African landscape turned to grassland. So the early Australopiths and early Homo lived in wooded places, whereas Homo ergaster and H. erectus lived in areas of Africa that were more open. Early human populations encountered many new and different environments when they spread beyond Africa, including colder temperatures in the Near East and bamboo forests in Southeast Asia. By about 1.4 million years ago, populations had moved into the temperate zone of northeast Asia, and by 800,000 years ago they had dispersed into the temperate latitudes of Europe. Although these first excursions to latitudes of 400 north and higher may have occurred during warm climate phases, these populations also must have encountered long seasons of cold weather.
All of these changes,-dramatic shifts in the landscape, changing rainfall and drying patterns, and temperature fluctuations posed challenges to the immediate and long-term survival of early human populations. Populations in different environments evolved different adaptations, which in part explains why more than one species existed at the same time during much of human evolution.
Some early human adaptations to new climates involved changes in physical (anatomical) form. For example, the physical adaptation of having a tall, lean body such as that of H. ergaster,-with lots of skin exposed to cooling winds-would have dissipated heat very well. This adaptation probably helped the species to survive in the hotter, more open environments of Africa around 1.7 million years ago. Conversely, the short, wide bodies of the Neanderthals would have conserved heat, helping them to survive in the ice age climates of Europe and western Asia
Increases in the size and complexity of the brain, however, made early humans progressively better at adapting through changes in cultural behaviour. The largest of these brain-size increases occurred over the past 700,000 years, a period during which global climates and environments fluctuated dramatically. Human cultural behaviour also evolved more quickly during this period, most likely in response to the challenges of coping with unpredictable and changeable surroundings
Humans have always adapted to their environments by adjusting their behaviour. For instance, early Australopiths moved both in the trees and on the ground, which probably helped them survive environmental fluctuations between wooded and more open habitats. Early Homo adapted by making stone tools and transporting their food over long distances, thereby increasing the variety and quantities of different foods they could eat. An expanded and flexible diet would have helped these Toolmaker survive unexpected changes in their environment and food supply
When populations of H. erectus moved into the temperate regions of Eurasia, but they faced new challenges to survival. During the colder seasons they had to either move away or seek shelter, such as in caves. Some of the earliest definitive evidence of cave dwellers dates from around 800,000 years ago at the site of Atapuerca in northern Spain. This site may have been home too early H. heidelbergensis populations. H. erectus also used caves for shelter.
Eventually, early humans learned to control fire and to use it to create warmth, cook food, and protect themselves from other animals. The oldest known fire hearths date from between 450,000 and 300,000 years ago, at sites such as Bilzingsleben, Germany; Verteszöllös, Hungary; and Zhoukoudian (Chou-k'ou-tien), China. African sites as old as 1.6 million to 1.2 million years contain burned bones and reddened sediments, but many scientists find such evidence too ambiguous to prove that humans controlled fire. Early populations in Europe and Asia may also have worn animal hides for warmth during glacial periods. The oldest known bone needles, which indicate the development of sewing and tailored clothing, date from about 30,000 to 26,000 years ago.
Behaviour relates directly to the development of the human brain, and particularly the cerebral cortex, the part of the brain that allows abstract thought, beliefs, and expression through language. Humans communicate through the use of symbols-ways of referring to things, ideas, and feelings that communicate meaning from one individual to another but that need not have any direct connection to what they identify. For instance, a word-one types of symbol-does not usually relate directly or actualized among the things or indexical to its held idea, but by its representation, it has only of itself for being abstractive.
People can also paint abstract pictures or play pieces of music that evoke emotions or ideas, even though emotions and ideas have no form or sound. In addition, people can conceive of and believe in supernatural beings and powers-abstract concepts that symbolize real-world events such as the creation of Earth and the universe, the weather, and the healing of the sick. Thus, symbolic thought lies at the heart of three hallmarks of modern human culture: language, art, and religion.
In language, people creatively join words together in an endless variety of sentences,-each with a noun, verb and with the collective distinction in meanings, according to a set of mental rules, or grammar. Language provides the ability to communicate complex concepts. It also allows people to exchange information about both past and future events, about objects that are not present, and about complex philosophical or technical concepts
Language gives people many adaptive advantages, including the ability to plan, to communicate the location of food or dangers to other members of a social group, and to tell stories that unify a group, such as mythologies and histories. However, words, sentences, and languages cannot be preserved like bones or tools, so the evolution of language is one of the most difficult topics to investigate through scientific study.
It appears that modern humans have an inborn instinct for language. Under normal conditions not developing language is almost impossible for a person, and people everywhere go through the same stages of increasing language skill at about the same ages. While people appear to have inborn genetic information for developing language, they learn specific languages based on the cultures from which they come and the experiences they have in life.
The ability of humans to have language depends on the complex structure of the modern brain, which has many interconnected, specific areas dedicated to the development and control of language. The complexity of the brain structures necessary for language suggests that it probably took a long time to evolve. While paleoanthropologists would like to know when these important parts of the brain evolved, endocasts (inside impressions) of early human skulls do not provide enough detail to show this.
Some scientists think that even the early Australopiths had some ability to understand and use symbols. Support for this view comes from studies with chimpanzees. A few chimps and other apes have been taught to use picture symbols or American Sign Language for simple communication. Nevertheless, it appears that language, -as well as art and religious rituals became vital aspects of human life only during the past 100,000 years, primarily within our own species.
Humans also express symbolic thought through many forms of art, including painting, sculpture, and music. The oldest known object of possible symbolic and artistic value dates from about 250,000 years ago and comes from the site of Berekhat Ram, Israel. Scientists have interpreted this object, a figure carved into a small piece of volcanic rock, as a representation of the outline of a female body. Only a few other possible art objects are known from between 200,000 and 50,000 years ago. These items, from western Europe and usually attributed to Neanderthals, include two simple pendants-a tooth and a bone with bored holes,-and several grooved or polished fragments of tooth and bone.
Sites dating from at least 400,000 years ago contain fragments of red and black pigment. Humans might have used these pigments to decorate bodies or perishable items, such as wooden tools or clothing of animal hides, but this evidence would not have survived to today. Solid evidence of the sophisticated use of pigments for symbolic purposes,-such as in religious rituals comes only from after 40,000 years ago. From early in this period, researchers have found carefully made types of crayons used in painting and evidence that humans burned pigments to create a range of colours.
People began to create and use advanced types of symbolic objects between about 50,000 and 30,000 years ago. Much of this art appears to have been used in rituals-possibly ceremonies to ask spirit beings for a successful hunt. The archaeological record shows a tremendous blossoming of art between 30,000 and 15,000 years ago. During this period people adorned themselves with intricate jewellery of ivory, bone, and stone. They carved beautiful figurines representing animals and human forms. Many carvings, sculptures, and paintings depict stylized images of the female body. Some scientists think such female figurines represent fertility.
Early wall paintings made sophisticated use of texture and colour. The area upon which is now Southern France contains many famous sites of such paintings. These include the caves of Chauvet, which contain art more than 30,000 years old, and Lascaux, in which paintings date from as much as 18,000 years ago. In some cases, artists painted on walls that can be reached only with special effort, such as by crawling. The act of getting to these paintings gives them a sense of mystery and ritual, as it must have to the people who originally viewed them, and archaeologists refer to some of the most extraordinary painted chambers as sanctuaries. Yet no one knows for sure what meanings these early paintings and engravings had for the people who made them.
Graves from Europe and western Asia indicate that the Neanderthals were the first humans to bury their dead. Some sites contain very shallow graves, which group or family members may have dug simply to remove corpses from sight. In other cases it appears that groups may have observed rituals of grieving for the dead or communicating with spirits. Some researchers have claimed that grave goods, such as meaty animal bones or flowers, had been placed with buried bodies, suggesting that some Neanderthal groups might have believed in an afterlife. In a large proportion of Neanderthal burials, the corpse had its legs and arms drawn in close to its chest, which could indicate a ritual burial position.
Other researchers have challenged these interpretations, however. They suggest that perhaps the Neanderthals had practically rather than religious reasons for positioning dead bodies. For instance, a body manipulated into a fetal position would need only a small hole for burial, making the job of digging a grave easier. In addition, the animal bones and flower pollen near corpses could have been deposited by accident or without religious intention.
Many scientists once thought that fossilized bones of cave bears (a now-extinct species of large bear) found in Neanderthal caves indicated that these people had what has been referred to as a cave bear cult, in which they worshipped the bears as powerful spirits. However, after careful study researchers concluded that the cave bears probably died while hibernating and that Neanderthals did not collect their bones or worship them. Considering current evidence, the case for religion among Neanderthals remains controversial.
One of the most important developments in human cultural behaviour occurred when people began to domesticate (control the breeding of) plants and animals.  and the advent of agriculture led to the development of dozens of staple crops (foods that forms the basis of an entire diet) in temperate and tropical regions around the world. Almost the entire population of the world today depends on just four of these major crops: wheat, rice, corn, and potatoes.
The growth of farming and animal herding initiated one of the most remarkable changes ever in the relationship between humans and the natural environment. The change first began just 10,000 years ago in the Near East and has accelerated very rapidly since then. It also occurred independently in other places, including areas of Mexico, China, and South America. Since the first of plants and animals, many species over large areas of the planet have come under human control. The overall number of plant and animal species has decreased, while the populations of a few species needed to support large human populations have grown immensely. In areas dominated by people, interactions between plants and animals usually fall under the control of a single species,-Homo sapiens.
The rise of civilizations-the large and complex types of societies in which most people still live today-developed along with surplus food production. People of high status eventually used food surpluses as a way to pay for labour and to create alliances among groups, often against other groups. In this way, large villages could grow into city-states (urban centres that governed them) and eventually empires covering vast territories. With surplus food production, many people could work exclusively in political, religious, or military positions, or in artistic and various skilled vocations. Command of food surpluses also enabled rulers to control labourers, such as in slavery. All civilizations developed based on such hierarchical divisions of status and vocation.
The earliest civilization arose more than 7,000 years ago in Sumer in what is now Iraq. Sumer grew powerful and prosperous by 5,000 years ago, when it entered on the city-state of Ur. The region containing Sumer, known as Mesopotamia, was the same area in which people had first domesticated animals and plants. Other centres of early civilizations include the Nile Valley of Northeast Africa, the Indus. Valley of South Asia, the Yellow River Valley of East Asia, the Oaxaca and Mexico valleys and the Yucatán region of Central America, and the Andean region of South America, China and Inca Empire.
All early civilizations had some common features. Some of these included a bureaucratic political body, a military, a body of religious leadership, large urban centres, monumental buildings and other works of architecture, networks of trade, and food surpluses created through extensive systems of farming. Many early civilizations also had systems of writing, numbers and mathematics, and astronomy (with calendars); road systems; a formalized body of law; and facilities for education and the punishment of crimes. With the rise of civilizations, human evolution entered a phase vastly different from all before which came. Before this time, humans had lived in small, family-entered groups essentially exposed to and controlled by forces of nature. Several thousand years after the rise of the first civilizations, most people now live in societies of millions of unrelated people, all separated from the natural environment by houses, buildings, automobiles, and numerous other inventions and technologies. Culture will continue to evolve quickly and in unforeseen directions, and these changes will, in turn, influence the physical evolution of Homo sapiens and any other human species to come,-attempt to base ethical reasoning on the presumed fact about evolution. The movement is particularly associated with Spencer, the premise that later elements in an evolutionary path are better than earlier ones, the application of the principle then requires seeing western society, laissez faire capitalism, or another object of approval  as more evolved than more ‘primitive' social forms. Neither the principle nor the application commands much respect. The version of evolutionary ethics called ‘social Darwinism, emphasised the struggle for natural selection, and drew the conclusion that we should glorify and help such struggles, usually by enchaining competitive and aggressive relations between people in society, or between societies themselves. More recently subjective matters and opposing physical theories have rethought the relations between evolution and ethics in the light of biological discoveries concerning altruism and kin-selection.
It is, nevertheless, and, least of mention, that Sociobiology (the academic discipline best known through the work of Edward O. Alison who coined the tern in his Sociobiology: the New Synthesise, 1975). The approach to human behaviour is based on the premise that all social behaviour has a biological basis, and seeks to understand that logical basis as to genetic encoding for features that are themselves selected for through evolutionary history. The philosophical problem is essentially of methodology of finding criteria for identifying features that are objectively manifest in that they can usefully identify features, which classical epistemology can usefully explain in this way, and for finding criteria for assessing various genetic stories that might provide useful explanations among the features proposed for this kind of explanation are such things as male dominance, male promiscuity versus female fidelity, propensities to sympathy and other emotions, and the limited altruism characteristics accused of ignoring the influence of environmental and social factors in moulding people's characteristics, e.g., at the limit of silliness, by postulating a ‘gene for poverty, however there is no need for the approach to committing such errors, since the feature explained sociobiologically may be indexical to environmental considerations: For instance, it may be a propensity to develop some feature in some social or order environment, or even a propensity to develop propensities . . . That man's problem was to separate genuine explanation from speculatively methodological morally stories, which may or may not identify really selective mechanisms
Scientists are unbiased observers who use the scientific method to confirm conclusively and falsify various theories. These experts have no preconceptions in gathering the data and logically derive theories from these objective observations. One great strength of science is that its self-correcting, because scientists readily abandon theories when their use has been forfeited, and then again they have shown them to be irrational, although many people have accepted such eminent views of science, they are almost completely untrue. Data can neither conclusively confirm nor conclusively falsify theories, there really is no such thing as the scientific method, data become subjective in practice, and scientists have displayed a surprising fierce loyalty to their theories. There have been many misconceptions of what science is and what science is not.
Science, is, and should be the systematic study of anything that breathes, walk of its own locomotion, in a bipedal orthogonality, and has some effectual regard for its own responsibility of Beingness, and, of course, have to some degreeable form in living personal manner. In that others of science can examine, test, and verify. Not-knowing or knowing has derived the word science from the Latin word scribe meaning ‘to know.' From its beginnings, science has developed into one of the greatest and most influential fields of human endeavour. Today different branches of science investigate almost everything that thumps in the night in that can observe or detect, and science as the whole shape in the way we understand the universe, our planet, ourselves, and other living things.
Science develops through objective analysis, instead of through personal belief. Knowledge gained in science accumulates as time goes by, building to a turn of work through with what has ben foregoing. Some of this knowledge, such as our understanding of numbers, stretches back to the time of ancient civilizations, when scientific thought first began. Other scientific knowledge,-such as our understanding of genes that cause cancer or of quarks (the smallest known building block of matter), dates back to less than fifty years. However, in all fields of science, old or new, researchers use the same systematic approach, known as the scientific method, to add to what governing evolutionary principles have known.
During scientific investigations, scientists put together and compare new discoveries and existing knowledge. Commonly, new discoveries extend what continuing phenomenons have currently accepted, providing further evidence that existing idea are correct. For example, in 1676 the English physicist Robert Hooke discovered those elastic objects, such as metal springs, stretches in proportion to the force that acts on them. Despite all the advances made in physics since 1676, this simple law still holds true.
Scientists use existing knowledge in new scientific investigations to predict how things will behave. For example, a scientist who knows the exact dimensions of a lens can predict how the lens will focus a beam of light. In the same way, by knowing the exact makeup and properties of two chemicals, a researcher can predict what will happen when they combine. Sometimes scientific predictions go much further by describing objects or events those existing object relations have not yet known. An outstanding instance occurred in 1869, when the Russian chemist Dmitry Mendeleyev drew up a periodic table of the elements arranged to illustrate patterns of recurring chemical and physical properties. Mendeleyev used this table to predict the existence and describe the properties of several elements unknown in his day, and when the mysteriousness of science began the possibilities of experimental simplicities in the discovering enactments whose elements, under which for the several years past, the later, predictions were correct.
In science, and only through experimentation can we find the sublime simplicities of our inherent world, however, by this similarity to theoretical implications can we manifest of what can also be made important as when current ideas are shown to be wrong. A classic case of this occurred early in the 20th century, when the German geologist Alfred Wegener suggested that the continents were at once connected, a theory known as continental drift. At the time, most geologists discounted Wegener's ideas, because the Earth's crust may be fixed. However, following the discovery of plate tectonics in the 1960's, in which scientists found that the Earth's crust is made of moving plates, continental drift became an important part of geology.
Through advances like these, scientific knowledge is constantly added to and refined. As a result, science gives us an ever more detailed insight into the way the world around us works.
For a large part of recorded history, science had little bearing on people's everyday lives. Scientific knowledge was gathered for its own sake, and it had few practical applications. However, with the dawn of the Industrial Revolution in the 18th century, this rapidly changed. Today, science affects the way we live, largely through technology-the use of scientific knowledge for practical purposes.
Some forms of technology have become so well established that forgetting the great scientific achievements that they represent is easy. The refrigerator, for example, owes its existence to a discovery that liquids take in energy when they evaporate, a phenomenon known as latent heat. The principle of latent heat was first exploited in a practical way in 1876, and the refrigerator has played a major role in maintaining public health ever since. The first automobile, dating from the 1880's, used many advances in physics and engineering, including reliable ways of generating high-voltage sparks, while the first computers emerged in the 1940's from simultaneous advances in electronics and mathematics.
Other fields of science also play an important role in the things we use or consume every day. Research in food technology has created new ways of preserving and flavouring what we eat. Research in industrial chemistry has created a vast range of plastics and other synthetic materials, which have thousands of uses in the home and in industry. Synthetic materials are easily formed into complex shapes and can be used to make machine, electrical, and automotive parts, scientific and industrial instruments, decorative objects, containers, and many other items. Alongside these achievements, science has also caused technology that helps save human life. The kidney dialysis machine enables many people to survive kidney diseases that would once have proved fatal, and artificial valves allow sufferers of coronary heart disease to return to active living. Biochemical research is responsible for the antibiotics and vaccinations that protect us from infectious diseases, and for a wide range of other drugs used to combat specific health problems. As a result, the majority of people on the planet now live longer and healthier lives than ever before.
However, scientific discoveries can also have a negative impact in human affairs. Over the last hundred years, some technological advances that make life easier or more enjoyable have proved to have unwanted and often unexpected long-term effects. Industrial and agricultural chemicals pollute the global environment, even in places as remote as Antarctica, and city air is contaminated by toxic gases from vehicle exhausts. The increasing pace of innovation means that products become rapidly obsolete, adding to a rising tide of waste. Most significantly of all, the burning of fossil fuels such as coal, oil, and natural gas releases into the atmosphere carbon dioxide and other substances knew as greenhouse gases. These gases have altered the composition of the entire atmosphere, producing global warming and the prospect of major climate change in years to come.
Science has also been used to develop technology that raises complex ethical questions. This is particularly true in the fields of biology and medicine. Research involving genetic engineering, cloning, and in vitro fertilization gives scientists the unprecedented power to cause new life, or to devise new forms of living things. At the other extreme, science can also generate technology that is deliberately designed to harm or to kill. The fruits of this research include chemical and biological warfare, and nuclear weapons, by far the most destructive weapons that the world has ever known.
Scientific research can be divided into basic science, also known as pure science, and applied science. In basic science, scientists working primarily at academic institutions pursue research simply to satisfy the thirst for knowledge. In applied science, scientists at industrial corporations conduct research to achieve some kind of practical or profitable gain.
In practice, the division between basic and applied science is not always clear-cut. This is because discoveries that initially seem to have no practical use often develop one as time goes away. For example, superconductivity, the ability to conduct electricity with no resistance, was little more than a laboratory curiosity when Dutch physicist Heike Kamerlingh Omnes discovered it in 1911. Today superconducting electromagnets are used in several of important applications, from diagnostic medical equipment to powerful particle accelerators.
Scientists study the origin of the solar system by analysing meteorites and collecting data from satellites and space probes. They search for the secrets of life processes by observing the activity of individual molecules in living cells. They observe the patterns of human relationships in the customs of aboriginal tribes. In each of these varied investigations the questions asked and the means employed to find answers are different. All the inquiries, however, share a common approach to problem solving known as the scientific method. Scientists may work alone or they may collaborate with other scientists. Always, a scientist's work must measure up to the standards of the scientific community. Scientists submit their findings to science forums, such as science journals and conferences, to subject the findings to the scrutiny of their peers.
Whatever the aim of their work, scientists use the same underlying steps to organize their research: (1) they make detailed observations about objects or processes, either as they occur in nature or as they take place during experiments; (2) they collect and analyse the information observed; and (3) they formulate a hypothesis that explains the behaviour of the phenomena observed.
A scientist begins an investigation by observing an object or an activity. Observations typically involve one or more of the human senses, like hearing, sight, smells, tastes, and touch. Scientists typically use tools to aid in their observations. For example, a microscope helps view objects too small to be seen with the unaided human eye, while a telescope views objects too far away to be seen by the unaided eye.
Scientists typically implement their observation skills to an experiment. An experiment is any kind of trial that enables scientists to control and change at will the conditions under which events occur. It can be something extremely simple, such as heating a solid to see when it melts, or the periodical perception to differences of complexity, such as bouncing a radio signal off the surface of a distant planet. Scientists typically repeat experiments, sometimes many times, in order to be sure that the results were not affected by unforeseen factors.
Most experiments involve real objects in the physical world, such as electric circuits, chemical compounds, or living organisms. However, with the rapid progress in electronics, computer simulations can now carry out some experiments instead. If they are carefully constructed, these simulations or models can accurately predict how real objects will behave.
One advantage of a simulation is that it allows experiments to be conducted without any risks. Another is that it can alter the apparent passage of time, speeding up or slowing natural processes. This enables scientists to investigate things that happen very gradually, such as evolution in simple organisms, or ones that happen almost instantaneously, such as collisions or explosions.
During an experiment, scientists typically make measurements and collect results as they work. This information, known as data, can take many forms. Data may be a set of numbers, such as daily measurements of the temperature in a particular location or a description of side effects in an animal that has been given an experimental drug. Scientists typically use computers to arrange data in ways that make the information easier to understand and analysed data may be arranged into a diagram such as a graph that shows how one quantity (body temperature, for instance) varies in relation to another quantity (days since starting a drug treatment). A scientist flying in a helicopter may collect information about the location of a migrating herd of elephants in Africa during different seasons of a year. The data collected maybe in the form of geographic coordinates that can be plotted on a map to provide the position of the elephant herd at any given time during a year.
Scientists use mathematics to analyse the data and help them interpret their results. The types of mathematical use that include statistics, which is the analysis of numerical data, and probability, which calculates the likelihood that any particular event will occur.
Once an experiment has been carried out, data collected and analysed, scientists look for whatever pattern their results produce and try to formulate a hypothesis that explains all the facts observed in an experiment. In developing a hypothesis, scientists employ methods of induction to generalize from the experiment's results to predict future outcomes, and deduction to infer new facts from experimental results.
Formulating a hypothesis may be difficult for scientists because there may not be enough information provided by a single experiment, or the experiment's conclusion may not fit old theories. Sometimes scientists do not have any prior idea of a hypothesis before they start their investigations, but often scientists start out with a working hypothesis that will be proved or disproved by the results of the experiment. Scientific hypotheses can be useful, just as hunches and intuition can be useful in everyday life. Still, they can also be problematic because they tempt scientists, either deliberately or unconsciously, to favour data that support their ideas. Scientists generally take great care to avoid bias, but it remains an ever-present threat. Throughout the history of science, numerous researchers have fallen into this trap, either in the promise of self-advancement that perceive to be the same or that they firmly believe their ideas to be true.
If a hypothesis is borne out by repeated experiments, it becomes a theory-an explanation that seems to fit with the facts consistently. The ability to predict new facts or events is a key test of a scientific theory. In the 17th century German astronomer Johannes Kepler proposed three theories concerning the motions of planets. Kepler's theories of planetary orbits were confirmed when they were used to predict the future paths of the planets. On the other hand, when theories fail to provide suitable predictions, these failures may suggest new experiments and new explanations that may lead to new discoveries. For instance, in 1928 British microbiologist Frederick Griffith discovered that the genes of dead virulent bacteria could transform harmless bacteria into virulent ones. The prevailing theory at the time was that genes were made of proteins. Nevertheless, studies succeeded by Canadian-born American bacteriologist Oswald Avery and colleagues in the 1930's repeatedly showed that the transforming gene was active even in bacteria from which protein was removed. The failure to prove that genes were composed of proteins spurred Avery to construct different experiments and by 1944 Avery and his colleagues had found that genes were composed of deoxyribonucleic acid (DNA), not proteins.
If other scientists do not have access to scientific results, the research may as well not have had the liberated amounts of time at all. Scientists need to share the results and conclusions of their work so that other scientists can debate the implications of the work and use it to spur new research. Scientists communicate their results with other scientists by publishing them in science journals and by networking with other scientists to discuss findings and debate issues.
In science, publication follows a formal procedure that has set rules of its own. Scientists describe research in a scientific paper, which explains the methods used, the data collected, and the conclusions that can be drawn. In theory, the paper should be detailed enough to enable any other scientist to repeat the research so that the findings can be independently checked.
Scientific papers usually begin with a brief summary, or abstract, that describes the findings that follow. Abstracts enable scientists to consult papers quickly, without having to read them in full. At the end of most papers is a list of citations-bibliographic references that acknowledge earlier work that has been drawn on in the course of the research. Citations enable readers to work backwards through a chain of research advancements to verify that each step is soundly based.
Scientists typically submit their papers to the editorial board of a journal specializing in a particular field of research. Before the paper is accepted for publication, the editorial board sends it out for peer review. During this procedure a panel of experts, or referees, assesses the paper, judging whether or not the research has been carried out in a fully scientific manner. If the referees are satisfied, publication goes ahead. If they have reservations, some of the research may have to be repeated, but if they identify serious flaws, the entire paper may be rejected from publication.
The peer-review process plays a critical role because it ensures high standards of scientific method. However, it can be a contentious area, as it allows subjective views to become involved. Because scientists are human, they cannot avoid developing personal opinions about the value of each other's work. Furthermore, because referees tend to be senior figures, they may be less than welcoming to new or unorthodox ideas.
Once a paper has been accepted and published, it becomes part of the vast and ever-expanding body of scientific knowledge. In the early days of science, new research was always published in printed form, but today scientific information spreads by many different means. Most major journals are now available via the Internet (a network of linked computers), which makes them quickly accessible to scientists all over the world.
When new research is published, it often acts as a springboard for further work. Its impact can then be gauged by seeing how often the published research appears as a cited work. Major scientific breakthroughs are cited thousands of times a year, but at the other extreme, obscure pieces of research may be cited rarely or not at all. However, citation is not always a reliable guide to the value of scientific work. Sometimes a piece of research will go largely unnoticed, only to be rediscovered in subsequent years. Such was the case for the work on genes done by American geneticist Barbara McClintock during the 1940s. McClintock discovered a new phenomenon in corn cells known as ‘transposable genes', sometimes referred to as jumping genes. McClintock observed that a gene could move from one chromosome to another, where it would break the second chromosome at a particular site, insert itself there, and influence the function of an adjacent gene. Her work was largely ignored until the 1960s when scientists found that transposable genes were a primary means for transferring genetic material in bacteria and more complex organisms. McClintock was awarded the 1983 Nobel Prize in physiology or medicine for her work in transposable genes, more than thirty-five years after doing the research.
In addition to publications, scientists form associations with other scientists from particular fields. Many scientific organizations arrange conferences that bring together scientists to share new ideas. At these conferences, scientists present research papers and discuss their implications. In addition, science organizations promote the work of their members by publishing newsletters and Web sites; networking with journalists at newspapers, magazines, and television stations to help them understand new findings; and lobbying lawmakers to promote government funding for research.
The oldest surviving science organization is the Academia dei Lincei, in Italy, which was established in 1603. The same century also saw the inauguration of the Royal Society of London, founded in 1662, and the Académie des Sciences de Paris, founded in 1666. American scientific societies date back to the 18th century, when American scientist and diplomat Benjamin Franklin founded a philosophical club in 1727. In 1743 this organization became the American Philosophical Society, which still exists today.
In the United States, the American Association for the Advancement of Science (AAAS) plays a key role in fostering the public understanding of science and in promoting scientific research. Founded in 1848, it has nearly 300 affiliated organizations, many of which originally developed from AAAS special-interest groups.
Since the late 19th century, communication among scientists has also been improved by international organizations, such as the International Bureau of Weights and Measures, founded in 1873, the International Council of Research, founded in 1919, and the World Health Organization, founded in 1948. Other organizations act as international forums for research in particular fields. For example, the Intergovernmental Panel on Climate Change (IPCC), established in 1988, as research on how climate change occurs, and what affects change is likely to have on humans and their environment.
Classifying sciences involves arbitrary decisions because the universe is not easily split into separate compartments. This article divides science into five major branches: mathematics, physical sciences, earth sciences, life sciences, and social sciences. A sixth branch, technology, draws on discoveries from all areas of science and puts them to practical use. Each of these branches itself consists of numerous subdivisions. Many of these subdivisions, such as astrophysics or biotechnology, combine overlapping disciplines, creating yet more areas of research.
The 20th century mathematics made rapid advances on all fronts. The foundations of mathematics became more solidly grounded in logic, while at the same time mathematics advanced the development of symbolic logic. Philosophy was not the only field to progress with the help of mathematics. Physics, too, benefited from the contributions of mathematicians to relativity theory and quantum theory. In fact, mathematics achieved broader applications than ever before, as new fields developed within mathematics (computational mathematics, game theory, and chaos theory) and other branches of knowledge, including economics and physics, achieved firmer grounding through the application of mathematics. Even the most abstract mathematics seemed to find application, and the boundaries between pure mathematics and applied mathematics grew ever fuzzier Mathematicians searched for unifying principles and general statements that applied to large categories of numbers and objects. In algebra, the study of structure continued with a focus on structural units called rings, fields, and groups, and at mid-century it extended to the relationships between these categories. Algebra became an important part of other areas of mathematics, including analysis, number theory, and topology, as the search for unifying theories moved ahead. Topology—the studies of the properties of objects that remain constant during transformation, or stretching-became a fertile research field, bringing together geometry, algebra, and analysis. Because of the abstract and complex nature of most 20th-century mathematics, most of the remaining sections of this article will discuss practical developments in mathematics with applications in more familiar fields.
Until the 20th century the centres of mathematics research in the West were all located in Europe. Although the University of Göttingen in Germany, the University of Cambridge in England, the French Academy of Sciences and the University of Paris, and the University of Moscow in Russia retained their importance, the United States rose in prominence and reputation for mathematical research, especially the departments of mathematics at Princeton University and the University of Chicago.
At the Second International Congress of Mathematicians held in Paris in 1900, German mathematician David Hilbert spoke to the assembly. Hilbert was a professor at the University of Göttingen, the former academic home of Gauss and Riemann. Hilbert's speech at Paris was a survey of twenty-three mathematical problems that he felt would guide the work being done in mathematics during the coming century. These problems stimulated a great deal of the mathematical research of the 20th century, and many of the problems were solved. When news breaks that another ‘Hilbert problem' has been solved, mathematicians worldwide impatiently await further details.
Hilbert contributed to most areas of mathematics, starting with his classic Grundlagen der Geometric (Foundations of Geometry), published in 1899. Hilbert's work created the field of functional analysis (the analysis of functions as a group), a field that occupied many mathematicians during the 20th century. He also contributed to mathematical physics. From 1915 on he fought to have Emmy Noether, a noted German mathematician, hired at Göttingen. When the university refused to hire her because of objections to the presence of a woman in the faculty senate, Hilbert countered that the senate was not the changing room for a swimming pool. Noether later made major contributions to ring theory in algebra and wrote a standard text on abstract algebra.
In some ways pure mathematics became more abstract in the 20th century, as it joined forces with the field of symbolic logic in philosophy. The scholars who bridged the fields of mathematics and philosophy early in the century were Alfred North Whitehead and Bertrand Russell, who worked together at Cambridge University. They published their major work, Principia Mathematica (Principles of Mathematics), in three volumes from 1910 to 1913. In it they demonstrated the principles of mathematical logic and attempted to show that all of the mathematics could be deduced from a few premises and definitions by the rules of formal logic. In the late 19th century, German mathematician Gottlob Frége had provided the system of notation for mathematical logic and paved the way for the work of Russell and Whitehead. Mathematical logic influenced the direction of 20th-century mathematics, including the work of Hilbert.
Hilbert proposed that the underlying consistency of all mathematics could be demonstrated within mathematics. Nevertheless, logician Kurt Gödel in Austria proved that the goal of establishing the completeness and consistency of every mathematical theory is impossible. Despite its negative conclusion Gödel's Theorem, published in 1931, opened new areas in mathematical logic. One area, known as recursion theory, played a major role in the development of computers.
Several revolutionary theories, including relativity and quantum theory, challenged existing assumptions in physics in the early 20th century. The work of a number of mathematicians contributed to these theories. Among them was Noether, whose gender had denied her a paid position at the University of Göttingen. Noether's mathematical formulations on invariants (quantities that remain unchanged as other quantities change) contributed to Einstein's theory of relativity. Russian mathematician Hermann Minkowski contributed to relativity the notion of the space-time continuum, with time as a fourth dimension. Hermann Weyl, a student of Hilbert's, investigated the geometry of relativity and applied group theory to quantum mechanics. Weyl's investigations helped advance the field of topology. Early in the century Hilbert quipped, "Physics is getting too difficult for physicists."
Hungarian-born American mathematician John von Neumann built a solid mathematical basis for quantum theory with his text Mathematische Grundlagen der Quantenmechanik (1932, Mathematical Foundations of Quantum Mechanics). This investigation led him to explore algebraic operators and groups associated with them, opening a new area now known as Neumann algebra. Von Neumann, however, is probably best known for his work in game theory and computers.
During World War II (1939-1945) mathematicians and physicists worked together on developing radar, the atomic bomb, and other technology that helped defeat the Axis powers. Polish-born mathematician Stanislaw Ulam solved the problem of how to initiate fusion in the hydrogen bomb. Von Neumann participated in numerous US defence projects during the war.
Mathematics plays an important role today in cosmology and astrophysics, especially in research into big bang theory and the properties of black holes, antimatter, elementary particles, and other unobservable objects and events. Stephen Hawking, among the best-known cosmologists of the 20th century, in 1979 was appointed Lucasian Professor of Mathematics at Trinity College, Cambridge, a position once held by Newton.
Mathematics formed an alliance with economics in the 20th century as the tools of mathematical analysis, algebra, probability, and statistics illuminated economic theories. A specialty called econometrics links enormous numbers of equations to form mathematical models for use as forecasting tools.
Game theory began in mathematics but had immediate applications in economics and military strategy. This branch of mathematics deals with situations in which some sort of decision must be made to maximize a profit-that is, too win. Its theoretical foundations were supplied by von Neumann in a series of papers written during the 1930s and 1940s. Von Neumann and economist Oskar Morgenstern published results of their investigations in The Theory of Games and Economic Behaviour (1944). John Nash, the Princeton mathematician profiled in the motion picture A Beautiful Mind, shared the 1994 Nobel Prize in economics for his work in game theory.
Mathematicians, physicists, and engineers contributed to the development of computers and computer science. Nevertheless, the early, theoretical work came from mathematicians. English mathematician Alan Turing, working at Cambridge University, introduced the idea of a machine that could considerably equate of equal value the mathematical operations and solve equations. The Turing machine, as it became known, was a precursor of the modern computer. Through his work Turing brought together the elements that form the basis of computer science: symbolic logic, numerical analysis, electrical engineering, and a mechanical vision of human thought processes.
Computer theory is the third area with which von Neumann is associated, in addition to mathematical physics and game theory. He established the basic principles on which computers operate. Turing and von Neumann both recognized the usefulness of the binary arithmetic system for storing computer programs.
The first large-scale digital computers were pioneered in the 1940s. Von Neumann completed the EDVAC (Electronic Discrete Variable Automatic Computer) at the Institute of Advanced Study in Princeton in 1945. Engineers John Eckert and John Mauchly built ENIAC (Electronic Numerical Integrator and Calculator), which began operation at the University of Pennsylvania in 1946. As increasingly complex computers are built, the field of artificial intelligence has drawn attention. Researchers in this field attempt to develop computer systems that can mimic human thought processes.
Mathematician Norbert Wiener, working at the Massachusetts Institute of Technology (MIT), also became interested in automatic computing and developed the field known as cybernetics. Cybernetics grew out of Wiener's work on increasing the accuracy of bombsights during World War II. From this came a broader investigation of how information can be translated into improved performance. Cybernetics is now applied to communication and control systems in living organisms.
Computers have exercised an enormous influence on mathematics and its applications. As ever more complex computers are developed, their applications proliferate. Computers have given great impetus to areas of mathematics such as numerical analysis and finite mathematics. Computer science has suggested new areas for mathematical investigation, such as the study of algorithms. Computers also have become powerful tools in areas as diverse as number theory, differential equations, and abstract algebra. In addition, the computer has made possible the solution of several long-standing problems in mathematics, such as the four-colours theorem first proposed in the mid-19th century.
The four-colour theorem stated that four colours are sufficient to colour any map, given that any two countries with a contiguous boundary require different colours. Mathematicians at the University of Illinois finally confirmed the theorem in 1976 by means of a large-scale computer that reduced the number of possible maps too less than 2,000. The program they wrote ran thousands of lines in length and took more than 1,200 hours to run. Many mathematicians, however, do not accept the result as a proof because it has not been checked. Verification by hand would require far too many human hours. Some mathematicians object to the solution's lack of elegance. This complaint has been paraphrased, "a good mathematical proof is like a poem-this are a telephone directory."
Hilbert inaugurated the 20th century by proposing twenty-three problems that he expected to occupy mathematicians for the next 100 years. A number of these problems, such as the Riemann hypothesis about prime numbers, remain unsolved in the early 21st century. Hilbert claimed, "If I were to awaken after having slept for a thousand years, my first question would be: Has the Riemann hypothesis been proven?"
The existence of old problems, along with new problems that continually arise, ensures that mathematics research will remain challenging and vital through the 21st century. Influenced by Hilbert, the Clay Mathematics Institute at Harvard University announced the Millennium Prize in 2000 for solutions to mathematics problems that have long resisted solution. Among the seven problems is the Riemann hypothesis. An award of $1 million awaits the mathematician who solves any of these problems.
Minkowski, Hermann (1864-1909), Russian mathematician, who developed the concept of the space-time continuum. He was born in Russia and attended and then taught at German universities. To the three dimensions of space, Minkowski added the concept of a fourth dimension, time. This concept developed from Albert Einstein's 1905 relativity theory, and became, in turn, the framework for Einstein's 1916 general theory of relativity.
Gravitation is one of the four fundamental forces of nature, along with electromagnetism and the weak and strong nuclear forces, which hold together the particles that make up atoms. Gravitation is by far the weakest of these forces and, as a result, is not important in the interactions of atoms and nuclear particles or even of moderate-sized objects, such as people or cars. Gravitation is important only when very large objects, such as planets, are involved. This is true for several reasons. First, the force of gravitation reaches great distances, while nuclear forces operate only over extremely short distances and decrease in strength very rapidly as distance increases. Second, gravitation is always attractive. In contrast, electromagnetic forces between particles can be repulsive or attractive depending on whether the particles both have a positive or negative electrical charge, or they have opposite electrical charges. These attractive and repulsive forces tend to cancel each other out, leaving only a weak net force. Gravitation has no repulsive force and, therefore, no such cancellation or weakening.
After presenting his general theory of relativity in 1915, German-born American physicist Albert Einstein tried in vain to unify his theory of gravitation with one that would include all the fundamental forces in nature. Einstein discussed his special and general theories of relativity and his work toward a unified field theory in a 1950 Scientific American article. At the time, he was not convinced that he had discovered a valid solution capable of extending his general theory of relativity to other forces. He died in 1955, leaving this problem unsolved. open sidebar.
Gravitation plays a crucial role in most processes on the earth. The ocean tides are caused by the gravitational attraction of the moon and the sun on the earth and its oceans. Gravitation drives whether patterns by making cold air sink and displace less dense warm air, forcing the warm air to rise. The gravitational pull of the earth on all objects holds the objects to the surface of the earth. Without it, the spin of the earth would send them floating off into space.
The gravitational attraction of every bit of matter in the earth for every other bit of matter amounts to an inward pull that holds the earth together against the pressure forces tending to push it outward. Similarly, the inward pull of gravitation holds stars together. When a star's fuel nears depletion, the processes producing the outward pressure weaken and the inward pull of gravitation eventually compresses the star to a very compact size.
Freefall Falling objects accelerate in response to the force exerted on them by Earth's gravity. Different objects accelerate at the same rate, regardless of their mass. This illustration shows the speed at which a ball and a cat would be moving and the distance each would have fallen at intervals of a tenth of a second during a short fall
If an object held near the surface of the earth is released, it will fall and accelerate, or pick up speed, as it descends. This acceleration is caused by gravity, the force of attraction between the object and the earth. The force of gravity on an object is also called the object's weight. This force depends on the object's mass, or the amount of matter in the object. The weight of an object is equal to the mass of the object multiplied by the acceleration due to gravity.
A bowling ball that weighs 16 lb. is being pulled toward the earth with a force of 16 lb? In the metric system, the bowling ball is pulled toward the earth with a force of seventy-one newtons (a metric unit of force abbreviated N). The bowling ball also pulls on the earth with a force of 16 lb. (71 N), but the earth is so massive that it does not move appreciably. In order to hold the bowling ball up and keep it from falling, a person must exert an upward force of 16 lb (71 N) on the ball. This upward force acts to oppose the 16 lb. (71 N) downward weight force, leaving a total force of zero. The total force on an object determines the object's acceleration.
If the pull of gravity is the only force acting on an object, then all objects, regardless of their weight, size, or shape, will accelerate in the same manner. At the same place on the earth, the 16 lb. (71 N) bowling ball and a 500 lb. (2200 N) boulder will fall with the same rate of acceleration. As each second passes, each object will increase its downward speed by about 9.8 m. sec.(thirty-two ft./sec.), resulting in an acceleration of 9.8 m/sec/sec (32 ft. sec/sec). In principle, a rock and a feather both would fall with this acceleration if there were no other forces acting. In practice, however, air friction exerts a greater upward force on the falling feather than on the rock and makes the feather fall more slowly than the rock.
The mass of an object does not change as it is moved from place to place, but the acceleration due to gravity, and therefore the object's weight, will change because the strength of the earth's gravitational pull is not the same everywhere. The earth's pull and the acceleration due to gravity decrease as an object moves farther away from the centre of the earth. At an altitude of 4000 miles (6400 km) above the earth's surface, for instance, the bowling ball that weighed 16 lb (71 N) at the surface would weigh only about 4 lb. (18 N). Because of the reduced weight force, the rate of acceleration of the bowling ball at that altitude would be only one quarter of the acceleration rate at the surface of the earth. The pull of gravity on an object also changes slightly with latitude. Because the earth is not perfectly spherical, but bulges at the equator, the pull of gravity is about 0.5 percent stronger at the earth's poles than at the equator.
The special theory of relativity dealt only with constant, as opposed to accelerated, motion of the frames of reference, and the Lorentz transformations apply to frames moving with uniform motion with respect to each other. In 1915-1916, Einstein extended relativity to account for the more general case of accelerated frames of reference in his general theory of relativity, the central idea in general relativity theory, which accounts for accelerated motion, is that distinguishing between the effects of gravity is impossible and of nonuniform motion, if we did not know, for example, that we were on a spacecraft accelerating at a constant speed and dropped a cup of coffee, we cold not determined whether the mess on the floor was due to the effects of gravity or the accelerated motion, this inability to distinguish between a nonuniform motion, like an acceleration, and gravity is known as the ‘principle of equivalence'.
In this context, Einstein posited the laws elating space and time measurements carried out by two observers moving uniformly, as of one observer in an accelerating spacecraft and another on Earth. Force fields, like gravity, cause space-like, Einstein concluded, to become warped or curved and hence non-Euclidean in form. In the general theory the motion of material points, including light, is not along straight lines, as in Euclidean space, but along geodesics was confirmed in an experiment performed during a total eclipse of the Sun by Arthur Eddington in 1919.
Here, as in the special theory, visualization may help to understand the situation but does not really describe it. This is nicely illustrated in the typical visual analogy used to illustrate what spatial geodesics base. In this tremendous sheet of paper, extends infinitely in all directions. The inhabitants of this flatland, the flatlanders, are not only aware of the third dimension. Since the world here is perfectly Euclidean, any measurement of the sum of lines, no mater how far expended, would never meet.
We are then asked to mov e our flatlanders to a new land on the surface of a large sphere. Initially, our relocated population would perceive their new world as identical to the old, or as Euclidean and flat.  Next we suppose them to send a kind of laser light along the surface of their two world for thousands of mile s. the discovery is then made that if the two beams of light are sent in parallel directions, the come together after travelling a thousand miles.
After experiencing utter confusion in the face of these results, the flatlaners eventually realize that their world is non-Euclidean or curved and invert Riemannian geometry to describe the curved space. The analogy normally concludes with the suggestion that we are the flatlanders, with the difference being that our story takes place in three, rather than two, dimensions in space. Just as the shadow creatures could not visualize the curved two-dimensional surface of their world, so we cannot visualize a three-dimensional curved space.
Thus a visual analogy to illustrate the reality described by the general theory is useful only to the extent that it entices us into an acceptance of the proposition that the reality is unvisualizable. Yet here, as in the special theory, there is no ambiguity in the mathematical description of this reality. Although curved geodesics are not any more unphysical than straight lines, visualizing the three spatial dimensions as a ‘surface' in' in the higher four-dimensional space-time cannot be done. Visualization may help us better understand what is implied by the general theory, but it dies no t disclose what is really meant by the theory,
The ancient Greek philosophers developed several theories about the force that caused objects to fall toward the earth. In the 4th century Bc, the Greek philosopher Aristotle proposed that all things were made from some combination of the four elements, earth, air, fire, and water. Objects that were similar in nature attracted one another, and as a result, objects with more earth in them were attracted to the earth. Fire, by contrast, was dissimilar and therefore tended to rise from the earth. Aristotle also developed a cosmology, that is, a theory describing the universe, that was geocentric, or earth-entered, with the moon, sun, planets, and stars moving around the earth on spheres. The Greek philosophers, however, did not propose a connection between the force behind planetary motion and the force that made objects fall toward the earth.
At the beginning of the 17th century, the Italian physicist and astronomer Galileo discovered that all objects fall toward the earth with the same acceleration, regardless of their weight, size, or shape, when gravity is the only force acting on them. Galileo also had a theory about the universe, which he based on the ideas of the Polish astronomer Nicolaus Copernicus. In the mid-16th century, Copernicus had proposed a heliocentric, or sun-centred system, in which the planets moved in circles around the sun, and Galileo agreed with this cosmology. However, Galileo believed that the planets moved in circles because this motion was the natural path of a body with no forces acting on it. Like the Greek philosophers, he saw no connection between the force behind planetary motion and gravitation on earth.
In the late 16th and early 17th centuries the heliocentric model of the universe gained support from observations by the Danish astronomer Tycho Brahe, and his student, the German astronomer Johannes Kepler. These observations, made without telescopes, were accurate enough to determine that the planets did not move in circles, as Copernicus had suggested. Kepler calculated that the orbits had to be ellipses (slightly elongated circles). The invention of the telescope made even more precise observations possible, and Galileo was one of the first to use a telescope to study astronomy. In 1609 Galileo observed that moons orbited the planet Jupiter, a fact that could not presumably fit into an earth-centred model of the heavens.
The new heliocentric theory changed scientists' views about the earth's place in the universe and opened the way for new ideas about the forces behind planetary motion. However, it was not until the late 17th century that Isaac Newton developed a theory of gravitation that encompassed both the attraction of objects on the earth and planetary motion.
Gravitational forces because the Moon has significantly less mass than Earth, the weight of an object on the Moon's surface is only one-sixth the object's weight on Earth's surface. This graph shows how much and object that weigh on Earth would weigh at different points between the Earth and Moon. Since the Earth and Moon pull in opposite directions, there is a point, about 346,000 km (215,000 mi) from Earth, where the opposite gravitational forces would cancel, and the object's weight would be zero.
To develop his theory of gravitation, Newton first had to develop the science of forces and motion called mechanics. Newton proposed that the natural motion of an object be motion at a constant speed on a straight line, and that it takes a force too slow, speed, or change the path of an object. Newton also invented calculus, a new branch of mathematics that became an important tool in the calculations of his theory of gravitation.
Newton proposed his law of gravitation in 1687 and stated that every particle in the universe attracts every other particle in the universe with a force that depends on the product of the two particles' masses divided by the square of the distance between them. The gravitational force between two objects can be expressed by the following equation: F= GMm/d2 where F is the gravitational force, G is a constant known as the universal constant of gravitation, M and m are the masses of each object, and d is the distance between them. Newton considered a particle to be an object with a mass that was concentrated in a small point. If the mass of one or both particles increases, then the attraction between the two particles increases. For instance, if the mass of one particle is doubled, the force of attraction between the two particles is doubled. If the distance between the particles increases, then the attraction decreases as the square of the distance between them. Doubling the distance between two particles, for instance, will make the force of attraction one quarter as great as it was.
According to Newton, the force acts along a line between the two particles. In the case of two spheres, it acts similar between their centres. The attraction between objects with irregular shapes is more complicated. Every bit of matter in the irregular object attracts every bit of matter in the other object. A simpler description is possible near the surface of the earth where the pull of gravity is approximately uniform in strength and direction. In this case there is a point in an object (even an irregular object) called the centre of gravity, at which all the force of gravity can be considered to be acting.
Newton's law affects all objects in the universe, from raindrops in the sky to the planets in the solar system. It is therefore known as the universal law of gravitation. In order to know the strength of gravitational forces overall, however, it became necessary to find the value of ‘G', the universal constant of gravitation. Scientists needed to re-enact an experiment, but gravitational forces are very weak between objects found in a common laboratory and thus hard to observe. In 1798 the English chemist and physicist Henry Cavendish finally measured G with a very sensitive experiment in which he nearly eliminated the effects of friction and other forces. The value he found was 6.754 x 10-11 N-m2/kg2-close to the currently accepted value of 6.670 x 10-11 N-m2/kg2 (a decimal point followed by ten zeros and then the number 6670). This value is so small that the force of gravitation between two objects with a mass of 1 metric ton each, 1 metre from each other, is about sixty-seven millionths of a newton, or about fifteen millionths of a pound.
Gravitation may also be described in a completely different way. A massive object, such as the earth, may be thought of as producing a condition in space around it called a gravitational field. This field causes objects in space to experience a force. The gravitational field around the earth, for instance, produces a downward force on objects near the earth surface. The field viewpoint is an alternative to the viewpoint that objects can affect each other across distance. This way of thinking about interactions has proved to be very important in the development of modern physics.
Newton's law of gravitation was the first theory to describe the motion of objects on the earth accurately as well as the planetary motion that astronomers had long observed. According to Newton's theory, the gravitational attraction between the planets and the sun holds the planets in elliptical orbits around the sun. The earth's moon and moons of other planets are held in orbit by the attraction between the moons and the planets. Newton's law led to many new discoveries, the most important of which was the discovery of the planet Neptune. Scientists had noted unexplainable variations in the motion of the planet Uranus for many years. Using Newton's law of gravitation, the French astronomer Urbain Leverrier and the British astronomer John Couch each independently predicted the existence of a more distant planet that was perturbing the orbit of Uranus. Neptune was discovered in 1864, in an orbit close to its predicted position.
Frames of Reference, as only a situation can appear different when viewed from different frames of reference. Try to imagine how an observer's perceptions could change from frame to frame in this illustration.
Scientists used Newton's theory of gravitation successfully for many years. Several problems began to arise, however, involving motion that did not follow the law of gravitation or Newtonian mechanics. One problem was the observed and unexplainable deviations in the orbit of Mercury (which could not be caused by the gravitational pull of another orbiting body).
Another problem with Newton's theory involved reference frames, that is, the conditions under which an observer measures the motion of an object. According to Newtonian mechanics, two observers making measurements of the speed of an object will measure different speeds if the observers are moving relative to each other. A person on the ground observing a ball that is on a train passing by will measure the speed of the ball as the same as the speed of the train. A person on the train observing the ball, however, will measure the ball's speed as zero. According to the traditional ideas about space and time, then, there could not be a constant, fundamental speed in the physical world because all speed is relative. However, near the end of the 19th century the Scottish physicist James Clerk Maxwell proposed a complete theory of electric and magnetic forces that contained just such a constant, which he called c. This constant speed was 300,000 km/sec (186,000 mi/sec) and was the speed of electromagnetic waves, including light waves. This feature of Maxwell's theory caused a crisis in physics because it indicated that speed was not always relative.
Albert Einstein resolved this crisis in 1905 with his special theory of relativity. An important feature of Einstein's new theory was that no particle, and even no information, could travel faster than the fundamental speed c. In Newton's gravitation theory, however, information about gravitation moved at infinite speed. If a star exploded into two parts, for example, the change in gravitational pull would be felt immediately by a planet in a distant orbit around the exploded star. According to Einstein's theory, such forces were not possible.
Though Newton's theory contained several flaws, it is still very practical for use in everyday life. Even today, it is sufficiently accurate for dealing with earth-based gravitational effects such as in geology (the study of the formation of the earth and the processes acting on it), and for most scientific work in astronomy. Only when examining exotic phenomena such as black holes (points in space with a gravitational force so strong that not even light can escape them) or in explaining the big bang (the origin of the universe) is Newton's theory inaccurate or inapplicable.
The gravitational attraction of objects for one another is the easiest fundamental force to observe and was the first fundamental force to be described with a complete mathematical theory by the English physicist and mathematician Sir Isaac Newton. A more accurate theory called general relativity was formulated early in the 20th century by the German-born American physicist Albert Einstein. Scientists recognize that even this theory is not correct for describing how gravitation works in certain circumstances, and they continue to search for an improved theory.
Gravitation plays a crucial role in most processes on the earth. The ocean tides are caused by the gravitational attraction of the moon and the sun on the earth and its oceans. Gravitation drives weather patterns by making cold air sink and displace less dense warm air, forcing the warm air to rise. The gravitational pull of the earth on all objects holds the objects to the surface of the earth. Without it, the spin of the earth would send them floating off into space.
The gravitational attraction of every bit of matter in the earth for every other bit of matter amounts to an inward pull that holds the earth together against the pressure forces tending to push it outward. Similarly, the inward pull of gravitation holds stars together. When a star's fuel nears depletion, the processes producing the outward pressure weaken and the inward pull of gravitation eventually compresses the star to a very compact size.
If the pull of gravity is the only force acting on an object, then all objects, regardless of their weight, size, or shape, will accelerate in the same manner. At the same place on the earth, the 16 lb (71 N) bowling ball and a 500 lb (2200 N) boulder will fall with the same rate of acceleration. As each second passes, each object will increase its downward speed by about 9.8 m/sec (32 ft/sec), resulting in an acceleration of 9.8 m/sec/sec (32 ft/sec/sec). In principle, a rock and a feather both would fall with this acceleration if there were no other forces acting. In practice, however, air friction exerts a greater upward force on the falling feather than on the rock and makes the feather fall more slowly than the rock.
The mass of an object does not change as it is moved from place to place, but the acceleration due to gravity, and therefore the object's weight, will change because the strength of the earth's gravitational pull is not the same everywhere. The earth's pull and the acceleration due to gravity decrease as an object moves farther away from the centre of the earth. At an altitude of 4000 miles (6400 km) above the earth's surface, for instance, the bowling ball that weighed 16 lb (71 N) at the surface would weigh only about 4 lb (18 N). Because of the reduced weight force, the rate of acceleration of the bowling ball at that altitude would be only one quarter of the acceleration rate at the surface of the earth. The pull of gravity on an object also changes slightly with latitude. Because the earth is not perfectly spherical, but bulges at the equator, the pull of gravity is about 0.5 percent stronger at the earth's poles than at the equator.
The ancient Greek philosophers developed several theories about the force that caused objects to fall toward the earth. In the 4th century Bc, the Greek philosopher Aristotle proposed that all things were made from some combination of the four elements, earth, air, fire, and water. Objects that were similar in nature attracted one another, and as a result, objects with more earth in them were attracted to the earth. Fire, by contrast, was dissimilar and therefore tended to rise from the earth. Aristotle also developed a cosmology, that is, a theory describing the universe, that was geocentric, or earth-entered, with the moon, sun, planets, and stars moving around the earth on spheres. The Greek philosophers, however, did not propose a connection between the force behind planetary motion and the force that made objects fall toward the earth.
At the beginning of the 17th century, the Italian physicist and astronomer Galileo discovered that all objects fall toward the earth with the same acceleration, regardless of their weight, size, or shape, when gravity is the only force acting on them. Galileo also had a theory about the universe, which he based on the ideas of the Polish astronomer Nicolaus Copernicus. In the mid-16th century, Copernicus had proposed a heliocentric, or sun-entered system, in which the planets moved in circles around the sun, and Galileo agreed with this cosmology. However, Galileo believed that the planets moved in circles because this motion was the natural path of a body with no forces acting on it. Like the Greek philosophers, he saw no connection between the force behind planetary motion and gravitation on earth.
In the late 16th and early 17th centuries the heliocentric model of the universe gained support from observations by the Danish astronomer Tycho Brahe, and his student, the German astronomer Johannes Kepler. These observations, made without telescopes, were accurate enough to determine that the planets did not move in circles, as Copernicus had suggested. Kepler calculated that the orbits had to be ellipses (slightly elongated circles). The invention of the telescope made even more precise observations possible, and Galileo was one of the first to use a telescope to study astronomy. In 1609 Galileo observed that moons orbited the planet Jupiter, a fact that could not presumably fit into an earth-centred model of the heavens.
The new heliocentric theory changed scientists' views about the earth's place in the universe and opened the way for new ideas about the forces behind planetary motion. However, it was not until the late 17th century that Isaac Newton developed a theory of gravitation that encompassed both the attraction of objects on the earth and planetary motion.
Gravitational Forces Because the Moon has significantly less mass than Earth, the weight of an object on the Moon's surface is only one-sixth the object's weight on Earth's surface. This graph shows how much an object that weighs ‘w' on Earth would weigh at different points between the Earth and Moon. Since the Earth and Moon pull in opposite directions, there is a point, about 346,000 km (215,000 mi) from Earth, where the opposite gravitational forces would cancel, and the object's weight would be zero.
To develop his theory of gravitation, Newton first had to develop the science of forces and motion called mechanics. Newton proposed that the natural motion of an object be motion at a constant speed on a straight line, and that it takes a force too slow, speed, or change the path of an object. Newton also invented calculus, a new branch of mathematics that became an important tool in the calculations of his theory of gravitation.
Newton proposed his law of gravitation in 1687 and stated that every particle in the universe attracts every other particle in the universe with a force that depends on the product of the two particles' masses divided by the square of the distance between them. The gravitational force between two objects can be expressed by the following equation: F= GMm/d2 where F is the gravitational force, ‘G' is a constant known as the universal constant of gravitation, ‘M' and ‘m' are the masses of each object, and d is the distance between them. Newton considered a particle to be an object with a mass that was concentrated in a small point. If the mass of one or both particles increases, then the attraction between the two particles increases. For instance, if the mass of one particle is doubled, the force of attraction between the two particles is doubled. If the distance between the particles increases, then the attraction decreases as the square of the distance between them. Doubling the distance between two particles, for instance, will make the force of attraction one quarter as great as it was.
According to Newton, the force acts along a line between the two particles. In the case of two spheres, it acts similarly between their centres. The attraction between objects with irregular shapes is more complicated. Every bit of matter in the irregular object attracts every bit of matter in the other object. A simpler description is possible near the surface of the earth where the pull of gravity is approximately uniform in strength and direction. In this case there is a point in an object (even an irregular object) called the centre of gravity, at which all the force of gravity can be considered to be acting.
Newton's law affects all objects in the universe, from raindrops in the sky to the planets in the solar system. It is therefore known as the universal law of gravitation. In order to know the strength of gravitational forces overall, however, it became necessary to find the value of G, the universal constant of gravitation. Scientists needed to re-enact an experiment, but gravitational forces are very weak between objects found in a common laboratory and thus hard to observe. In 1798 the English chemist and physicist Henry Cavendish finally measured ‘G' with a very sensitive experiment in which he nearly eliminated the effects of friction and other forces. The value he found was 6.754 x 10-11 N-m2/kg2-close to the currently accepted value of 6.670 x 10-11 N-m2/kg2 (a decimal point followed by ten zeros and then the number 6670). This value is so small that the force of gravitation between two objects with a mass of 1 metric ton each, 1 metre from each other, is about sixty-seven millionths of a newton, or about fifteen millionths of a pound.
Gravitation may also be described in a completely different way. A massive object, such as the earth, may be thought of as producing a condition in space around it called a gravitational field. This field causes objects in space to experience a force. The gravitational field around the earth, for instance, produces a downward force on objects near the earth surface. The field viewpoint is an alternative to the viewpoint that objects can affect each other across distance. This way of thinking about interactions has proved to be very important in the development of modern physics.
Newton's law of gravitation was the first theory to describe the motion of objects on the earth accurately as well as the planetary motion that astronomers had long observed. According to Newton's theory, the gravitational attraction between the planets and the sun holds the planets in elliptical orbits around the sun. The earth's moon and moons of other planets are held in orbit by the attraction between the moons and the planets. Newton's law led to many new discoveries, the most important of which was the discovery of the planet Neptune. Scientists had noted unexplainable variations in the motion of the planet Uranus for many years. Using Newton's law of gravitation, the French astronomer Urbain Leverrier and the British astronomer John Couch each independently predicted the existence of a more distant planet that was perturbing the orbit of Uranus. Neptune was discovered in 1864, in an orbit close to its predicted position.
Einstein's general relativity theory predicts special gravitational conditions. The Big Bang theory, which describes the origin and early expansion of the universe, is one conclusion based on Einstein's theory that has been verified in several independent ways.
Another conclusion suggested by general relativity, as well as other relativistic theories of gravitation, is that gravitational effects move in waves. Astronomers have observed a loss of energy in a pair of neutron stars (stars composed of densely packed neutrons) that are orbiting each other. The astronomers theorize that energy-carrying gravitational waves are radiating from the pair, depleting the stars of their energy. Very violent astrophysical events, such as the explosion of stars or the collision of neutron stars, can produce gravitational waves strong enough that they may eventually be directly detectable with extremely precise instruments. Astrophysicists are designing such instruments with the hope that they will be able to detect gravitational waves by the beginning of the 21st century.
Another gravitational effect predicted by general relativity is the existence of black holes. The idea of a star with a gravitational force so strong that light cannot escape from its surface can be traced to Newtonian theory. Einstein modified this idea in his general theory of relativity. Because light cannot escape from a black hole, for any object-a particle, spacecraft, or wave-to escape, it would have to move past light. Nevertheless, light moves outward at the speed c. According to relativity, c is the highest attainable speed, so nothing can pass it. The black holes that Einstein envisioned, then, allow no escape whatsoever. An extension of this argument shows that when gravitation is this strong, nothing can even stay in the same place, but must move inward. Even the surface of a star must move inward, and must continue the collapse that created the strong gravitational force. What remains then is not a star, but a region of space from which emerges a tremendous gravitational force.
Einstein's theory of gravitation revolutionized 20th-century physics. Another important revolution that took place was quantum theory. Quantum theory states that physical interactions, or the exchange of energy, cannot be made arbitrarily small. There is a minimal interaction that comes in a packet called the quantum of an interaction. For electromagnetism the quantum is called the photon. Like the other interactions, gravitation also must be quantized. Physicists call a quantum of gravitational energy a graviton. In principle, gravitational waves arriving at the earth would consist of gravitons. In practice, gravitational waves would consist of apparently continuous streams of gravitons, and individual gravitons could not be detected.
Einstein's theory did not include quantum effects. For most of the 20th century, theoretical physicists have been unsuccessful in their attempts to formulate a theory that resembles Einstein's theory but also includes gravitons. Despite the lack of a complete quantum theory, making some partial predictions about quantized gravitation is possible. In the 1970s, British physicist Stephen Hawking showed that quantum mechanical processes in the strong gravitational pull just outside of black holes would create particles and quanta that move away from the black hole, thereby robbing it of energy.
Astronomy, is the study of the universe and the celestial bodies, gas, and dust within it. Astronomy includes observations and theories about the solar system, the stars, the galaxies, and the general structure of space. Astronomy also includes cosmology, the study of the universe and its past and future. People whom analysis astronomy is called astronomers, and they use a wide variety of methods to achieve of what in finality is obtainably resolved through their research. These methods usually involve ideas of physics, so most astronomers are also astrophysicists, and the terms astronomer and astrophysicist are basically identical. Some areas of astronomy also use techniques of chemistry, geology, and biology.
Astronomy is the oldest science, dating back thousands of years to when primitive people noticed objects in the sky overhead and watched the way the objects moved. In ancient Egypt, he first appearance of certain stars each year marked the onset of the seasonal flood, an important event for agriculture. In 17th-century England, astronomy provided methods of keeping track of time that were especially useful for accurate navigation. Astronomy has a long tradition of practical results, such as our current understanding of the stars, day and night, the seasons, and the phases of the Moon. Much of today's research in astronomy does not address immediate practical problems. Instead, it involves basic research to satisfy our curiosity about the universe and the objects in it. One day such knowledge may be of practical use to humans.
Astronomers use tools such as telescopes, cameras, spectrographs, and computers to analyse the light that astronomical objects emit. Amateur astronomers observe the sky as a hobby, while professional astronomers are paid for their research and usually work for large institutions such as colleges, universities, observatories, and government research institutes. Amateur astronomers make valuable observations, but are often limited by lack of access to the powerful and expensive equipment of professional astronomers.
A wide range of astronomical objects is accessible to amateur astronomers. Many solar system objects-such as planets, moons, and comets - are bright enough to be visible through binoculars and small telescopes. Small telescopes are also sufficient to reveal some of the beautiful detail in nebulas-clouds of gas and dust in our galaxy. Many amateur astronomers observe and photograph these objects. The increasing availability of sophisticated electronic instruments and computers over the past few decades has made powerful equipment more affordable and allowed amateur astronomers to expand their observations too much fainter objects. Amateur astronomers sometimes share their observations by posting their photographs on the World Wide Web, a network of information based on connections between computers.
Amateurs often undertake projects that require numerous observations over days, weeks, months, or even years. By searching the sky over a long period of time, amateur astronomers may observe things in the sky that represent sudden change, such as new comets or novas (stars that brightens suddenly). This type of consistent observation is also useful for studying objects that change slowly over time, such as variable stars and double stars. Amateur astronomers observe meteor showers, sunspots, and groupings of planets and the Moon in the sky. They also participate in expeditions to places in which special astronomical events-such as solar eclipses and meteor showers-are most visible. Several organizations, such as the Astronomical League and the American Association of Variable Star Observers, provide meetings and publications through which amateur astronomers can communicate and share their observations.
Professional astronomers usually have access to powerful telescopes, detectors, and computers. Most work in astronomy includes three parts, or phases. Astronomers first observe astronomical objects by guiding telescopes and instruments to collect the appropriate information. Astronomers then analyse the images and data. After the analysis, they compare their results with existing theories to determine whether their observations match with what theories predict, or whether the theories can be improved. Some astronomers work solely on observation and analysis, and some work solely on developing new theories.
Astronomy is such a broad topic that astronomers specialize in one or more parts of the field. For example, the study of the solar system is a different area of specialization than the study of stars. Astronomers who study our galaxy, the Milky Way, often use techniques different from those used by astronomers who study distant galaxies. Many planetary astronomers, such as scientists who study Mars, may have geology backgrounds and not consider they astronomers at all. Solar astronomers use different telescopes than nighttime astronomers use, because the Sun is so bright. Theoretical astronomers may never use telescopes at all. Instead, these astronomers use existing data or sometimes only previous theoretical results to develop and test theories. An increasing field of astronomy is computational astronomy, in which astronomers use computers to simulate astronomical events. Examples of events for which simulations are useful include the formation of the earliest galaxies of the universe or the explosion of a star to make a supernova.
Astronomers learn about astronomical objects by observing the energy they emit. These objects emit energy in the form of electromagnetic radiation. This radiation travels throughout the universe in the form of waves and can range from gamma rays, which have extremely short wavelengths, to visible light, to radio waves, which are very long. The entire range of these different wavelengths makes up the electromagnetic spectrum.
Astronomers gather different wavelengths of electromagnetic radiation depending on the objects that are being studied. The techniques of astronomy are often very different for studying different wavelengths. Conventional telescopes work only for visible light and the parts of the spectrum near visible light, such as the shortest infrared wavelengths and the longest ultraviolet wavelengths. Earth's atmosphere complicates studies by absorbing many wavelengths of the electromagnetic spectrum. Gamma-ray astronomy, X-ray astronomy, infrared astronomy, ultraviolet astronomy, radio astronomy, visible-light astronomy, cosmic-ray astronomy, gravitational-wave astronomy, and neutrino astronomy all use different instruments and techniques.
Observational astronomers use telescopes or other instruments to observe the heavens. The astronomers who do the most observing, however, probably spend more time using computers than they do using telescopes. A few nights of observing with a telescope often provide enough data to keep astronomers busy for months analysing the data.
Until the 20th century, all observational astronomers studied the visible light that astronomical objects emit. Such astronomers are called optical astronomers, because they observe the same part of the electromagnetic spectrum that the human eye sees. Optical astronomers use telescopes and imaging equipment to study light from objects. Professional astronomers today hardly ever look through telescopes. Instead, a telescope sends an object's light to a photographic plate or to an electronic light-sensitive computer chip called a charge-coupled device, or CCD. CCDs are about fifty times more sensitive than film, so today's astronomers can record in a minute an image that would have taken about an hour to record on film.
Telescopes may use either lenses or mirrors to gather visible light, permitting direct observation or photographic recording of distant objects. Those that use lenses are called refracting telescopes, since they use the property of refraction, or bending, of light. The largest refracting telescope is the 40-in (1-m) telescope at the Yerkes Observatory in Williams Bay, Wisconsin, founded in the late 19th century. Lenses bend different colours of light by different amounts, so different colours focus differently. Images produced by large lenses can be tinged with colour, often limiting the observations to those made through filters. Filters limit the image to one colour of light, so the lens bends all of the light in the image the same amount and makes the image more accurate than an image that includes all colours of light. Also, because light must pass through lenses, lenses can only be supported at the very edges. Large, heavy lenses are so thick that all the large telescopes in current use are made with other techniques.
Reflecting telescopes, which use mirrors, are easier to make than refracting telescopes and reflect all colours of light equally. All the largest telescopes today are reflecting telescopes. The largest single telescopes are the Keck telescopes at Mauna Kea Observatory in Hawaii. The Keck telescope mirrors are 394 in (10.0 m) in diameter. Mauna Kea Observatory, at an altitude of 4,205 m (13,796 ft), is especially high. The air at the observatory is clear, so many major telescope projects are located there.
The Hubble Space Telescope (HST), a reflecting telescope that orbits Earth, has returned the clearest images of any optical telescope. The main mirror of the HST is only ninety-four in. (2.4 m.) across, far smaller than that of the largest ground-based reflecting telescopes. Turbulence in the atmosphere makes observing objects as clearly as the HST can see impossible for ground-based telescopes. HST images of visible light are about five times finer than any produced by ground-based telescopes. Giant telescopes on Earth, however, collect much more light than the HST can. Examples of such giant telescopes include the twin 32-ft (10-m) Keck telescopes in Hawaii and the four 26-ft (8-m) telescopes in the Very Large Telescope array in the Atacama Desert in northern Chile (the nearest city is Antofagasta, Chile). Often astronomers use space and ground-based telescopes in conjunction.
Astronomers usually share telescopes. Many institutions with large telescopes accept applications from any astronomer who wishes to use the instruments, though others have limited sets of eligible applicants. The institution then divides the available time between successful applicants and assigns each astronomer an observing period. Astronomers can collect data from telescopes remotely. Data from Earth-based telescopes can be sent electronically over computer networks. Data from space-based telescopes reach Earth through radio waves collected by antennas on the ground.
Gamma rays have the shortest wavelengths. Special telescopes in orbit around Earth, such as the National Aeronautics and Space Administration's (NASA's) Compton Gamma-Ray Observatory, gather gamma rays before Earth's atmosphere absorbs them. X rays, the next shortest wavelengths, also must be observed from space. NASA's Chandra x-ray Observatory (CXO) is a school-bus-sized spacecraft scheduled to begin studying X-rays from orbit in 1999. It is designed to make high-resolution images.
Ultraviolet light has wavelengths longer than X rays, but shorter than visible light. Ultraviolet telescopes are similar to visible-light telescopes in the way they gather light, but the atmosphere blocks most ultraviolet radiation. Most ultraviolet observations, therefore, must also take place in space. Most of the instruments on the Hubble Space Telescope (HST) are sensitive to ultraviolet radiation. Humans cannot see ultraviolet radiation, but astronomers can create visual images from ultraviolet light by assigning particular colours or shades to different intensities of radiation.
Infrared astronomers study parts of the infrared spectrum, which consists of electromagnetic waves with wavelengths ranging from just longer than visible light to 1,000 times longer than visible light. Earth's atmosphere absorbs infrared radiation, so astronomers must collect infrared radiation from places where the atmosphere is very thin, or from above the atmosphere. Observatories for these wavelengths are located on certain high mountaintops or in space. Most infrared wavelengths can be observed only from space. Every warm object emits some infrared radiation. Infrared astronomy is useful because objects that are not hot enough to emit visible or ultraviolet radiation may still emit infrared radiation. Infrared radiation also passes through interstellar and intergalactic gas and dusts more easily than radiation with shorter wavelengths. Further, the brightest part of the spectrum from the farthest galaxies in the universe is shifted into the infrared. The Next Generation Space Telescope, which NASA plans to launch in 2006, will operate especially in the infrared.
Radio waves have the longest wavelengths. Radio astronomers use giant dish antennas to collect and focus signals in the radio part of the spectrum. These celestial radio signals, often from hot bodies in space or from objects with strong magnetic fields, come through Earth's atmosphere to the ground. Radio waves penetrate dust clouds, allowing astronomers to see into the centre of our galaxy and into the cocoons of dust that surround forming stars.
Sometimes astronomers study emissions from space that are not electromagnetic radiation. Some of the particles of interest to astronomers are neutrinos, cosmic rays, and gravitational waves. Neutrinos are tiny particles with no electric charge and very little or no mass. The Sun and supernovas emit neutrinos. Most neutrino telescopes consist of huge underground tanks of liquid. These tanks capture a few of the many neutrinos that strike them, while the vast majority of neutrinos pass right through the tanks.
Cosmic rays are electrically charged particles that come to Earth from outer space at almost the speed of light. They are made up of negatively charged particles called electrons and positively charged nuclei of atoms. Astronomers do not know where most cosmic rays come from, but they use cosmic-ray detectors to study the particles. Cosmic-ray detectors are usually grids of wires that produce an electrical signal when a cosmic ray passes close to them.
Gravitational waves are a predicted consequence of the general theory of relativity developed by German-born American physicist Albert Einstein. Set off up in the 1960s astronomers have been building detectors for gravitational waves. Older gravitational-wave detectors were huge instruments that surrounded a carefully measured and positioned massive object suspended from the top of the instrument. Lasers trained on the object were designed to measure the object's movement, which theoretically would occur when a gravitational wave hit the object. At the end of the 20th century, these instruments had picked up no gravitational waves. Gravitational waves should be very weak, and the instruments were probably not yet sensitive enough to register them. In the 1970s and 1980s American physicists Joseph Taylor and Russell Hulse observed indirect evidence of gravitational waves by studying systems of double pulsars. A new generation of gravitational-wave detectors, developed in the 1990s, used interferometers to measure distortions of space that would be caused by passing gravitational waves.
Some objects emit radiation more strongly in one wavelength than in another, but a set of data across the entire spectrum of electromagnetic radiation is much more useful than observations in anyone wavelength. For example, the supernova remnant known as the Crab Nebula has been observed in every part of the spectrum, and astronomers have used all the discoveries together to make a complete picture of how the Crab Nebula is evolving.
Whether astronomers take data from a ground-based telescope or have data radioed to them from space, they must then analyse the data. Usually the data are handled with the aid of a computer, which can carry out various manipulations the astronomer requests. For example, some of the individual picture elements, or pixels, of a CCD may be more sensitive than others. Consequently, astronomers sometimes take images of blank sky to measure which pixels appear brighter. They can then take these variations into account when interpreting the actual celestial images. Astronomers may write their own computer programs to analyse data or, as is increasingly the case, use certain standard computer programs developed at national observatories or elsewhere.
Often an astronomer uses observations to test a specific theory. Sometimes, a new experimental capability allows astronomers to study a new part of the electromagnetic spectrum or to see objects in greater detail or through special filters. If the observations do not verify the predictions of a theory, the theory must be discarded or, if possible, modified.
Up to about 3,000 stars are visible at a time from Earth with the unaided eye, far away from city lights, on a clear night. A view at night may also show several planets and perhaps a comet or a meteor shower. Increasingly, human-made light pollution is making the sky less dark, limiting the number of visible astronomical objects. During the daytime the Sun shines brightly. The Moon and bright planets are sometimes visible early or late in the day but are rarely seen at midday.
Earth moves in two basic ways: It turns in place, and it revolves around the Sun. Earth turns around its axis, an imaginary line that runs down its centre through its North and South poles. The Moon also revolves around Earth. All of these motions produce day and night, the seasons, the phases of the Moon, and solar and lunar eclipses.
Earth is about 12,000 km. (about 7,000 mi.) in diameter. As it revolves, or moves in a circle, around the Sun, Earth spins on its axis. This spinning movement is called rotation. Earth's axis is tilted 23.5° with respect to the plane of its orbit. Each time Earth rotates on its axis, its corrective velocity to enable it of travelling, or free falling through into a new day, in other words, its rotational inertia or axial momentum carries it through one day, a cycle of light and dark. Humans artificially divide the day into 24 hours and then divide the hours into 60 minutes and the minutes into 60 seconds.
Earth revolves around the Sun once every year, or 365.25 days (most people use a 365-day calendar and take care of the extra 0.25 day by adding a day to the calendar every four years, creating a leap year). The orbit of Earth is almost, but not quite, a circle, so Earth is sometimes a little closer to the Sun than at other times. If Earth were upright as it revolved around the Sun, each point on Earth would have exactly twelve hours of light and twelve hours of dark each day. Because Earth is tilted, however, the northern hemisphere sometimes points toward the Sun and sometimes points away from the Sun. This tilt is responsible for the seasons. When the northern hemisphere points toward the Sun, the northernmost regions of Earth see the Sun 24 hours a day. The whole northern hemisphere gets more sunlight and gets it at a more direct angle than the southern hemisphere does during this period, which lasts for half of the year. The second half of this period, when the northern hemisphere points most directly at the Sun, is the northern hemisphere's summer, which corresponds to winter in the southern hemisphere. During the other half of the year, the southern hemisphere points more directly toward the Sun, so it is spring and summer in the southern hemisphere and fall and winters in the northern hemisphere.
One revolution of the Moon around Earth takes a little more than twenty-seven days seven hours. The Moon rotates on its axis in this same period of time, so the same face of the Moon is always presented to Earth. Over a period a little longer than twenty-nine days twelve hours, the Moon goes through a series of phases, in which the amount of the lighted half of the Moon we see from Earth changes. These phases are caused by the changing angle of sunlight hitting the Moon. (The period of phases is longer than the period of revolution of the Moon, because the motion of Earth around the Sun changes the angle at which the Sun's light hits the Moon from night to night.)
The Moon's orbit around Earth is tilted five from the plane of Earth's orbit. Because of this tilt, when the Moon is at the point in its orbit when it is between Earth and the Sun, the Moon is usually a little above or below the Sun. At that time, the Sun lights the side of the Moon facing away from Earth, and the side of the Moon facing toward Earth is dark. This point in the Moon's orbit corresponds to a phase of the Moon called the new moon. A quarter moon occurs when the Moon is at right angles to the line formed by the Sun and Earth. The Sun lights the side of the Moon closest to it, and half of that side is visible from Earth, forming a bright half-circle. When the Moon is on the opposite side of Earth from the Sun, the face of the Moon visible from Earth is lit, showing the full moon in the sky
Because of the tilt of the Moon's orbit, the Moon usually passes above or below the Sun at new moon and above or below Earth's shadow at full moon. Sometimes, though, the full moon or new moon crosses the plane of Earth's orbit. By a coincidence of nature, even though the Moon is about 400 times smaller than the Sun, it is also about 400 times closer to Earth than the Sun is, so the Moon and Sun look almost the same size from Earth. If the Moon lines up with the Sun and Earth at new moon (when the Moon is between Earth and the Sun), it blocks the Sun's light from Earth, creating a solar eclipse. If the Moon lines up with Earth and the Sun at the full moon (when Earth is between the Moon and the Sun), Earth's shadow covers the Moon, making a lunar eclipse.
A total solar eclipse is visible from only a small region of Earth. During a solar eclipse, the complete shadow of the Moon that falls on Earth is only about 160 km. (about 100 mi.) wide. As Earth, the Sun, and the Moon move, however, the Moon's shadow sweeps out a path up to 16,000 km. (10,000 mi.) long. The total eclipse can only be seen from within this path. A total solar eclipse occurs about every eighteen months. Off to the sides of the path of a total eclipse, a partial eclipse, in which the Sun is only partly covered, is visible. Partial eclipses are much less dramatic than total eclipses. The Moon's orbit around Earth is elliptical, or egg-shaped. The distance between Earth and the Moon varies slightly as the Moon orbits Earth. When the Moon is farther from Earth than usual, it appears smaller and may not cover the entire Sun during an eclipse. A ring, or annulus, of sunlight remains assimilated through visibility. Making an annular eclipse. An annular solar eclipse also occurs about every eighteen months. Additional partial solar eclipses are also visible from Earth in between.
At a lunar eclipse, the Moon is existent in Earth's shadow. When the Moon is completely in the shadow, the total lunar eclipse is visible from everywhere on the half of Earth from which the Moon is visible at that time. As a result, more people see total lunar eclipses than see total solar eclipses.
In an open place on a clear dark night, streaks of light may appear in a random part of the sky about once every ten minutes. These streaks are meteors-bits of rock-turning up in Earth's atmosphere. The bits of rock are called meteoroids, and when these bits survive Earth's atmosphere intact and land on Earth, they are known as meteorites.
Every month or so, Earth passes through the orbit of a comet. Dust from the comet remains in the comet's orbit. When Earth passes through the band of dust, the dust and bits of rock burn up in the atmosphere, creating a meteor shower. Many more meteors are visible during a meteor shower than on an ordinary night. The most observed meteor shower is the Perseid shower, which occurs each year on August 11th or 12th.
Humans have picked out landmarks in the sky and mapped the heavens for thousands of years. Maps of the sky helped to potentially lost craft in as much as sailors have navigated using the celestially fixed stars to find refuge away from being lost. Now astronomers methodically map the sky to produce a universal format for the addresses of stars, galaxies, and other objects of interest.
Some of the stars in the sky are brighter and more noticeable than others are, and some of these bright stars appear to the eye to be grouped together. Ancient civilizations imagined that groups of stars represented figures in the sky. The oldest known representations of these groups of stars, called constellations, are from ancient Sumer (now Iraq) from about 4000 Bc. The constellations recorded by ancient Greeks and Chinese resemble the Sumerian constellations. The northern hemisphere constellations that astronomers recognize today are based on the Greek constellations. Explorers and astronomers developed and recorded the official constellations of the southern hemisphere in the 16th and 17th centuries. The International Astronomical Union (IAU) officially recognizes eighty-eight constellations. The IAU defined the boundaries of each constellation, so the eighty-eight constellations divide the sky without overlapping.
A familiar group of stars in the northern hemisphere is called the Big Dipper. The Big Dipper is part of an official constellation-Ursa Major, or the Great Bear. Groups of stars that are not official constellations, such as the Big Dipper, are called asterisms. While the stars in the Big Dipper appear in approximately the same part of the sky, they vary greatly in their distance from Earth. This is true for the stars in all constellations or asterisms-the stars accumulating of the group do not really occur close to each other in space, they merely appear together as seen from Earth. The patterns of the constellations are figments of humans' imagination, and different artists may connect the stars of a constellation in different ways, even when illustrating the same myth.
Astronomers use coordinate systems to label the positions of objects in the sky, just as geographers use longitude and latitude to label the positions of objects on Earth. Astronomers use several different coordinate systems. The two most widely used are the altazimuth system and the equatorial system. The altazimuth system gives an object's coordinates with respect to the sky visible above the observer. The equatorial coordinate system designates an object's location with respect to Earth's entire night sky, or the celestial sphere.
One of the ways astronomers give the position of a celestial object is by specifying its altitude and its azimuth. This coordinate system is called the altazimuth system. The altitude of an object is equal to its angle, in degrees, above the horizon. An object at the horizon would have an altitude of zero, and an object directly overhead would have an altitude of ninety. The azimuth of an object is equal to its angle in the horizontal direction, with north at zero, east at ninety, south at 180°, and west at 270°. For example, if an astronomer were looking for an object at twenty-three altitude and eighty-seven azimuth, the astronomer would know to look low in the sky and almost directly east.
As Earth rotates, astronomical objects appear to rise and set, so their altitudes and azimuths are constantly changing. An object's altitude and azimuth also vary according to an observer's location on Earth. Therefore, astronomers almost never use altazimuth coordinates to record an object's position. Instead, astronomers with altazimuth telescopes translate coordinates from equatorial coordinates to find an object. Telescopes that use an altazimuth mounting system may be simple to set up, but they require many calculated movements to keep them pointed at an object as it moves across the sky. These telescopes fell out of use with the development of the equatorial coordinate and mounting system in the early 1800s. However, computers have made the return to popularity possible for altazimuth systems. Altazimuth mounting systems are simple and inexpensive, and-with computers to do the required calculations and control the motor that moves the telescope-they are practical.
The equatorial coordinate system is a coordinate system fixed on the sky. In this system, a star keeps the same coordinates no matter what the time is or where the observer is located. The equatorial coordinate system is based on the celestial sphere. The celestial sphere is a giant imaginary globe surrounding Earth. This sphere has north and south celestial pole directly above Earth's North and South poles. It has a celestial equator, directly above Earth's equator. Another important part of the celestial sphere is the line that marks the movement of the Sun with respect to the stars throughout the year. This path is called the ecliptic. Because Earth is tilted with respect to its orbit around the Sun, the ecliptic is not the same as the celestial equator. The ecliptic is tilted 23.5° to the celestial equator and crosses the celestial equator at two points on opposite sides of the celestial sphere. The crossing points are called the vernal (or spring) equinox and the autumnal equinox. The vernal equinox and autumnal equinox mark the beginning of spring and fall, respectively. The points at which the ecliptic and celestial equator are farthest apart are called the summer solstice and the winter solstice, which mark the beginning of summer and winter, respectively.
As Earth rotates on its axis each day, the stars and other distant astronomical objects appear to rise in the eastern part of the sky and set in the west. They seem to travel in circles around Earth's North or South poles. In the equatorial coordinate system, the celestial sphere turns with the stars (but this movement is really caused by the rotation of Earth). The celestial sphere makes one complete rotation every twenty-three hours fifty-six minutes, which is four unexpected moments than a day measured by the movement of the Sun. A complete rotation of the celestial sphere is called a sidereal day. Because the sidereal day is shorter than a solar day, the stars that an observer sees from any location on Earth change slightly from night to night. The difference between a sidereal day and a solar day occurs because of Earth's motion around the Sun.
The equivalent of longitude on the celestial sphere is called right ascension and the equivalent of latitude is declination. Specifying the right ascension of a star is equivalent to measuring the east-west distance from a line called the prime meridian that runs through Greenwich, England, for a place on Earth. Right ascension starts at the vernal equinox. Longitude on Earth is given in degrees, but right ascension is given in units of time-hours, minutes, and seconds. This is because the celestial equator is divided into 24 equal parts-each called an hour of right ascension instead of fifteen. Each hour is made up of 60 minutes, each of which is equal to 60 seconds. Measuring right ascension in units of time makes determine when will be the best time for observing an object easier for astronomers. A particular line of right ascension will be at its highest point in the sky above a particular place on Earth four minutes earlier each day, so keeping track of the movement of the celestial sphere with an ordinary clock would be complicated. Astronomers have special clocks that keep sidereal time (24 sidereal hours are equal to twenty-three hours fifty-six minutes of familiar solar time). Astronomers compare the current sidereal time with the right ascension of the object they wish to view. The object will be highest in the sky when the sidereal time equals the right ascension of the object.
The direction perpendicular to right ascension-and the equivalent to latitude on Earth-is declination. Declination is measured in degrees. These degrees are divided into arcminutes and arcseconds. One arcminute is equal to 1/60 of a degree, and one arcsecond is equal to 1/60 of an arcminute, or 1/360 of a degree. The celestial equator is at declination zero, the north celestial pole is at declination ninety, and the south celestial pole has a declination of -90°. Each star has a right ascension and a declination that mark its position in the sky. The brightest star, Sirius, for example, has right ascension six hours forty-five minutes (abbreviated as 6h. 45m.) and declination-16 degrees forty-three arcminutes
Stars are so far away from Earth that the main star motion we see results from Earth's rotation. Stars do move in space, however, and these proper motions slightly change the coordinates of the nearest stars over time. The effects of the Sun and the Moon on Earth also cause slight changes in Earth's axis of rotation. These changes, called precession, cause a slow drift in right ascension and declination. To account for precession, astronomers redefine the celestial coordinates every fifty years or so.
Solar systems, both our own and those located around other stars, are a major area of research for astronomers. A solar system consists of a central star orbited by planets or smaller rocky bodies. The gravitational force of the star holds the system together. In our solar system, the central star is the Sun. It holds all the planets, including Earth, in their orbits and provides light and energy necessary for life. Our solar system is just one of many. Astronomers are just beginning to be able to study other solar systems.
Our solar system contains the Sun, nine planets (of which Earth is third from the Sun), and the planets' satellites. It also contains asteroids, comets, and interplanetary dust and gases.
Until the end of the 18th century, humans knew of five planets-Mercury, Venus, Mars, Jupiter, and Saturn-in addition to Earth. When viewed without a telescope, planets appear to be dots of light in the sky. They shine steadily, while stars seem to twinkle. Twinkling results from turbulence in Earth's atmosphere. Stars are so far away that they appear as tiny points of light. A moment of turbulence can change that light for a fraction of a second. Even though they look the same size as stars to unaided human eyes, planets are close enough that they take up more space in the sky than stars do. The disks of planets are big enough to average out variations in light caused by turbulence and therefore do not twinkle.
Between 1781 and 1930, astronomers found three more planets-Uranus, Neptune, and Pluto. This brought the total number of planets in our solar system to nine. In order of increasing distance from the Sun, the planets in our solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
Astronomers call the inner planets-Mercury, Venus, Earth, and Mars-the terrestrial planets. Terrestrial (from the Latin word terra, meaning ‘Earth') planets are Earthlike in that they have solid, rocky surfaces. The next group of planets-Jupiter, Saturn, Uranus, and Neptune-is called the Jovian planets, or the giant planets. The word Jovian has the same Latin root as the word Jupiter. Astronomers call these planets the Jovian planets because they resemble Jupiter in that they are giant, massive planets made almost entirely of gas. The mass of Jupiter, for example, is 318 times the mass of Earth. The Jovian planets have no solid surfaces, although they probably have rocky cores several times more massive than Earth. Rings of chunks of ice and rock surround each of the Jovian planets. The rings around Saturn are the most familiar.
Pluto, the outermost planet, is tiny, with a mass about one five-hundredth the mass of Earth. Pluto seems out of place, with its tiny, solid body out beyond the giant planets. Many astronomers believe that Pluto is really just the largest, or one of the largest, of a group of icy objects in the outer solar system. These objects orbit in a part of the solar system called the Kuiper Belt. Even if astronomers decide that Pluto belongs to the Kuiper Belt objects, it will probably still be called a planet for historical reasons.
Most of the planets have moons, or satellites. Earth's Moon has a diameter about one-fourth the diameter of Earth. Mars has two tiny chunks of rock, Phobos and Deimos, each only about 10 km (about 6 mi) across. Jupiter has at least seventeen satellites. The largest four, known as the Galilean satellites, are Io, Europa, Ganymede, and Callisto. Ganymede is even larger than the planet Mercury. Saturn has at least eighteen satellites. Saturn's largest moon, Titan, is also larger than the planet Mercury and is enshrouded by a thick, opaque, smoggy atmosphere. Uranus has at least seventeen moons, and Neptune has at least eight moons. Pluto had one moon, called Charon. Charon is more than half as big as Pluto.
Comets and asteroids are rocky and icy bodies that are smaller than planets. The distinction between comets, asteroids, and other small bodies in the solar system is a little fuzzy, but generally a comet is icier than an asteroid and has a more elongated orbit. The orbit of a comet takes it close to the Sun, then back into the outer solar system. When comets near the Sun, some of their ice turns from solid material into gas, releasing some of their dust. Comets have long tails of glowing gas and dust when they are near the Sun. Asteroids are rockier bodies and usually have orbits that keep them at always about the same distance from the Sun.
Both comets and asteroids have their origins in the early solar system. While the solar system was forming, many small, rocky objects called planetesimals condensed from the gas and dust of the early solar system. Millions of planetesimals remain in orbit around the Sun. A large spherical cloud of such objects out beyond Pluto forms the Oort cloud. The objects in the Oort cloud are considered comets. When our solar system passes close to another star or drifts closer than usual to the centre of our galaxy, the change in gravitational pull may disturb the orbit of one of the icy comets in the Oort cloud. As this comet falls toward the Sun, the ice turns into vapour, freeing dust from the object. The gas and dust form the tail or tails of the comet. The gravitational pull of large planets such as Jupiter or Saturn may swerve the comet into an orbit closer to the Sun. The time needed for a comet to make a complete orbit around the Sun is called the comet's period. Astronomers believe that comets with periods longer than about 200 years come from the Oort Cloud. Short-period comets, those with periods less than about 200 years, probably come from the Kuiper Belt, a ring of planetesimals beyond Neptune. The material in comets is probably from the very early solar system, so astronomers study comets to find out more about our solar system's formation.
When the solar system was forming, some of the planetesimals came together more toward the centre of the solar system. Gravitational forces from the giant planet Jupiter prevented these planetesimals from forming full-fledged planets. Instead, the planetesimals broke up to create thousands of minor planets, or asteroids, that orbit the Sun. Most of them are in the asteroid belt, between the orbits of Mars and Jupiter, but thousands are in orbits that come closer to Earth or even cross Earth's orbit. Scientists are increasingly aware of potential catastrophes if any of the largest of these asteroids hits Earth. Perhaps 2,000 asteroids larger than 1 km. (0.6 mi.) in diameter are potential hazards.
The Sun is the nearest star to Earth and is the centre of the solar system. It is only eight light-minutes away from Earth, meaning light takes only eight minutes to travel from the Sun to Earth. The next nearest star is four light-years away, so light from this star, Proxima Centauri (part of the triple star Alpha Centauri), takes four years to reach Earth. The Sun's closeness means that the light and other energy we get from the Sun dominate Earth's environment and life. The Sun also provides a way for astronomers to study stars. They can see details and layers of the Sun that are impossible to see on more distant stars. In addition, the Sun provides a laboratory for studying hot gases held in place by magnetic fields. Scientists would like to create similar conditions (hot gases contained by magnetic fields) on Earth. Creating such environments could be useful for studying basic physics.
The Sun produces its energy by fusing hydrogen into helium in a process called nuclear fusion. In nuclear fusion, two atoms merge to form a heavier atom and release energy. The Sun and stars of similar mass start off with enough hydrogen to shine for about ten billion years. The Sun is less than halfway through its lifetime.
Although most telescopes are used mainly to collect the light of faint objects so that they can be studied, telescopes for planetary and other solar system studies are also used to magnify images. Astronomers use some of the observing time of several important telescopes for planetary studies. Overall, planetary astronomers must apply and compete for observing time on telescopes with astronomers seeking to study other objects. Some planetary objects can be studied as they pass in front of, or occult, distant stars. The atmosphere of Neptune's moon Triton and the shapes of asteroids can be investigated in this way, for example. The fields of radio and infrared astronomy are useful for measuring the temperatures of planets and satellites. Ultraviolet astronomy can help astronomers study the magnetic fields of planets.
During the space age, scientists have developed telescopes and other devices, such as instruments to measure magnetic fields or space dust, that can leave Earth's surface and travel close to other objects in the solar system. Robotic spacecraft have visited all of the planets in the solar system except Pluto. Some missions have targeted specific planets and spent much time studying a single planet, and some spacecraft have flown past a number of planets.
Astronomers use different telescopes to study the Sun than they use for nighttime studies because of the extreme brightness of the Sun. Telescopes in space, such as the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE), are able to study the Sun in regions of the spectrum other than visible light. X-rays, ultraviolet, and radio waves from the Sun are especially interesting to astronomers. Studies in various parts of the spectrum give insight into giant flows of gas in the Sun, into how the Sun's energy leaves the Sun to travel to Earth, and into what the interior of the Sun is like. Astronomers also study solar-terrestrial relations-the relation of activity on the Sun with magnetic storms and other effects on Earth. Some of these storms and effects can affect radio reception, cause electrical blackouts, or damage satellites in orbit.
Our solar system began forming about five billion years ago, when a cloud of gas and dust between the stars in our Milky Way Galaxy began contracting. A nearby supernova-an exploding star-may have started the contraction, but most astronomers believe a random change in density in the cloud caused the contraction. Once the cloud-known as the solar nebula-began to contract, the contraction occurred faster and faster. The gravitational energy caused by this contraction heated the solar nebula. As the cloud became smaller, it began to spin faster, much as a spinning skater will spin faster by pulling in his or her arms. This spin kept the nebula from forming a sphere; instead, it settled into a disk of gas and dust.
In this disk, small regions of gas and dust began to draw closer and stick together. The objects that resulted, which were usually less than 500 km (300 mi) across, are the planetesimals. Eventually, some planetesimals stuck together and grew to form the planets. Scientists have made computer models of how they believe the early solar system behaved. The models show that for a solar system to produce one or two huge planets like Jupiter and several other, much smaller planets is usual.
The largest region of gas and dust wound up in the centre of the nebula and formed the protosun (proto is Greek for ‘before' and is used to distinguish between an object and its forerunner). The increasing temperature and pressure in the middle of the protosun vaporized the dust and eventually allowed nuclear fusion to begin, marking the formation of the Sun. The young Sun gave off a strong solar wind that drove off most of the lighter elements, such as hydrogen and helium, from the inner planets. The inner planets then solidified and formed rocky surfaces. The solar wind lost strength. Jupiter's gravitational pull was strong enough to keep its shroud of hydrogen and helium gas. Saturn, Uranus, and Neptune also kept their layers of light gases.
The theory of solar system formation described above accounts for the appearance of the solar system as we know it. Examples of this appearance include the fact that the planets all orbit the Sun in the same direction and that almost all the planets rotate on their axes in the same direction. The recent discoveries of distant solar systems with different properties could lead to modifications in the theory, however
Studies in the visible, the infrared, and the shortest radio wavelengths have revealed disks around several young stars in our galaxy. One such object, Beta Pictoris (about sixty-two light-years from Earth), has revealed a warp in the disk that could be a sign of planets in orbit. Astronomers are hopeful that, in the cases of these young stars, they are studying the early stages of solar system formation.
Although astronomers have long assumed that many other stars have planets, they have been unable to detect these other solar systems until recently. Planets orbiting around stars other than the Sun are called extra solar planets. Planets are small and dim compared with stars, so they are lost in the glare of their parent stars and are invisible to direct observation with telescopes.
Astronomers have tried to detect other solar systems by searching for the way a planet affects the movement of its parent star. The gravitational attraction between a planet and its star pulls the star slightly toward the planet, so the star wobbles slightly as the planet orbits it. Throughout the mind and late 1900s, several observatories tried to detect wobbles in the nearest stars by watching the stars' movement across the sky. Wobbles were reported in several stars, but later observations showed that the results were false.
In the early 1990s, studies of a pulsar revealed at least two planets orbiting it. Pulsars are compact stars that give off pulses of radio waves at very regular intervals. The pulsar, designated PSR 1257+12, is about 1,000 light-years from Earth. This pulsar's pulses sometimes came a little early and sometimes a little late in a periodic pattern, revealing that an unseen object was pulling the pulsar toward and away from Earth. The environment of a pulsar, which emits X rays and other strong radiation that would be harmful to life on Earth, is so extreme that these objects would have little resemblance to planets in our solar system.
The wobbling of a star changes the star's light that reaches Earth. When the star moves away from Earth, even slightly, each wave of light must travel farther to Earth than the wave before it. This increases the distance between waves (called the wavelength) as the waves reach Earth. When a star's planet pulls the star closer to Earth, each successive wavefront has less distance to travel to reach Earth. This shortens the wavelength of the light that reaches Earth. This effect is called the Doppler effect. No star moves fast enough for the change in wavelength to result in a noticeable change in colour, which depends on wavelength, but the changes in wavelength can be measured with precise instruments. Because the planet's effect on the star is very small, astronomers must analyse the starlight carefully to detect a shift in wavelength. They do this by first using a technique called spectroscopy to separate the white starlight into its component colours, as water vapour does to sunlight in a rainbow. Stars emit light in a continuous range. The range of wavelengths a star emits is called the star's spectrum. This spectrum had dark lines, called absorption lines, at wavelengths at which atoms in the outermost layers of the star absorb light.
Astronomers know what the exact wavelength of each absorption line is for a star that is not moving. By seeing how far the movement of a star shifts the absorption lines in its spectrum, astronomers can calculate how fast the star is moving. If the motion fits the model of the effect of a planet, astronomers can calculate the mass of the planet and how close it is to the star. These calculations can only provide the lower limit to the planet's mass, because telling at what angle the planet orbits. The star is impossible for astronomers. Astronomers need to know the angle at which the planet orbits the star to calculate the planet's mass accurately. Because of this uncertainty, some of the giant extra solar planets may be a type of failed star called a brown dwarf instead of planets. Most astronomers believe that many of the suspected planets are true planets.
Between 1995 and 1999 astronomers discovered more than a dozen extra solar planets. Astronomers now know of far more planets outside our solar system than inside our solar system. Most of these planets, surprisingly, are more massive than Jupiter and are orbiting so close to their parent stars that some of them have ‘years' (the time it takes to orbit the parent star once) as long as only a few days on Earth. These solar systems are so different from our solar system that astronomers are still trying to reconcile them with the current theory of solar system formation. Some astronomers suggest that the giant extra solar planets formed much farther away from their stars and were later thrown into the inner solar systems by some gravitational interaction.
Stars are an important topic of astronomical research. Stars are balls of gas that shine or used to shine because of nuclear fusion in their cores. The most familiar star is the Sun. The nuclear fusion in stars produces a force that pushes the material in a star outward. However, the gravitational attraction of the star's material for itself pulls the material inward. A star can remain stable as long as the outward pressure and gravitational force balance. The properties of a star depend on its mass, its temperature, and its stage in evolution.
Astronomers study stars by measuring their brightness or, with more difficulty, their distances from Earth. They measure the ‘colour' of a star-the differences in the star's brightness from one part of the spectrum to another-to determine its temperature. They also study the spectrum of a star's light to determine not only the temperature, but also the chemical makeup of the star's outer layers.
Many different types of stars exist. Some types of stars are really just different stages of a star's evolution. Some types are different because the stars formed with much more or much less mass than other stars, or because they formed close to other stars. The Sun is a type of star known as a main-sequence star. Eventually, main-sequence stars such as the Sun swell into giant stars and then evolve into tiny, dense, white dwarf stars. Main-sequence stars and giants have a role in the behaviour of most variable stars and novas. A star much more massive than the Sun will become a supergiant star, then explode as a supernova. A supernova may leave behind a neutron star or a black hole.
In about 1910 Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell independently worked out a way to graph basic properties of stars. On the horizontal axis of their graphs, they plotted the temperatures of stars. On the vertical axis, they plotted the brightness of stars in a way that allowed the stars to be compared. (One plotted the absolute brightness, or absolute magnitude, of a star, a measurement of brightness that takes into account the distance of the star from Earth. The other plotted stars in a nearby galaxy, all about the same distance from Earth.)
On an H-R diagram, the brightest stars are at the top and the hottest stars are at the left. Hertzsprung and Russell found that most stars fell on a diagonal line across the H-R diagram from upper left lower to right. This line is called the main sequence. The diagonal line of main-sequence stars indicates that temperature and brightness of these stars are directly related. The hotter a main-sequence stars is, the brighter it is. The Sun is a main-sequence star, located in about the middle of the graph. More faint, cool stars exist than hot, bright ones, so the Sun is brighter and hotter than most of the stars in the universe.
At the upper right of the H-R diagram, above the main sequence, stars are brighter than main-sequence stars of the same colour. The only way stars of a certain colour can be brighter than other stars of the same colour is if the brighter stars are also bigger. Bigger stars are not necessarily more massive, but they do have larger diameters. Stars that fall in the upper right of the H-R diagram are known as giant stars or, for even brighter stars, supergiant stars. Supergiant stars have both larger diameters and larger masses than giant stars.
Giant and supergiant stars represent stages in the lives of stars after they have burned most of their internal hydrogen fuel. Stars swell as they move off the main sequence, becoming giants and—for more massive stars-supergiants.
A few stars fall in the lower left portion of the H-R diagram, below the main sequence. Just as giant stars are larger and brighter than main-sequences stars, these stars are smaller and dimmer. These smaller, dimmer stars are hot enough to be white or blue-white in colour and are known as white dwarfs.
White dwarf stars are only about the size of Earth. They represent stars with about the mass of the Sun that have burned as much hydrogen as they can. The gravitational force of a white dwarf's mass is pulling the star inward, but electrons in the star resist being pushed together. The gravitational force is able to pull the star into a much denser form than it was in when the star was burning hydrogen. The final stage of life for all stars like the Sun is the white dwarf stage.
Many stars vary in brightness over time. These variable stars come in a variety of types. One important type is called a Cepheid variable, named after the star delta Cepheid, which is a prime example of a Cepheid variable. These stars vary in brightness as they swell and contract over a period of weeks or months. Their average brightness depends on how long the period of variation takes. Thus astronomers can determine how bright the star is merely by measuring the length of the period. By comparing how intrinsically bright these variable stars are with how bright they look from Earth, astronomers can calculate how far away these stars are from Earth. Since they are giant stars and are very bright, Cepheid variables in other galaxies are visible from Earth. Studies of Cepheid variables tell astronomers how far away these galaxies are and are very useful for determining the distance scale of the universe. The Hubble Space Telescope (HST) can determine the periods of Cepheid stars in galaxies farther away than ground-based telescopes can see. Astronomers are developing a more accurate idea of the distance scale of the universe with HST data.
Cepheid variables are only one type of variable star. Stars called long-period variables vary in brightness as they contract and expand, but these stars are not as regular as Cepheid variables. Mira, a star in the constellation Cetus (the whale), is a prime example of a long-period variable star. Variable stars called eclipsing binary stars are really pairs of stars. Their brightness varies because one member of the pair appears to pass in front of the other, as seen from Earth. A type of variable star called R Coronae Borealis stars varies because they occasionally give off clouds of carbon dust that dim these stars.
Sometimes stars brighten drastically, becoming as much as 100 times brighter than they were. These stars are called novas (Latin for ‘new stars'). They are not really new, just much brighter than they were earlier. A nova is a binary, or double, star in which one member is a white dwarf and the other is a giant or supergiant. Matter from the large star falls onto the small star. After a thick layer of the large star's atmosphere has collected on the white dwarf, the layer burns off in a nuclear fusion reaction. The fusion produces a huge amount of energy, which, from Earth, appears as the brightening of the nova. The nova gradually returns to its original state, and material from the large star again begins to collect on the white dwarf.
Sometimes stars brighten many times more drastically than novas do. A star that had been too dim to see can become one of the brightest stars in the sky. These stars are called supernovas. Sometimes supernovas that occur in other galaxies are so bright that, from Earth, they appear as bright as their host galaxy.
There are two types of supernovas. One type is an extreme case of a nova, in which matter falls from a giant or supergiant companion onto a white dwarf. In the case of a supernova, the white dwarf gains so much fuel from its companion that the star increases in mass until strong gravitational forces cause it to become unstable. The star collapses and the core explodes, vaporizing a lot of the white dwarves and producing an immense amount of light. Only bits of the white dwarf remain after this type of supernova occurs.
The other type of supernova occurs when a supergiant star uses up all its nuclear fuel in nuclear fusion reactions. The star uses up its hydrogen fuel, but the core is hot enough that it provides the initial energy necessary for the star to begin ‘burning' helium, then carbon, and then heavier elements through nuclear fusion. The process stops when the core is mostly iron, which is too heavy for the star to ‘burn' in a way that gives off energy. With no such fuel left, the inward gravitational attraction of the star's material for itself has no outward balancing force, and the core collapses. As it collapses, the core releases a shock wave that tears apart the star's atmosphere. The core continues collapsing until it forms either a neutron star or a black hole, depending on its mass
Only a handfuls of supernovas are known in our galaxy. The last Milky Way supernova seen from Earth was observed in 1604. In 1987 astronomers observed a supernova in the Large Magellanic Cloud, one of the Milky Way's satellite galaxies. This supernova became bright enough to be visible to the unaided eye and is still under careful study from telescopes on Earth and from the Hubble Space Telescope. A supernova in the process of exploding emits radiation in the X-ray range and ultraviolet and radio radiation studies in this part of the spectrum are especially useful for astronomers studying supernova remnants.
Neutron stars are the collapsed cores sometimes left behind by supernova explosions. Pulsars are a special type of neutron star. Pulsars and neutron stars form when the remnant of a star left after a supernova explosion collapses until it is about 10 km. (about 6 mi.) in radius. At that point, the neutrons-electrically neutral atomic particles-of the star resists being pressed together further. When the force produced by the neutrons, balances, the gravitational force, the core stops collapsing. At that point, the star is so dense that a teaspoonful has the mass of a billion metric tons.
Neutron stars become pulsars when the magnetic field of a neutron star directs a beam of radio waves out into space. The star is so small that it rotates from one to a few hundred times per second. As the star rotates, the beam of radio waves sweeps out a path in space. If Earth is in the path of the beam, radio astronomers see the rotating beam as periodic pulses of radio waves. This pulsing is the reason these stars are called pulsars.
Some neutron stars are in binary systems with an ordinary star neighbour. The gravitational pull of a neutron star pulls material off its neighbour. The rotation of the neutron star heats the material, causing it to emit X-rays. The neutron star's magnetic field directs the X-rays into a beam that sweeps into space and may be detected from Earth. Astronomers call these stars X-ray pulsars.
Gamma-ray spacecraft detect bursts of gamma rays about once a day. The bursts come from sources in distant galaxies, so they must be extremely powerful for us to be able to detect them. A leading model used to explain the bursts are the merger of two neutron stars in a distant galaxy with a resulting hot fireball. A few such explosions have been seen and studied with the Hubble and Keck telescopes.
Black holes are objects that are so massive and dense that their immense gravitational pull does not even let light escape. If the core left over after a supernova explosion has a mass of more than about fives times that of the Sun, the force holding up the neutrons in the core is not large enough to balance the inward gravitational force. No outward force is large enough to resist the gravitational force. The core of the star continues to collapse. When the core's mass is sufficiently concentrated, the gravitational force of the core is so strong that nothing, not even light, can escape it. The gravitational force is so strong that classical physics no longer applies, and astronomers use Einstein's general theory of relativity to explain the behaviour of light and matter under such strong gravitational forces. According to general relativity, space around the core becomes so warped that nothing can escape, creating a black hole. A star with a mass ten times the mass of the Sun would become a black hole if it were compressed to 90 km. (60 mi.) or less in diameter.
Astronomers have various ways of detecting black holes. When a black hole is in a binary system, matter from the companion star spirals into the black hole, forming a disk of gas around it. The disk becomes so hot that it gives off X rays that astronomers can detect from Earth. Astronomers use X-ray telescopes in space to find X-ray sources, and then they look for signs that an unseen object of more than about five times the mass of the Sun is causing gravitational tugs on a visible object. By 1999 astronomers had found about a dozen potential black holes.
The basic method that astronomers use to find the distance of a star from Earth uses parallax. Parallax is the change in apparent position of a distant object when viewed from different places. For example, imagine a tree standing in the centre of a field, with a row of buildings at the edge of the field behind the tree. If two observers stand at the two front corners of the field, the tree will appear in front of a different building for each observer. Similarly, a nearby star's position appears different when seen from different angles.
Parallax also allows human eyes to judge distance. Each eye sees an object from a different angle. The brain compares the two pictures to judge the distance to the object. Astronomers use the same idea to calculate the distance to a star. Stars are very far away, so astronomers must look at a star from two locations as far apart as possible to get a measurement. The movement of Earth around the Sun makes this possible. By taking measurements six months apart from the same place on Earth, astronomers take measurements from locations separated by the diameter of Earth's orbit. That is a separation of about 300 million km (186 million mi). The nearest stars will appear to shift slightly with respect to the background of more distant stars. Even so, the greatest stellar parallax is only about 0.77 seconds of arc, an amount 4,600 times smaller than a single degree. Astronomers calculate a star's distance by dividing one by the parallax. Distances of stars are usually measured in parsecs. A parsec is 3.26 light-years, and a light-year is the distance that light travels in a year, or about 9.5 trillion km (5.9 trillion mi). Proxima Centauri, the Sun's nearest neighbour, has a parallax of 0.77 seconds of arc. This measurement indicates that Proxima Centauri's distance from Earth is about 1.3 parsecs, or 4.2 light -years. Because Proxima Centauri is the Sun's nearest neighbours, it has a larger parallax than any other star.
Astronomers can measure stellar parallaxes for stars up to about 500 light-years away, which is only about 2 percent of the distance to the centre of our galaxy. Beyond that distance, the parallax angle is too small to measure.
A European Space Agency spacecraft named Hipparcos (an acronym for High Precision Parallax Collecting Satellite), launched in 1989, gave a set of accurate parallaxes across the sky that was released in 1997. This set of measurements has provided a uniform database of stellar distances for more than 100,000 stars and to some degree less accurate database of more than one million stars. These parallax measurements provide the base for measurements of the distance scale of the universe. Hipparcos data are leading to more accurate age calculations for the universe and for objects in it, especially globular clusters of stars.
Astronomers use a star's light to determine the star's temperature, composition, and motion. Astronomers analyse a star's light by looking at its intensity at different wavelengths. Blue light has the shortest visible wavelengths, at about 400 nanometres. (A nanometre, abbreviated ‘nm', is one billionth of a metre, or about one forty-thousandth of an inch.) Red light has the longest visible wavelengths, at about 650 nm. A law of radiation known as Wien's displacement law (developed by German physicist Wilhelm Wien) links the wavelength at which the most energy is given out by an object and its temperature. A star like the Sun, whose surface temperature is about 6000 K (about 5730°C or about 10,350°F), gives off the most radiation in yellow-green wavelengths, with decreasing amounts in shorter and longer wavelengths. Astronomers put filters of different standard colours on telescopes to allow only light of a particular colour from a star to pass. In this way, astronomers determine the brightness of a star at particular wavelengths. From this information, astronomers can use Wien's law to determine the star's surface temperature.
Astronomers can see the different wavelengths of light of a star in more detail by looking at its spectrum. The continuous rainbow of colour of a star's spectrum is crossed by dark lines, or spectral lines. In the early 19th century, German physicist Josef Fraunhofer identified such lines in the Sun's spectrum, and they are still known as Fraunhofer lines. American astronomer Annie Jump Cannon divided stars into several categories by the appearance of their spectra. She labelled them with capital letters according to how dark their hydrogen spectral lines were. Later astronomers reordered these categories according to decreasing temperature. The categories are O, B, A, F, G, K, and M, where O stars are the hottest and M stars are the coolest. The Sun is a G star. An additional spectral type, L stars, was suggested in 1998 to accommodate some cool stars studied using new infrared observational capabilities. Detailed study of spectral lines shows the physical conditions in the atmospheres of stars. Careful study of spectral lines shows that some stars have broader lines than others of the same spectral type. The broad lines indicate that the outer layers of these stars are more diffuse, meaning that these layers are larger, but spread more thinly, than the outer layers of other stars. Stars with large diffuse atmospheres are called giants. Giant stars are not necessarily more massive than other stars-the outer layers of giant stars are just more spread out.
Many stars have thousands of spectral lines from iron and other elements near iron in the periodic table. Other stars of the same temperature have very few spectral lines from such elements. Astronomers interpret these findings to mean that two different populations of stars exist. Some formed long ago, before supernovas produced the heavy elements, and others formed more recently and incorporated some heavy elements. The Sun is one of the more recent stars.
Spectral lines can also be studied to see if they change in wavelength or are different in wavelength from sources of the same lines on Earth. These studies tell us, according to the Doppler effect, how much the star is moving toward or away from us. Such studies of starlight can tell us about the orbits of stars in binary systems or about the pulsations of variable stars, for example.
Astronomers study galaxies to learn about the structure of the universe. Galaxies are huge collections of billions of stars. Our Sun is part of the Milky Way Galaxy. Galaxies also contain dark strips of dust and may contain huge black holes at their centres. Galaxies exist in different shapes and sizes. Some galaxies are spirals, some are oval, or elliptical, and some are irregular. The Milky Way is a spiral galaxy. Galaxies tend to group together in clusters.
Our Sun is only one of about 400 billion stars in our home galaxy, the Milky Way. On a dark night, far from outdoor lighting, a faint, hazy, whitish band spans the sky. This band is the Milky Way Galaxy as it appears from Earth. The Milky Way looks splotchy, with darker regions interspersed with lighter ones.
The Milky Way Galaxy is a pinwheel-shaped flattened disk about 75,000 light-years in diameter. The Sun is located on a spiral arm about two-thirds of the way out from the centre. The galaxy spins, but the centre spins faster than the arms. At Earth's position, the galaxy makes a complete rotation about every 200 million years.
When observers on Earth look toward the brightest part of the Milky Way, which is in the constellation Sagittarius, they look through the galaxy's disk toward its centre. This disk is composed of the stars, gas, and dust between Earth and the galactic centre. When observers look in the sky in other directions, they do not see as much of the galaxy's gas and dust, and so can see objects beyond the galaxy more clearly.
The Milky Way Galaxy has a core surrounded by its spiral arms. A spherical cloud containing about 100 examples of a type of star cluster known as a globular cluster surrounds the galaxy. Still, farther out is a galactic corona. Astronomers are not sure what types of particles or objects occupy the corona, but these objects do exert a measurable gravitational force on the rest of the galaxy. Galaxies contain billions of stars, but the space between stars is not empty. Astronomers believe that almost every galaxy probably has a huge black hole at its centre.
The space between stars in a galaxy consists of low
- density gas and dust. The dust is largely carbon given off by red-giant stars. The gas is largely hydrogen, which accounts for 90 percent of the atoms in the universe. Hydrogen exists in two main forms in the universe. Astronomers give complete hydrogen atoms, with a nucleus and an electron, a designation of the Roman numeral I, or HI. Ionized hydrogen, hydrogen made up of atoms missing their electrons, is given the designation II, or HII. Clouds, or regions, of both types of hydrogen exist between the stars. HI regions are too cold to produce visible radiation, but they do emit radio waves that are useful in measuring the movement of gas in our own galaxy and in distant galaxies. The HII regions form around hot stars. These regions emit diffuse radiation in the visual range, as well as in the radio, infrared, and ultraviolet ranges. The cloudy light from such regions forms beautiful nebulas such as the Great Orion Nebula.
Astronomers have located more than 100 types of molecules in interstellar space. These molecules occur only in trace amounts among the hydrogens. Still, astronomers can use these molecules to map galaxies. By measuring the density of the molecules throughout a galaxy, astronomers can get an idea of the galaxy's structure. interstellar dust sometimes gathers to form dark nebulae, which appear in silhouette against background gas or stars from Earth. The Horsehead Nebula, for example, is the silhouette of interstellar dust against a background HI region.
The first known black holes were the collapsed cores of supernova stars, but astronomers have since discovered signs of much larger black holes at the centres of galaxies. These galactic black holes contain millions of times as much mass as the Sun. Astronomers believe that huge black holes such as these provide the energy of mysterious objects called quasars. Quasars are very distant objects that are moving away from Earth at high speed. The first ones discovered were very powerful radio sources, but scientists have since discovered quasars that don't strongly emit radio waves. Astronomers believe that almost every galaxy, whether spiral or elliptical, has a huge black hole at its centre.
Astronomers look for galactic black holes by studying the movement of galaxies. By studying the spectrum of a galaxy, astronomers can tell if gas near the centre of the galaxy is rotating rapidly. By measuring the speed of rotation and the distance from various points in the galaxy to the centre of the galaxy, astronomers can determine the amount of mass in the centre of the galaxy. Measurements of many galaxies show that gas near the centre is moving so quickly that only a black hole could be dense enough to concentrate so much mass in such a small space. Astronomers suspect that a significant black hole occupies even the centre of the Milky Way. The clear images from the Hubble Space Telescope have allowed measurements of motions closer to the centres of galaxies than previously possible, and have led to the confirmation in several cases that giant black holes are present.
Galaxies are classified by shape. The three types are spiral, elliptical, and irregular. Spiral galaxies consist of a central mass with one, two, or three arms that spiral around the centre. An elliptical galaxy is oval, with a bright centre that gradually, evenly dims to the edges. Irregular galaxies are not symmetrical and do not look like spiral or elliptical galaxies. Irregular galaxies vary widely in appearance. A galaxy that has a regular spiral or elliptical shape but has, some special oddity is known as a peculiar galaxy. For example, some peculiar galaxies are stretched and distorted from the gravitational pull of a nearby galaxy.
Spiral galaxies are flattened pinwheels in shape. They can have from one to three spiral arms coming from a central core. The Great Andromeda Spiral Galaxy is a good example of a spiral galaxy. The shape of the Milky Way is not visible from Earth, but astronomers have measured that the Milky Way is also a spiral galaxy. American astronomer Edwin Hubble further classified spirals galaxies by the tightness of their spirals. In order of increasingly open arms, Hubble's types are Sa, Sb., and Sc. Some galaxies have a straight, bright, bar-shaped feature across their centre, with the spiral arms coming off the bar or off a ring around the bar. With a capital B for the bar, the Hubble types of these galaxies are SBa, SBb, and Sbc.
Many clusters of galaxies have giant elliptical galaxies at their centres. Smaller elliptical galaxies, called dwarf elliptical galaxies, are much more common than giant ones. Most of the two dozen galaxies in the Milky Way's Local Group of galaxies are dwarf elliptical galaxies.
Astronomers classify elliptical galaxies by how oval they look, ranging from E0 for very round to E3 for intermediately oval to E7 for extremely elongated. The galaxy class E7 is also called S0, which is also known as a lenticular galaxy, a shape with an elongated disk but no spiral arms. Because astronomers can see other galaxies only from the perspective of Earth, the shape astronomers see is not necessarily the exact shape of a galaxy. For instance, they may be viewing it from an end, and not from above or below.
Some galaxies have no structure, while others have some trace of structure but do not fit the spiral or elliptical classes. All of these galaxies are called irregular galaxies. The two small galaxies that are satellites to the Milky Way Galaxy are both irregular. They are known as the Magellanic Clouds. The Large Magellanic Cloud shows signs of having a bar in its centre. The Small Magellanic Cloud is more formless. Studies of stars in the Large and Small Magellanic Clouds have been fundamental for astronomers' understanding of the universe. Each of these galaxies provides groups of stars that are all at the same distance from Earth, allowing astronomers to compare the absolute brightness of these stars.
In the late 1920s American astronomer Edwin Hubble discovered that all but the nearest galaxies to us are receding, or moving away from us. Further, he found that the farther away from Earth a galaxy is, the faster it is receding. He made his discovery by taking spectra of galaxies and measuring the amount by which the wavelengths of spectral lines were shifted. He measured distance in a separate way, usually from studies of Cepheid variable stars. Hubble discovered that essentially all the spectra of all the galaxies were shifted toward the red, or had red-shifts. The red-shifts of galaxies increased with increasing distance from Earth. After Hubble's work, other astronomers made the connection between red-shift and velocity, showing that the farther a galaxy is from Earth, the faster it moves away from Earth. This idea is called Hubble's law and is the basis for the belief that the universe is uniformly expanding. Other uniformly expanding three-dimensional objects, such as a rising cake with raisins in the batter, also demonstrate the consequence that the more distant objects (such as the other raisins with respect to any given raisin) appear to recede more rapidly than nearer ones. This consequence is the result of the increased amount of material expanding between these more distant objects.
Hubble's law state that there is a straight-line, or linear, relationship between the speed at which an object is moving away from Earth and the distance between the object and Earth. The speed at which an object is moving away from Earth is called the object's velocity of recession. Hubble's law indicates that as velocity of recession increases, distance increases by the same proportion. Using this law, astronomers can calculate the distance to the most-distant galaxies, given only measurements of their velocities calculated by observing how much their light is shifted. Astronomers can accurately measure the red-shifts of objects so distant that the distance between Earth and the objects cannot be measured by other means.
The constant of proportionality that relates velocity to distance in Hubble's law is called Hubble's constant, or H. Hubble's law is often written v Hd, or velocity equals Hubble's constant multiplied by distance. Thus determining Hubble's constant will give the speed of the universe's expansion. The inverse of Hubble's constant, or 1/H, theoretically provides an estimate of the age of the universe. Astronomers now believe that Hubble's constant has changed over the lifetime of the universe, however, so estimates of expansion and age must be adjusted accordingly.
The value of Hubble's constant probably falls between sixty-four and 78 kilometres per second per mega-parsec (between forty and 48 miles per second per mega-parsec). A mega-parsec is one million parsecs and a parsec is 3.26 light-years. The Hubble Space Telescope studied Cepheid variables in distant galaxies to get an accurate measurement of the distance between the stars and Earth to refine the value of Hubble's constant. The value they found is 72 kilometres per second per mega-parsec (45 miles per second per mega-parsec), with an uncertainty of only 10 percent
The actual age of the universe depends not only on Hubble's constant but also on how much the gravitational pull of the mass in the universe slows the universe's expansion. Some data from studies that use the brightness of distant supernovas to assess distance indicate that the universe's expansion is speeding up instead of slowing. Astronomers invented the term ‘dark energy' for the unknown cause of this accelerating expansion and are actively investigating these topics. The ultimate goal of astronomers is to understand the structure, behaviour, and evolution of all of the matter and energy that exist. Astronomers call the set of all matter and energy the universe. The universe is infinite in space, but astronomers believe it does have a finite age. Astronomers accept the theory that about fourteen billion years ago the universe began as an explosive event resulting in a hot, dense, expanding sea of matter and energy. This event is known as the big bang Astronomers cannot observe that far back in time. Many astronomers believe, however, the theory that within the first fraction of a second after the big bang, the universe went through a tremendous inflation, expanding many times in size, before it resumed a slower expansion.
As the universe expanded and cooled, various forms of elementary particles of matter formed. By the time the universe was one second old, protons had formed. For approximately the next 1,000 seconds, in the era of nucleosynthesis, all the nuclei of deuterium (hydrogen with both a proton and neutron in the nucleus) that are present in the universe today formed. During this brief period, some nuclei of lithium, beryllium, and helium formed as well.
When the universe was about one million years old, it had cooled to about 3000 K (about 3300°C or about 5900°F). At that temperature, the protons and heavier nuclei formed during nucleosynthesis could combine with electrons to form atoms. Before electrons combined with nuclei, the travel of radiation through space was very difficult. Radiation in the form of photons (packets of light energy) could not travel very far without colliding with electrons. Once protons and electrons combined to form hydrogen, photons became able to travel through space. The radiation carried by the photons had the characteristic spectrum of a hot gas. Since the time this radiation was first released, it has cooled and is now 3 K (-270°C or-450°F). It is called the primeval background radiation and has been definitively detected and studied, first by radio telescopes and then by the Cosmic Background Explorer (COBE) and Wilkinson Microwave Anisotropy Probe (WMAP) spacecrafts. COBE, WMAP, and ground-based radio telescopes detected tiny deviations from uniformity in the primeval background radiation; these deviations may be the seeds from which clusters of galaxies grew.
The gravitational force from invisible matter, known as dark matter, may have helped speed the formation of structure in the universe. Observations from the Hubble Space Telescope have revealed older galaxies than astronomers expected, reducing the interval between the big bang and the formation of galaxies or clusters of galaxies.
From about two billion years after the big bang for another two billion years, quasars formed as active giant black holes in the cores of galaxies. These quasars gave off radiation as they consumed matter from nearby galaxies. Few quasars appear close to Earth, so quasars must be a feature of the earlier universe.
A population of stars formed out of the interstellar gas and dust that contracted to form galaxies. This first population, known as Population II, was made up almost entirely of hydrogen and helium. The stars that formed evolved and gave out heavier elements that were made through fusion in the stars' cores or that was formed as the stars exploded as supernovas. The later generation of stars, to which the Sun belongs, is known as Population I and contains heavy elements formed by the earlier population. The Sun formed about five billion years ago and is almost halfway through its 11-billion-year lifetime
About 4.6 billion years ago, our solar system formed. The oldest fossils of a living organism date from about 3.5 billion years ago and represent Cyanobacteria. Life evolved, and sixty-five million years ago, the dinosaurs and many other species were extinguished, probably from a catastrophic meteor impact. Modern humans evolved no earlier than a few hundred thousand years ago, a blink of an eye on the cosmic timescale.
Will the universe expand forever or eventually stop expanding and collapse in on itself? Jay M. Pasachoff, professor of astronomy at Williams College in Williamstown, Massachusetts, confronts this question in this discussion of cosmology. Whether the universe will go on expanding forever, depends on whether there is enough critical density to halt or reverse the expansion, and the answer to that question may, in turn, depend on the existence of something the German-born American physicist Albert Einstein once labelled the cosmological constant.
New technology allows astronomers to peer further into the universe than ever before. The science of cosmology, the study of the universe as a whole, has become an observational science. Scientists may now verify, modify, or disprove theories that were partially based on guesswork.
In the 1920s, the early days of modern cosmology, it took an astronomer all night at a telescope to observe a single galaxy. Current surveys of the sky will likely compile data for a million different galaxies within a few years. Building upon advances in cosmology over the past century, our understanding of the universe should continue to accelerate
Modern cosmology began with the studies of Edwin Hubble, who measured the speeds that galaxies move toward or away from us in the mid-1920s. By observing red-shift-the change in wavelength of the light that galaxies give off as they move away from us-Hubble realized that though the nearest galaxies are approaching us, all distant galaxies are receding. The most-distant galaxies are receding most rapidly. This observation is consistent with the characteristics of an expanding universe. Since 1929 an expanding universe has been the first and most basic pillar of cosmology.
In 1990 the National Aeronautics and Space Administration (NASA) launched the Hubble Space Telescope (HST), named to honour the pioneer of cosmology. Appropriately, determining the rate at which the universe expands was one of the telescope's major tasks.
One of the HST's key projects was to study Cepheid variables (stars that varies greatly in brightness) and to measure distances in space. Another set of Hubble's observations focuses on supernovae, exploding stars that can be seen at very great distances because they are so bright. Studies of supernovae in other galaxies reveal the distances to those galaxies.
The term big bang refers to the idea that the expanding universe can be traced back in time to an initial explosion. In the mid-1960s, physicists found important evidence of the big bang when they detected faint microwave radiation coming from every part of the sky. Astronomers think this radiation originated about 300,000 years after the big bang, when the universe thinned enough to become transparent. The existence of cosmic microwave background radiation, and its interpretation, is the second pillar of modern cosmology.
Also in the 1960s, astronomers realized that the lightest of the elements, including hydrogen, helium, lithium, and boron, were formed mainly at the time of the big bang. What is most important, deuterium (the form of hydrogen with an extra neutron added to normal hydrogen's single proton) was formed only in the era of nucleosynthesis? This era started about one second after the universe was formed and made up the first three minutes or so after the big bang. No sources of deuterium are known since that early epoch. The current ratio of deuterium to regular hydrogen depends on how dense the universe was at that early time, so studies of the deuterium that can now be detected indicate how much matter the universe contains. These studies of the origin of the light elements are the third pillar of modern cosmology.
Until recently many astronomers disagreed on whether the universe was expected to expand forever or eventually stop expanding and collapse in on itself in a ‘big crunch.'
At the General Assembly of the International Astronomical Union (IAU) held in August 2000, a consistent picture of cosmology emerged. This picture depends on the current measured value for the expansion rate of the universe and on the density of the universe as calculated from the abundances of the light elements. The most recent studies of distant supernovae seem to show that the universe's expansion is accelerating, not slowing. Astronomers have recently proposed a theoretical type of negative energy-which would provide a force that opposes the attraction of gravity-to explain the accelerating universe.
For decades scientists have debated the rate at which the universe is expanding. We know that the further away a galaxy is, the faster it moves away from us. The question is: How fast are galaxies receding for each unit of distance they are away from us? The current value, as announced at the IAU meeting, is 75 km/s/Mpc, that is, for each mega-parsec of distance from us (where each mega-parsec is 3.26 million light-years), the speed of expansion increases by 75 kilometres per second.
What's out there, exactly?
In the picture of expansion held until recently, astronomers thought the universe contained just enough matter and energy so that it would expand forever but expand at a slower and slower rate as time went on. The density of matter and energy necessary for this to happen is known as the critical density.
Astronomers now think that only 5 percent or so of the critical density of the universe is made of ordinary matter. Another 25 percent or so of the critical density is made of dark matter, a type of matter that has gravity but that has not been otherwise detected. The accelerating universe, further, shows that the remaining 70 percent of the critical density is made of a strange kind of energy, perhaps that known as the cosmological constant, an idea tentatively invoked and then abandoned by Albert Einstein in equations for his general theory of relativity.
Some may be puzzled: Didn't we learn all about the foundations of physics when we were still at school? The answer is ‘yes' or ‘no', depending on the interpretation. We have become acquainted with concepts and general relations that enable us to comprehend an immense range of experiences and make them accessible to mathematical treatment. In a certain sense these concepts and relations are probably even final. This is true, for example, of the laws of light refraction, of the relations of classical thermodynamics as far as it is based on the concepts of pressure, volume, temperature, heat and work, and of the hypothesis of the nonexistence of a perpetual motion machine.
What, then, impels us to devise theory after theory? Why do we devise theories at all? The answer to the latter question is simple: Because we enjoy ‘comprehending', i.e., reducing phenomena by the process of logic to something already known or (apparently) evident. New theories are first of all necessary when we encounter new facts that cannot be ‘explained' by existing theories. Nevertheless, this motivation for setting up new theories is, so to speak, trivial, imposed from without. There is another, more subtle motive of no less importance. This is the striving toward unification and simplification of the premises of the theory as a whole (i.e., Mach's principle of economy, interpreted as a logical principle).
There exists a passion for comprehension, just as there exists a passion for music. That passion is altogether common in children, but gets lost in most people later on. Without this passion, there would be neither mathematics nor natural science. Time and again the passion for understanding has led to the illusion that man is able to comprehend the objective world rationally, by pure thought, without any empirical foundations-in short, by metaphysics. I believe that every true theorist is a kind of tamed metaphysicist, no matter how pure a
‘positivist', he may fancy himself. The metaphysicist believes that the logically simple are also the real. The tamed metaphysicist believes that not all that is logically simple is embodied in experienced reality, but that the totality of all sensory experience can be ‘comprehended' on the basis of a conceptual system built on premises of great simplicity. The skeptic will say that this is a ‘miracle creed'. Admittedly so, but it is a miracle creed that has been borne out to an amazing extent by the development of science.
The rise of atomism is a good example. How may Leucippus have conceived this bold idea? When water freezes and becomes ice-apparently something entirely different from water-why is it that the thawing of the ice forms something that seems indistinguishable from the original water? Leucippus is puzzled and looks for an ‘explanation'. He is driven to the conclusion that in these transitions the ‘essence', of the thing has not changed at all. Maybe the thing consists of immutable particles and the change is only a change in their spatial arrangement. Could it not be that the same is true of all material objects that emerge again and again with nearly identical qualities?
This idea is not entirely lost during the long hibernation of occidental thought. Two thousand years after Leucippus, Bernoulli wonders why gas exerts pressure on the walls of a container. Should this be ‘explained' by mutual repulsion of the parts of the gas, in the sense of Newtonian mechanics? This hypothesis appears absurd, for the gas pressure depends on the temperature, all other things being equal. To assume that the Newtonian forces of interaction depend on temperature is contrary to the spirit of Newtonian mechanics. Since Bernoulli is aware of the concept of atomism, he is bound to conclude that the atoms (or molecules) collide with the walls of the container and in doing so exert pressure. After all, one has to assume that atoms are in motion; how else can one account for the varying temperature of gases?
A simple mechanical consideration shows that this pressure depends only on the kinetic energy of the particles and on their density in space. This should have led the physicists of that age to the conclusion that heat consists in random motion of the atoms. Had they taken this consideration as seriously as it deserved to be taken, the development of the theory of heat-in particular the discovery of the equivalence of heat and mechanical energy-would have been considerably facilitated.
This example is meant to illustrate two things. The theoretical idea (atomism in this case) does not arise apart and independent of experience; nor can it be derived from experience by a purely logical procedure. It is produced by a creative act. Once a theoretical idea has been acquired, one does well to hold fast to it until it leads to an untenable conclusion.
In Newtonian physics the elementary theoretical concept on which the theoretical description of material bodies is based is the material point, or particle. Thus, matter is considered theoretically to be discontinuous. This makes it necessary to consider the action of material points on one another as ‘action at a distance'. Since the latter concept seems quite contrary to everyday experience, it is only natural that the contemporaries of Newton-and in fact, Newton himself found it difficult to accept. Owing to the almost miraculous success of the Newtonian system, however, the succeeding generations of physicists became used to the idea of action at a distance. Any doubt was buried for a long time to come.
All the same, when, in the second half of the 19th century, the laws of electrodynamics became known, it turned out that these laws could not be satisfactorily incorporated into the Newtonian system. It is fascinating to muse: Would Faraday have discovered the law of electromagnetic induction if he had received a regular college education? Unencumbered by the traditional way of thinking, he felt that the introduction of the ‘field' as an independent element of reality helped him to coordinate the experimental facts. It was Maxwell who fully comprehended the significance of the field concept; he made the fundamental discovery that the laws of electrodynamics found their natural expression in the differential equations for the electric and magnetic fields. These equations implied the existence of waves, whose properties corresponded to those of light as far as they were known at that time.
This incorporation of optics into the theory of electromagnetism represents one of the greatest triumphs in the striving toward unification of the foundations of physics; Maxwell achieved this unification by purely theoretical arguments, long before it was corroborated by Hertz' experimental work. The new insight made it possible to dispense with the hypothesis of action at a distance, at least in the realm of electromagnetic phenomena; the intermediary field now appeared as the only carrier of electromagnetic interaction between bodies, and the field's behaviour was completely determined by contiguous processes, expressed by differential equations.
Now a question arose: Since the field exists even in a vacuum, should one conceive of the field as a state of a ‘carrier', or should it be endowed with an independent existence not reducible to anything else? In other words, is there an ‘ether' which carries the field; the ether being considered in the undulatory state, for example, when it carries light waves?
The question has a natural answer: Because one cannot dispense with the field concept, not introducing in addition a carrier with hypothetical properties is preferable. However, the pathfinder who first recognized the indispensability of the field concept were still too strongly imbued with the mechanistic tradition of thought to accept unhesitatingly this simple point of view. Nevertheless, in the course of the following decades this view imperceptibly took hold.
The introduction of the field as an elementary concept gave rise to an inconsistency of the theory as a whole. Maxwell's theory, although adequately describing the behaviour of electrically charged particles in their interaction with one another, does not explain the behaviours of electrical densities, i.e., it does not provide a theory of the particles themselves. They must therefore be treated as mass points on the basis of the old theory. The combination of the idea of a continuous field with that of material points discontinuous in space appears inconsistent. A consistent field theory requires continuity of all elements of the theory, not only in time but also in space, and in all points of space. Hence the material particle has no place as a fundamental concept in a field theory. Thus, even apart from the fact that gravitation is not included. Maxwell's electrodynamics cannot be considered a complete theory.
Maxwell's equations for empty space remain unchanged if the spatial coordinates and the time are subjected to a particular linear transformations-the Lorentz transformations (‘covariance' with respect to Lorentz transformations). Covariance also holds, of course, for a transformation that is composed of two or more such transformations; this is called the ‘group' property of Lorentz transformations.
Maxwell's equations imply the ‘Lorentz group', but the Lorentz group does not imply Maxwell's equations. The Lorentz group may effectively be defined independently of Maxwell's equations as a group of linear transformations that leave a particular value of the velocity-the velocity of light-invariant. These transformations hold for the transition from one ‘inertial system to another that is in uniform motion relative to the first. The most conspicuous novel property of this transformation group is that it does away with the absolute character of the concept of simultaneity of events distant from each other in space. On this account it is to be expected that all equations of physics are covariant with respect to Lorentz transformations (special theory of relativity). Thus it came about that Maxwell's equations led to a heuristic principle valid far beyond the range of the applicability or even validity of the equations themselves.
Special relativity has this in common with Newtonian mechanics: The laws of both theories are supposed to hold only with respect to certain coordinate systems: those known as ‘inertial systems'. An inertial system is a system in a state of motion such that ‘force-free' material points within it are not accelerated with respect to the coordinate system. However, this definition is empty if there is no independent means for recognizing the absence of forces. Nonetheless, such a means of recognition does not exist if gravitation is considered as a ‘field'.
Let ‘A' be a system uniformly accelerated with respect to an ‘inertial system' I. Material points, not accelerated with respect to me, are accelerated with respect to ‘A', the acceleration of all the points being equal in magnitude and direction. They behave as if a gravitational field exists with respect to ‘A', for it is a characteristic property of the gravitational field that the acceleration is independent of the particular nature of the body. There is no reason to exclude the possibility of interpreting this behaviour as the effect of a ‘true' gravitational field (principle of equivalence). This interpretation implies that ‘A' is an ‘inertial system,' even though it is accelerated with respect to another inertial system. (It is essential for this argument that the introduction of independent gravitational fields is considered justified even though no masses generating the field are defined. Therefore, to Newton such an argument would not have appeared convincing.) Thus the concepts of inertial system, the law of inertia and the law of motion are deprived of their concrete meaning-not only in classical mechanics but also in special relativity. Moreover, following up this train of thought, it turns out that with respect to A time cannot be measured by identical clocks; effectively, even the immediate physical significance of coordinate differences is generally lost. In view of all these difficulties, should one not try, after all, to hold on to the concept of the inertial system, relinquishing the attempt to explain the fundamental character of the gravitational phenomena that manifest themselves in the Newtonian system as the equivalence of inert and gravitational mass? Those who trust in the comprehensibility of nature must answer: No.
This is the gist of the principle of equivalence: In order to account for the equality of inert and gravitational mass within the theory admitting nonlinear transformations of the four coordinates is necessary. That is, the group of Lorentz transformations and hence the set of the "permissible" coordinate systems has to be extended.
What group of coordinate transformations can then be substituted for the group of Lorentz transformations? Mathematics suggests an answer that is based on the fundamental investigations of Gauss and Riemann: namely, that the appropriate substitute is the group of all continuous (analytical) transformations of the coordinates. Under these transformations the only thing that remains invariant is the fact that neighbouring points have nearly the same coordinates; the coordinate system expresses only the topological order of the points in space (including its four-dimensional character). The equations expressing the laws of nature must be covariant with respect to all continuous transformations of the coordinates. This is the principle of general relativity.
The procedure just described overcomes a deficiency in the foundations of mechanics that had already been noticed by Newton and was criticized by Leibnitz and, two centuries later, by Mach: Inertia resists acceleration, but acceleration relative to what? Within the frame of classical mechanics the only answer is: Inactivity resists velocity relative to distances. This is a physical property of space-space acts on objects, but objects do not act on space. Such is probably the deeper meaning of Newton's assertion spatium est absolutum (space is absolute). Nevertheless, the idea disturbed some, in particular Leibnitz, who did not ascribe an independent existence to space but considered it merely a property of ‘things' (contiguity of physical objects). Had his justified doubts won out at that time, it hardly would have been a boon to physics, for the empirical and theoretical foundations necessary to follow up his idea was not available in the 17th century.
According to general relativity, the concept of space detached from any physical content does not exist. The physical reality of space is represented by a field whose components are continuous functions of four independent variables—the coordinates of space and time. It is just this particular kind of dependence that expresses the spatial character of physical reality.
Since the theory of general relativity implies the representation of physical reality by a continuous field, the concept of particles or material points cannot . . . play a fundamental part, nor can the concept of motion. The particle can only appear as a limited region in space in which the field strength or the energy density is particularly high.
A relativistic theory has to answer two questions: (1) What is the mathematical character of the field? What equations hold for this field?
Concerning the first question: From the mathematical point of view the field is essentially characterized by the way its components transform if a coordinate transformation is applied. Concerning the second (2) question: The equations must determine the field to a sufficient extent while satisfying the postulates of general relativity. Whether or not this requirement can be satisfied, depends on the choice of the field-type.
The attempts to comprehend the correlations among the empirical data on the basis of such a highly abstract program may at first appear almost hopeless. The procedure amounts, in fact, to putting the question: What most simple property can be required from what most simple object (field) while preserving the principle of general relativity? Viewed in formal logic, the dual character of the question appears calamitous, quite apart from the vagueness of the concept ‘simple'. Moreover, as for physics there is nothing to warrant the assumption that a theory that is ‘logically simple' should also be ‘true'.
Yet every theory is speculative. When the basic concepts of a theory are comparatively ‘close to experience' (e.g., the concepts of force, pressures, mass), its speculative character is not so easily discernible. If, however, a theory is such as to require the application of complicated logical processes in order to reach conclusions from the premises that can be confronted with observation, everybody becomes conscious of the speculative nature of the theory. In such a case an almost irresistible feeling of aversion arises in people who are inexperienced in epistemological analysis and who are unaware of the precarious nature of theoretical thinking in those fields with which they are familiar.
On the other hand, it must be conceded that a theory has an important advantage if its basic concepts and fundamental hypotheses are ‘close to experience', and greater confidence in such a theory is justifiable. There is less danger of going completely astray, particularly since it takes so much less time and effort to disprove such theories by experience. Yet ever more, as the depth of our knowledge increases, we must give up this advantage in our quest for logical simplicity and uniformity in the foundations of physical theory. It has to be admitted that general relativity has gone further than previous physical theories in relinquishing ‘closeness to experience' of fundamental concepts in order to attain logical simplicity. This holds all ready for the theory of gravitation, and it is even more true of the new generalization, which is an attempt to comprise the properties of the total field. In the generalized theory the procedure of deriving from the premises of the theory conclusions that can be confronted with empirical data is so difficult that so far no such result has been obtained. In favour of this theory are, at this point, its logical simplicity and its ‘rigidity'. Rigidity means here that the theory is either true or false, but not modifiable.
The greatest inner difficulty impeding the development of the theory of relativity is the dual nature of the problem, indicated by the two questions we have asked. This duality is the reason the development of the theory has taken place in two steps so widely separated in time. The first of these steps, the theory of gravitation, is based on the principle of equivalence discussed above and rests on the following consideration: According to the theory of special relativity, light has a constant velocity of propagation. If a light ray in a vacuum starts from a point, designated by the coordinates x1, x2 and x3 in a three-dimensional coordinate system, at the time x4, it spreads as a spherical wave and reaches a neighbouring point (x1 + dx1, x2 + dx2, x3 + dx3) at the time x4 + dx4. Introducing the velocity of light, c, we write the expression:
This expression represents an objective relation between neighbouring space-time points in four dimensions, and it holds for all inertial systems, provided the coordinate transformations are restricted to those of special relativity. The relation loses this form, however, if arbitrary continuous transformations of the coordinates are admitted in accordance with the principle of general relativity. The relation then assumes the more general form:
 ik gik dxi dxk=0
The gik are certain functions of the coordinates that transform in a definite way if a continuous coordinate transformation is applied. According to the principle of equivalence, these gik functions describe a particular kind of gravitational field: a field that can be obtained by transformation of ‘field-free' space. The gik satisfies a particular law of transformation. Mathematically speaking, they are the components of a ‘tensor' with a property of symmetry that is preserved in all transformations; the symmetrical property is expressed as follows:
gik=gki
The idea suggests itself: May we not ascribe objective meaning to such a symmetrical tensor, even though the field cannot be obtained from the empty space of special relativity by a mere coordinate transformation? Although we cannot expect that such a symmetrical tensor will describe the most general field, it may describe the particular case of the ‘pure gravitational field'. Thus it is evident what kind of field, at least for a special case, general relativity has to postulate: a symmetrical tensor field.
Hence only the second question is left: What kind of general covariant field law can be postulated for a symmetrical tensor field?
This question has not been difficult to answer in our time, since the necessary mathematical conceptions were already here in the form of the metric theory of surfaces, created a century ago by Gauss and extended by Riemann to manifolds of an arbitrary number of dimensions. The result of this purely formal investigation has been amazing in many respects. The differential equations that can be postulated as field law for gik cannot be of lower than second order, i.e., they must at least contain the second derivatives of the gik with respect to the coordinates. Assuming that no higher than second derivatives appear in the field law, it is mathematically determined by the principle of general relativity. The system of equations can be written in the form: Rik = 0. The Rik transforms in the same manner as the gik, i.e., they too form a symmetrical tensor.
These differential equations completely replace the Newtonian theory of the motion of celestial bodies provided the masses are represented as singularities of the field. In other words, they contain the law of force as well as the law of motion while eliminating ‘inertial systems'.
The fact that the masses appear as singularities indicate that these masses themselves cannot be explained by symmetrical gik fields, or ‘gravitational fields'. Not even the fact that only positive gravitating masses exist can be deduced from this theory. Evidently a complete relativistic field theory must be based on a field of more complex nature, that is, a generalization of the symmetrical tensor field.
The first observation is that the principle of general relativity imposes exceedingly strong restrictions on the theoretical possibilities. Without this restrictive principle hitting on the gravitational equations would be practically impossible for anybody, not even by using the principle of special relativity, even though one knows that the field has to be described by a symmetrical tensor. No amount of collection of facts could lead to these equations unless the principles of general relativity were used. This is the reason that all attempts to obtain a deeper knowledge of the foundations of physics seem doomed to me unless the basic concepts are in accordance with general relativity from the beginning. This situation makes it difficult to use our empirical knowledge, however comprehensive, in looking for the fundamental concepts and relations of physics, and it forces us to apply free speculation to a much greater extent than is presently assumed by most physicists. One may not see any reason to assume that the heuristic significance of the principle of general relativity is restricted to gravitation and that the rest of physics can be dealt with separately on the basis of special relativity, with the hope that later as a resultant circumstance brings about the whole that may be fitted consistently into a general relativistic scheme. One is to think that such an attitude, although historically understandable, can be objectively justified. The comparative smallness of what we know today as gravitational effects is not a conclusive reason for ignoring the principle of general relativity in theoretical investigations of a fundamental character. In other words, I do not believe that asking it is justifiable: What would physics look like without gravitation?
The second point we must note is that the equations of gravitation are ten differential equations for the ten components of the symmetrical tensor gik. In the case of a non-generalized relativity theory, a system is ordinarily not over determined if the number of equations is equal to the number of unknown functions. The manifold of solutions is such that within the general solution a certain number of functions of three variables can be chosen arbitrarily. For a general relativistic theory this cannot be expected as a matter of course. Free choice with respect to the coordinate system implies that out of the ten functions of a solution, or components of the field, four can be made to assume prescribed values by a suitable choice of the coordinate system. In other words, the principle of general relativity implies that the number of functions to be determined by differential equations is not ten but 10-4=6. For these six functions only six independent differential equations may be postulated. Only six out of the ten differential equations of the gravitational field ought to be independent of each other, while the remaining four must be connected to those six by means of four relations (identities). In earnest there exist among the left-hand sides, Rik, of the ten gravitational equations four identities 'Bianchi's identities'-which assure their ‘compatibility'.
In a case like this-when the number of field variables is equal to the number of differential equations-compatibility is always assured if the equations can be obtained from a variational principle. This is unquestionably the case for the gravitational equations.
However, the ten differential equations cannot be entirely replaced by six. The system of equations is verifiably ‘over determined', but due to the existence of the identities it is over determined in such a way that its compatibility is not lost,i.e., the manifold of solutions is not critically restricted. The fact that the equations of gravitation imply the law of motion for the masses is intimately connected with this (permissible) over determination.
After this preparation understanding the nature of the present investigation without entering into the details of its mathematics is now easy. The problem is to set up a relativistic theory for the total field. The most important clue to its solution is that there exists already the solution for the special case of the pure gravitational field. The theory we are looking for must therefore be a generalization of the theory of the gravitational field. The first question is: What is the natural generalization of the symmetrical tensor field?
This question cannot be answered by itself, but only in connection with the other question: What generalization of the field is going to provide the most natural theoretical system? The answer on which the theory under discussion is based is that the symmetrical tensor field must be replaced by a non-symmetrical one. This means that the condition gik = gki for the field components must be dropped. In that case the field has sixteen instead of ten independent components.
There remains the task of setting up the relativistic differential equations for a non-symmetrical tensor field. In the attempt to solve this problem one meets with a difficulty that does not arise in the case of the symmetrical field. The principle of general relativity does not suffice to determine completely the field equations, mainly because the transformation law of the symmetrical part of the field alone does not involve the components of the anti-symmetrical part or vice versa. Probably this is the reason that this kind of generalization of the field has been hardly ever tried before. The combination of the two parts of the field can only be shown to be a natural procedure if in the formalism of the theory only the total field plays a role, and not the symmetrical and anti-symmetrical parts separately.
It turned out that this requirement can actively be satisfied in a natural way. Nonetheless, even this requirement, together with the principle of general relativity, is still not sufficient to determine uniquely the field equations. Let us remember that the system of equations must satisfy a further condition: the equations must be compatible. It has been mentioned above that this condition is satisfied if the equations can be derived from a variational principle.
This has rightfully been achieved, although not in so natural a way as in the case of the symmetrical field. It has been disturbing to find that it can be achieved in two different ways. These variational principles furnished two systems of equations-let us denote them by E1 and E2-which were different from each other (although only so), each of them exhibiting specific imperfections. Consequently even the condition of compatibility was insufficient to determine the system of equations uniquely.
It was, in fact, the formal defects of the systems E1 and E2 out whom indicated a possible way. There exists a third system of equations, E3, which is free of the formal defects of the systems E1 and E2 and represents a combination of them in the sense that every solution of E3 is a solution of E1 as well as of E2. This suggests that E3 may be the system for which we have been looking. Why not postulate E3, then, as the system of equations? Such a procedure is not justified without further analysis, since the compatibility of E1 and that of E2 does not imply compatibility of the stronger system E3, where the number of equations exceeds the number of field components by four.
An independent consideration shows that irrespective of the question of compatibility the stronger system, E3, is the only really natural generalization of the equations of gravitation.
It seems, nonetheless, that E3 is not a compatible system in the same sense as are the systems E1 and E2, whose compatibility is assured by a sufficient number of identities, which means that every field that satisfies the equations for a definite value of the time has a continuous extension representing a solution in four-dimensional space. The system E3, however, is not extensible in the same way. Using the language of classical mechanics, we might say: In the case of the system E3 the ‘initial condition' cannot be freely chosen. What really matter is the answer to the question: Is the manifold of solutions for the system E3 as extensive as must be required for a physical theory? This purely mathematical problem is as yet unsolved.
The skeptic will say: "It may be true that this system of equations is reasonable from a logical standpoint. However, this does not prove that it corresponds to nature." You are right, dear skeptic. Experience alone can decide on truth. Yet we have achieved something if we have succeeded in formulating a meaningful and precise question. Affirmation or refutation will not be easy, in spite of an abundance of known empirical facts. The derivation, from the equations, of conclusions that can be confronted with experience will require painstaking efforts and probably new mathematical methods.
Schrödinger's mathematical description of electron waves found immediate acceptance. The mathematical description matched what scientists had learned about electrons by observing them and their effects. In 1925, a year before Schrödinger published his results, German-British physicist Max Born and German physicist Werner Heisenberg developed a mathematical system called matrix mechanics. Matrix mechanics also succeeded in describing the structure of the atom, but it was totally theoretical. It gave no picture of the atom that physicists could verify observationally. Schrödinger's vindication of de Broglie's idea of electron waves immediately overturned matrix mechanics, though later physicists showed that wave mechanics are equivalent to matrix mechanics.
To solve these problems, mathematicians use calculus, which deals with continuously changing quantities, such as the position of a point on a curve. Its simultaneous development in the 17th century by English mathematician and physicist Isaac Newton and German philosopher and mathematician Gottfried Wilhelm Leibniz enabled the solution of many problems that had been insoluble by the methods of arithmetic, algebra, and geometry. Among the advances that calculus helped develop were the determinations of Newton's laws of motion and the theory of electromagnetism.
The physical sciences investigate the nature and behaviour of matter and energy on a vast range of size and scale. In physics itself, scientists study the relationships between matter, energy, force, and time in an attempt to explain how these factors shape the physical behaviour of the universe. Physics can be divided into many branches. Scientists study the motion of objects, a huge branch of physics known as mechanics that involves two overlapping sets of scientific laws. The laws of classical mechanics govern the behaviour of objects in the macroscopic world, which includes everything from billiard balls to stars, while the laws of quantum mechanics govern the behaviour of the particles that make up individual atoms.
The new math is new only in that the material is introduced at a much lower level than heretofore. Thus geometry, which was and is commonly taught in the second year of high school, is now frequently introduced, in an elementary fashion, in the fourth grade-in fact, naming and recognition of the common geometric figures, the circle and the square, occurs in kindergarten. At an early stage, numbers are identified with points on a line, and the identification is used to introduce, much earlier than in the traditional curriculum, negative numbers and the arithmetic processes involving them.
The elements of set theory constitute the most basic and perhaps the most important topic of the new math. Even a kindergarten child can understand, without formal definition, the meaning of a set of red blocks, the set of fingers on the left hand, and the set of the child's ears and eyes. The technical word set is merely a synonym for many common words that designate an aggregate of elements. The child can understand that the set of fingers on the left hand and the set on the right-hand match-that is, the elements, fingers, can be put into a one-to-one correspondence. The set of fingers on the left hand and the set of the child's ears and eyes do not match. Some concepts that are developed by this method are counting, equality of number, more than, and less then. The ideas of union and intersection of sets and the complement of a set can be similarly developed without formal definition in the early grades. The principles and formalism of set theory are extended as the child advances; upon graduation from high school, the student's knowledge is quite comprehensive.
The amount of new math and the particular topics taught vary from school to school. In addition to set theory and intuitive geometry, the material is usually chosen from the following topics: a development of the number systems, including methods of numeration, binary and other bases of notation, and modular arithmetic; measurement, with attention to accuracy and precision, and error study; studies of algebraic systems, including linear algebra, modern algebra, vectors, and matrices, with an axiomatically delegated approach; logic, including truth tables, the nature of proof, Venn or Euler diagrams, relations, functions, and general axiomatic; probability and statistics; linear programming; computer programming and language; and analytic geometry and calculus. Some schools present differential equations, topology, and real and complex analysis.
Cosmology, of an evolution, is the study of the general nature of the universe in space and in time-what it is now, what it was in the past and what it is likely to be in the future. Since the only forces at work between the galaxies that makes up the material universe are the forces of gravity, the cosmological problem is closely connected with the theory of gravitation, in particular with its modern version as comprised in Albert Einstein's general theory of relativity. In the frame of this theory the properties of space, time and gravitation are merged into one harmonious and elegant picture.
The basic cosmological notion of general relativity grew out of the work of great mathematicians of the 19th century. In the middle of the last century two inquisitive mathematical minds-Russian named Nikolai Lobachevski and a Hungarian named János Bolyai-discovered that the classical geometry of Euclid was not the only possible geometry: in fact, they succeeded in constructing a geometry that was fully as logical and self-consistent as the Euclidean. They began by overthrowing Euclid's axiom about parallel lines: namely, that only one parallel to a given straight line can be drawn through a point not on that line. Lobachevski and Bolyai both conceived a system of geometry in which a great number of lines parallel to a given line could be drawn through a point outside the line.
To illustrate the differences between Euclidean geometry and their non-Euclidean system considering just two dimensions are simplest-that is, the geometry of surfaces. In our schoolbooks this is known as ‘plane geometry', because the Euclidean surface is a flat surface. Suppose, now, we examine the properties of a two-dimensional geometry constructed not on a plane surface but on a curved surface. For the system of Lobachevski and Bolyai we must take the curvature of the surface to be ‘negative', which means that the curvature is not like that of the surface of a sphere but like that of a saddle. Now if we are to draw parallel lines or any figure (e.g., a triangle) on this surface, we must decide first of all how we will define a ‘straight line', equivalent to the straight line of plane geometry. The most reasonable definition of a straight line in Euclidean geometry is that it is the path of the shortest distance between two points. On a curved surface the line, so defined, becomes a curved line known as a ‘geodesic'.
Considering a surface curved like a saddle, we find that, given a ‘straight' line or geodesic, we can draw through a point outside that line a great many geodesics that will never intersect the given line, no matter how far they are extended. They are therefore parallel to it, by the definition of parallel. The possible parallels to the line fall within certain limits, indicated by the intersecting lines.
As a consequence of the overthrow of Euclid's axiom on parallel lines, many of his theorems are demolished in the new geometry. For example, the Euclidean theorem that the sum of the three angles of a triangle is 180 degrees no longer holds on a curved surface. On the saddle-shaped surface the angles of a triangle formed by three geodesics always add up to less than 180 degrees, the actual sum depending on the size of the triangle. Further, a circle on the saddle surface does not have the same properties as a circle in plane geometry. On a flat surface the circumference of a circle increases in proportion to the increase in diameter, and the area of a circle increases in proportion to the square of the increase in diameter. Still, on a saddle surface both the circumference and the area of a circle increase at faster rates than on a flat surface with increasing diameter.
After Lobachevski and Bolyai, the German mathematician Bernhard Riemann constructed another non-Euclidean geometry whose two-dimensional model is a surface of positive, rather than negative, curvature-that is, the surface of a sphere. In this case a geodesic line is simply a great circle around the sphere or a segment of such a circle, and since any two great circles must intersect at two points (the poles), there are no parallel lines at all in this geometry. Again the sum of the three angles of a triangle is not 180 degrees: in this case it is always more than 180. The circumference of a circle now increases at a rate slower than in proportion to its increase in diameter, and its area increases more slowly than the square of the diameter.
Now all this is not merely an exercise in abstract reasoning but bears directly on the geometry of the universe in which we live. Is the space of our universe ‘flat', as Euclid assumed, or is it curved negatively (per Lobachevski and Bolyai) or curved positively (Riemann)? If we were two-dimensional creatures living in a two-dimensional universe, we could tell whether we were living on a flat or a curved surface by studying the properties of triangles and circles drawn on that surface. Similarly as three-dimensional beings living in three-dimensional space, in that we should be capably able by way of studying geometrical properties of that space, to decide what the curvature of our space is. Riemann in fact developed mathematical formulas describing the properties of various kinds of curved space in three and more dimensions. In the early years of this century Einstein conceived the idea of the universe as a curved system in four dimensions, embodying time as the fourth dimension, and he proceeded to apply Riemann's formulas to test his idea.
Einstein showed that time can be considered a fourth coordinate supplementing the three coordinates of space. He connected space and time, thus establishing a ‘space-time continuum', by means of the speed of light as a link between time and space dimensions. However, recognizing that space and time are physically different entities, he employed the imaginary number Á, or me, to express the unit of time mathematically and make the time coordinate formally equivalent to the three coordinates of space.
In his special theory of relativity Einstein made the geometry of the time-space continuum strictly Euclidean, that is, flat. The great idea that he introduced later in his general theory was that gravitation, whose effects had been neglected in the special theory, must make it curved. He saw that the gravitational effect of the masses distributed in space and moving in time was equivalent to curvature of the four-dimensional space-time continuum. In place of the classical Newtonian statement that ‘the sun produces a field of forces that impel the earth to deviate from straight-line motion and to move in a circle around the sun'. Einstein substituted a statement to the effect that ‘the presence of the sun causes a curvature of the space-time continuum in its neighbourhood'.
The motion of an object in the space-time continuum can be represented by a curve called the object's ‘world line'. Einstein declared, in effect: ‘The world line of the earth is a geodesic trajectory in the curved four-dimensional space around the sun'. In other words, the . . . earth's ‘world line' . . . corresponds to the shortest four-dimensional distance between the position of the earth in January . . . and its position in October . . .
Einstein's idea of the gravitational curvature of space-time was, of course, triumphantly affirmed by the discovery of perturbations in the motion of Mercury at its closest approach to the sun and of the deflection of light rays by the sun's gravitational field. Einstein next attempted to apply the idea to the universe as a whole. Does it have a general curvature, similar to the local curvature in the sun's gravitational field? He now had to consider not a single centre of gravitational force but countless focal points in a universe full of matter concentrated in galaxies whose distribution fluctuates considerably from region to region in space. However, in the large-scale view the galaxies are spread uniformly throughout space as far out as our biggest telescopes can see, and we can justifiably ‘smooth out' its matter to a general average (which comes to about one hydrogen atom per cubic metre). On this assumption the universe as a whole has a smooth general curvature.
Nevertheless, if the space of the universe is curved, what is the sign of this curvature? Is it positive, as in our two-dimensional analogy of the surface of a sphere, or is it negative, as in the case of a saddle surface? Since we cannot consider space alone, how is this space curvature related to time?
Analysing the pertinent mathematical equations, Einstein came to the conclusion that the curvature of space must be independent of time, i.e., that the universe as a whole must be unchanging (though it changes internally). However, he found to his surprise that there was no solution of the equations that would permit a static cosmos. To repair the situation, Einstein was forced to introduce an additional hypothesis that amounted to the assumption that a new kind of force was acting among the galaxies. This hypothetical force had to be independent of mass (being the same for an apple, the moon and the sun) and to gain in strength with increasing distance between the interacting objects (as no other forces ever do in physics).
Einstein's new force, called ‘cosmic repulsion',  allowed two mathematical models of a static universe. One solution, which was worked out by Einstein himself and became known as, Einstein's spherical universe, gave the space of the cosmos a positive curvature. Like a sphere, this universe was closed and thus had a finite volume. The space coordinates in Einstein's spherical universe were curved in the same way as the latitude or longitude coordinates on the surface of the earth. However, the time axis of the space-time continuum ran quite straight, as in the good old classical physics. This means that no cosmic event would ever recur. The two-dimensional analogy of Einstein's space-time continuum is the surface of a cylinder, with the time axis running parallel to the axis of the cylinder and the space axis perpendicular to it.
The other static solution based on the mysterious repulsion forces was discovered by the Dutch mathematician Willem de Sitter. In his model of the universe both space and time were curved. Its geometry was similar to that of a globe, with longitude serving as the space coordinate and latitude as time. Unhappily astronomical observations contradicted by both Einstein and de Sitter's static models of the universe, and they were soon abandoned.
In the year 1922 a major turning point came in the cosmological problem. A Russian mathematician, Alexander A. Friedman (from whom the author of this article learned his relativity), discovered an error in Einstein's proof for a static universe. In carrying out his proof Einstein had divided both sides of an equation by a quantity that, Friedman found, could become zero under certain circumstances. Since division by zero is not permitted in algebraic computations, the possibility of a nonstatic universe could not be excluded under the circumstances in question. Friedman showed that two nonstatic models were possible. One depiction as afforded by the efforts as drawn upon the imagination can see that the universe as expanding with time, others, by contrast, are less neuronally excited and cannot see beyond any celestial attempt for looking.
Einstein quickly recognized the importance of this discovery. In the last edition of his book The Meaning of Relativity he wrote: "The mathematician Friedman found a way out of this dilemma. He showed that having a finite density in the whole is possible, according to the field equations, (three-dimensional) space, without enlarging these field equations. Einstein remarked to me many years ago that the cosmic repulsion idea was the biggest blunder that he ever made in his entire life
Almost at the very moment that Friedman was discovering the possibility of an expanding universe by mathematical reasoning, Edwin P. Hubble at the Mount Wilson Observatory on the other side of the world found the first evidence of actual physical expansion through his telescope. He made a compilation of the distances of a number of far galaxies, whose light was shifted toward the red end of the spectrum, and it was soon found that the extent of the shift was in direct proportion to a galaxy's distance from us, as estimated by its faintness. Hubble and others interpreted the red-shift as the Doppler effect-the well-known phenomenon of lengthening of wavelengths from any radiating source that is moving rapidly away (a train whistle, a source of light or whatever). To date there has been no other reasonable explanation of the galaxies' red-shift. If the explanation is correct, it means that the galaxies are all moving away from one another with increasing velocity as they move farther apart. Thus, Friedman and Hubble laid the foundation for the theory of the expanding universe. The theory was soon developed further by a Belgian theoretical astronomer, Georges Lemaître. He proposed that our universe started from a highly compressed and extremely hot state that he called the ‘primeval atom'. (Modern physicists would prefer the term ‘primeval nucleus'.) As this matter expanded, it gradually thinned out, cooled down and reaggregated in stars and galaxies, giving rise to the highly complex structure of the universe as we now know it to be.
Not until a few years ago the theory of the expanding universe lay under the cloud of a very serious contradiction. The measurements of the speed of flight of the galaxies and their distances from us indicated that the expansion had started about 1.8 billion years ago. On the other hand, measurements of the age of ancient rocks in the earth by the clock of radioactivity (i.e., the decay of uranium to lead) showed that some of the rocks were at least three billion years old; more recent estimates based on other radioactive elements raise the age of the earth's crust to almost five billion years. Clearly a universe 1.8 billion years old could not contain five-billion-year-old rocks! Happily the contradiction has now been disposed of by Walter Baade's recent discovery that the distance yardstick (based on the periods of variable stars) was faulty and that the distances between galaxies are more than twice as great as they were thought to be. This change in distances raises the age of the universe to five billion years or more.
Friedman's solution of Einstein's cosmological equation, permits two kinds of universe. We can call one the ‘pulsating' universe. This model says that when the universe has reached a certain maximum permissible expansion, it will begin to contract; that it will shrink until its matter has been compressed to a certain maximum density, possibly that of atomic nuclear material, which is a hundred million times denser than water; that it will then begin to expand again-and so on through the cycle ad infinitum. The other model is a ‘hyperbolic' one: it suggests that from an infinitely thin state an eternity ago the universe contracted until it reached the maximum density, from which it rebounded to an unlimited expansion that will go on indefinitely in the future.
The question whether our universe is ‘pulsating' or ‘hyperbolic' should be decidable from the present rate of its expansion. The situation is analogous to the case of a rocket shot from the surface of the earth. If the velocity of the rocket is less than seven miles per second-the ‘escape velocity'-the rocket will climb only to a certain height and then fall back to the earth. (If it were completely elastic, it would bounce up again, . . . and so on.) On the other hand, a rockets shot with a velocity of more than seven miles per second will escape from the earth's gravitational field and disappeared in space. The case of the receding system of galaxies is very similar to that of an escape rocket, except that instead of just two interacting bodies: the rocket and the earth, we have an unlimited number of them escaping from one another. We find that the galaxies are fleeing from one another at seven times the velocity necessary for mutual escape.
Thus we may conclude that our universe corresponds to the ‘hyperbolic' model, so that its present expansion will never stop. We must make one reservation. The estimate of the necessary escape velocity is based on the assumption that practically all the mass of the universe is concentrated in galaxies. If intergalactic space contained matter whose total mass was more than seven times that in the galaxies, we would have to reverse our conclusion and decide that the universe is pulsating. There has been no indication so far, however, that any matter exists in intergalactic space. It could have escaped detection only if it were in the form of pure hydrogen gas, without other gases or dust.
Is the universe finite or infinite? This resolves itself into the question: Is the curvature of space positive or negative-closed like that of a sphere, or open like that of a saddle? We can look for the answer by studying the geometrical properties of its three-dimensional space, just as we examined the properties of figures on two-dimensional surfaces. The most convenient property to investigate astronomically is the relation between the volume of a sphere and its radius.
We saw that, in the two-dimensional case, the area of a circle increases with increasing radius at a faster rate on a negatively curved surface than on a Euclidean or flat surface; and that on a positively curved surface the relative rate of increase is slower. Similarly the increase of volume is faster in negatively curved space, slower in positively curved space. In Euclidean space the volume of a sphere would increase in proportion to the cube, or third power, of the increase in radius. In negatively curved space the volume would increase faster than this, in undisputably curved space, slower. Thus if we look into space and find that the volume of successively larger spheres, as measured by a count of the galaxies within them, increases faster than the cube of the distance to the limit of the sphere (the radius), we can conclude that the space of our universe has negative curvature, and therefore is open and infinite. Similarly, if the number of galaxies increases at a rate slower than the cube of the distance, we live in a universe of positive curvature-closed and finite.
Following this idea, Hubble undertook to study the increase in number of galaxies with distance. He estimated the distances of the remote galaxies by their relative faintness: galaxies vary considerably in intrinsic brightness, but over a very large number of galaxies these variations are expected to average out. Hubble's calculations produced the conclusion that the universe is a closed system-a small universe only a few billion light-years in radius.
We know now that the scale he was using was wrong: with the new yardstick the universe would be more than twice as large as he calculated. Nevertheless, there is a more fundamental doubt about his result. The whole method is based on the assumption that the intrinsic brightness of a galaxy remains constant. What if it changes with time? We are seeing the light of the distant galaxies as it was emitted at widely different times in the past-500 million, a billion, two billion years ago. If the stars in the galaxies are burning out, the galaxies must dim as they grow older. A galaxy two billion light-years away cannot be put on the same distance scale with a galaxy 500 million light-years away unless we take into account the fact that we are seeing the nearer galaxy at an older, and less bright, age. The remote galaxy is farther away than a mere comparison of the luminosity of the two would suggest.
When a correction is made for the assumed decline in brightness with age, the more distant galaxies are spread out to farther distances than Hubble assumed. In fact, the calculations of volume are nonetheless drastically that we may have to reverse the conclusion about the curvature of space. We are not sure, because we do not yet know enough about the evolution of galaxies. Even so, if we find that galaxies wane in intrinsic brightness by only a few per cent in a billion years, we will have to conclude that space is curved negatively and the universe is infinite.
Effectively there is another line of reasoning which supports the side of infinity. Our universe seems to be hyperbolic and ever-expanding. Mathematical solutions of fundamental cosmological equations indicate that such a universe is open and infinite.
We have reviewed the questions that dominated the thinking of cosmologists during the first half of this century: the conception of a four-dimensional space-time continuum, of curved space, of an expanding universe and of a cosmos that is either finite or infinite. Now we must consider the major present issue in cosmology: Is the universe in truth evolving, or is it in a steady state of equilibrium that has always existed and will go on through eternity? Most cosmologists take the evolutionary view. All the same, in 1951 a group at the University of Cambridge, whose chief official representative has been Fred Hoyle, advanced the steady-state idea. Essentially their theory is that the universe is infinite in space and time that it has neither a beginning nor an end, that the density of its matter remains constant, that new matter is steadily being created in space at a rate that exactly compensates for the thinning of matter by expansion, that as a consequence new galaxies are continually being born, and that the galaxies of the universe therefore range in age from mere youngsters to veterans of 5, 10, 20 and more billions of years. In my opinion this theory must be considered very questionable because of the simple fact (apart from other reasons) that the galaxies in our neighbourhood all seem to be of the same age as our own Milky Way. However, the issue is many-sided and fundamental, and can be settled only by extended study of the universe as far as we can observe it . . . Thus coming to summarize the evolutionary theory.
We assume that the universe started from a very dense state of matter. In the early stages of its expansion, radiant energy was dominant over the mass of matter. We can measure energy and matter on a common scale by means of the well-known equation E=mc2, which says that the energy equivalent of matter is the mass of the matter multiplied by the square of the velocity of light. Energy can be translated into mass, conversely, by dividing the energy quantity by c2. Thus, we can speak of the ‘mass density' of energy. Now at the beginning the mass density of the radiant energy was incomparably greater than the density of the matter in the universe. Yet in an expanding system the density of radiant energy decreases faster than does the density of matter. The former thins out as the fourth power of the distance of expansion: as the radius of the system doubles, the density of radiant energy drops to one sixteenth. The density of matter declines as the third power; a doubling of the radius means an eightfold increase in volume, or eightfold decrease in density.
Assuming that the universe at the beginning was under absolute rule by radiant energy, we can calculate that the temperature of the universe was 250 million degrees when it was one hour old, dropped to 6,000 degrees (the present temperature of our sun's surface) when it was 200,000 years old and had fallen to about 100 degrees below the freezing point of water when the universe reached its 250-millionth birthday.
This particular birthday was a crucial one in the life of the universe. It was the point at which the density of ordinary matter became greater than the mass density of radiant energy, because of the more rapid fall of the latter. The switch from the reign of radiation to the reign of matter profoundly changed matter's behaviours. During the eons of its subjugation to the will of radiant energy (i.e., light), it must have been spread uniformly through space in the form of thin gas. Nevertheless, as soon as matter became gravitationally more important than the radiant energy, it began to acquire a more interesting character. James Jeans, in his classic studies of the physics of such a situation, proved half a century ago that a gravitating gas filling a very large volume is bound to break up into individual ‘gas balls', the size of which is determined by the density and the temperature of the gas. Thus in the year 250,000,000 A.B.E. (after the beginning of expansion), when matter was freed from the dictatorship of radiant energy, the gas broke up into giant gas clouds, slowly drifting apart as the universe continued to expand. Applying Jeans's mathematical formula for the process to the gas filling the universe at that time, in that these primordial balls of gas would have had just about the mass that the galaxies of stars possess today. They were then only ‘proto galaxies'-cold, dark and chaotic. However, their gas soon condensed into stars and formed the galaxies as we see them now.
A central question in this picture of the evolutionary universe is the problem of accounting for the formation of the varied kinds of matter composing it, i.e., the chemical elements . . . Its belief is that at the start matter was composed simply of protons, neutrons and electrons. After five minutes the universe must have cooled enough to permit the aggregation of protons and neutrons into larger units, from deuterons (one neutron and one proton) up to the heaviest elements. This process must have ended after about thirty minutes, for by that time the temperature of the expanding universe must have dropped below the threshold of thermonuclear reactions among light elements, and the neutrons must have been used up in element-building or been converted to protons.
To many, the statement that the present chemical constitution of our universe was decided in half an hour five billion years ago will sound nonsensical. However, consider a spot of ground on the atomic proving ground in Nevada where an atomic bomb was exploded three years ago. Within one microsecond the nuclear reactions generated by the bomb produced a variety of fission products. Today, 100 million-million microseconds later, the site is still ‘hot' with the surviving fission products. The ratio of one microsecond to three years is the same as the ratio of half an hour to five billion years! If we can accept a time ratio of this order in the one case, why not in the other?
The late Enrico Fermi and Anthony L. Turkevich at the Institute for Nuclear Studies of the University of Chicago undertook a detailed study of thermonuclear reactions such as must have taken place during the first half hour of the universe's expansion. They concluded that the reactions would have produced about equal amounts of hydrogen and helium, making up 99 per cent of the total material, and about 1 per cent of deuterium. We know that hydrogen and helium do in fact make up about 99 per cent of the matter of the universe. This leaves us with the problem of building the heavier elements. Hold to opinion, that some of them were built by capture of neutrons. However, since the absence of any stable nucleus of atomic weight five makes it improbable that the heavier elements could have been produced in the first half hour in the abundances now observed, and, yet agreeing that the lion's share of the heavy elements may have been formed later in the hot interiors of stars.
All the theories-of the origin, age, extent, composition and nature of the universe-are becoming more subject to test by new instruments and new techniques . . . Nevertheless, we must not forget that the estimate of distances of the galaxies is still founded on the debatable assumption that the brightness of galaxies does not change with time. If galaxies diminish in brightness as they age, the calculations cannot be depended upon. Thus the question whether evolution is or is not taking place in the galaxies is of crucial importance at the present stage of our outlook on the universe.
In addition certain branches of physical science focus on energy and its large-scale effects. Thermodynamics is the study of heat and the effects of converting heat into other kinds of energy. This branch of physics has a host of highly practical applications because heat is often used to power machines. Physicists also investigate electrical energy and energy that are carried in electromagnetic waves. These include radio waves, light rays, and X-rays-forms of energy that are closely related and that all obey the same set of rules. Chemistry is the study of the composition of matter and the way different substances interact-subjects that involve physics on an atomic scale. In physical chemistry, chemists study the way physical laws govern chemical change, while in other branches of chemistry the focus is on particular chemicals themselves. For example, inorganic chemistry investigates substances found in the nonliving world and organic chemistry investigates carbon-based substances. Until the 19th century, these two areas of chemistry were thought to be separate and distinct, but today chemists routinely produce organic chemicals from inorganic raw materials. Organic chemists have learned how to synthesize many substances that are found in nature, together with hundreds of thousands that are not, such as plastics and pesticides. Many organic compounds, such as reserpine, a drug used to treat hypertension, cost less to produce by synthesizing from inorganic raw materials than to isolate from natural sources. Many synthetic medicinal compounds can be modified to make them more effective than their natural counterparts, with fewer harmful side effects.
The branch of chemistry known as biochemistry deals solely with substances found in living things. It investigates the chemical reactions that organisms use to obtain energy and the reactions up which they use to build themselves. Increasingly, this field of chemistry has become concerned not simply with chemical reactions themselves but also with how the shape of molecules influences the way they work. The result is the new field of molecular biology, one of the fastest-growing sciences today.
Physical scientists also study matter elsewhere in the universe, including the planets and stars. Astronomy is the science of the heavens usually, while astrophysics is a branch of astronomy that investigates the physical and chemical nature of stars and other objects. Astronomy deals largely with the universe as it appears today, but a related science called cosmology looks back in time to answer the greatest scientific questions of all: how the universe began and how it came to be as it is today
The life sciences include all those areas of study that deal with living things. Biology is the general study of the origin, development, structure, function, evolution, and distribution of living things. Biology may be divided into botany, the study of plants; zoology, the study of animals; and microbiology, the study of the microscopic organisms, such as bacteria, viruses, and fungi. Many single-celled organisms play important roles in life processes and thus are important to more complex forms of life, including plants and animals.
Genetics is the branch of biology that studies the way in which characteristics are transmitted from an organism to its offspring. In the latter half of the 20th century, new advances made it easier to study and manipulate genes at the molecular level, enabling scientists to catalogue all the genes finds in each cell of the human body. Exobiology, a new and still speculative field, is the study of possible extraterrestrial life. Although Earth remains the only place known to support life, many believe that it is only a matter of time before scientists discover life elsewhere in the universe.
While exobiology is one of the newest life sciences, anatomy is one of the oldest. It is the study of plant and animal structures, carried out by dissection or by using powerful imaging techniques. Gross anatomy deals with structures that are large enough to see, while microscopic anatomy deals with much smaller structures, down to the level of individual cells.
Physiology explores how living things' work. Physiologists study processes such as cellular respiration and muscle contraction, as well as the systems that keep these processes under control. Their work helps to answer questions about one of the key characteristics of life, the fact that most living things maintain a steady internal state when the environment around them constantly changes.
Together, anatomy and physiology form two of the most important disciplines in medicine, the science of treating injury and human disease. General medical practitioners have to be familiar with human biology as a whole, but medical science also includes a host of clinical specialties. They include sciences such as cardiology, urology, and oncology, which investigate particular organs and disorders, and pathology, the general study of disease and the changes that it causes in the human body.
As well as working with individual organisms, life scientists also investigate the way living things interact. The study of these interactions, known as ecology, has become a key area of study in the life sciences as scientists become increasingly concerned about the disrupting effects of human activities on the environment.
The social sciences explore human society past and present, and the way human beings behave. They include sociology, which investigates the way society is structured and how it functions, as well as psychology, which is the study of individual behaviour and the mind. Social psychology draws on research in both these fields. It examines the way society influence's people's behaviour and attitudes.
Another social science, anthropology, looks at humans as a species and examines all the characteristics that make us what we are. These include not only how people relate to each other but also how they interact with the world around them, both now and in the past. As part of this work, anthropologists often carry out long-term studies of particular groups of people in different parts of the world. This kind of research helps to identify characteristics that all human beings share. That there are those that are the products of some non-regional culture, in that have been taught by others in sharing their knowledge as given up from generation to generation.
The social sciences also include political science, law, and economics, which are products of human society. Although far removed from the world of the physical sciences, all these fields can be studied in a scientific way. Political science and law are uniquely human concepts, but economics has some surprisingly close parallels with ecology. This is because the laws that govern resource use, productivity, and efficiency do not operate only in the human world, with its stock markets and global corporations, but in the nonhuman world as well in technology, scientific knowledge is put to practical ends. This knowledge comes chiefly from mathematics and the physical sciences, and it is used in designing machinery, materials, and industrial processes. Overall, this work is known as engineering, a word dating back to the early days of the Industrial Revolution, when an ‘engine' was any kind of machine.
Engineering has many branches, calling for a wide variety of different skills. For example, aeronautical engineers need expertise in the science of fluid flow, because aeroplanes fly through air, which is a fluid. Using wind tunnels and computer models, aeronautical engineers strive to minimize the air resistance generated by an aeroplane, while at the same time maintaining a sufficient amount of lift. Marine engineers also need detailed knowledge of how fluids behave, particularly when designing submarines that have to withstand extra stresses when they dive deep below the water's surface. In civil engineering, stress calculations ensure that structures such as dams and office towers will not collapse, particularly if they are in earthquake zones. In computing, engineering takes two forms: hardware design and software design. Hardware design refers to the physical design of computer equipment (hardware). Software design is carried out by programmers who analyse complex operations, reducing them to a series of small steps written in a language recognized by computers.
In recent years, a completely new field of technology has developed from advances in the life sciences. Known as biotechnology, it involves such varied activities as genetic engineering, the manipulation of genetic material of cells or organisms, and cloning, the formation of genetically uniform cells, plants, or animals. Although still in its infancy, many scientists believe that biotechnology will play a major role in many fields, including food production, waste disposal, and medicine. Science exists because humans have a natural curiosity and an ability to organize and record things. Curiosity is a characteristic shown by many other animals, but organizing and recording knowledge is a skill demonstrated by humans alone.
During prehistoric times, humans recorded information in a rudimentary way. They made paintings on the walls of caves, and they also carved numerical records on bones or stones. They may also have used other ways of recording numerical figures, such as making knots in leather cords, but because these records were perishable, no traces of them remain. Even so, with the invention of writing about 6,000 years ago, a new and much more flexible system of recording knowledge appeared.
The earliest writers were the people of Mesopotamia, who lived in a part of present-day Iraq. Initially they used a pictographic script, inscribing tallies and lifelike symbols on tablets of clay. With the passage of time, these symbols gradually developed into cuneiform, a much more stylized script composed of wedge-shaped marks.
Because clay is durable, many of these ancient tablets still survive. They show that when writing first appeared. The Mesopotamians already had a basic knowledge of mathematics, astronomy, and chemistry, and that they used symptoms to identify common diseases. During the following 2,000 years, as Mesopotamian culture became increasingly sophisticated, mathematics in particular became a flourishing science. Knowledge accumulated rapidly, and by 1000 Bc the earliest private libraries had appeared.
Southwest of Mesopotamia, in the Nile Valley of northeastern Africa, the ancient Egyptians developed their own form of a pictographic script, writing on papyrus, or inscribing text in stone. Written records from 1500 Bc. shows that, like the Mesopotamians, the Egyptians had a detailed knowledge of diseases. They were also keen astronomers and skilled mathematicians-a fact demonstrated by the almost perfect symmetry of the pyramids and by other remarkable structures they built.
For the peoples of Mesopotamia and ancient Egypt, knowledge was recorded mainly for practical needs. For example, astronomical observations enabled the development of early calendars, which helped in organizing the farming year. Yet in ancient Greece, often recognized as the birthplace of Western science, a new scientific enquiry began. Here, philosophers sought knowledge largely for its own sake.
Thales of Miletus were one of the first Greek philosophers to seek natural causes for natural phenomena. He travelled widely throughout Egypt and the Middle East and became famous for predicting a solar eclipse that occurred in 585 Bc. At a time when people regarded eclipses as ominous, inexplicable, and frightening events, his prediction marked the start of rationalism, a belief that the universe can be explained by reason alone. Rationalism remains the hallmark of science to this day.
Thales and his successors speculated about the nature of matter and of Earth itself. Thales himself believed that Earth was a flat disk floating on water, but the followers of Pythagoras, one of ancient Greece's most celebrated mathematicians, believed that Earth was spherical. These followers also thought that Earth moved in a circular orbit-not around the Sun but around a central fire. Although flawed and widely disputed, this bold suggestion marked an important development in scientific thought: the idea that Earth might not be, after all, the centre of the universe. At the other end of the spectrum of scientific thought, the Greek philosopher Leucippus and his student Democritus of Abdera proposed that all matter be made up of indivisible atoms, more than 2,000 years before the idea became a part of modern science.
As well as investigating natural phenomena, ancient Greek philosophers also studied the nature of reasoning. At the two great schools of Greek philosophy in Athens-the Academy, founded by Plato, and the Lyceum, founded by Plato's pupil Aristotle-students learned how to reason in a structured way using logic. The methods taught at these schools included induction, which involve taking particular cases and using them to draw general conclusions, and deduction, the process of correctly inferring new facts from something already known.
In the two centuries that followed Aristotle's death in 322 Bc, Greek philosophers made remarkable progress in a number of fields. By comparing the Sun's height above the horizon in two different places, the mathematician, astronomer, and geographer Eratosthenes calculated Earth's circumference, producing the figure of an accurate overlay within one percent. Another celebrated Greek mathematician, Archimedes, laid the foundations of mechanics. He also pioneered the science of hydrostatics, the study of the behaviour of fluids at rest. In the life sciences, Theophrastus founded the science of botany, providing detailed and vivid descriptions of a wide variety of plant species as well as investigating the germination process in seeds.
By the 1st century Bc, Roman power was growing and Greek influence had begun to wane. During this period, the Egyptian geographer and astronomer Ptolemy charted the known planets and stars, putting Earth firmly at the centre of the universe, and Galen, a physician of Greek origin, wrote important works on anatomy and physiology. Although skilled soldiers, lawyers, engineers, and administrators, the Romans had little interest in basic science. As a result, scientific growth made little advancement in the days of the Roman Empire. In Athens, the Lyceum and Academy were closed down in AD. 529, bringing the first flowering of rationalism to an end.
For more than nine centuries, from about ad 500 to 1400, Western Europe made only a minor contribution to scientific thought. European philosophers became preoccupied with alchemy, a secretive and mystical pseudoscience that held out the illusory promise of turning inferior metals into gold. Alchemy did lead to some discoveries, such as sulfuric acid, which was first described in the early 1300's, but elsewhere, particularly in China and the Arab world, much more significant progress in the sciences was made.
Chinese science developed in isolation from Europe, and followed a different pattern. Unlike the Greeks, who prized knowledge as an end, the Chinese excelled at turning scientific discoveries to practical ends. The list of their technological achievements is dazzling: it includes the compass, invented in about AD. 270; wood-block printing, developed around 700, and gunpowder and movable type, both invented around the year 1000. The Chinese were also capable mathematicians and excellent astronomers. In mathematics, they calculated the value of   (pi) to within seven decimal places by the year 600, while in astronomy, one of their most celebrated observations was that of the supernova, or stellar explosion, that took place in the Crab Nebula in 1054. China was also the source of the world's oldest portable star map, dating from about 940 Bc.
The Islamic world, which in medieval times extended as far west as Spain, also produced many scientific breakthroughs. The Arab mathematician Muhammad al-Khwarizmi introduced Hindu-Arabic numerals to Europe many centuries after they had been devised in southern Asia. Unlike the numerals used by the Romans, Hindu-Arabic numerals include zero, a mathematical device unknown in Europe at the time. The value of Hindu-Arabic numerals depends on their place: in the number 300, for example, the numeral three is worth ten times as much as in thirty. Al-Khwarizmi also wrote on algebra (it derived from the Arab word al-jabr), and his name survives in the word algorithm, a concept of great importance in modern computing.
In astronomy, Arab observers charted the heavens, giving many of the brightest stars the names we use today, such as Aldebaran, Altair, and Deneb. Arab scientists also explored chemistry, developing methods to manufacture metallic alloys and test the quality and purity of metals. As in mathematics and astronomy, Arab chemists left their mark in some of the names they used-alkali and alchemy, for example, are both words of Arabic origin. Arab scientists also played a part in developing physics. One of the most famous Egyptian physicists, Alhazen, published a book that dealt with the principles of lenses, mirrors, and other devices used in optics. In this work, he rejected the then-popular idea that eyes give out light rays. Instead, he correctly deduced that eyes work when light rays enter the eye from outside.
In Europe, historians often attribute the rebirth of science to a political event-the capture of Constantinople (now Istanbul) by the Turks in 1453. At the time, Constantinople was the capital of the Byzantine Empire and a major seat of learning. Its downfall led to an exodus of Greek scholars to the West. In the period that followed, many scientific works, including those originally from the Arab world, were translated into European languages. Through the invention of the movable type printing press by Johannes Gutenberg around 1450, copies of these texts became widely available.
The Black Death, a recurring outbreak of bubonic plague that began in 1347, disrupted the progress of science in Europe for more than two centuries. However, in 1543 two books were published that had a profound impact on scientific progress. One was De Corporis Humani Fabrica (On the Structure of the Human Body, seven volumes, 1543), by the Belgian anatomist Andreas Vesalius. Vesalius studied anatomy in Italy, and his masterpiece, which was illustrated by superb woodcuts, corrected errors and misunderstandings about the body before which had persisted since the time of Galen more than 1,300 years. Unlike Islamic physicians, whose religion prohibited them from dissecting human cadavers, Vesalius investigated the human body in minute detail. As a result, he set new standards in anatomical science, creating a reference work of unique and lasting value.
The other book of great significance published in 1543 was De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres), written by the Polish astronomer. In it, Copernicus rejected the idea that Earth was the centre of the universe, as proposed by Ptolemy in the 1st century Bc. Instead, he set out to prove that Earth, together with the other planets, follows orbits around the Sun. Other astronomers opposed Copernicus's ideas, and more ominously, so did the Roman Catholic Church. In the early 1600's, the church placed the book on a list of forbidden works, where it remained for more than two centuries. Despite this ban and despite the book's inaccuracies (for instance, Copernicus believed that Earth's orbit was circular rather than elliptical), De Revolutionibus remained a momentous achievement. It also marked the start of a conflict between science and religion that has dogged Western thought ever since
In the first decade of the 17th century, the invention of the telescope provided independent evidence to support Copernicus's views. Italian physicist and astronomer Galileo Galilei used the new device to remarkable effect. He became the first person to observe satellites circling Jupiter, the first to make detailed drawings of the surface of the Moon, and the first to see how Venus waxes and wanes as it circles the Sun.
These observations of Venus helped to convince Galileo that Copernicus's Sun-entered view of the universe had been correct, but he fully understood the danger of supporting such heretical ideas. His Dialogue on the Two Chief World Systems, Ptolemaic and Copernican, published in 1632, was carefully crafted to avoid controversy. Even so, he was summoned before the Inquisition (tribunal established by the pope for judging heretics) the following year and, under threat of torture, forced to recant.
Nicolaus Copernicus (1473-1543), the first developed heliocentric theory of the Universes in the modern era presented in De Revolutioniv bus Coelestium, published in the year of Copernicus's death. The system is entirely mathematical, in the sense of predicting the observed position of celestial bodies on te basis of an underlying geometry, without exploring the mechanics of celestial motion. Its mathematical and scientific superiority over the Ptolemaic system was not as direct as poplar history suggests: Copernicus's system adhered to circular planetary motion and let the planets run on forty-eight epicycles and eccentrics. It was not until the work of Kepler and Galileo that the system became markedly simpler than Ptolemaic astronomy.
The publication of Nicolaus Copernicus's De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres) in 1543 is traditionally considered the inauguration of the scientific revolution. Ironically, Copernicus had no intention of introducing radical ideas into cosmology. His aim was only to restore the purity of ancient Greek astronomy by eliminating novelties introduced by Ptolemy. With such an aim in mind he modelled his own book, which would turn astronomy upside down, on Ptolemy's Almagest. At the core of the Copernican system, as with that of Aristarchus before him, is the concept of the stationary Sun at the centre of the universe, and the revolution of the planets, Earth included, around the Sun. The Earth was ascribed, in addition to an annual revolution around the Sun, a daily rotation around its axis.
Copernicus's greatest achievement is his legacy. By introducing mathematical reasoning into cosmology, he dealt a severe blow to Aristotelian commonsense physics. His concept of an Earth in motion launched the notion of the Earth as a planet. His explanation that he had been unable to detect stellar parallax because of the enormous distance of the sphere of the fixed stars opened the way for future speculation about an infinite universe. Nevertheless, Copernicus still clung to many traditional features of Aristotelian cosmology. He continued to advocate the entrenched view of the universe as a closed world and to see the motion of the planets as uniform and circular. Thus, in evaluating Copernicus's legacy, it should be noted that he set the stage for far more daring speculations than he himself could make. The heavy metaphysical underpinning of Kepler's laws, combined with an obscure style and a demanding mathematics, caused most contemporaries to ignore his discoveries. Even his Italian contemporary Galileo Galilei, who corresponded with Kepler and possessed his books, never referred to the three laws. Instead, Galileo provided the two important elements missing from Kepler's work: a new science of dynamics that could be employed in an explanation of planetary motion, and a staggering new body of astronomical observations. The observations were made possible by the invention of the telescope in Holland c.1608 and by Galileo's ability to improve on this instrument without having ever seen the original. Thus equipped, he turned his telescope skyward, and saw some spectacular sights.
The results of his discoveries were immediately published in the Sidereus nuncius (The Starry Messenger) of 1610. Galileo observed that the Moon was very similar to the Earth, with mountains, valleys, and oceans, and not at all that perfect, smooth spherical body it was claimed to be. He also discovered four moons orbiting Jupiter. As for the Milky Way, instead of being a stream of light, it was, alternatively a large aggregate of stars. Later observations resulted in the discovery of sunspots, the phases of Venus, and that strange phenomenon that would later be designated as the rings of Saturn.
Having announced these sensational astronomical discoveries which reinforced his conviction of the reality of the heliocentric theory-Galileo resumed his earlier studies of motion. He now attempted to construct a comprehensive new science of mechanics necessary in a Copernican world, and the results of his labours were published in Italian in two epoch
- making books: Dialogue Concerning the Two Chief World Systems (1632) and Discourses and Mathematical Demonstrations Concerning the Two New Sciences (1638). His studies of projectiles and free-falling bodies brought him very close to the full formulation of the laws of inertia and acceleration (the first two laws of Isaac Newton). Galileo's legacy includes both the modern notion of ‘laws of nature' and the idea of mathematics as nature's true language. He contributed to the mathematization of nature and the geometrization of space, as well as to the mechanical philosophy that would dominate the 17th and 18th centuries. Perhaps most important, it is largely due to Galileo that experiments and observations serve as the cornerstone of scientific reasoning.
Today, Galileo is remembered equally well because of his conflict with the Roman Catholic church. His uncompromising advocacy of Copernicanism after 1610 was responsible, in part, for the placement of Copernicus's De Revolutionibus on the Index of Forbidden Books in 1616. At the same time, Galileo was warned not to teach or defend Copernicanism in public. The election of Galileo's friend Maffeo Barbering as Pope Urban VIII in 1624 filled Galileo with the hope that such a verdict could be revoked. With perhaps some unwarranted optimism, Galileo set to work to complete his Dialogue (1632). However, Galileo underestimated the power of the enemies he had made during the previous two decades, particularly some Jesuits who had been the target of his acerbic tongue. The outcome was that Galileo was summoned to Rome and there forced to abjure, on his knees, the views he had expressed in his book. Ever since, Galileo has been portrayed as a victim of a repressive church and a martyr in the cause of freedom of thought; as such, he has become a powerful symbol.
Despite his passionate advocacy of Copernicanism and his fundamental work in mechanics, Galileo continued to accept the age-old views that planetary orbits were circular and the cosmos an enclosed world. These beliefs, as well as a reluctance rigorously to apply mathematics to astronomy as he had previously applied it to terrestrial mechanics, prevented him from arriving at the correct law of inertia. Thus, it remained for Isaac Newton to unite heaven and Earth in his immense intellectual achievement, the Philosophiae Naturalis principia mathematica (Mathematical Principles of Natural Philosophy), which was published in 1687. The first book of the Principia contained Newton's three laws of motion. The first expounds the law of inertia: everybody persists in a state of rest or uniform motion in a straight line unless compelled to change such a state by an impressing force. The second is the law of acceleration, according to which the change of motion of a body is proportional to the force acting upon it and takes place in the direction of the straight line along which that force is impressed. The third, and most original, law ascribes to every action an opposite and equal reaction. These laws governing terrestrial motion were extended to include celestial motion in book three of the Principia, where Newton formulated his most famous law, the law of gravitation: everybody in the universe attracts any other body with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
The Principia is deservedly considered one of the greatest scientific masterpieces of all time. Nevertheless, in 1704, Newton published his second great work, the Opticks, in which he formulated his corpuscular theory of light and his theory of colours. In later editions Newton appended a series of ‘queries' concerning various related topics in natural philosophy. These speculative, and sometimes metaphysical, statements on such issues as light, heat, ether, and matter became most productive during the 18th century, when the book and the experimental method it propagated became immensely popular.
The 17th century French scientist and mathematician René Descartes was also one of the most influential thinkers in Western philosophy. Descartes stressed the importance of skepticism in thought and proposed the idea that existence had a dual nature: one physical, the other mental. The latter concept, known as Cartesian dualism, continues to engage philosophers today. This passage from Discourse on Method (first published in his Philosophical Essays in 1637) contains a summary of his thesis, which includes the celebrated phrase "I think, therefore I am."
Then examining attentively what I was, and seeing that I could pretend that I had no body and that there was no world or place that I [was] in, but that I could not, for all that, pretend that I did not exist, and that, on the contrary, from the very fact that I thought of doubting the truth of other things, it followed very evidently and very conveniently that I existed; while, on the other hand, if I had only ceased to think, although all the rest of what I had ever imagined had been true, I would have had no reason to believe that I existed; I thereby concluded that I was a substance, of which the whole essence or nature consists in thinking, and which, in order to exist, needs no place and depends on no material thing; so that this ‘I', which is to say, the mind, by which I am what I am, is distinct entirely from the body, and even that knowing is easier than the body, and moreover that even if the body were not, it would not cease to be all that it is.
It is, nonetheless, as considered overall of what is needed for a proposition to be true and certain; for, I had morally justified, in finding of one that so happens that I knew it to be so. I thought too, that I had morally justified by reason alone, in that to know of what is of this necessitates a narrative coherence as availed to a set-order of governing principles. Having marked and noted that there is nothing in at all that in this, I think, therefore I am, which assures me that I am speaking the truth, except that I see very clearly that in order to think one must exist, I judged that I could take it to be a general rule that the things we conceive very clearly and very distinctly is nevertheless some difficulty in being able to recognize for certain that are the things we see distinctly.
Following this, reflecting on the fact that I had doubts, and that consequently my being was not perfect, for I saw clearly that it was a greater perfection to know than to doubt, I decided to inquire from what place I had learned to think of some thing perfect than myself; and I clearly recognized that this must have been from some nature that was in fact perfect. As for the notions I had of several other things outside myself, such as the sky, the earth, light, heat and a thousand others, I had not the same concern to know their source, because, seeing nothing in them that seemed to make them superior to me. I could believe that, if they were true, they were dependencies of my nature, in as much as it. One perfection; and, if they were not, that I held them from nothing, that is to say that they were in me because of an imperfection in my nature. Nevertheless, I could not make the same judgement concerning the idea of a being perfect than myself; for to hold it from nothing was something manifestly impossible; and because it is no less contradictory that the perfect should proceed from and depend on the less perfect, than it is that something should emerge out of nothing, I could not hold it from myself; with the result that it remained that it must have been put into me by a being whose nature was truly perfect than mine and which even had in it all the perfection of which I could have any idea, which is to say, in a word, which was God. To which I added that, since I knew some perfections that I did not have, I was not the only being that existed (I will freely use here, with your permission, the terms of the School) but that there must be another perfect, upon whom I depended, and from whom I had acquired all I had; for, if I had been alone and independent of all other, so as to have had from myself this small portion of perfection that I had by participation in the perfection of God, I could have given myself, by the same reason, all the remainder of perfection that I knew myself to lack, and thus to be myself infinite, eternal, immutable, omniscient, all powerful, and finally to have all the perfections that I could observe to be in God. For, consequentially upon the reasoning by which I had proved the existence of God, in order to understand the nature of God as far as my own nature was capable of doing, I had only to consider, concerning all the things of which I found in myself some idea, whether it was a perfection or not to have them: and I was assured that none of those that indicated some imperfection was in him, but that all the others were. So I saw that doubt, inconstancy, sadness and similar things could not be in him, seeing that I myself would have been very pleased to be free from them. Then, further, I had ideas of many sensible and bodily things; for even supposing that I was dreaming, and that everything I saw or imagined was false, I could not, nevertheless, deny that the ideas were really in my thoughts. However, because I had already recognized in myself very clearly that intelligent nature is distinct from the corporeal, considering that all composition is evidence of dependency, and that dependency is manifestly a defect, I thence judged that it could not be a perfection in God to be composed of these two natures, and that, consequently, he was not so composed, but that, if there were any bodies in the world or any intelligence or other natures that were not wholly perfect, their existence must depend on his power, in such a way that they could not subsist without him for a single instant.
I set out after that to seek other truths; and turning to the object of the geometers [geometry], which I conceived as a continuous body, or a space extended indefinitely in length, width and height or depth, divisible into various parts, which could have various figures and sizes and be moved or transposed in all sorts of ways-for the geometers take all that to be in the object of their study-I went through some of their simplest proofs. Having observed that the great certainty that everyone attributes to them is based only on the fact that they are clearly conceived according to the rule I spoke of earlier, I noticed also that they had nothing at all in them that might assure me of the existence of their object. Thus, for example, I very well perceived that, supposing a triangle to be given, its three angles must be equal to two right-angles, but I saw nothing, for all that, which assured me that any such triangle existed in the world, whereas regressing to the examination of the idea I had of a perfect Being. In that of its finding it was found that existence was comprised in the idea in the same way that the equality of the three angles of a triangle to two right angles is comprised in the idea of a triangle or, as in the idea of a sphere, the fact that all its parts are equidistant from its centre, or even more obviously so; and that consequently it is at least as certain that God, who is this perfect Being, is, or exists, as any geometric demonstration can be.
The impact of the Newtonian accomplishment was enormous. Newton's two great books resulted in the establishment of two traditions that, though often mutually exclusive, nevertheless permeated into every area of science. The first was the mathematical and reductionist tradition of the Principia, which, like René Descartes's mechanical philosophy, propagated a rational, well-regulated image of the universe. The second was the experimental tradition of the Opticks, in a measure less demanding than the mathematical tradition and, owing to the speculative and suggestive queries appended to the Opticks, highly applicable to chemistry, biology, and the other new scientific disciplines that began to flourish in the 18th century. This is not to imply that everyone in the scientific establishment was, or would be, a Newtonian. Newtonianism had its share of detractors. Instead, the Newtonian achievement was so great, and its applicability to other disciplines so strong, that although Newtonian science could be argued against, it could not be ignored. In fact, in the physical sciences an initial reaction against universal gravitation occurred. For many, the concept of action at a distance seemed to hark back to those occult qualities with which the mechanical philosophy of the 17th century had done away. By the second half of the 18th century, however, universal gravitation would be proved correct, thanks to the work of Leonhard Euler, A. C. Clairaut, and Pierre Simon de LaPlace, the last of whom announced the stability of the solar system in his masterpiece Celestial Mechanics (1799-1825).
Newton's influence was not confined to the domain of the natural sciences. The philosophes of the 18th-century Enlightenment sought to apply scientific methods to the study of human society. To them, the empiricist philosopher John Locke was the first person to attempt this. They believed that in his Essay on Human Understanding (1690) Locke did for the human mind what Newton had done for the physical world. Although Locke's psychology and epistemology were to come under increasing attack as the 18th century advanced, other thinkers such as Adam Smith, David Hume, and Abbé de Condillac would aspire to become the Newtons of the mind or the moral realm. These confident, optimistic men of the Enlightenment argued that there must exist universal human laws that transcend differences of human behaviour and the variety of social and cultural institutions. Labouring under such an assumption, they sought to uncover these laws and apply them to the new society about which they hoped to bring.
As the 18th century progressed, the optimism of the philosophes waned and a reaction began to set in. Its first manifestation occurred in the religious realm. The mechanistic interpretation of the world-shared by Newton and Descartes -had, in the hands of the philosophes, led to materialism and atheism. Thus, by mid-century the stage was set for a revivalist movement, which took the form of Methodism in England and pietism in Germany. By the end of the century the romantic reaction had begun. Fuelled in part by religious revivalism, the romantics attacked the extreme rationalism of the Enlightenment, the impersonalization of the mechanistic universe, and the contemptuous attitude of "mathematicians" toward imagination, emotions, and religion.
The romantic reaction, however, was not anti-scientific; its adherents rejected a specific type of the mathematical science, not the entire enterprise. In fact, the romantic reaction, particularly in Germany, would give rise to a creative movement-the Naturphilosophie -that in turn would be crucial for the development of the biological and life sciences in the 19th century, and would nourish the metaphysical foundation necessary for the emergence of the concepts of energy, forces, and conservation.
Thus and so, in classical physics, external reality consisted of inert and inanimate matter moving in accordance with wholly deterministic natural laws, and collections of discrete atomized parts constituted wholes. Classical physics was also premised, however, on a dualistic conception of reality as consisting of abstract disembodied ideas existing in a domain separate from and superior to sensible objects and movements. The motion that the material world experienced by the senses was inferior to the immaterial world experiences by mind or spirit has been blamed for frustrating the progress of physics up too at least the time of Galileo. Nevertheless, in one very important respect it also made the fist scientific revolution possible. Copernicus, Galileo, Kepler and Newton firmly believed that the immaterial geometrical mathematical ides that inform physical reality had a prior existence in the mind of God and that doing physics was a form of communion with these ideas.
Even though instruction at Cambridge was still dominated by the philosophy of Aristotle, some freedom of study was permitted in the student's third year. Newton immersed himself in the new mechanical philosophy of Descartes, Gassendi, and Boyle; in the new algebra and analytical geometry of Vieta, Descartes, and Wallis; and in the mechanics and Copernican astronomy of Galileo. At this stage Newton showed no great talent. His scientific genius emerged suddenly when the plague closed the University in the summer of 1665 and he had to return to Lincolnshire. There, within eighteen months he began revolutionary advances in mathematics, optics, physics, and astronomy.
During the plague years Newton laid the foundation for elementary differential and integral Calculus, several years before its independent discovery by the German philosopher and mathematician Leibniz. The ‘method of fluxions', as he termed it, was based on his crucial insight that the integration of a function (or finding the area under its curve) is merely the inverse procedure to differentiating it (or finding the slope of the curve at any point). Taking differentiation as the basic operation, Newton produced simple analytical methods that unified a host of disparate techniques previously developed on a piecemeal basis to deal with such problems as finding areas, tangents, the lengths of curves, and their maxima and minima. Even though Newton could not fully justify his methods -rigorous logical foundations for the calculus were not developed until the 19th century-he receives the credit for developing a powerful tool of problem solving and analysis in pure mathematics and physics. Isaac Barrow, a Fellow of Trinity College and Lucasian Professor of Mathematics in the University, was so impressed by Newton's achievement that when he resigned his chair in 1669 to devote himself to theology, he recommended that the 27-year-old Newton take his place.
Newton's initial lectures as Lucasian Professor dealt with optics, including his remarkable discoveries made during the plague years. He had reached the revolutionary conclusion that white light is not a simple, homogeneous entity, as natural philosophers since Aristotle had believed. When he passed a thin beam of sunlight through a glass prism, he noted the oblong spectrum of colours-red, yellow, green, blue, violet -that formed on the wall opposite. Newton showed that the spectrum was too long to be explained by the accepted theory of the bending (or refraction) of light by dense media. The old theory said that all rays of white light striking the prism at the same angle would be equally refracted. Newton argued that white light is really a mixture of many different types of rays, that the different types of rays are refracted at different angles, and that each different type of ray is responsible for producing a given spectral colour. A so-called crucial experiment confirmed the theory. Newton selected out of the spectrum a narrow band of light of one colour. He sent it through a second prism and observed that no further elongation occurred. All the selected rays of one colour were refracted at the same angle.
These discoveries led Newton to the logical, but erroneous, conclusion that telescopes using refracting lenses could never overcome the distortions of chromatic dispersion. He therefore proposed and constructed a reflecting telescope, the first of its kind, and the prototype of the largest modern optical telescopes. In 1671 he donated an improved version to the Royal Society of London, the foremost scientific society of the day. As a consequence, he was elected a fellow of the society in 1672. Later that year Newton published his first scientific paper in the Philosophical Transactions of the society. It dealt with the new theory of light and colour and is one of the earliest examples of the short research paper.
Newton's paper was well received, but two leading natural philosophers, Robert Hooke and Christian Huygens rejected Newton's naive claim that his theory was simply derived with certainty from experiments. In particular they objected to what they took to be Newton's attempt to prove by experiment alone that light consists in the motion of small particles, or corpuscles, rather than in the transmission of waves or pulses, as they both believed. Although Newton's subsequent denial of the use of hypotheses was not convincing, his ideas about scientific method won universal assent, along with his corpuscular theory, which reigned until the wave theory was revived in the early 19th century.
The debate soured Newton's relations with Hooke. Newton withdrew from public scientific discussion for about a decade after 1675, devoting himself to chemical and alchemical researches. He delayed the publication of a full account of his optical researches until after the death of Hooke in 1703. Newton's Opticks appeared the following year. It dealt with the theory of light and colour and with Newton's investigations of the colours of thin sheets, of ‘Newton's rings', and of the phenomenon of diffraction of light. To explain some of his observations he had to graft elements of a wave theory of light onto his basically corpuscular theory. q
Newton's greatest achievement was his work in physics and celestial mechanics, which culminated in the theory of universal gravitation. Even though Newton also began this research in the plague years, the story that he discovered universal gravitation in 1666 while watching an apple fall from a tree in his garden is a myth. By 1666, Newton had formulated early versions of his three Laws of motion. He had also discovered the law stating the centrifugal force (or force away from the centre) of a body moving uniformly in a circular path. However, he still believed that the earth's gravity and the motions of the planets might be caused by the action of whirlpools, or vortices, of small corpuscles, as Descartes had claimed. Moreover, although he knew the law of centrifugal force, he did not have a correct understanding of the mechanics of circular motion. He thought of circular motion as the result of a balance between two forces. One centrifugal, the other centripetal (toward the centre)-than as the result of one force, a centripetal force, which constantly deflects the body away from its inertial path in a straight line.
Newton's great insight of 1666 was to imagine that the Earth's gravity extended to the Moon, counterbalancing its centrifugal force. From his law of centrifugal force and Kepler's third law of planetary motion, Newton deduced that the centrifugal (and hence centripetal) forces of the Moon or of any planet must decrease as the inverse square of its distance from the centre of its motion. For example, if the distance is doubled, the force becomes one-fourth as much; if distance is trebled, the force becomes one-ninth as much. This theory agreed with Newton's data to within about 11%.
In 1679, Newton returned to his study of celestial mechanics when his adversary Hooke drew him into a discussion of the problem of orbital motion. Hooke is credited with suggesting to Newton that circular motion arises from the centripetal deflection of inertially moving bodies. Hooke further conjectured that since the planets move in ellipses with the Sun at one focus (Kepler's first law), the centripetal force drawing them to the Sun should vary as the inverse square of their distances from it. Hooke could not prove this theory mathematically, although he boasted that he could. Not to be shown up by his rival, Newton applied his mathematical talents to proving Hooke's conjecture. He showed that if a body obeys Kepler's second law (which states that the line joining a planet to the sun sweeps out equal areas in equal times), then the body is being acted upon by a centripetal force. This discovery revealed for the first time the physical significance of Kepler's second law. Given this discovery, Newton succeeded in showing that a body moving in an elliptical path and attracted to one focus must truly be drawn by a force that varies as the inverse square of the distance. Later even these results were set aside by Newton.
In 1684 the young astronomer Edmond Halley, tired of Hooke's fruitless boasting, asked Newton whether he could prove Hooke's conjecture and to his surprise was told that Newton had solved the problem a full five years before but had now mislaid the paper. At Halley's constant urging Newton reproduced the proofs and expanded them into a paper on the laws of motion and problems of orbital mechanics. Finally Halley persuaded Newton to compose a full-length treatment of his new physics and its application to astronomy. After eighteen months of sustained effort, Newton published (1687) the Philosophiae Naturalis principia Mathematica (The Mathematical Principles of Natural Philosophy), or Principia, as it is universally known.
By common consent the Principia is the greatest scientific book ever written. Within the framework of an infinite, homogeneous, three-dimensional, empty space and a uniformly and eternally flowing ‘absolute' time, Newton fully analysed the motion of bodies in resisting and nonresisting media under the action of centripetal forces. The results were applied to orbiting bodies, projectiles, pendula, and free-fall near the Earth. He further demonstrated that the planets were attracted toward the Sun by a force varying as the inverse square of the distance and generalized that all heavenly bodies mutually attract one another. By further generalization, he reached his law of universal gravitation: every piece of matter attracts every other piece with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Given the law of gravitation and the laws of motion, Newton could explain a wide range of hitherto disparate phenomena such as the eccentric orbits of comets, the causes of the tides and their major variations, the precession of the Earth's axis, and the perturbation of the motion of the Moon by the gravity of the Sun. Newton's one general law of nature and one system of mechanics reduced to order most of the known problems of astronomy and terrestrial physics. The work of Galileo, Copernicus, and Kepler was united and transformed into one coherent scientific theory. The new Copernican world-picture finally had a firm physical basis.
Because Newton repeatedly used the term ‘attraction' in the Principia, mechanical philosophers attacked him for reintroducing into science the idea that mere matter could act at a distance upon other matter. Newton replied that he had only intended to show the existence of gravitational attraction and to discover its mathematical law, not to inquire into its cause. He no more than his critics believed that brute matter could act at a distance. Having rejected the Cartesian vortices, he reverted in the early 1700s to the idea that some material medium, or ether, caused gravity. However, Newton's ether was no longer a Cartesian-type ether acting solely by impacts among particles. The ether had to be extremely rare so it would not obstruct the motions of the planets, and yet very elastic or springy so it could push large masses toward one another. Newton postulated that the new ether consisted of particles endowed with very powerful short-range repulsive forces. His unreconciled ideas on forces and ether deeply influenced later natural philosophers in the 18th century when they turned to the phenomena of chemistry, electricity and magnetism, and physiology.
With the publication of the Principia, Newton was recognized as the leading natural philosopher of the age, but his creative career was effectively over. After suffering a nervous breakdown in 1693, he retired from research to seek a government position in London. In 1696 he became Warden of the Royal Mint and in 1699 its Master, an extremely lucrative position. He oversaw the great English recoinage of the 1690s and pursued counterfeiters with ferocity. In 1703 he was elected president of the Royal Society and was reelected each year until his death. He was knighted (1708) by Queen Anne, the first scientist to be so honoured for his work.
As any overt appeal to metaphysics became unfashionable, the science of mechanics was increasingly regarded, says Ivor Leclerc, as ‘an autonomous science,' and any alleged role of God as ‘deus ex machina'. At the beginning of the nineteenth century, Pierre-Simon LaPlace, along with a number of other great French mathematicians and, advanced the view that the science of mechanics constituted a complex view of nature. Since this science, by observing its epistemology, had revealed itself to be the fundamental science, the hypothesis of God as, they concluded unnecessary.
Pierre de Simon LaPlace (1749-1827) is recognized for eliminating not only the theological component of classical physics but the ‘entire metaphysical component' as well. The epistemology of science requires, had that we proceeded by inductive generalisations from observed facts to hypotheses that are ‘tested by observed conformity of the phenomena.' What was unique out LaPlace's view of hypotheses as insistence that we cannot attribute reality to them. Although concepts like force, mass, notion, cause, and laws are obviously present in classical physics, they exist in LaPlace's view only as quantities. Physics is concerned, he argued, with quantities that we associate as a matter of convenience with concepts, and the truths abut nature are only quantities.
The seventeenth-century view of physics s a philosophy of nature or as natural philosophy was displaced by the view of physics as an autonomous science that was: The science of nature. This view, which was premised on the doctrine e of positivism, promised to subsume all of the nature with a mathematical analysis of entities in motion and claimed that the true understanding of nature was revealed only in the mathematical descriptions. Since the doctrine of positivism, assumed that the knowledge we call physics resides only in the mathematical formalisms of physical theory, it disallows the prospect that the vision of physical reality revealed in physical theory can have any other meaning. In the history of science, the irony is that positivism, which was intended to banish metaphysical concerns from the domain of science, served to perpetuate a seventeenth-century metaphysical assumption about the relationship between physical reality and physical theory.
So, then, the decision was motivated by our conviction that our discoveries have more potential to transform our conception of the ‘way thing are' than any previous discovery in the history of science, as these implications of discovery extend well beyond the domain of the physical sciences, and the best efforts of large numbers of thoughtfully convincing in others than I will be required to understand them.
In fewer contentious areas, European scientists made rapid progress on many fronts in the 17th century. Galileo himself investigated the laws governing falling objects, and discovered that the duration of a pendulum's swing is constant for any given length. He explored the possibility of using this to control a clock, an idea that his son put into practice in 1641. Two years later another Italian, mathematician and physicist Evangelists Torricelli, made the first barometer. In doing so he discovered atmospheric pressure and produced the first artificial vacuum known to science. In 1650 German physicist Otto von Guericke invented the air pump. He is best remembered for carrying out a demonstration of the effects of atmospheric pressure. Von Guericke joined two large, hollow bronze hemispheres, and then pumped out the air within them to form a vacuum. To illustrate the strength of the vacuum, von Guericke showed how two teams of eight horses pulling in opposite directions could not separate the hemispheres. Yet the hemispheres fell apart as soon as air was let in.
Throughout the 17th century major advances occurred in the life sciences, including the discovery of the circulatory system by the English physician William Harvey and the discovery of microorganisms by the Dutch microscope maker Antoni van Leeuwenhoek. In England, Robert Boyle established modern chemistry as a full-fledged science, while in France, philosopher and scientist René Descartes made numerous discoveries in mathematics, as well as advancing the case for rationalism in scientific research.
However, the century's greatest achievements came in 1665, when the English physicist and mathematician Isaac Newton fled from Cambridge to his rural birthplace in Woolsthorpe to escape an epidemic of the plague. There, in the course of a single year, he made a series of extraordinary breakthroughs, including new theories about the nature of light and gravitation and the development of calculus. Newton is perhaps best known for his proof that the force of gravity extends throughout the universe and that all objects attract each other with a precisely defined and predictable force. Gravity holds the Moon in its orbit around the Earth and is the principal cause of the Earth's tides. These discoveries revolutionized how people viewed the universe and they marked the birth of modern science.
Newton's work demonstrated that nature was governed by basic rules that could be identified using the scientific method. This new approach to nature and discovery liberated 18th-century scientists from passively accepting the wisdom of ancient writings or religious authorities that had never been tested by experiment. In what became known as the Age of Reason, or the Age of Enlightenment, scientists in the 18th century began to apply rational thought actively, careful observation, and experimentation to solve a variety of problems.
Advances in the life sciences saw the gradual erosion of the theory of spontaneous generation, a long-held notion that life could spring from nonliving matter. It also brought the beginning of scientific classification, pioneered by the Swedish naturalist Carolus Linnaeus, who classified close to 12,000 living plants and animals into a systematic arrangement.
By 1700 the first steam engine had been built. Improvements in the telescope enabled German-born British astronomer Sir William Herschel to discover the planet Uranus in 1781. Throughout the 18th century science began to play an increasing role in everyday life. New manufacturing processes revolutionized the way that products were made, heralding the Industrial Revolution. In An Inquiry Into the Nature and Causes of the Wealth of Nations, published in 1776, British economist Adam Smith stressed the advantages of division of labour and advocated the use of machinery to increase production. He urged governments to allow individuals to compete within a free market in order to produce fair prices and maximum social benefit. Smith's work for the first time gave economics the stature of an independent subject of study and his theories greatly influenced the course of economic thought for more than a century.
With knowledge in all branches of science accumulating rapidly, scientists began to specialize in particular fields. Specialization did not necessarily mean that discoveries were specializing as well: From the 19th century onward, research began to uncover principles that unite the universe as a whole.
In chemistry, one of these discoveries was a conceptual one: that all matter is made of atoms. Originally debated in ancient Greece, atomic theory was revived in a modern form by the English chemist John Dalton in 1803. Dalton provided clear and convincing chemical proof that such particles exist. He discovered that each atom has a characteristic mass and that atoms remain unchanged when they combine with other atoms to form compound substances. Dalton used atomic theory to explain why substances always combine in fixed proportions-a field of study known as quantitative chemistry. In 1869 Russian chemist Dmitry Mendeleyev used Dalton's discoveries about atoms and their behaviour to draw up his periodic table of the elements.
Other 19th-century discoveries in chemistry included the world's first synthetic fertilizer, manufactured in England in 1842. In 1846 German chemist Christian Schoenbein accidentally developed the powerful and unstable explosive nitrocellulose. The discovery occurred after he had spilled a mixture of nitric and sulfuric acids and then mopped it up with a cotton apron. After the apron had been hung up to dry, it exploded. He later learned that the cellulose in the cotton apron combined with the acids to form a highly flammable explosive.
In 1828 the German chemist Friedrich Wöhler showed that making carbon-containing was possible, organic compounds from inorganic ingredients, a breakthrough that opened an entirely new field of research. By the end of the 19th century, hundreds of organic compounds had been synthesized, including mauve, magenta, and other synthetic dyes, as well as aspirin, still one of the world's most useful drugs.
In physics, the 19th century is remembered chiefly for research into electricity and magnetism, which were pioneered by physicists such as Michael Faraday and James Clerk Maxwell of Great Britain. In 1821 Faraday demonstrated that a moving magnet could set an electric current flowing in a conductor. This experiment and others he carried as a process, led to the development of electric motors and generators. While Faraday's genius lay in discovery by experiment, Maxwell produced theoretical breakthroughs of even greater note. Maxwell's development of the electromagnetic theory of light took many years. It began with the paper ‘On Faraday's Lines of Force' (1855–1856), in which Maxwell built on the ideas of British physicist Michael Faraday. Faraday explained that electric and magnetic effects result from lines of forces that surround conductors and magnets. Maxwell drew an analogy between the behaviour of the lines of force and the flow of a liquid, deriving equations that represented electric and magnetic effects. The next step toward Maxwell's electromagnetic theory was the publication of the paper, On Physical Lines of Force (1861-1862). Here Maxwell developed a model for the medium that could carry electric and magnetic effects. He devised a hypothetical medium that consisted of a fluid in which magnetic effects created whirlpool-like structures. These whirlpools were separated by cells created by electric effects, so the combination of magnetic and electric effects formed a honeycomb pattern.
Maxwell could explain all known effects of electromagnetism by considering how the motion of the whirlpools, or vortices, and cells could produce magnetic and electric effects. He showed that the lines of force behave like the structures in the hypothetical fluid. Maxwell went further, considering what would happen if the fluid could change density, or be elastic. The movement of a charge would set up a disturbance in an elastic medium, forming waves that would move through the medium. The speed of these waves would be equal to the ratio of the value for an electric current measured in electrostatic units to the value of the same current measured in electromagnetic units. German physicists Friedrich Kohlrausch and Wilhelm Weber had calculated this ratio and found it the same as the speed of light. Maxwell inferred that light consists of waves in the same medium that causes electric and magnetic phenomena.
Maxwell found supporting evidence for this inference in work he did on defining basic electrical and magnetic quantities in terms of mass, length, and time. In the paper, On the Elementary Regulations of Electric Quantities (1863), he wrote that the ratio of the two definitions of any quantity based on electric and magnetic forces is always equal to the velocity of light. He considered that light must consist of electromagnetic waves but first needed to prove this by abandoning the vortex analogy and developing a mathematical system. He achieved this in ‘A Dynamical Theory of the Electromagnetic Field' (1864), in which he developed the fundamental equations that describe the electromagnetic field. These equations showed that light is propagated in two waves, one magnetic and the other electric, which vibrate perpendicular to each other and perpendicular to the direction in which they are moving (like a wave travelling along a string). Maxwell first published this solution in Note on the Electromagnetic Theory of Light (1868) and summed up all of his work on electricity and magnetism in Treatise on Electricity and Magnetism in 1873.
The treatise also suggested that a whole family of electromagnetic radiation must exist, of which visible light was only one part. In 1888 German physicist Heinrich Hertz made the sensational discovery of radio waves, a form of electromagnetic radiation with wavelengths too long for our eyes to see, confirming Maxwell's ideas. Unfortunately, Maxwell did not live long enough to see this vindication of his work. He also did not live to see the ether (the medium in which light waves were said to be propagated) disproved with the classic experiments of German-born American physicist Albert Michelson and American chemist Edward Morley in 1881 and 1887. Maxwell had suggested an experiment much like the Michelson-Morley experiment in the last year of his life. Although Maxwell believed the ether existed, his equations were not dependent on its existence, and so remained valid.
Maxwell's other major contribution to physics was to provide a mathematical basis for the kinetic theory of gases, which explains that gases behave as they do because they are composed of particles in constant motion. Maxwell built on the achievements of German physicist Rudolf Clausius, who in 1857 and 1858 had shown that a gas must consist of molecules in constant motion colliding with each other and with the walls of their container. Clausius developed the idea of the mean free path, which is the average distance that a molecule travels between collisions.
Maxwell's development of the kinetic theory of gases was stimulated by his success in the similar problem of Saturn's rings. It dates from 1860, when he used a statistical treatment to express the wide range of velocities (speeds and the directions of the speeds) that the molecules in a quantity of gas must inevitably possess. He arrived at a formula to express the distribution of velocity in gas molecules, relating it to temperature. He showed that gases store heat in the motion of their molecules, so the molecules in a gas will speed up as the gasses temperature increases. Maxwell then applied his theory with some success to viscosity (how much a gas resists movement), diffusion (how gas molecules move from an area of higher concentration to an area of lower concentration), and other properties of gases that depend on the nature of the molecules' motion.
Maxwell's kinetic theory did not fully explain heat conduction (how heat travels through a gas). Austrian physicist Ludwig Boltzmann modified Maxwell's theory in 1868, resulting in the Maxwell-Boltzmann distribution law, showing the number of particles (n) having an energy (E) in a system of particles in thermal equilibrium. It has the form:
n = n0 exp(-E/kT),
where n0 is the number of particles having the lowest energy, ‘k' the Boltzmann constant, and ‘T' the thermodynamic temperature.
If the particles can only have certain fixed energies, such as the energy levels of atoms, the formula gives the number (Ei) above the ground state energy. In certain cases several distinct states may have the same energy and the formula then becomes:
ni = gin0 exp(-Ki/kT),
where (g)i is the statistical weight of the level of energy ‘Ei',
i.e., the number of states having energy Ei. The distribution of energies obtained by the formula is called a Boltzmann distribution.
Both Maxwell' s thermodynamic relational equations and the Boltzmann formulation to a contributional successive succession of refinements of kinetic theory, and it proved fully applicable to all properties of gases. It also led Maxwell to an accurate estimate of the size of molecules and to a method of separating gases in a centrifuge. The kinetic theory was derived using statistics, so it also revised opinions on the validity of the second law of thermodynamics, which states that heat cannot flow from a colder to a hotter body of its own accord. In the case of two connected containers of gases at the same temperature, it is statistically possible for the molecules to diffuse so that the faster-moving molecules all concentrate in one container while the slower molecules gather in the other, making the first container hotter and the second colder. Maxwell conceived this hypothesis, which is known as Maxwell's demon. Although this event is very unlikely, it is possible, and the second law is therefore not absolute, but highly probable.
These sources provide additional information on James Maxwell Clerk: Maxwell is generally considered the greatest theoretical physicist of the 1800s. He combined a rigorous mathematical ability with great insight, which enabled him to make brilliant advances in the two most important areas of physics at that time. In building on Faraday's work to discover the electromagnetic nature of light, Maxwell not only explained electromagnetism but also paved the way for the discovery and application of the whole spectrum of electromagnetic radiation that has characterized modern physics. Physicists now know that this spectrum also includes radio, infrared, ultraviolet, and X-ray waves, to name a few. In developing the kinetic theory of gases, Maxwell gave the final proof that the nature of heat resides in the motion of molecules.
With Maxwell's famous equations, as devised in 1864, uses mathematics to explain the intersaction between electric and magnetic fields. His work demonstrated the principles behind electromagnetic waves created when electric and magnetic fields oscillate simultaneously. Maxwell realized that light was a form of electromagnetic energy, but he also thought that the complete electromagnetic spectrum must include many other forms of waves as well.
With the discovery of radio waves by German physicist Heinrich Hertz in 1888 and X-rays by German physicist Wilhelm Roentgen in 1895, Maxwell's ideas were proved correct. In 1897 British physicist Sir Joseph J. Thomson discovered the electron, a subatomic particle with a negative charge. This discovery countered the long-held notion that atoms were the basic unit of matter.
As in chemistry, these 19th-century discoveries in physics proved to have immense practical value. No one was more adept at harnessing them than American physicist and prolific inventor Thomas Edison. Working from his laboratories in Menlo Park, New Jersey, Edison devised the carbon-granule microphone in 1877, which greatly improved the recently invented telephone. He also invented the phonograph, the electric light bulb, several kinds of batteries, and the electric metre. Edison was granted more than 1,000 patents for electrical devices, a phenomenal feat for a man who had no formal schooling.
In the earth sciences, the 19th century was a time of controversy, with scientists debating Earth's age. Estimated ranges may be as far as from less than 100,000 years to several hundred million years. In astronomy, greatly improved optical instruments enabled important discoveries to be made. The first observation of an asteroid, Ceres, took place in 1801. Astronomers had long noticed that Uranus exhibited an unusual orbit. French astronomer Urbain Jean Joseph Leverrier predicted that another planet nearby caused Uranus's odd orbit. Using mathematical calculations, he narrowed down where such a planet would be located in the sky. In 1846, with the help of German astronomer Johann Galle, Leverrier discovered Neptune. The Irish astronomer William Parsons, the third Earl of Rosse, became the first person to see the spiral form of galaxies beyond our own solar system. He did this with the Leviathan, a 183-cm. (72-in.) reflecting telescopes, built on the grounds of his estate in Parsonstown (now Birr), Ireland, in the 1840s. His observations were hampered by Ireland's damp and cloudy climate, but his gigantic telescope remained the world's largest for more than 70 years.
In the 19th century the study of microorganisms became increasingly important, particularly after French biologist Louis Pasteur revolutionized medicine by correctly deducing that some microorganisms are involved in disease. In the 1880's Pasteur devised methods of immunizing people against diseases by deliberately treating them with weakened forms of the disease-causing organisms themselves. Pasteur's vaccine against rabies was a milestone in the field of immunization, one of the most effective forms of preventive medicine the world has yet seen. In the area of industrial science, Pasteur invented the process of pasteurization to help prevent the spread of disease through milk and other foods.
Pasteur's work on fermentation and spontaneous generation had considerable implications for medicine, because he believed that the origin and development of disease are analogous to the origin and process of fermentation. That is, disease arises from germs attacking the body from outside, just as unwanted microorganisms invade milk and cause fermentation. This concept, called the germ theory of disease, was strongly debated by physicians and scientists around the world. One of the main arguments against it was the contention that the role germs played during the course of disease was secondary and unimportant; the notion that tiny organisms could kill vastly larger ones seemed ridiculous to many people. Pasteur's studies convinced him that he was right, however, and in the course of his career he extended the germ theory to explain the causes of many diseases.
Pasteur also determined the natural history of anthrax, a fatal disease of cattle. He proved that anthrax is caused by a particular bacillus and suggested that animals could be given anthrax in a mild form by vaccinating them with attenuated (weakened) bacilli, thus providing immunity from potentially fatal attacks. In order to prove his theory, Pasteur began by inoculating twenty-five sheep; a few days later he inoculated these and twenty-five more sheep with an especially strong inoculant, and he left ten sheep untreated. He predicted that the second twenty-five sheep would all perish and concluded the experiment dramatically by showing, to a sceptical crowd, the carcasses of the twenty-five sheep lying side by side.
Pasteur spent the rest of his life working on the causes of various diseases, including septicaemia, cholera, diphtheria, fowl cholera, tuberculosis, and smallpox-and their prevention by means of vaccination. He is best known for his investigations concerning the prevention of rabies, otherwise known in humans as hydrophobia. After experimenting with the saliva of animals suffering from this disease, Pasteur concluded that the disease rests in the nerve centres of the body; when an extract from the spinal column of a rabid dog was injected into the bodies of healthy animals, symptoms of rabies were produced. By studying the tissues of infected animals, particularly rabbits, Pasteur was able to develop an attenuated form of the virus that could be used for inoculation.
In 1885, a young boy and his mother arrived at Pasteur's laboratory; the boy had been bitten badly by a rabid dog, and Pasteur was urged to treat him with his new method. At the end of the treatment, which lasted ten days, the boy was being inoculated with the most potent rabies virus known; he recovered and remained healthy. Since that time, thousands of people have been saved from rabies by this treatment.
Pasteur's research on rabies resulted, in 1888, in the founding of a special institute in Paris for the treatment of the disease. This became known as the Instituted Pasteur, and it was directed by Pasteur himself until he died. (The institute still flourishes and is one of the most important centres in the world for the study of infectious diseases and other subjects related to microorganisms, including molecular genetics.) By the time of his death in Saint-Cloud on September 28, 1895, Pasteur had long since become a national hero and had been honoured in many ways. He was given a state funeral at the Cathedral of Nôtre Dame, and his body was placed in a permanent crypt in his institute.
Also during the 19th century, the Austrian monk Gregor Mendel laid the foundations of genetics, although his work, published in 1866, was not recognized until after the century had closed. Nevertheless, the British scientist Charles Darwin towers above all other scientists of the 19th century. His publication of On the Origin of Species in 1859 marked a major turning point for both biology and human thought. His theory of evolution by natural selection (independently and simultaneously developed by British naturalist Alfred Russel Wallace) initiated a violent controversy that until it has not subsided. Particularly controversial was Darwin's theory that humans resulted from a long process of biological evolution from apelike ancestors. The greatest opposition to Darwin's ideas came from those who believed that the Bible was an exact and literal statement of the origin of the world and of humans. Although the public initially castigated Darwin's ideas, by the late 1800s most biologists had accepted that evolution occurred, although not all agreed on the mechanism, known as natural selection, that Darwin proposed.
In the 20th century, scientists achieved spectacular advances in the fields of genetics, medicine, social sciences, technology, and physics.
At the beginning of the 20th century, the life sciences entered a period of rapid progress. Mendel's work in genetics was rediscovered in 1900, and by 1910 biologists had become convinced that genes are located in chromosomes, the threadlike structures that contain proteins and deoxyribonucleic acid (DNA). During the 1940's American biochemists discovered that DNA taken from one kind of bacterium could influence the characteristics of another. From these experiments, DNA is clearly the chemical that makes up genes and thus the key to heredity.
After American biochemist James Watson and British biophysicist Francis Crick established the structure of DNA in 1953, geneticists became able to understand heredity in chemical terms. Since then, progress in this field has been astounding. Scientists have identified the complete genome, or genetic catalogue, of the human body. In many cases, scientists now know how individual genes become activated and what affects they have in the human body. Genes can now be transferred from one species to another, sidestepping the normal processes of heredity and creating hybrid organisms that are unknown in the natural world.
At the turn of the 20th century, Dutch physician Christian Eijkman showed that disease can be caused not only by microorganisms but by a dietary deficiency of certain substances now called vitamins. In 1909 German bacteriologist Paul Ehrlich introduced the world's first bactericide, a chemical designed to kill specific kinds of bacteria without killing the patient's cells as well. Following the discovery of penicillin in 1928 by British bacteriologist Sir Alexander Fleming, antibiotics joined medicine's chemical armoury, making the fight against bacterial infection almost a routine matter. Antibiotics cannot act against viruses, but vaccines have been used to great effect to prevent some of the deadliest viral diseases. Smallpox, once a worldwide killer, was completely eradicated by the late 1970's, and in the United States the number of polio cases dropped from 38,000 in the 1950's to less than ten a year by the 21st century. By the middle of the 20th century scientists believed they were well on the way to treating, preventing, or eradicating many of the most deadly infectious diseases that had plagued humankind for centuries. Nevertheless, by the 1980's the medical community's confidence in its ability to control infectious diseases had been shaken by the emergence of new types of disease-causing microorganisms. New cases of tuberculosis developed, caused by bacteria strains that were resistant to antibiotics. New, deadly infections for which there was no known cure also appeared, including the viruses that cause haemorrhagic fever and the human immunodeficiency virus (HIV), the cause of acquired immunodeficiency syndrome.
In other fields of medicine, the diagnosis of disease has been revolutionized by the use of new imaging techniques, including magnetic resonance imaging and computed tomography. Scientists were also on the verge of success in curing some diseases using gene therapy, in which the insertion of normal or genetically an altered gene into a patient's cells replaces nonfunctional or missing genes.
Improved drugs and new tools have made surgical operations that were once considered impossible now routine. For instance, drugs that suppress the immune system enable the transplant of organs or tissues with a reduced risk of rejection Endoscopy permits the diagnosis and surgical treatment of a wide variety of ailments using minimally invasive surgery. Advances in high-speed fiberoptic connections permit surgery on a patient using robotic instruments controlled by surgeons at another location. Known as ‘telemedicine', this form of medicine makes it possible for skilled physicians to treat patients in remote locations or places that lack medical help.
In the 20th century the social sciences emerged from relative obscurity to become prominent fields of research. Austrian physician Sigmund Freud founded the practice of psychoanalysis, creating a revolution in psychology that led him to be called the ‘Copernicus of the mind'. In 1948 the American biologist Alfred Kinsey published Sexual Behaviour in the Human Male, which proved to be one of the best-selling scientific works of all time. Although criticized for his methodology and conclusions, Kinsey succeeded in making human sexuality an acceptable subject for scientific research.
The 20th century also brought dramatic discoveries in the field of anthropology, with new fossil finds helping to piece together the story of human evolution. A completely new and surprising source of anthropological information became available from studies of the DNA in mitochondria, cell structures that provide energy to fuel the cell's activities. Mitochondrial DNA has been used to track certain genetic diseases and to trace the ancestry of a variety of organisms, including humans.
In the field of communications, Italian electrical engineer Guglielmo Marconi sent his first radio signal across the Atlantic Ocean in 1901. American inventor Lee De Forest invented the triode, or vacuum tube, in 1906. The triode eventually became a key component in nearly all early radio, radar, television, and computer systems. In 1920 Scottish engineer John Logie Baird developed the Baird Televisor, a primitive television that provided the first transmission of a recognizable moving image. In the 1920's and 1930's American electronic engineer Vladimir Kosma Zworykin significantly improved the television's picture and reception. In 1935 British physicist Sir Robert Watson-Watt used reflected radio waves to locate aircraft in flight. Radar signals have since been reflected from the Moon, planets, and stars to learn their distance from Earth and to track their movements.
In 1947 American physicists John Bardeen, Walter Brattain, and William Shockley invented the transistor, an electronic device used to control or amplify an electrical current. Transistors are much smaller, far less expensive, require less power to operate, and are considerably more reliable than triodes. Since their first commercial use in hearing aids in 1952, transistors have replaced triodes in virtually all applications.
During the 1950's and early 1960's minicomputers were developed using transistors rather than triodes. Earlier computers, such as the electronic numerical integrator and computer (ENIAC), first introduced in 1946 by American physicist John W. Mauchly and American electrical engineer John Presper Eckert, Jr., used as many as 18,000 triodes and filled a large room. However, the transistor initiated a trend toward microminiaturization, in which individual electronic circuits can be reduced to microscopic size. This drastically reduced the computer's size, cost, and power requirements and eventually enabled the development of electronic circuits with processing speeds measured in billionths of a second
Further miniaturization led in 1971 to the first microprocessor-a computer on a chip. When combined with other specialized chips, the microprocessor becomes the central arithmetic and logic unit of a computer smaller than a portable typewriter. With their small size and a price less than that of a used car, today's personal computers are many times more powerful than the physically huge, multimillion-dollar computers of the 1950's. Once used only by large businesses, computers are now used by professionals, small retailers, and students to complete a wide variety of everyday tasks, such as keeping data on clients, tracking budgets, and writing school reports. People also use computers to understand each other with worldwide communications networks, such as the Internet and the World Wide Web, to send and receive E-mail, to shop, or to find information on just about any subject.
During the early 1950's public interest in space exploration developed. The focal event that opened the space age was the International Geophysical Year from July 1957 to December 1958, during which hundreds of scientists around the world coordinated their efforts to measure the Earth's near-space environment. As part of this study, both the United States and the Soviet Union announced that they would launch artificial satellites into orbit for nonmilitary space activities.
When the Soviet Union launched the first Sputnik satellite in 1957, the feat spurred the United States to intensify its own space exploration efforts. In 1958 the National Aeronautics and Space Administration (NASA) was founded for the purpose of developing human spaceflight. Throughout the 1960's NASA experienced its greatest growth. Among its achievements, NASA designed, manufactured, tested, and eventually used the Saturn rocket and the Apollo spacecraft for the first manned landing on the Moon in 1969. In the 1960's and 1970's, NASA also developed the first robotic space probes to explore the planet's Mercury, Venus, and Mars. The success of the Mariner probes paved the way for the unmanned exploration of the outer planets in Earth's solar system.
In the 1970's through 1990's, NASA focussed its space exploration efforts on a reusable space shuttle, which was first deployed in 1981. In 1998 the space shuttle, along with its Russian counterpart known as Soyuz, became the workhorses that enabled the construction of the International Space Station.
In 1900 the German physicist Max Planck proposed the then sensational idea that energy be not divisible but is always given off in set amounts, or quanta. Five years later, German-born American physicist Albert Einstein successfully used quanta to explain the photoelectric effect, which is the release of electrons when metals are bombarded by light. This, together with Einstein's special and general theories of relativity, challenged some of the most fundamental assumptions of the Newtonian era.
Unlike the laws of classical physics, quantum theory deals with events that occur on the smallest of scales. Quantum theory explains how subatomic particles form atoms, and how atoms interact when they combine to form chemical compounds. Quantum theory deals with a world where the attributes of any single particle can never be completely known-an idea known as the uncertainty principle, put forward by the German physicist Werner Heisenberg in 1927, whereby, the principle, that the product of the uncertainty in measured value of a component of momentum (p ) and the uncertainty in the corresponding co-ordinates of ( ) is of the equivalent set-order of magnitude, as the Planck constant. In its most precise form:
 p2 x       h/4
where    represents the root-mean-square value of the uncertainty. For mot purposes one can assume:
 p  x    = h/2
the principle can be derived exactly from quantum mechanics, a physical theory that grew out of Planck's quantum theory and deals with the mechanics of atomic and related systems in terms of quantities that an be measured mathematical forms, including ‘wave mechanics' (Schrödinger) and ‘matrix mechanics' (Born and Heisenberg), all of which are equivalent.
Nonetheless, it is most easily understood as a consequence of the fact that any measurement of a system mist disturbs the system under investigation, with a resulting lack of precision in measurement. For example, if seeing an electron was possible and thus measures its position, photons would have to be reflected from the electron. If a single photon could be used and detected with a microscope, the collision between the electron and photon would change the electron's momentum, as to its effectuality Compton Effect as a result to wavelengths of the photon is increased by an amount   , whereby:
   = (2h/m0c) sin2 ½  .
This is the Compton equation, h is the Planck constant, m0 the rest mass of the particle, c the speed of light, and   the angle between the directions of the incident and scattered photon. The quantity h/m0c is known as the Compton wavelength, symbol:  C, to which for an electron is equal to 0.002  43 nm.
A similar relationship applies to the determination of energy and time, thus:
 E x  t   h/4 .
The effects of the uncertainty principle are not apparent with large systems because of the small size of h. However, the principle is of fundamental importance in the behaviour of systems on the atomic scale. For example, the principle explains the inherent width of spectral lines, if the lifetime of an atom in an excited state is very short there is a large uncertainty in its energy and line resulting from a transition is broad.
One consequence of the uncertainty principle is that predicting the behaviour of a system and the macroscopic principle of causality cannot apply at the atomic level is impossible fully. Quantum mechanics give a statistical description of the behaviour of physical systems.
Nevertheless, while there is uncertainty on the subatomic level, quantum physics successfully predicts the overall outcome of subatomic events, a fact that firmly relates it to the macroscopic world, that is, the one in which we live.
In 1934 Italian-born American physicist Enrico Fermi began a series of experiments in which he used neutrons (subatomic particles without an electric charge) to bombard atoms of various elements, including uranium. The neutrons combined with the nuclei of the uranium atoms to produce what he thought were elements heavier than uranium, known as transuranium elements. In 1939 other scientists demonstrated that in these experiments' Fermi had not formed heavier elements, but instead had achieved the splitting, or fission, of the uranium atom's nucleus. These early experiments led to the development of fission as both energy sources.
These fission studies, coupled with the development of particle accelerators in the 1950's, initiated a long and remarkable journey into the nature of subatomic particles that continues today. Far from being indivisible, scientists now know that atoms are made up of twelve fundamental particles known as quarks and leptons, which combine in different ways to make all the kinds of matter currently known.
Advances in particle physics have been closely linked to progress in cosmology. From the 1920's onward, when the American astronomer Edwin Hubble showed that the universe is expanding, cosmologists have sought to rewind the clock and establish how the universe began. Today, most scientists believe that the universe started with a cosmic explosion some time between ten and twenty billion years ago. However, the exact sequence of events surrounding its birth, and its ultimate fate, are still matters of ongoing debate.
Apart from their assimilations affiliated within the paradigms of science, Descartes was to posit the existence of two categorically different domains of existence for immaterial ideas-the res extensa and the res cognitans or the ‘extended substance' and the ‘thinking substance. Descartes defined the extended substance as the realm of physical reality within primary mathematical and geometrical forms resides and thinking substance as the realm of human subjective reality. Given that Descartes distrusted the information from the senses to the point of doubting the perceived results of repeatable scientific experiments, how did he conclude that our knowledge of the mathematical ideas residing only in mind or in human subjectivity was accurate, much less the absolute truth? He did so by making a lap of faith-God constructed the world, said Descartes, in accordance with the mathematical ideas that our minds are capable of uncovering in their pristine essence. The truth of classical physics as Descartes viewed them were quite literally ‘revealed' truths, and it was this seventeenth-century metaphysical presupposition that became in the history of science what we term the ‘hidden ontology of classical epistemology.'
While classical epistemology would serve the progress of science very well, It also presented us with a terrible dilemma about the relationship between ‘mind' and the ‘world'. If there is no real or necessary correspondence between non-mathematical ideas in subjective reality and external physical reality, how do we now that the world in which we live, breath, and have our Being, then perish in so that we undeniably exist? Descartes's resolution of this dilemma took the form of an exercise. He asked us to direct our attention inward and to divest our consciousness of all awareness of eternal physical reality. If we do so, he concluded, the real existence of human subjective reality could be confirmed.
As it turned out, this resolution was considerably more problematic and oppressive than Descartes could have imaged. ‘I think, Therefore, I am' may be a marginally persuasive way of confirming the real existence e of the thinking self. However, the understanding of physical reality that obliged Descartes and others to doubt the existence of this self implied that the separation between the subjective world, or the world of life, and the real world of physical reality was ‘absolute.'
Our propped new understanding of the relationship between mind and world is framed within the larger context of the history of mathematical physics, the organs and extensions of the classical view of the foundations of scientific knowledge, and the various ways that physicists have attempted to obviate previous challenge s to the efficacy of classical epistemology, this was made so, as to serve as background for a new relationship between parts nd wholes in quantum physics, as well as similar view of the relationship that had emerged in the so-called ‘new biology' and in recent studies of the evolution of modern humans.
Nevertheless, at the end of such as this arduous journey lie two conclusions that should make possible that first, there is no basis in contemporary physics or biology for believing in the stark Cartesian division between mind and world, that some have alternatively given to describe as ‘the disease of the Western mind'. Secondly, there is a new basis for dialogue between two cultures that are now badly divided and very much un need of an enlarged sense of common understanding and shared purpose; let us briefly consider the legacy in Western intellectual life of the stark division between mind and world sanctioned by classical physics and formalized by Descartes.
The first scientific revolution of the seventeenth century freed Western civilization from the paralysing and demeaning forces of superstition, laid the foundations for rational understanding and control of the processes of nature, and ushered in an era of technological innovation and progress that provided untold benefits for humanity. Nevertheless, as classical physics progressively dissolved the distinction between heaven and earth and united the universe in a shared and communicable frame of knowledge, it presented us with a view of physical reality that was totally alien from the world of everyday life.
Philosophy, quickly realized that there was nothing in tis view of nature that could explain o provide a foundation for the mental, or for all that we know from direct experience cas distinctly human. In a mechanistic universe, he said, there is no privileged place or function for mind, and the separation between mind and matter is absolute. Descartes was also convinced, however, that the immaterial essences that gave form and structure to this universe were coded in geometrical and mathematical ideas, and this insight led to invent ‘algebraic geometry'.
A scientific understanding of these ideas could be derived, said Descartes, with the aid of precise deduction, and him also claimed that the contours of physical reality could be laid out in three-dimensional co-ordinates. Following the publication of Isaac Newton's Principia Mathematica. In 1687, reductionism and mathematical modelling became the most powerful tools of modern science. The dream that the entire physical world would be known and mastered though the extension and refinement of mathematical theory became the central feature and guiding principle of scientific knowledge.
Descartes's theory of knowledge starts with the quest for certainty, for an indubitable starting-point or foundation on the basis alone of which progress is possible. This is the method of investigating the extent of knowledge and its basis in reason or experience, it attempts to put knowledge upon a secure formation by first inviting us to suspend judgement on any proposition whose truth can be doubted, even as a bare possibility. The standards of acceptance are gradually raised as we are asked to doubt the deliverance of memory, the senses, and even reason, all of which are in principle capable of letting us down. The process is eventually dramatized in the figure of the evil-demon, or malin génie, whose aim is to deceive us, so that our sense, memories, and seasonings lead us astray. The task then becomes one of finding a demon-proof points of certainty, and Descartes produces this in the famous ‘Cogito ergo sum', I think therefore I am'. It is on this slender basis that the correct use of our faculties has to be reestablished, but it seems as though Descartes has denied himself any materials to use in reconstructing the edifice of knowledge. He has a basis, but any way of building on it without invoking principles tat will not be demon-proof, and so will not meet the standards he had apparently set himself. It vis possible to interpret him as using ‘clear and distinct ideas' to prove the existence of God, whose benevolence then justifies our use of clear and distinct ideas (‘God is no deceiver'): This is the notorious Cartesian circle. Descartes's own attitude to this problem is not quite clear, at timers he seems more concerned with providing a stable body of knowledge, that our natural faculties will endorse, rather than one that meets the more severe standards with which he starts. For example, in the second set of Replies he shrugs off the possibility of ‘absolute falsity' of our natural system of belief, in favour of our right to retain ‘any conviction so firm that it is quite incapable of being destroyed'. The need to add such natural belief to anything certified by reason Events eventually the cornerstone of Hume ‘s philosophy, and the basis of most 20th-century reactionism, to the method of doubt.
In his own time Rene Descartes' conception of the entirely separate substance of the mind was recognized to give rise to insoluble problems of the nature of the causal efficacy to the action of God. Events in the world merely form occasions on which God acts so as to bring about the events normally accompanying them, and thought of as their effects, although the position is associated especially with Malebrallium, it is much older, many among the Islamic philosophies, their processes for adducing philosophical proofs to justify elements of religious doctrine. It plays the parallel role in Islam to that which scholastic philosophy played in the development of Christianity. The practitioners of kalam were known as the Mutakallimun. It also gives rise to the problem, insoluble in its own terms, of ‘other minds'. Descartes's notorious denial that nonhuman animals are conscious is a stark illustration of th problem.
In his conception of matter Descartes also gives preference to rational cogitation over anything derived from the senses., since we can conceive of the nature of a ‘ball of wax' surviving changes to its sensible qualities, matter is not an empirical concept, but eventually an entirely geometrical one, with extension and motion as its only physical nature. Descartes's thought here is reflected in Leibniz's view, as held later by Russell, that the qualities of sense experience have no resemblance to qualities of things, so that knowledge of the external world is essentially knowledge of structure rather than of filling. On this basis Descartes erects a remarkable physics. Since matter is in effect the same as extension there can be no empty space or ‘void', since there is no empty space motion is not a question of occupying previously empty space, but is to be thought of in terms of vortices (like the motion of a liquid).
Although the structure of Descartes's epistemology, theory of mind, and theory of matter have been rejected many times, their relentless exposure of the hardest issues, their exemplary clarity, and even their initial plausibility all contrive to make him the central point of reference for modern philosophy.
It seems, nonetheless, that the radical separation between mind and nature formalized by Descartes served over time to allow scientists to concentrate on developing mathematical descriptions of matter as pure mechanisms without any concerns about is spiritual dimension or ontological foundations. In the meantime, attempts to rationalize, reconcile, or eliminate Descartes's stark division between mind and matter became perhaps te most cental feature of Western intellectual life.
Philosophers in the like of John Locke, Thomas Hobbes, and David Hume tried to articulate some basis for linking the mathematical describable motions of mater with linguistic representations of external reality in the subjective space of mind. Descartes' compatriot Jean-Jacques Rousseau reified nature as the ground of human consciousness in a state of innocence and proclaimed that "Liberty, Equality, Fraternity" are the guiding principles of this consciousness. Rousseau also made godlike the ideas o the ‘general will' of the people to achieve these goals and declare that those who do not conform to this will were social deviants.
Evenhandedly, Rousseau's attempt to posit a ground for human consciousness by reifying nature was revived in a measure more different in form by the nineteenth-century Romantics in Germany, England, and the United Sates. Goethe and Friedrich Schelling proposed a natural philosophy premised on ontological monism (the idea that God, man, and nature are grounded in an indivisible spiritual Oneness) and argued for the reconciliation of mind and matter with an appeal to sentiment, mystical awareness, and quasi-scientific musing. In Goethe's attempt to wed mind and matter, nature became a mindful agency that ‘loves illusion'. Shrouds man in mist, ‘ presses him to her heart', and punishes those who fail to see the ‘light'. Schelling, in his version of cosmic unity, argued that scientific facts were at best partial truths and that the mindful creative spirit that unifies mind and matter is progressively moving toward self-realization and undivided wholeness.
Descartes believed there are two basic kinds of things in the world, a belief known as substance dualism. For Descartes, the principles of existence for these two groups of things -bodies and minds-are completely different from one another: Bodies exist by being extended in space, while minds exist by being conscious. According to Descartes, nothing can be done to give a body thought and consciousness. No matter how we shape a body or combine it with other bodies, we cannot turn the body into a mind, a thing that is conscious, because being conscious is not a way of being extended.
For Descartes, a person consists of a human body and a human mind causally interacting with one another. For example, the intentions of a human being might have awaken that person's limbs to move. In this way, the mind can affect the body. In addition, the sense organs of a human being may be affected by light, pressure, or sound, external sources that in turn affect the brain, affecting mental states. Thus, the body may affect the mind. Exactly how mind can affect body, and vice versa, is a central issue in the philosophy of mind, and is known as the mind-body problem. According to Descartes, this interaction of mind and body is peculiarly intimate. Unlike the interaction between a pilot and his ship, the connection between mind and body more closely resembles two substances that have been thoroughly mixed.
Because of the diversity of positions associated with existentialism, the term is impossible to define precisely. Certain themes common to virtually all existentialist writers can, however, be identified. The term itself suggests one major theme: the stress on concrete individual existence and, consequently, on subjectivity, individual freedom, and choice.
Most philosophers since Plato have held that the highest ethical good is the same for everyone; insofar as one approaches moral perfection, one resembles other morally perfect individuals. The 19th-century Danish philosopher Søren Kierkegaard, who was the first writer to call himself existential, reacted against this tradition by insisting that the highest good for the individual is to find his or her own unique vocation. As he wrote in his journal, "I must find a truth that is true for me . . . the idea for which I can live or die." Other existentialist writers have echoed Kierkegaard's belief that one must choose one's own way without the aid of universal, objective standards. Against the traditional view that moral choice involves an objective judgment of right and wrong, existentialists have argued that no objective, rational basis can be found for moral decisions. The 19th-century German philosopher Friedrich Nietzsche further contended that the individual must decide which situations are to count as moral situations.
All existentialists have followed Kierkegaard in stressing the importance of passionate individual action in deciding questions of both morality and truth. They have insisted, accordingly, that personal experience and acting on one's own convictions are essential in arriving at the truth. Thus, the understanding of a situation by someone involved in that situation is superior to that of a detached, objective observer. This emphasis on the perspective of the individual agent has also made existentialists suspicious of systematic reasoning. Kierkegaard, Nietzsche, and other existentialist writers have been deliberately unsystematic in the exposition of their philosophies, preferring to express themselves in aphorisms, dialogues, parables, and other literary forms. Despite their antirationalist position, however, most existentialists cannot be said to be irrationalists in the sense of denying all validity to rational thought. They have held that rational clarity is desirable wherever possible, but that the most important questions in life are not accessible to reason or science. Furthermore, they have argued that even science is not as rational as is commonly supposed. Nietzsche, for instance, asserted that the scientific assumption of an orderly universe is for the most part a useful fiction.
Perhaps the most prominent theme in existentialist writing is that of choice. Humanity's primary distinction, in the view of most existentialists, is the freedom to choose. Existentialists have held that human beings do not have a fixed nature, or essence, as other animals and plants do; each human being makes choices that create his or her own nature. In the formulation of the 20th-century French philosopher Jean-Paul Sartre, existence precedes essence. Choice is therefore central to human existence, and it is inescapable; even the refusal to choose is a choice. Freedom of choice entails commitment and responsibility. Because individuals are free to choose their own path, existentialists have argued, they must accept the risk and responsibility of following their commitment wherever it leads.
Kierkegaard held that recognizing that one experiences is spiritually crucial not only a fear of specific objects but also a feeling of general apprehension, which he called dread. He interpreted it as God's way of calling each individual to make a commitment to a personally valid way of life. The word anxiety (German Angst) has a similarly crucial role in the work of the 20th-century German philosopher Martin Heidegger; anxiety leads to the individual's confrontation with nothingness and with the impossibility of finding ultimate justification for the choices he or she must make. In the philosophy of Sartre, the word nausea is used for the individual's recognition of the pure contingency of the universe, and the word anguish is used for the recognition of the total freedom of choice that confronts the individual at every moment.
Existentialism as a distinct philosophical and literary movement belongs to the 19th and 20th centuries, but elements of existentialism can be found in the thought (and life) of Socrates, in the Bible, and in the work of many premodern philosophers and writers.
The first to anticipate the major concerns of modern existentialism was the 17th-century French philosopher Blaise Pascal. Pascal rejected the rigorous rationalism of his contemporary René Descartes, asserting, in his Pensées (1670), that a systematic philosophy that presumes to explain God and humanity is a form of pride. Like later existentialist writers, he saw human life in terms of paradoxes: The human self, which combines mind and body, is itself a paradox and contradiction.
Kierkegaard, generally regarded as the founder of modern existentialism, reacted against the systematic absolute idealism of the 19th-century German philosopher Georg Wilhelm Friedrich Hegel, who claimed to have worked out a total rational understanding of humanity and history. Kierkegaard, on the contrary, stressed the ambiguity and absurdity of the human situation. The individual's response to this situation must be to live a totally committed life, and this commitment can only be understood by the individual who has made it. The individual therefore must always be prepared to defy the norms of society for the sake of the higher authority of a personally valid way of life. Kierkegaard ultimately advocated a ‘leap of faith' into a Christian way of life, which, although incomprehensible and full of risk, was the only commitment he believed could save the individual from despair.
Nietzsche, who was not acquainted with the work of Kierkegaard, influenced subsequent existentialist thought through his criticism of traditional metaphysical and moral assumptions and through his espousal of tragic pessimism and the life-affirming individual will that opposes itself to the moral conformity of the majority. In contrast to Kierkegaard, whose attack on conventional morality led him to advocate a radically individualistic Christianity, Nietzsche proclaimed the "Death of God" and went on to reject the entire Judeo-Christian moral tradition in favour of a heroic pagan ideal.
Heidegger, like Pascal and Kierkegaard, reacted against an attempt to put philosophy on a conclusive rationalistic basis-in this case the phenomenology of the 20th-century German philosopher Edmund Husserl. Heidegger argued that humanity finds itself in an incomprehensible, indifferent world. Human beings can never hope to understand why they are here; instead, each individual must choose a goal and follow it with passionate conviction, aware of the certainty of death and the ultimate meaninglessness of one's life. Heidegger contributed to existentialist thought an original emphasis on being and ontology as well as on language.
Sartre first gave the term existentialism general currency by using it for his own philosophy and by becoming the leading figure of a distinct movement in France that became internationally influential after World War II. Sartre's philosophy is explicitly atheistic and pessimistic; he declared that human beings require a rational basis for their lives but are unable to achieve one, and thus human life is a ‘futile passion'. Sartre nevertheless insisted that his existentialism be a form of humanism, and he strongly emphasized human freedom, choice, and responsibility. He eventually tried to reconcile these existentialist concepts with a Marxist analysis of society and history.
Although existentialist thought encompasses the uncompromising atheism of Nietzsche and Sartre and the agnosticism of Heidegger, its origin in the intensely religious philosophies of Pascal and Kierkegaard foreshadowed its profound influence on 20th-century theology. The 20th-century German philosopher Karl Jaspers, although he rejected explicit religious doctrines, influenced contemporary theology through his preoccupation with transcendence and the limits of human experience. The German Protestant theologians Paul Tillich and Rudolf Bultmann, the French Roman Catholic theologian Gabriel Marcel, the Russian Orthodox philosopher Nikolay Berdyayev, and the German Jewish philosopher Martin Buyer inherited many of Kierkegaard's concerns, especially that a personal sense of authenticity and commitment is essential to religious faith.
A number of existentialist philosophers used literary forms to convey their thought, and existentialism has been as vital and as extensive a movement in literature as in philosophy. The 19th-century Russian novelist Fyodor Dostoyevsky is probably the greatest existentialist literary figure. In Notes from the Underground (1864), the alienated antihero rages against the optimistic assumptions of rationalist humanism. The view of human nature that emerges in this and other novels of Dostoyevsky is that it is unpredictable and perversely self-destructive; only Christian love can save humanity from itself, but such love cannot be understood philosophically. As the character Alyosha says in The Brothers Karamazov (1879-80), "We must love life more than the meaning of it."
In the 20th century, the novels of the Austrian Jewish writer Franz Kafka, such as The Trial (1925; trans. 1937) and The Castle (1926; trans. 1930), present isolated men confronting vast, elusive, menacing bureaucracies; Kafka's themes of anxiety, guilt, and solitude reflect the influence of Kierkegaard, Dostoyevsky, and Nietzsche. The influence of Nietzsche is also discernible in the novels of the French writers André Malraux and in the plays of Sartre. The work of the French writer Albert Camus is usually associated with existentialism because of the prominence in it of such themes as the apparent absurdity and futility of life, the indifference of the universe, and the necessity of engagement in a just cause. Existentialist themes are also reflected in the th eater of the absurd, notably in the plays of Samuel Beckett and Eugène Ionesco. In the United States, the influence of existentialism on literature has been more indirect and diffuse, but traces of Kierkegaard's thought can be found in the novels of Walker Percy and John Updike, and various existentialist themes are apparent in the work of such diverse writers as Norman Mailer, John Barth, and Arthur Miller.
The fatal flaw of pure reason is, of course, the absence of emotion, and purely rational explanations of the division between subjective reality and external reality had limited appeal outside the community of intellectuals. The figure most responsible for infusing our understanding of Cartesian dualism with emotional content was the death of God theologian Friedrich Nietzsche. After declaring that God and ‘divine will' do not exist, Nietzsche reified the ‘essences' of consciousness in the domain of subjectivity as the ground for individual ‘will' and summarily dismissed all pervious philosophical attempts to articulate the ‘will to truth'. The problem, claimed Nietzsche, is that earlier versions of the ‘will to power' disguise the fact that all allege truths were arbitrarily created in the subjective reality of the individual and are expression or manifestations of individual ‘will'.
In Nietzsche's view, the separation between mind and mater is more absolute and total than had previously been imagined. Based on the assumption that there is no real or necessary correspondences between linguistic constructions of reality in human subjectivity and external reality, he declared that we are all locked in ‘a prison house of language'. The prison as he conceived it, however, it was also a ‘space' where the philosopher can examine the ‘innermost desires of his nature' and articulate a new massage of individual existence founded on will.
Those who fail to enact their existence in this space, says Nietzsche, are enticed into sacrificing their individuality on the nonexistent altars of religious beliefs and democratic or socialist ideals and become, therefore, members of the anonymous and docile crowd. Nietzsche also invalidated the knowledge claims of science in the examination of human subjectivity. Science, he said, not only exalted natural phenomena and favours reductionistic examinations of phenomena at the expense of mind. It also seeks to educe mind to a mere material substance, and thereby to displace or subsume the separateness and uniqueness of mind with mechanistic description that disallow any basis for te free exerciser of individual will.
Nietzsche's emotionally charged defence of intellectual freedom and his radical empowerment of mind as the maker and transformer of the collective fictions that shape human reality in a soulful mechanistic inverse proved terribly influential on twentieth-century thought. Nietzsche sought to reinforce his view of the subjective character of scientific knowledge by appealing to an epistemological crisis over the foundations of logic and arithmetic that arose during the last three decades of the nineteenth century. Though a curious course of events, attempts by Edmund Husserl, a philosopher trained in higher math and physics, to resolve this crisis resulted in a view of the character of human consciousness that closely resembled that of Nietzsche.
Friedrich Nietzsche is openly pessimistic about the possibility of knowledge. ‘We simply lack any organ for knowledge, for ‘truth': we know (or believe or imagine) just as much as may be useful in the interests of the human herd, the species: and even what is called ‘utility' is ultimately also a mere belief, something imaginary and perhaps precisely that most calamitous stupidity of which we will not perish some day' (The Gay Science).
This position is very radical, Nietzsche does not simply deny that knowledge, construed as the adequate representation of the world by the intellect, exists. He also refuses the pragmatist identification of knowledge and truth with usefulness: he writes that we think we know what we think is useful, and that we can be quite wrong about the latter.
Nietzsche's view, his ‘Perspectivism', depends on his claim that there is no sensible conception of a world independent of human interpretation and to which interpretations would correspond if hey were to constitute knowledge. He sum up this highly controversial position in The Will to Power: ‘Facts are precisely what there is not. Only interpretation'.
It is often claimed that Perspectivism is self-undermining. If the thesis that all views are interpretations is true then, it is argued there is at least one view that is not an interpretation. If, on the other hand, the thesis is itself an interpretation, then there is no reason to believe that it is true, and it follows again that nit every view is an interpretation.
Yet this refutation assume that if a view, like Perspectivism itself, is an interpretation it is wrong. This is not the case. To call any view, including Perspectivism, an interpretation is to say that it can be wrong, which is true of all views, and that is not a sufficient refutation. To show the Perspectivism is literally false producing another view superior to it on specific epistemological grounds is necessary.
Perspectivism does not deny that particular views can be true. Like some versions of cotemporary anti-realism, it attributes to specific approaches truth in relation t o facts specified internally those approaches themselves. Bu t it refuses to envisage a single independent set of facts, To be accounted for by all theories. Thus Nietzsche grants the truth of specific scientific theories does, however, deny that a scientific interpretation can possibly be ‘the only justifiable interpretation of the world' (The Gay Science): Neither t h fact science addresses nor the methods it employs are privileged. Scientific theories serve the purposes for which hey have been devised, but these have no priority over the many other purposes of human life. The existence of many purposes and needs relative to which the value of theories is established-another crucial element of Perspectivism is sometimes thought to imply a reason relative, according to which no standards for evaluating purposes and theories can be devised. This is correct only in that Nietzsche denies the existence of single set of standards for determining epistemic value, but holds that specific views can be compared with and evaluated in relation to one another the ability to use criteria acceptable in particular circumstances does not presuppose the existence of criteria applicable in all. Agreement is therefore not always possible, since individuals may sometimes differ over the most fundamental issues dividing them.
Still, Nietzsche would not be troubled by this fact, which his opponents too also have to confront only he would argue, to suppress it by insisting on the hope that all disagreements are in particular eliminable even if our practice falls woefully short of the ideal. Nietzsche abandons that ideal. He considers irresoluble disagreement and essential part of human life.
Knowledge for Nietzsche is again material, but now based on desire and bodily needs more than social refinements Perspectives are to be judged not from their relation to the absolute but on the basis of their effects in a specific era. The possibility of any truth beyond such a local, pragmatic one becomes a problem in Nietzsche, since either a noumenal realm or an historical synthesis exists to provide an absolute criterion of adjudication for competing truth claims: what get called truths are simply beliefs that have been for so long that we have forgotten their genealogy? In this Nietzsche reverses the Enlightenment dictum that truth is the way to liberation by suggesting that trying classes in as far as they are considered absolute for debate and conceptual progress and cause as opposed to any ambient behaviour toward the ease of which backwardness and unnecessary misery. Nietzsche moves back and forth without revolution between the positing of trans-histories; truth claims, such as his claim about the will to power, and a kind of epistemic nihilism that calls into question not only the possibility of truth but the need and desire of it as well. However, perhaps what is most important, Nietzsche introduces the notion that truth is a kind of human practice, in a game whose rules are contingent rather than necessary it. The evaluation of truth claims should be based of their strategic efforts, not their ability to represent a reality conceived of as separate as of an autonomous of human influence, for Nietzsche the view that all truth is truth from or within a particular perspective. The perspective may be a general human pin of view, set by such things as the nature of our sensory apparatus, or it may be thought to be bound by culture, history, language, class or gender. Since there may be many perspectives, there are also different families of truth. The term is frequently applied to, of course Nietzsche's philosophy.
The best-known disciples of Husserl was Martin Heidegger, and the work of both figures greatly influenced that of the French atheistic existentialist Jean-Paul Sartre. The work of Husserl, Heidegger and Sartre became foundational to that of the principle architects of philosophical postmodernism, the deconstructionists Jacques Lacan, Roland Bathes, Michel Foucault and Jacques Derrida, this direct linkage among the nineteenth-century crises about epistemological foundations of physics and the origins of philosophical postmodernism served to perpetuate the Cartesian two-world dilemma in an even more oppressive form
Of Sartre's main philosophical work, Being and Nothingness, Sartre examines the relationships between Being For-itself (consciousness) and Being In-itself (the non-conscious world). He rejects central tenets of the rationalalist and empiricist traditions, calling the view that the mind or self is a thing or substance. ‘Descartes's substantialist illusion', and claiming also that consciousness dos not contain ideas or representations . . . are idolist invented by the psychologists. Sartre also attacks idealism in the forms associated with Berkeley and Kant, and concludes that his account of the relationship between consciousness and the world is neither realist nor idealist.
Sartre also discusses Being For-others, which comprises the aspects of experience about interactions with other minds.. His views are subtle: roughly, he holds that one's awareness of others is constituted by feelings of shame, pride, and so on.
Sartre's rejection of ideas, and the denial of idealism, appear to commit him to direct realism in the theory of perception. This is neither inconsistent with his claim as been non-realist nor idealist, since by ‘realist' he means views that allow for the mutual independence or in-principle separability of mind and world. Against this Sartre emphasizes, after Heidegger, that perceptual experience has an active dimension, in hat it is a way of interacting and dealing with the world, than a way of merely contemplating it (‘activity, as spontaneous, unreflecting consciousness, constitutes a certain existential stratum in the world'). Consequently, he holds that experience is richer, and open to more aspects of the world, than empiricist writers customarily claim:
When I run after a streetcar . . . there is consciousness of-the-streetcar-having-to-be-overtaken, etc., . . . I am then plunged into the world of objects, it is they that constitute the unity of my consciousness, it is they that present themselves with values, with attractive nd repellent qualities . . .
Relatedly, he insists that I experience material things as having certain potentialities -for-me ('nothingness'). I see doors and bottles as openable, bicycles as ridable (these matters are linked ultimately to the doctrine of extreme existentialist freedom). Similarly, if my friend is not where I expect to meet her, then I experience her absence ‘as a real event'.
These Phenomenological claims are striking and compelling, but Sartre pay insufficient attention to such things as illusions and hallucinations, which are normally cited as problems for direct realists. In his discussion of mental imagery, however, he describes the act of imaging as a ‘transformation' of ‘psychic material'. This connects with his view that even a physical image such as a photograph of a tree does not figure as an object of consciousness when it is experienced as a tree-representation (than as a piece of coloured cards). Nonetheless, the fact remains that the photograph continues to contribute to the character of the experience. Given this, seeing how Sartre avoids positing a mental analogue of a photograph for episodes of mental imaging is hard, and harder still to reconcile this with his rejection of visual representations. If ones image is regarded as debased and the awareness of awakening is formally received as a differential coefficient of perceptual knowledge, but this merely rises once more the issue of perceptual illusion and hallucination, and the problem of reconciling them are dialectally the formalization built upon realism.
Much of Western religion and philosophical thought since the seventeenth century has sought to obviate this prospect with an appeal to ontology or to some conception of God or Being. Yet we continue to struggle, as philosophical postmodernism attests, with the terrible prospect by Nietzsche-we are locked in a prison house of our individual subjective realities in a universe that is as alien to our thought as it is to our desires. This universe may seem comprehensible and knowable in scientific terms, and science does seek in some sense, as Koyré puts it, to ‘find a place for everything.' Nonetheless, the ghost of Descartes lingers in the widespread conviction that science does not provide a ‘place for man' or for all that we know as distinctly human in subjective reality.
Nonetheless, after The Gay Science (1882) began the crucial exploration of self-mastery. The relations between reason and power, and the revelation of the unconscious striving after power that provide the actual energy for the apparent self-denial of the ascetic and the martyred was during this period that Nietzsche's failed relationship with Lou Salome resulted in the emotional crisis from which Also sprach Zarathustra (1883-5, trans., as Thus Spoke Zarathustra) signals a recovery. This work is frequently regarded as Nietzsche's masterpiece. It was followed by Jenseits von Gut and Böse (1887), trans., as Beyond Good and Evil); Zur Genealogie der Moral (1887, trans., as, The Genealogy of Moral.)
In Thus Spake Zarathustra (1883-85), Friedrich Nietzsche introduced in eloquent poetic prose the concepts of the death of God, the superman, and the will to power. Vigorously attacking Christianity and democracy as moralities for the ‘weak herd', he argued for the ‘natural aristocracy' of the superman who, driven by the ‘will to power', celebrates life on earth rather than sanctifying it for some heavenly reward. Such a heroic man of merit has the courage to ‘live dangerously' and thus rise above the masses, developing his natural capacity for the creative use of passion.
Also known as radical theology, this movement flourished in the mid 1960s. As a theological movement it never attracted a large following, did not find a unified expression, and passed off the scene as quickly and dramatically as it had arisen. There is even disagreement as to whom its major representatives were. Some identify two, and others three or four. Although small, the movement attracted attention because it was a spectacular symptom of the bankruptcy of modern theology and because it was a journalistic phenomenon. The very statement "God is dead" was tailor-made for journalistic exploitation. The representatives of the movement effectively used periodical articles, paperback books, and the electronic media. This movement gave expression to an idea that had been incipient in Western philosophy and theology for some time, the suggestion that the reality of a transcendent God at best could not be known and at worst did not exist at all. Philosopher Kant and theologian Ritschl denied that one could have a theoretical knowledge of the being of God. Hume and the empiricist for all practical purposes restricted knowledge and reality to the material world as perceived by the five senses. Since God was not empirically verifiable, the biblical world view was said to be mythological and unacceptable to the modern mind. Such atheistic existentialist philosophers as Nietzsche despaired even of the search of God; it was he who coined the phrase "God is dead" almost a century before the death of God theologians.
Mid-twentieth century theologians not associated with the movement also contributed to the climate of opinion out of which death of God theology emerged. Rudolf Bultmann regarded all elements of the supernaturalistic, theistic world view as mythological and proposed that Scripture be demythologized so that it could speak its message to the modern person.
Paul Tillich, an avowed anti supernaturalist, said that the only nonsymbiotic statement that could be made about God was that he was being itself. He is beyond essence and existence; therefore, to argue that God exists is to deny him. It is more appropriate to say God does not exist. At best Tillich was a pantheist, but his thought borders on atheism. Dietrich Bonhoeffer (whether rightly understood or not) also contributed to the climate of opinion with some fragmentary but tantalizing statements preserved in Letters and Papers from Prison. He wrote of the world and man ‘coming of age', of ‘religionless Christianity', of the ‘world without God', and of getting rid of the ‘God of the gaps' and getting along just as well as before. It is not always certain what Bonhoeffer meant, but if nothing else, he provided a vocabulary that later radical theologians could exploit.
It is clear, then, that as startling as the idea of the death of God was when proclaimed in the mid 1960s, it did not represent as radical a departure from recent philosophical and theological ideas and vocabulary as might superficially appear.
Just what was death of God theology? The answers are as varied as those who proclaimed God's demise. Since Nietzsche, theologians had occasionally used "God is dead" to express the fact that for an increasing number of people in the modern age God seems to be unreal. Nonetheless, the idea of God's death began to have special prominence in 1957 when Gabriel Vahanian published a book entitled God is Dead. Vahanian did not offer a systematic expression of death of God theology. Instead, he analysed those historical elements that contributed to the masses of people accepting atheism not so much as a theory but as a way of life. Vahanian himself did not believe that God was dead. Still, he urged that there be a form of Christianity that would recognize the contemporary loss of God and exert its influence through what was left. Other proponents of the death of God had the same assessment of God's status in contemporary culture, but were to draw different conclusions.
Thomas J. J. Altizer believed that God had really died. Nonetheless, Altizer often spoke in exaggerated and dialectic language, occasionally with heavy overtones of Oriental mysticism. Sometimes knowing exactly what Altizer meant when he spoke in dialectical opposites is difficult such as "God is dead, thank God" Apparently the real meaning of Altizer's belief that God had died is to be found in his belief in God's immanence. To say that God has died is to say that he has ceased to exist as a transcendent, supernatural being. Alternately, he has become fully immanent in the world. The result is an essential identity between the human and the divine. God died in Christ in this sense, and the process has continued time and again since then. Altizer claims the church tried to give God life again and put him back in heaven by its doctrines of resurrection and ascension. However, the traditional doctrines about God and Christ must be repudiated because man has discovered after nineteen centuries that God does not exist. Christians must even now will the death of God by which the transcendent becomes immanent.
For William Hamilton the death of God describes the event many have experienced over the last two hundred years. They no longer accept the reality of God or the meaningfulness of language about him. non theistic explanations have been substituted for theistic ones. This trend is irreversible, and everyone must come to terms with the historical-cultural -death of God. God's death must be affirmed and the secular world embraced as normative intellectually and good ethically. Doubtlessly, Hamilton was optimistic about the world, because he was optimistic about what humanity could do and was doing to solve its problems.
Paul van Buren is usually associated with death of God theology, although he himself disavowed this connection. Yet, his disavowal seems hollow in the light of his book The Secular Meaning of the Gospel and his article "Christian Education Post Mortem Dei." In the former he accepts empiricism and the position of Bultmann that the world view of the Bible is mythological and untenable to modern people. In the latter he proposes an approach to Christian education that does not assume the existence of God but does assume ‘the death of God' and that ‘God is gone'.
Van Buren was concerned with the linguistic aspects of God's existence and death. He accepted the premise of empirical analytic philosophy that real knowledge and meaning can be conveyed only by language that is empirically verifiable. This is the fundamental principle of modern secularists and is the only viable option in this age. If only empirically verifiable language is meaningful, ipso facto all language that refers to or assumes the reality of God is meaningless, since one cannot verify God's existence by any of the five senses. Theism, belief in God, is not only intellectually untenable, it is meaningless. In The Secular Meaning of the Gospel van Buren seeks to reinterpret the Christian faith without reference to God. One searches the book in vain for even one clue that van Buren is anything but a secularist trying to translate Christian ethical values into that language game. There is a decided shift in van Buren's later book Discerning the Way, however.
In retrospect, there was clearly no single death of God theology, only death of God theologies. Their real significance was that modern theologies, by giving up the essential elements of Christian belief in God, had logically led to what were really antitheologies. When the death of God theologies passed off the scene, the commitment to secularism remained and manifested itself in other forms of secular theology in the late 1960s and the 1970s.
Nietzsche is unchallenged as the most insightful and powerful critic of the moral climate of the 19th century (and of what of it remains in ours). His exploration of unconscious motivation anticipated Freud. He is notorious for stressing the ‘will to power' that is the basis of human nature, the ‘resentment' that comes when it is denied its basis in action, and the corruptions of human nature encouraged by religion, such as Christianity, that feed on such resentment. Yet the powerful human beings who escapes all this, the Ubermensch, is not the ‘blood beast' of later fascism: It is a human being who has mastered passion, risen above the senseless flux, and given creative style to his or her character. Nietzsche's free spirits recognize themselves by their joyful attitude to eternal return. He frequently presents the creative artist rather than the warlord as his best exemplar of the type, but the disquieting fact remains that he seems to leave himself no words to condemn any uncaged beasts of pre y who best find their style by exerting repulsive power find their style by exerting repulsive power over others. This problem is no t helped by Nietzsche's frequently expressed misogyny, although in such matters the interpretation of his many-layered and ironic writings is no always straightforward. Similarly y, such
Anti-Semitism as has been found in his work is balanced by an equally vehement denunciation of anti-Semitism, and an equal or greater dislike of the German character of his time.
Nietzsche's current influence derives not only from his celebration of will, but more deeply from his scepticism about the notions of truth and act. In particular, he anticipated any of the central tenets of postmodernism: an aesthetic attitude toward the world that sees it as a ‘text'; the denial of facts; the denial of essences; the celebration of the plurality of interpretation and of the fragmented self, as well as the downgrading of reason and the politicization of discourse. All awaited rediscovery in the late 20th century. Nietzsche also has the incomparable advantage over his followers of being a wonderful stylist, and his Perspectivism is echoed in the shifting array of literary devices-humour, irony, exaggeration, aphorisms, verse, dialogue, parody-with that he explores human life and history.
Yet, it is nonetheless, that we have seen, the origins of the present division that can be traced to the emergence of classical physics and the stark Cartesian division between mind and bodily world are two separate substances, the self is as it happened associated with a particular body, but is self-subsisting, and capable of independent existence, yet Cartesian duality, much as the ‘ego' that we are tempted to imagine as a simple unique thing that makes up our essential identity, but, seemingly  sanctioned by this physics. The tragedy of the Western mind, well represented in the work of a host of writers, artists, and intellectual, is that the Cartesian division was perceived as uncontrovertibly real.
Beginning with Nietzsche, those who wished to free the realm of the mental from the oppressive implications of the mechanistic world-view sought to undermine the alleged privileged character of the knowledge called physicians with an attack on its epistemological authority. After Husserl tried and failed to save the classical view of correspondence by grounding the logic of mathematical systems in human consciousness, this not only resulted in a view of human consciousness that became characteristically postmodern. It also represents a direct link with the epistemological crisis about the foundations of logic and number in the late nineteenth century that foreshadowed the epistemological crisis occasioned by quantum physics beginning in the 1920's. This, as a result in disparate views on the existence of oncology and the character of scientific knowledge that fuelled the conflict between the two.
If there were world enough and time enough, the conflict between each that both could be viewed as an interesting artifact in the richly diverse coordinative systems of higher education. Nevertheless, as the ecological crisis teaches us, the ‘old enough' capable of sustaining the growing number of our life firms and the ‘time enough' that remains to reduce and reverse the damage we are inflicting on this world ae rapidly diminishing. Therefore, put an end to the absurd ‘betweeness' and go on with the business of coordinate human knowledge in the interest of human survival in a new age of enlightenment that could be far more humane and much more enlightened than any has gone before.
It now, which it is, nonetheless, that there are significant advances in our understanding to a purposive mind. Cognitive science is an interdisciplinary approach to cognition that draws primarily on ideas from cognitive psychology, artificial intelligence, linguistics and logic. Some philosophers may be cognitive scientists, and others concern themselves with the philosophy of cognitive psychology and cognitive science. Since inauguration of cognitive science these disciplines have attracted much attention from certain philosophers of mind. This has changed the character of philosophy of mind, and there are areas where philosophical work on the nature of mind is continuous with scientific work. Yet, the problems that make up this field concern the ways of ‘thinking' and ‘mental properties' are those that these problems are standardly and traditionally regarded within philosophy of mind than those that emerge from the recent developments in cognitive science. The cognitive aspect is what has to be understood is to know what would make the sentence true or false. It is frequently identified with the truth cognition of the sentence. Justly as the scientific study of precesses of awareness, thought, and mental organization, often by means of a computer modelling or artificial intelligence research. Contradicted by the evidence, it only has to do with is structure and the way it functioned, that is just because a theory does not mean that the scientific community currently accredits it. Generally, there are many theories, though technically scientific, have been rejected because the scientific evidence is strangely against it. The historical enquiry into the evolution of self-consciousness, developing from elementary sense experience to fully rational, free, thought processes capable of yielding knowledge the presented term, is associated with the work and school of Husserl. Following Brentano, Husserl realized that intentionality was the distinctive mark of consciousness, and saw in it a concept capable of overcoming traditional mind-body dualism. The stud y of consciousness, therefore, maintains two sides: a conscious experience can be regarded as an element in a stream of consciousness, but also as a representative of one aspect or ‘profile' of an object. In spite of Husserl's rejection of dualism, his belief that there is a subject-matter remaining after epoch or bracketing of the content of experience, associates him with the priority accorded to elementary experiences in the parallel doctrine of phenomenalism, and phenomenology has partly suffered from the eclipse of that approach to problems of experience and reality. However, later phenomenologists such as Merleau-Ponty do full justice to the world-involving nature of Phenomenological theories are empirical generalizations of data experience, or manifest in experience. More generally, the phenomenal aspects of things are the aspects that show themselves, than the theoretical aspects that are inferred or posited in order to account for them. They merely described the recurring process of nature and do not refer to their cause or that, in the words of J.S. Mill, ‘objects are the permanent possibilities of sensation'. To inhabit a world of independent, external objects is, on this view, to be the subject of actual and possible orderly experiences. Espoused by Russell, the view issued in a programme of translating talk about physical objects and their locations into talking about possible experience. The attempt is widely supposed to have failed, and the priority the approach gives to experience has been much criticized. It is more common in contemporary philosophy to see experience as itself a construct from the actual way of the world, than the other way round.
Phenomenological theories are also called ‘scientific laws' ‘physical laws' and ‘natural laws.' Newton's third law is one example. It says that every action ha an equal and opposite reaction. ‘Explanatory theories' attempt to explain the observations rather than generalized them. Whereas laws are descriptions of empirical regularities, explanatory theories are conceptual constrictions to explain why the data exit, for example, atomic theory explains why we see certain observations, the same could be said with DNA and relativity, Explanatory theories are particularly helpful in such cases where the entities (like atoms,
DNA . . . ) cannot be directly observed.
What is knowledge? How does knowledge get to have the content it has? The problem of defining knowledge in terms of true belief plus some favoured relation between the believer and the facts begun with Plato, in that knowledge is true belief plus logos, as it is what enables us to apprehend the principle and firms, i.e., an aspect of our own reasoning.
What makes a belief justified for what measures of belief is knowledge? According to most epistemologists, knowledge entails belief, so that to know that such and such is the case. None less, there are arguments against all versions of the thesis that knowledge requires having a belief-like attitude toward the known. These arguments are given by philosophers who think that knowledge and belief or facsimile, are mutually incompatible (the incompatibility thesis) or by ones who say that knowledge does not entail belief, or vice versa, so that each may exist without the other, but the two may also coexist (the separability thesis). The incompatibility thesis that hinged on the equation of knowledge with certainty. The assumption that we believe in the truth of  claim we are not certain about its truth. Given that belief always involves uncertainty, while knowledge never does, believing something rules out the possibility of knowledge knowing it. Again, given to no reason to grant that states of belief are never ones involving confidence. Conscious beliefs clearly involve some level of confidence, to suggest otherwise, that we cease to believe things about which we are completely confident is bizarre.
A. D. Woozley (1953) defends a version of the separability thesis. Woozley's version that deal with psychological certainty than belief per se, is that knowledge can exist without confidence about the item known, although knowledge might also be accompanied by confidence as well. Woozley says, ‘what I can Do, where what I can do may include answering questions.' on the basis of this remark he suggests that even when people are unsure of the truth of a claim, they might know that the claim is true. We unhesitatingly attribute knowledge to people that correct responses on examinations if those people show no confidence in their answers. Woozley acknowledges however, that it would be odd for those who lack confidence to claim knowledge. Saying it would be peculiar, ‘I know it is correct.' but this tension; still ‘I know is correct.' Woozley explains, using a distinction between condition under which are justified in making a claim (such as a claim to know something) and conditioned under which the claim we make is true. While ‘I know such and such' might be true even if I answered whether such and such holds, nonetheless claiming that ‘I know that such should be inappropriate for me and such unless I was sure of the truth of my claim.'
Colin Redford (1966) extends Woozley's defence of the separability thesis. In Redford's view, not only in knowledge compatible with the lacking of certainty, it is also compatible with a complete lack of belief. He argues by example, in this one example, Jean had forgotten that he learned some English history years prior and yet he is able to give several correct responses to questions such as, ‘When did the Battle of Hastings occur?' since he forgot that the battle of Hastings took place in 1066 in history, he considers his correct response to be no more than guesses. Thus when he says that the Battle of Hastings took place in 1066 he would deny having the belief that the Battle of Hasting took place in 1066.
Those who agree with Radford's defence of the separation thesis will probably think of belief as an inner state that can be directed through introspection. That Jean lacks' beliefs out English history are plausible on this Cartesian picture since Jean does not find himself with the belief out of which the English history when with any beliefs about English history when he seeks them out. One might criticize Radford, however, by rejecting the Cartesian view of belief. One could argue that some beliefs are thoroughly unconscious. For example, (1859), according to which having beliefs is a matter of the way people are disposed to behave (and has not Radford already adopted a behaviourist conception of knowledge?). since Jean gives the correct response when queried, a form of verbal behaviour, a behaviourist would be tempted to credit him with the belief that the battle of Hastings occurred in 1066.
Once, again, but the jargon is attributable to different attitudinal values. AS, D. M. Armstrong (1973) makes a different task against Radford. Jean does know that the Battle of Hastings took place in 1066. Armstrong will grant Radford that points, which in fact, Armstrong suggests that Jean believe that 1066 is not the date the Battle of Hastings occur. For Armstrong equates the belief of such and such is just possible bu t no more than just possible with the belief that such and such is not the case. However, Armstrong insists Jean also believe that the Battle did occur in 1066. After all, had Jean been mistaught that the Battle occurred in 1066, and had he forgotten being ‘taught' this and subsequently ‘guessed' that it took place in 10690, we would surely describe the situation as one in which Jean' false belief about te Battle became a memory trace that was causally responsible or his guess. Thus while Jean consciously believes that the Battle did not occur in 1066, unconsciously he does believe it occurred in 1066. So after all, Radford does not have a counterexample to the claim that knowledge entails belief.
Suppose that Jean's memory had been sufficiently powerful to produce the relevant belief. As Radford says, Jan has every reason to suppose that his response is mere guesswork, and so he has every reason to consider his belief false. His belief would be an irrational one, and hence one about whose truth Jean would be ignorant.
The attempt to understand the concepts involved in religious belief, existence, necessity, fate, creation, sun, justice, Mercy, Redemption, God. Until the 20th century the history of western philosophy is closely intertwined with attempts to make sense of aspect of pagan, Jewish or Christian religion, while in other tradition such as Hinduism, Buddhism or Taoism, there is even less distinction between religious and philosophical enquiry. The classic problem of conceiving an appropriate object of religious belief is that of understanding whether any term can be predicated of it: Does it make to any sense of talking about its creating to things, willing event, or being one thing or many? The via negativa of theology is to claim that God can only be known by denying ordinary terms of any application (or them); another influential suggestion is that ordinary term only apply metaphorically, sand that there is in hope of cashing the metaphors. Once a description of a Supreme Being is hit upon, there remains the problem of providing any reason for supposing that anything answering to the description exists. The medieval period was the high-water mark-for purported proofs of the existence of God, such as the Five-Ays of Aquinas, or the ontological argument of such proofs have fallen out of general favour since the 18th century, although theories still sway many people and some philosophers.
Generally speaking, even religious philosophers (or perhaps, they especially) have  been wary of popular manifestations of religion. Kant, himself a friend of religious faith, nevertheless distinguishes various perversions: Theosophy (using transcendental conceptions that confuses reason), demonology (indulging an anthropomorphic, mode of representing the Supreme Being), theurgy (a fanatical delusion that feeling can be communicated from such a being, or that we can exert an influence on it), and idolatry, or a superstition's delusion the one can make oneself acceptable to his Supreme Being by order by means than that of having the moral law at heart (Critique of judgement) these warm conversational tendencies have, however, been increasingly important in modern theology.
Since Feuerbach there has been a growing tendency for philosophy of religion either to concentrate upon the social and anthropological dimension of religious belief, or to treat a manifestation of various explicable psychological urges. Another reaction is retreat into a celebration of purely subjective existential commitments. Still, the ontological arguments continue to attach attention. A modern anti-fundamentalists trends in epistemology are not entirely hostile to cognitive claims based on religious experience.
Still, the problem f reconciling the subjective or psychological nature of mental life with its objective and logical content preoccupied from of which is next of the problem was elephantine Logische untersuchungen (trans. as Logical Investigations, 1070). To keep a subjective and a naturalistic approach to knowledge together. Abandoning the naturalism in favour of a kind of transcendental idealism. The precise nature of his change is disguised by a pechant for new and impenetrable terminology, but the ‘bracketing' of eternal questions for which are to a great extent acknowledged implications of a solipistic, disembodied Cartesian ego s its starting-point, with it thought of as inessential that the thinking subject is ether embodied or surrounded by others. However by the time of Cartesian Meditations (trans. as, 1960, fist published in French as Méditations Carthusianness, 1931), a shift in priorities has begun, with the embodied individual, surrounded by others, than the disembodied Cartesian ego now returned to a fundamental position. The extent to which the desirable shift undermines the programme of phenomenology that is closely identical with Husserl's earlier approach remains unclear, until later phenomenologists such as Merleau -Ponty has worked fruitfully from the later standpoint.
Pythagoras established and was the central figure in school of philosophy, religion, and mathematics: He was apparently viewed by his followers as semi-divine. For his followers the regular solids (symmetrical three-dimensional forms in which all sides are the same regular polygon) with ordinary language.  The language of mathematical and geometric forms seem closed, precise and pure. Providing one understood the axioms and notations, and the meaning conveyed was invariant from one mind to another. The Pythagoreans following which was the language empowering the mind to leap beyond the confusion of sense experience into the realm of immutable and eternal essences. This mystical insight made Pythagoras the figure from antiquity must revered by the creators of classical physics, and it continues to have great appeal for contemporary physicists as they struggle with the epistemological of the quantum mechanical description of nature.
Pythagoras (570 Bc) was the son of Mn esarchus of Samos ut, emigrated (531 Bc) to Croton in southern Italy. Here he founded a religious society, but were forces into exile and died at Metapomtum. Membership of the society entailed self-disciplined, silence and the observance of his  taboos, especially against eating flesh and beans. Pythagoras taught the doctrine of metempsychosis or te cycle of reincarnation, and was supposed ale to remember former existence. The soul, which as its own divinity and may have existed as an animal or plant, can, however gain release by a religious dedication to study, after which it may rejoin the universal world-soul. Pythagoras is usually, but doubtfully, accredited with having discovered the basis of acoustics, the numerical ratios underlying the musical scale, thereby y intimating the arithmetical interpretation of nature. This tremendous success inspired the view that the whole of the cosmos should be explicable in terms of harmonia or number. the view represents a magnificent brake from the Milesian attempt to ground physics on a conception shared by all things, and to concentrate instead on form, meaning that physical nature receives an intelligible grounding in different geometric breaks. The view is vulgarized in the doctrine usually attributed to Pythagoras, that all things are number. However, the association of abstract qualitites with numbers, but reached remarkable heights, with occult attachments for instance, between justice and the number four, and mystical significance, especially of the number ten, cosmologically Pythagoras explained the origin of the universe in mathematical terms, as the imposition of limit on the limitless by a kind of injection of a unit. Followers of Pythagoras included Philolaus, the earliest cosmosologist known to have understood that the earth is a moving planet. It is also likely that the Pythagoreans discovered the irrationality of the square root of two.
The Pythagoreans considered numbers to be among te building blocks of the universe. In fact, one of the most central of the beliefs of Pythagoras mathematihoi, his inner circle, was that reality was mathematical in nature. This made numbers valuable tools, and over time even the knowledge of a number's name came to be associated with power. If you could name something you had a degree of control over it, and to have power over the numbers was to have power over nature.
One, for example, stood for the mind, emphasizing its Oneness. Two was opinion, taking a step away from the singularity of mind. Three was wholeness (a whole needs a beginning, a middle and its  ending to be more than a one-dimensional point), and four represented the stable squareness of justice. Five was marriage-being the sum of three and two, the first odd (male) and even (female) numbers. (Three was the first odd number because the number one was considered by the Greeks to be so special that it could not form part of an ordinary grouping of numbers).
The allocation of interpretations went on up to ten, which for the Pythagoreans was the number of perfections. Not only was it the sum of the first four numbers, but when a series of ten dots are arranged in the sequence 1, 2, 3, 4, . . . each above the next, it forms a perfect triangle, the simplest of the two-dimensional shapes. So convinced were the Pythagoreans of the importance of ten that they assumed there had to be a tenth body in the heavens on top of the known ones, an anti-Earth, never seen as it was constantly behind the Sun. This power of the number ten, may also have linked with ancient Jewish thought, where it appears in a number of guised the ten commandments, and the ten the components are of the Jewish mystical cabbala tradition.
Such numerology, ascribed a natural or supernatural significance to number, can also be seen in Christian works, and continued in some new-age tradition. In the Opus majus, written in 1266, the English scientist-friar Roger Bacon wrote that: ‘Moreover, although a manifold perfection of number is found according to which ten is said to be perfect, and seven, and six, yet most of all does three claim itself perfection'.
Ten, we have already seen, was allocated to perfection. Seven was the number of planets according to the ancient Greeks, while the Pythagoreans had designated the number as the universe. Six also has a mathematical significance, as Bacon points out, because if you break it down into te factor that can be multiplied together to make it-one, two, and three-they also add up to six:
1 x 2 x 3 = 6 = 1 + 2 + 3
Such was the concern of the Pythagoreans to keep irrational numbers to themselves, bearing in mind, it might seem amazing that the Pythagoreans could cope with the values involved in this discovery. After all, as the square root of 2 cannot be represented by a ratio, we have to use a decimal fraction to write it out. It would be amazing, were it true that the Greeks did have a grasp for the existence of irrational numbers as a fraction. In fact, though you might find it mentioned that the Pythagoreans did, to talk about them understanding numbers in its way, totally misrepresented the way they thought.
At this point, as occupied of a particular place in space, and giving the opportunity that our view presently becomes fused with Christian doctrine when logos are God's instrument in the development (redemption) of the world. The notion survives in the idea of laws of nature, if these conceived of as independent guides of the natural course of events, existing beyond the temporal world that they order. The theory of knowledge and its central questions include the origin of knowledge, the place of experience in generating knowledge, and the place of reason in doing so, the relationship between knowledge and certainty, not between knowledge and the impossibility of error, the possibility of universal scepticism, sand the changing forms of knowledge that arise from new conceptualizations of the world and its surrounding surfaces.
As, anyone group of problems concerns the relation between mental and physical properties. Collectively they are called ‘the mind-body problem ‘ this problem is of its central questioning of philosophy of mind since Descartes formulated in the three centuries past, for many people understanding the place of mind in nature is the greatest philosophical problem. Mind is often thought to be the last domain that stubbornly resists scientific understanding, and philosophers differ over whether they find that a cause for celebration or scandal, the mind-body problem in the modern era was given its definitive shape by Descartes, although the dualism that he espoused is far more widespread and far older, occurring in some form wherever there is a religious or philosophical tradition by which the soul may have an existence apart from the body. While most modern philosophers of mind would reject the imaginings that lead us to think that this makes sense, there is no consensus over the way to integrate our understanding people a bearer s of physical proper ties on the one hand and as subjects of mental lives on the other.
As the motivated convictions that discoveries of non-locality have more potential to transform our conceptions of the ‘way things are' than any previous discovery, it is, nonetheless, that these implications extend well beyond the domain of the physical sciences, and the best efforts of many thoughtful people will be required to understand them.
Perhaps the most startling and potentially revolutionary of these implications in human terms is the view in the relationship between mind and world that is utterly different from that sanctioned by classical physics. René Descartes, for reasons of the moment, was among the first to realize that mind or consciousness in the mechanistic world-view of classical physics appeared to exist in a realm separate and the distinction drawn upon ‘self-realisation' and ‘undivided wholeness' he lf within the form of nature. Philosophy quickly realized that there was nothing in this view of nature that could explain or provide a foundation for the mental, or for all that we know from direct experience and distinctly human. In a mechanistic universe, he said, there is no privileged place or function for mind, and the separation between mind and matter is absolute. Descartes was also convinced, however, that the immaterial essences that gave form and structure to this universe were coded in geometrical and mathematical ideas, and this insight led him to invent algebraic geometry.
Decanters' theory of knowledge starts with the quest for certainty, for an indubitable starting-point or foundation on the basis alone of which progress is possible, sometimes known as the use of hyperbolic (extreme) doubt, or Cartesian doubt. This is the method of investigating how much knowledge and its basis in reason or experience used by Descartes in the first two Meditations. This is eventually found in the celebrated ‘Cogito ergo sum': I think therefore I am. By finding the point of certainty in my own awareness of my own self, Descartes gives a first-person twist to the theory of knowledge that dominated the following centuries in spite of various counter attacks for social and public starting-point. The metaphysic associated with this priority is the famous Cartesian dualism, or separation of mind and matter into two different but interacting substances. Descartes rigorously and rightly understands the presence of divine dispensation to certify any relationship between the two realms thus divided, and to prove the reliability of the senses invoked a ‘clear and distinct perception' of highly dubious proofs of the existence of a benevolent deity. This has not met general acceptance: As Hume drily puts it, ‘to have recourse to the veracity of the supreme Being, to prove the veracity of our senses, is surely making a very unexpected circuit.'
In his own time Descartes's conception of the entirely separate substance of the mind was recognized to precede to insoluble problems of nature of the causal connection between the two systems running in parallel. When I stub my toe, this does not cause pain, but there is a harmony between the mental and the physical (perhaps, due to God) that ensure that there will be a simultaneous pain; when I form an intention and then act, the same benevolence ensures that my action is appropriate to my intention, or if it be to  desire its resultant intention be of an action to be performed on the causal effect of some unreasonable belief. The theory has never been wildly popular, and in its application to mind-body problems many philosophers would say that it was the result of a misconceived ‘Cartesian dualism,' it of ‘subjective knowledge' and ‘physical theory.'
It also produces the problem, insoluble in its own terms, of ‘other minds.' Descartes's notorious denial that nonhuman animals are conscious is a stark illustration of the problem. In his conception of matter Descartes also gives preference to rational cogitation over anything derived from the senses. Since we can conceive of the matter of a ball of wax surviving changes to its sensible qualities, matter is not an empirical concept, but eventually an entirely geometrical one, with extension and motion as its only physical nature. Descartes' s thought is reflected in Leibniz's view, held later by Russell, that the qualities of sense experience have no resemblance to qualities of things, so that knowledge of the external world is essentially knowledge of structure than of filling. On this basis Descartes builds a remarkable physics. Since matter is in effect the same as extension there can be no empty space or ‘void,' since there is no empty space motion is not a question of occupying previously empty space, but is to be thought of through vortices (like the motion of a liquid).
Although the structure of Descartes's epistemology, theory of mind, and theory of matter have been rejected often, their relentless exposures of the hardest issues, their exemplary clarity, and even their initial plausibility, all contrive to make him the central point of reference for modern philosophy.
A scientific understanding of these ideas could be derived, said, Descartes, with the aid of precise deduction, and has also claimed that the contours of physical reality could be laid out in three-dimensional coordinates. Following the publication of Isaac Newton's Principia Mathematica in 1687, reductionism and mathematical modelling became the most powerful tools of modern science. The dream that the entire physical world could be known and mastered through the extension and refinement of mathematical theory became the central feature and principle of scientific knowledge.
The radical separation between mind and nature formalized by Descartes served over time to allow scientists to concentrate on developing mathematical descriptions of matter as pure mechanisms without any concerns about its spiritual dimensions or ontological foundations. Meanwhile, attempts to rationalize, reconcile or eliminate Descartes's stark division between mind and matter became the most central feature of Western intellectual life.
Philosophers like John Locke, Thomas Hobbes, and David Hume tried to articulate some basis for linking the mathematical describable motions of matter with linguistic representations of external reality in the subjective space of mind. Descartes' compatriot Jean-Jacques Rousseau reified nature as the ground of human consciousness in a state of innocence and proclaimed that ‘Liberty, Equality, Fraternities' are the guiding principals of this consciousness. Rousseau also given rythum to cause an endurable god-like semblance so that the idea of the ‘general will' of the people to achieve these goals and declared that those who do no conform to this will were social deviants.
The Enlightenment idea of deism, which imaged the universe as a clockwork and God as the clockmaker, provided grounds for believing in a divine agency at the moment of creation. It also implied, however, that all the creative forces of the universe were exhausted at origins, that the physical substrates of mind were subject to the same natural laws as matter, and that the only means of mediating the gap between mind and matter was pure reason. Traditional Judeo-Christian theism, which had previously been based on both reason and revelation, responding to the challenge of deism by debasing rationality as a test of faith and embracing the idea that the truths of spiritual reality can be known only through divine revelation. This engendered a conflict between reason and revelation that persists to this day. It also laid the foundation for the fierce competition between the mega-narratives of science and religion as frame tales for mediating relations between mind and matter and the manner in which the special character of each should be ultimately defined.
Rousseau's attempt to posit a ground for human consciousness by reifying nature was revived in a different form by the nineteenth-century Romantics in Germany, England, and the United States. Goethe and Friedrich Schelling proposed a natural philosophy premised on ontological monism (the idea that God, man, and nature are grounded in an indivisible spiritual Oneness) and argued for the reconciliation of mind and matter with an appeal to sentiment, mystical awareness, and quasi-scientific musings. In Goethe's attempt to wed mind and matter, nature becomes a mindful agency that ‘loves illusion,' ‘shrouds man in mist,' ‘presses him to her heart,' and punishes those who fail to see the ‘light.' Schelling, in his version of cosmic unity, argues that scientific facts were at best partial truths and that the mindful dualism spirit that unites mind and matter is progressively moving toward self-realization and undivided wholeness.
The flaw of pure reason is, of course, the absence of emotion, an external reality had limited appeal outside the community of intellectuals. The figure most responsible for infusing our understanding of Cartesian dualism with emotional content was the death of God theologian Friedrich Nietzsche after declaring that God and ‘divine will' does not exist, verifiably, literature puts forward, it is the knowledge that God is dead. The death of God he calls the greatest events in modern history and the cause of extremer danger. Yet, the paradox contained in these words. He never said that there was no God, but the Eternal had been vanquished by Time and that the Immortal suffered death at the hands of mortals. ‘God is dead'. It is like a cry mingled of despair and triumph, reducing, by comparison, the whole story of atheism agnosticism before and after him to the level of respectable mediocrity and making it sound like a collection of announcements who in regret are unable to invest in an unsafe proposition:-this is the very essence of Nietzsche's spiritual core of existence, and what flows is despair and hope in a new generation of man, visions of catastrophe and glory, the icy brilliance of analytical reason, fathoming with affected irreverence those depths until now hidden by awe and fear, and side-by-side, with it, ecstatics invocations of as ritual healer.
Nietzsche reified for ‘existence' of consciousness in the domain of subjectivity as the ground for individual ‘will' and summarily dismissed all previous philosophical attempts to articulate the ‘will to truth.' The problem, claimed Nietzsche, is that earlier versions of the ‘will to truth' disguise the fact that all alleged truths were arbitrarily created in the subjective reality of the individual and are expressions or manifestations of individual ‘will.'
In Nietzsche's view, the separation between ‘mind' and ‘matter' is more absolute and total had previously been imagined. Based on the assumptions that there are no really necessary correspondences between linguistic constructions of reality in human subjectivity and external reality, he declared that we are all locked in ‘a prison house of language.' The prison as he conceived it, however, was also a ‘space' where the philosopher can examine the ‘innermost desires of his nature' and articulate a new message of individual existence founded on will.
Those who fail to enact their existence in this space, says Nietzsche, are enticed into sacrificing their individuality on the nonexistent altars of religious beliefs and democratic or socialist ideals and become, therefore, members of the anonymous and docile crowd. Nietzsche also invalidated the knowledge claims of science in the examination of human subjectivity. Science, he said, not only exalted natural phenomena and favours reductionistic examinations of phenomena at the expense of mind. It also seeks to reduce mind to a mere material substance, and by that to displace or subsume the separateness and uniqueness of mind with mechanistic descriptions that disallow a basis for the free exercise of individual will.
Nietzsche's emotionally charged defence of intellectual freedom and his radical empowerment of mind as the maker and transformer of the collective fictions that shape human reality in a soulless scientific universe proved terribly influential on twentieth-century thought. Nietzsche sought to reinforce his view on subjective character of scientific knowledge by appealing to an epistemological crisis over the foundations of logic and arithmetic that arose during the last three decades of the nineteenth century. As it turned out, these efforts resulted in paradoxes of recursion and self-reference that threatened to undermine both the efficacy of this correspondence and the privileged character of scientific knowledge.
Nietzsche appealed to this crisis in an effort to reinforce his assumption that, without onotology, all knowledge (including scientific knowledge) was grounded only in human consciousness. As the crisis continued, a philosopher trained in higher mathematics and physics, Edmund Husserl, attempted to preserve the classical view of correspondence between mathematical theory and physical reality by deriving the foundation of logic and number from consciousness in ways that would preserve self-consistency and rigour. It represented a direct link between these early challenges and the efficacy of classical epistemology and the tradition in philosophical thought that culminated in philosophical postmodernism.
Since Husserl's epistemology, like that of Descartes and Nietzsche, was grounded in human subjectivity, a better understanding of his attempt to preserve the classical view of correspondence not only reveals more about the legacy of Cartesian duality. It also suggests that the hidden onotology of classical epistemology was more responsible for the deep division and conflict between the two cultures of humanists-social scientists and scientists-engineers than we has preciously imagined. The central question in this late-nineteenth-century debate over the status of the mathematical description of nature as the following: Is the foundation of number and logic grounded in classical epistemology, or must we assume, without any ontology, that the rules of number and logic are grounded only in human consciousness? In order to frame this question in the proper context, it should first examine in more detail that intimate and ongoing dialogue between physics and metaphysics in Western thought.
Through a curious course of events, attempts by Edmund Husserl, a philosopher trained in higher math and physics to resolve this crisis resulted in a view of the character of human consciousness that closely resembled that of Nietzsche.
For Nietzsche, however, all the activities of human consciousness share the predicament of psychology. There can be, for him, no ‘pure' knowledge, only satisfaction, however sophisticated, of an ever-varying intellectual need of the will to know. He therefore demands that man should accept moral responsibility for the kind of questioned he asks, and that he should realize what values are implied in he answers he asks-and in this he was more Christian than all our post-Faustian Fausts of truth and scholarship. ‘The desire for truth,' he says, ‘is itself in need of critique. Let this be the definition of my philosophical task. By way of excrement, one will question for oneself the value of truth.' and does he not. He protests that, in an age that is as uncertain of its values as is his and ours, the search for truth will issue in the similarly of trivialities or-catastrophe. We might wonder how he would react to the pious hope of our day that the intelligence and moral conscience of politicians will save the world from the disastrous products of our scientific explorations and engineering skills. It is perhaps not too difficult to guess; for he knew that there was a fatal link between the moral resolution of scientists to follow the scientific search wherever, by its own momentum, it will take us, and te moral debility of societies not altogether disinclined to ‘apply' the results, however catastrophic, believing that there was a hidden identity among all the expressions of the ‘Will to Power', he saw the element of moral nihilism in the ethics of our science: Its determination not to let ‘higher values' interfere with its highest value -Truth (as it conceives it). Thus he said that the goal of knowledge pursued by the natural sciences means perdition.
In these regions of his mind dwells the terror that he may have helped to bring about the very opposite of what he desired. When this terror comes to the force, he is much afraid of the consequences of his teaching. Perhaps, the best will be driven to despair by it, the very worst accept it? Once he put into the mouth of some imaginary titanic genius what is his most terrible prophetic utterance: ‘Oh grant madness, your heavenly powers, madness that at last I may believe in myself,
. . . I am consumed by doubts, for I have killed the Law. If I am not more than the Law, then I am the most abject of all men'.
Still ‘God is dead,' and, sadly, that he had to think the meanest thought: He saw in the real Christ an illegitimate son of the Will to power, a flustrated rabbi sho set out to save himself and the underdog human from the intolerable strain of importantly resending the Caesars-not to be Caesar was now proclaimed a spiritual disjunction-a newly invented form of power, the power to be powerless.
It is the knowledge that God is dead, and suffered death at the hands of mortals: ‘God is dead': It is like a cry mingled of despair ad triumph, reducing the whole story of theism nd agnosticism before and after him to the level of respectable mediocrity nd masking it sound like a collection of announcement. Nietzsche, for the nineteenth century, brings to its perverse conclusion a line of religious thought and experience linked with the names of St. Paul, St. Augustin, Pascal, Kierkegaard, and Dostoevsky, minds for whom God was not simply the creator of an order of nature within which man has his clearly defined place, but to whom He came in order to challenge their natural being, masking demands that appeared absurd in the light of natural reason. These men are of the family of Jacob: Having wrestled with God for His blessing, they ever after limp through life with the framework of Nature incurably out of joint. Nietzsche is just a wrestler, except within him the shadow of Jacob merges with the shadow of Prometheus. Like Jacob, Nietzsche too believed that he prevailed against God in that struggle, and won a new name for himself, the name of Zarathustra. Yet the words he spoke on his mountain to the angle of the Lord were: ‘I will not let thee go, but thou curse me.' Or, in words that Nietzsche did in fact speak: ‘I have on purpose devoted my life to exploring the whole contrast to a truly religious nature. I know the Devil and all his visions of God.' ‘God is dead,' is the very core of Nietzsche's spiritual existence, and what follows is despair and hope in a new greatness of man.
Further to issues are the best-known disciple that Husserl was Martin Heidegger, and the work of both figures greatly influenced that of the French atheistic existentialist Jean-Paul Sartre. His first novel, La Nausée, was published in 1938 (trans. As Nausea, 1949). L  Imginaire (1940, trans. as The Psychology of the Imagination, 1948) is a contribution to phenomenal psychology. Briefly captured by the Germans, Sartre spent the ending of war years in Paris, where L  Être et le néant, his major purely philosophical work, was published in 1945 (trans. as Being and Nothingness, 1956). The lecture L  Existentialisme est un humanisme (1946, trans. as Existentialism is a Humanism, 1947) consolidated Sartre's position as France's leading existentialist philosopher.
Sartre's philosophy is concerned entirely with the nature of human life, and the structures of consciousness. As a result it gains expression in his novels and plays as well as in more orthodox academic treatises. Its immediate ancestors is the Phenomenological tradition of his teachers, and Sartre can most simply be seen as concerned to rebut the charge of idealism as it is laid at the door of phenomenology. The agent is not a spectator of the world, but, like everything in the world, constituted by acts of intentionality. The self constituted is historically situated, but as an agent whose own mode of finding itself in the world makes for responsibility and emotion. Responsibility is, however, a burden that we cannot frequently bear, and bad faith arises when we deny our own authorship of our actions, seeing then instead as forced responses to situations not of our own making.
Sartre thus locates the essential nature of human existence in the capacity for choice, although choice, being equally incompatible with determinism and with the existence of a Kantian moral law, implies a synthesis of consciousness (being for-itself) and the objective(being in-itself) that is forever unstable. The unstable and constantly disintegrating nature of free-will generates anguish. For Sartre our capacity to make negative judgement is one fundamental puzzles of consciousness. Like Heidegger he took the ‘ontological' approach of relating to the nature of nonbeing, a move that decisively differentiated him from the Anglo-American tradition of modern logic.
The work of Husserl, Heidegger and Sartre became foundational to that of the principal architects of philosophical postmodernism, Deconstructionists Jacques Lacan, Roland Barthes, Michel Foucault, and Jacqures Derrida. This direct linkage among the nineteenth-century crises about the epistemological foundations of mathematical physics and the origins of philosophical postmodernism served to perpetrate the Cartesian two world dilemmas in an even more oppressive form.
The American envisioned a unified spiritual reality that manifested itself as a personal ethos that sanctioned radical individualism and bred aversion to the emergent materialism of the Jacksonian era. They were also more inclined than their European counterpart, as the examples of Thoreau and Whitman attest, to embrace scientific descriptions of nature. However, the Americans also dissolved the distinction between mind and natter with an appeal to ontological monism and alleged that mind could free itself from all the constraint of assuming that by some sorted limitation of matter, in which such states have of them, some mystical awareness.
Since scientists, during the nineteenth century were engrossed with uncovering the workings of external reality and seemingly knew of themselves that these virtually overflowing burdens of nothing, in that were about the physical substrates of human consciousness, the business of examining the distributive contribution in dynamic functionality and structural foundation of mind became the province of social scientists and humanists. Adolphe Quételet proposed a ‘social physics' that could serve as the basis for a new discipline called sociology, and his contemporary Auguste Comte concluded that a true scientific understanding of the social reality was quite inevitable. Mind, in the view of these figures, was a separate and distinct mechanism subject to the lawful workings of a mechanical social reality.
Nonetheless, even like Planck and Einstein understood and embraced hoboism as an inescapable condition of our physical existence. According to Einstein's general relativity theory, wrote Planck, ‘each individual particle of a system in a certain sense, at any one time, exists simultaneously in every part of the space occupied by the system'. The system, as Planck made clear, is the entire cosmos. As Einstein put it, ‘physical reality must be described in terms of continuos functions in space. The material point, therefore, can hardly be conceived any more as the basic concept of the theory.'
More formal European philosophers, such as Immanuel Kant, sought to reconcile representations of external reality in mind with the motions of matter-based on the dictates of pure reason. This impulse was also apparent in the utilitarian ethics of Jerry Bentham and John Stuart Mill, in the historical materialism of Karl Marx and Friedrich Engels, and in the pragmatism of Charles Smith, William James and John Dewey. These thinkers were painfully aware, however, of the inability of reason to posit a self-consistent basis for bridging the gap between mind and matter, and each remains obliged to conclude that the realm of the mental exists only in the subjective reality of the individual.
The mechanistic paradigm of the late nineteenth century was the one Einstein came to know when he studied physics. Most physicists believed that it represented an eternal truth, but Einstein was open to fresh ideas. Inspired by Mach's critical mind, he demolished the Newtonian ideas of space and time and replaced them with new, ‘relativistic' notions.
As for Newton, a British mathematician , whereupon the man Hume called Newton, ‘the greatest and rarest genius that ever arose for the ornament and instruction of the species.' His mathematical discoveries are usually dated to between 1665 and 1666, when he was secluded in Lincolnshire, the university being closed because of the plague His great work, the Philosophae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy, usually referred to as the Principia), was published in 1687.
Yet throughout his career, Newton engaged in scientific correspondence and controversy. The often-quoted remark, ‘If I have seen further it is by standing on the shoulders of Giant's occurs in a conciliatory letter to Robert Hooke (1635-1703). Newton was in fact echoing the remark of Bernard of Chartres in 1120: ‘We are dwarfs standing on the shoulders of giants'. The dispute with Leibniz over the invention of the calculus is his best-known quarrel, and abound with restrictive limitation that gave specified allowances given to spiritual insight under which for Newton himself, did appoint the committee of the Royal Society that judged the question of precedence, and then writing the report, the Commercium Epistolicum, awarding himself the victory. Although was himself of the ‘age of reason,' Newton was himself interested in alchemy, prophesy, gnostic, wisdom and theology,