PHYSICAL SCIENCE

the systematic study of the inorganic world, as distinct from the study of the organic world, which is the province of biological science. Physical science is ordinarily thought of as consisting of four broad areas: astronomy, physics, chemistry, and the Earth sciences. Each of these is in turn divided into fields and subfields. Physics is the basic physical science. It deals with the structure and behaviour of individual atoms and their components, as well as with the different forces of nature and their relationships. It also is concerned with the physical properties of matter and with such phenomena as electricity and magnetism. Its goal is the formulation of comprehensive principles that summarize disparate phenomena in the most general way possible and that are expressed with economy and precision in the language of mathematics. The principal subject areas of physics are mechanics, gravitation, thermodynamics and heat, electricity and magnetism, optics, atomic and chemical physics, condensed-matter physics, nuclear physics, particle physics, quantum mechanics, relativistic mechanics, conservation laws and symmetry, and fundamental fields and forces. Chemistry focuses on the properties and reactions of molecules. Broadly speaking, it tends to concentrate on the specific properties of different elements and compounds, as opposed to physics which is chiefly concerned with the general properties of matter as a whole. The principal divisions of chemistry are analytical chemistry, inorganic chemistry, organic chemistry, biochemistry, polymer chemistry, physical chemistry, and industrial chemistry. Astronomy entails the study of the entire universe beyond the Earth. It includes investigations of the gross physical properties of the Earth (e.g., mass and rotation period) primarily as they relate to interactions with other components of the solar system. Most other aspects are dealt with by the Earth sciences (q.v.). Modern astronomy includes astrophysics, the application of physical and chemical knowledge to the study of cosmic objects and the physical processes that control their formation, evolution, and emission of radiant energy. It also encompasses cosmology, the study of the structure and evolution of the universe. the systematic study of the inorganic world, as distinct from the study of the organic world, which is the province of biological science. Physical science is ordinarily thought of as consisting of four broad areas: astronomy, physics, chemistry, and the Earth sciences. Each of these is in turn divided into fields and subfields. This article discusses the historical developmentwith due attention to the scope, principal concerns, and methodsof the first three of these areas. The Earth sciences are discussed in a separate article. Physics, in its modern sense, was founded in the mid-19th century as a synthesis of several older sciencesnamely, those of mechanics, optics, acoustics, electricity, magnetism, heat, and the physical properties of matter. The synthesis was based in large part on the recognition that the different forces of nature are related and are, in fact, interconvertible because they are forms of energy. The boundary between physics and chemistry is somewhat arbitrary. As it has developed in the 20th century, physics is concerned with the structure and behaviour of individual atoms and their components, while chemistry deals with the properties and reactions of molecules. These latter depend on energy, especially heat, as well as on atoms; hence, there is a strong link between physics and chemistry. Chemists tend to be more interested in the specific properties of different elements and compounds, whereas physicists are concerned with general properties shared by all matter. Astronomy is the science of the entire universe beyond the Earth; it includes the Earth's gross physical properties, such as its mass and rotation, insofar as they interact with other bodies in the solar system. Until the 18th century, astronomers were concerned primarily with the Sun, Moon, planets, and comets. During the last two centuries, however, the study of stars, galaxies, nebulas, and the interstellar medium has become increasingly important. Celestial mechanics, the science of the motion of planets and other solid objects within the solar system, was the first testing ground for Newton's laws of motion and thereby helped to establish the fundamental principles of classical (that is, pre-20th-century) physics. Astrophysics, the study of the physical properties of celestial bodies, arose during the 19th century and is closely connected with the determination of the chemical composition of those bodies. In the 20th century physics and astronomy have become more intimately linked through cosmological theories, especially those based on the theory of relativity. Additional reading A. Rupert Hall and Marie Boas Hall, A Brief History of Science (1964, reprinted 1988), provides a good introduction to the subject; A.E.E. McKenzie, The Major Achievements of Science, 2 vol. (1960, reprinted 1988), concentrates on developments from the 16th century, with brief extracts from original sources; and Cecil J. Schneer, The Search for Order: The Development of the Major Ideas in the Physical Sciences from the Earliest Times to the Present (1960, reissued 1984 as The Evolution of Physical Science), accounts for the developments from the 17th through the 19th centuries. Comprehensive surveys include Stephen F. Mason, A History of the Sciences, new rev. ed. (1962); and Stephen Toulmin and June Goodfield, The Architecture of Matter (1962, reissued 1982). Thomas S. Kuhn, The Structure of Scientific Revolutions, 2nd enl. ed. (1970), presents the paradigm theory of science, based on historical examples; Gerald Holton, Thematic Origins of Scientific Thought: Kepler to Einstein, rev. ed. (1988), offers a new interpretation of the history of science, with case studies of the work of Einstein and others; and I. Bernard Cohen, Revolution in Science (1985), is a comparative study. See also Stephen G. Brush, The History of Modern Science: A Guide to the Second Scientific Revolution, 18001950 (1988). Charles Coulston Gillispie (ed.), Dictionary of Scientific Biography, 16 vol. (197080), is an excellent source for authoritative biographical data. For references to the scholarly literature in the history of science, consult the Critical Bibliography of the History of Science and Its Cultural Influences, an annual feature in Isis, an international review of the history of science. Cumulations of this bibliography appeared as Magda Whitrow (ed.), Isis Cumulative Bibliography: 191365, 6 vol. (197184); and John Neu (ed.), Isis Cumulative Bibliography 19661975, 2 vol. (198085). Developments and trends of the 20th century Astronomy Some of the most spectacular advances in modern astronomy have come from research on the large-scale structure and development of the universe. This research goes back to William Herschel's observations of nebulas at the end of the 18th century. Some astronomers considered them to be island universeshuge stellar systems outside of and comparable to the Milky Way Galaxy, to which the solar system belongs. Others, following Herschel's own speculations, thought of them simply as gaseous cloudsrelatively small patches of diffuse matter within the Milky Way Galaxy, which might be in the process of developing into stars and planetary systems, as described in Laplace's nebular hypothesis. In 1912 Vesto Melvin Slipher began at the Lowell Observatory in Arizona an extensive program to measure the velocities of nebulas, using the Doppler shift of their spectral lines. (Doppler shift is the observed change in wavelength of the radiation from a source that results from the relative motion of the latter along the line of sight.) By 1925 he had studied about 40 nebulas, most of which were found to be moving away from the Earth according to the red shift (displacement toward longer wavelengths) of their spectra. Although the nebulas were apparently so far away that their distances could not be measured directly by the stellar parallax method, an indirect approach was developed on the basis of a discovery made in 1908 by Henrietta Swan Leavitt at the Harvard College Observatory. Leavitt studied the magnitudes (apparent brightnesses) of a large number of variable stars, including the type known as Cepheid variables. Some of them were close enough to have measurable parallaxes so that their distances and thus their intrinsic brightnesses could be determined. She found a correlation between brightness and period of variation. Assuming that the same correlation holds for all stars of this kind, their observed magnitudes and periods could be used to estimate their distances. In 1923 the American astronomer Edwin P. Hubble identified a Cepheid variable in the so-called Andromeda Nebula. Using Leavitt's periodbrightness correlation, Hubble estimated its distance to be approximately 900,000 light-years. Since this was much greater than the size of the Milky Way system, it appeared that the Andromeda Nebula must be another galaxy (island universe) outside of our own. In 1929 Hubble combined Slipher's measurements of the velocities of nebulas with further estimates of their distances and found that on the average such objects are moving away from the Earth with a velocity proportional to their distance. Hubble's velocitydistance relation suggested that the universe of galactic nebulas is expanding, starting from an initial state about 2,000,000,000 years ago in which all matter was contained in a fairly small volume. Revisions of the distance scale in the 1950s and later increased the Hubble age of the universe to more than 10,000,000,000 years. Calculations by Aleksandr A. Friedmann in the Soviet Union, Willem de Sitter in The Netherlands, and Georges Lematre in Belgium, based on Einstein's general theory of relativity, showed that the expanding universe could be explained in terms of the evolution of space itself. According to Einstein's theory, space is described by the non-Euclidean geometry proposed in 1854 by the German mathematician G.F. Bernhard Riemann. Its departure from Euclidean space is measured by a curvature that depends on the density of matter. The universe may be finite, though unbounded, like the surface of a sphere. Thus the expansion of the universe refers not merely to the motion of extragalactic stellar systems within space but also to the expansion of the space itself. The beginning of the expanding universe was linked to the formation of the chemical elements in a theory developed in the 1940s by the physicist George Gamow, a former student of Friedmann who had emigrated to the United States. Gamow proposed that the universe began in a state of extremely high temperature and density and exploded outwardthe so-called big bang. Matter was originally in the form of neutrons, which quickly decayed into protons and electrons; these then combined to form hydrogen and heavier elements. Gamow's students Ralph Alpher and Robert Herman estimated in 1948 that the radiation left over from the big bang should by now have cooled down to a temperature just a few degrees above absolute zero (0 K, or -459 F). In 1965 the predicted cosmic background radiation was discovered by Arno A. Penzias and Robert W. Wilson of the Bell Telephone Laboratories as part of an effort to build sensitive microwave-receiving stations for satellite communication. Their finding provided unexpected evidence for the idea that the universe was in a state of very high temperature and density sometime between 10,000,000,000 and 20,000,000,000 years ago. Evolution of stars and formation of chemical elements Just as the development of cosmology relied heavily on ideas from physics, especially Einstein's general theory of relativity, so did theories of stellar structure and evolution depend on discoveries in atomic physics. These theories also offered a fundamental basis for chemistry by showing how the elements could have been synthesized in stars. The idea that stars are formed by the condensation of gaseous clouds was part of the 19th-century nebular hypothesis (see above). The gravitational energy released by this condensation could be transformed into heat, but calculations by Hermann von Helmholtz and Lord Kelvin indicated that this process would provide energy to keep the Sun shining for only about 20,000,000 years. Evidence from radiometric dating, starting with the work of the British physicist Ernest Rutherford in 1905, showed that the Earth is probably several billion years old. Astrophysicists were perplexed: what source of energy has kept the Sun shining for such a long time? In 1925 Cecilia Payne, a graduate student from Britain at Harvard College Observatory, analyzed the spectra of stars using statistical atomic theories that related them to temperature, density, and composition. She found that hydrogen and helium are the most abundant elements in stars, though this conclusion was not generally accepted until it was confirmed four years later by the noted American astronomer Henry Norris Russell. By this time Prout's hypothesis that all the elements are compounds of hydrogen had been revived by physicists in a somewhat more elaborate form. The deviation of atomic weights from exact integer values (expressed as multiples of hydrogen) could be explained partly by the fact that some elements are mixtures of isotopes with different atomic weights and partly by Einstein's relation between mass and energy (taking account of the binding energy of the forces that hold together the atomic nucleus). The German physicist Werner Heisenberg proposed in 1932 that, whereas the hydrogen nucleus consists of just one proton, all heavier nuclei contain protons and neutrons. Since a proton can be changed into a neutron by fusing it with an electron, this meant that all the elements could be built up from protons and electronsi.e., from hydrogen atoms. In 1938 the German-born physicist Hans Bethe proposed the first satisfactory theory of stellar energy generation based on the fusion of protons to form helium and heavier elements. He showed that once elements as heavy as carbon had been formed, a cycle of nuclear reactions could produce even heavier elements. Fusion of hydrogen into heavier elements would also provide enough energy to account for the Sun's energy generation over a period of billions of years. Although Bethe's theory, as extended by Fred Hoyle, Edwin E. Salpeter, and William A. Fowler, is the best one available, there is still some doubt about its accuracy because the neutrinos supposedly produced by the fusion reactions have not been observed in the amounts predicted. According to the theory of stellar evolution developed by the Indian-born American astrophysicist Subrahmanyan Chandrasekhar and others, a star will become unstable after it has converted most of its hydrogen to helium and may go through stages of rapid expansion and contraction. If the star is much more massive than the Sun, it will explode violently, giving rise to a supernova. The explosion will synthesize heavier elements and spread them throughout the surrounding interstellar medium, where they provide the raw material for the formation of new stars and eventually of planets and living organisms. After a supernova explosion, the remaining core of the star may collapse further under its own gravitational attraction to form a dense star composed mainly of neutrons. This so-called neutron star, predicted theoretically in the 1930s by the astronomers Walter Baade and Fritz Zwicky, is apparently the same as the pulsar (a source of rapid, very regular pulses of radio waves), discovered in 1967 by Jocelyn Bell of the British radio astronomy group under Antony Hewish at Cambridge University. More massive stars may undergo a further stage of evolution beyond the neutron star: they may collapse to a black hole, in which the gravitational force is so strong that even light cannot escape. The black hole as a singularity in an idealized space-time universe was predicted from the general relativity theory by the German astronomer Karl Schwarzschild in 1916. Its role in stellar evolution was later described by the American physicists J. Robert Oppenheimer and John Wheeler. During the 1980s, possible black holes were thought to have been located in X-ray sources and at the centre of certain galaxies.