Physics This entry includes 4 subentries:
Overview
High-Energy Physics
Nuclear Physics
Solid-State Physics
Overview
From the colonial period through the early nineteenth century, physics, which was then a branch of natural philosophy, was practiced by only a few Americans, virtually none of whom earned his living primarily in research. Some, like John Winthrop at Harvard, were college professors who were expected and encouraged only to teach. Others were gentlemanly amateurs with private laboratories. The physics of that day ranged from astronomy and navigation to pneumatics, hydrostatics, mechanics, and optics. In virtually all these subjects Americans followed the intellectual lead of Europeans, especially the British. As well as the practitioners of other sciences, they were also inspired by the English philosopher Francis Bacon, who had urged scholars to study the facts of nature and had taught that knowledge led to power. Thus, American physicists emphasized the accumulation of experimental facts rather than mathematical theorizing, and they made no distinction between abstract and practical research, or what a later generation would call pure and applied science. The archetypal American physicist was Benjamin Franklin, the retired printer, man of affairs, and deist, who was celebrated for his practical lightning rod as well as for his speculative and experimental contributions to electrical science.
Nineteenth Century
From the Jacksonian era through the Civil War, American physics became more specialized, with its subject matter narrowing to geophysics, meteorology, and such topics of physics proper as the older mechanics and the newer heat, light, electricity, and magnetism. The leading American physicist of the period was Joseph Henry, who discovered electromagnetic induction while teaching at the Albany Academy in Albany, New York. Later he became a professor at Princeton and then the first secretary of the Smithsonian Institution. Imbibing the nationalism of the day, Henry worked to advance the study of physics and, indeed, of all science in America. With Henry's support, Alexander Dallas Bache, Franklin's great-grandson and the director of the U.S. Coast Survey, enlarged the scope of that agency to include studies in the geodesy and geophysics of the entire continent. In the 1850s the survey was the largest single employer of physicists in the country. Henry also channeled part of the Smithsonian's income into fundamental research, including research in meteorology. During Henry's lifetime, American physics became more professional; the gentlemanly amateur was gradually superseded by the college-trained physicist who was employed on a college faculty or by the government.
In the quarter-century after the Civil War, many physicists set themselves increasingly apart from utilitarian concerns and embraced the new ethic of "pure" science. At the same time, the reform of higher education gave physics a considerable boost by permitting students to major in the sciences, making laboratory work a standard part of the curriculum, creating programs of graduate studies, and establishing the advancement of knowledge, at least nominally, as an important function of the university and its professors. Between 1865 and 1890 the number of physicists in the United States doubled, to about 150. The profession included Albert A. Michelson, the first American to win the Nobel Prize in physics (1907), who measured the speed of light with unprecedented accuracy and invented the Michelson interferometer during his famed ether drift experiment in 1881. During the late 1870s and the 1880s, Henry A. Rowland won an international reputation for his invention of the Rowland spectral grating and for his painstakingly accurate determinations of the value of the ohm and of the mechanical equivalent of heat. Generally, American physics remained predominantly experimental, with the notable exception of the brilliant theorist Josiah Willard Gibbs of Yale, an authority in thermo dynamics and statistical mechanics.
Professionalization by the Early-Twentieth Century
In 1893 Edward L. Nichols of Cornell University inaugurated the Physical Review, the first journal devoted to the discipline in the United States. Six years later Arthur Gordon Webster of Clark University helped found the American Physical Society, which in 1913 assumed publication of the Review. After the turn of the century, a sharp rise in electrical engineering enrollments created an increased demand for college teachers of physics. Employment opportunities for physicists rose elsewhere also. Some of the major corporations, notably General Electric Company and American Telephone and Telegraph Company, opened industrial research laboratories; and the federal government established the National Bureau of Standards, whose charter permitted it to enter a wide area of physical research. Before World War I, the graduation of physics Ph.D.s climbed steadily, reaching 23 in 1914, when membership in the American Physical Society was close to 700.
Americans had not been responsible for any of the key discoveries of the 1890s—X rays, radioactivity, and the electron—that introduced the age of atomic studies.
Like many of their colleagues in Europe, the older American members of the profession were disturbed by the development in the early twentieth century of the quantum theory of radiation and the theory of relativity. But the younger scientists turned to the new atomic research fields, although not immediately to the new theories, with growing interest and enthusiasm. At the University of Chicago, Robert A. Millikan demonstrated that all electrons are identically charged particles (1909) and then more accurately measured the electronic charge (1913). Richard Tolman of the University of Illinois and Gilbert N. Lewis of the Massachusetts Institute of Technology delivered the first American paper on the theory of relativity (1908). By the beginning of World War I, modernist physicists like Millikan were moving into the front rank of the profession, which was focusing increasingly, at its meetings and in its publications, on the physics of the quantized atom.
During the war, physicists worked for the military in various ways, most notably in the development of systems and devices for the detection of submarines and for the location of artillery. Their success in this area helped bolster the argument that physics, like chemistry, could produce practical and, hence, economically valuable results. Partly in recognition of that fact, industrial research laboratories hired more physicists in the 1920s. Moreover, the funding for physical research rose considerably in both state and private universities. During the 1920s about 650 Americans received doctorates in physics; a number of them received postdoctoral fellowships from the International Education Board of the Rockefeller Foundation and from the National Research Council. After studying with the leading physicists in the United States and Europe, where the revolution in quantum mechanics was proceeding apace, many of these young scientists were well prepared for the pursuit of theoretical research.
World Class Physics by the 1930s
By the end of the 1920s the United States had more than 2,300 physicists, including a small but significant influx of Europeans, including Paul Epstein, Fritz Zwicky, Samuel Goudsmit, and George Uhlenbeck, who had joined American university faculties. During that decade, Nobel Prizes in physics were awarded to Millikan (1923), director of the Norman Bridge Laboratory of Physics (1921) and chief executive at the California Institute of Technology, and to Arthur H. Compton (1927) of the University of Chicago for his quantum interpretation of the collision of X rays and electrons. At the Bell Telephone Laboratories, Clinton J. Davisson performed the research in electron diffraction for which he became a Nobel laureate in 1937. By the early 1930s the American physics profession compared favorably in experimental achievement with its counterparts in Europe; and in theoretical studies its potential, although not yet its accomplishment, had also reached the first rank.
During the 1930s the interest of physicists shifted from the atom to the nucleus and to what were later called elementary particles. In 1932, while conducting research for which they later won Nobel Prizes, Carl Anderson of the California Institute of Technology identified the positron in cosmic rays and, at the University of California at Berkeley, Ernest O. Lawrence successfully accelerated protons to one million volts of energy with his new cyclotron. Despite the depression, which at first reduced the funds available for physics research, U.S. physicists managed to construct cyclotrons, arguing that the exploration of the nucleus might yield the secret of atomic energy or that the radioactive products of cyclotron bombardment might be medically useful, especially in the treatment of cancer. All the while, more Americans earned Ph.D.s in physics, and the profession was further enriched by such refugees from the Soviet Union as George Gamow, and from Nazi Europe as Albert Einstein, Hans Bethe, Felix Bloch, Victor Weisskopf, Enrico Fermi, Emilio Segrè, Leo Szilard, Eugene Wigner, and Edward Teller. By the end of the 1930s, the American physics profession, with more than 3,500 members, led the world in both theoretical and experimental research.
During World War II, physicists, mobilized primarily under the Office of Scientific Research and Development, contributed decisively to the development of microwave radar, the proximity fuse, and solid-fuel rockets. They also worked on the atomic bomb in various laboratories of the Manhattan Project, notably Los Alamos, New Mexico, which was directed by J. Robert Oppenheimer. Equally important, physicists began advising the military how best to use the new weapons tactically and, in some cases, strategically.
After World War II, American physicists became prominent figures in the government's strategic advisory councils, and they played a central role in the debates over nuclear and thermonuclear weapons programs in the 1950s and 1960s. Recognized as indispensable to the national defense and welfare, physics and physicists received massive governmental support in the postwar decades, notably from the National Science Foundation, the Atomic Energy Commission, and the Office of Naval Research. Thus, the profession expanded rapidly, totaling more than 32,000 by 1972. About half of all American physicists were employed in industry, most of the rest in universities and colleges, and the remainder in federal laboratories.
Big Science
Many academic physicists did their research in groups organized around large, highly energetic particle accelerators, notably those at the Stanford Linear Accelerator Center and the Fermi National Accelerator Laboratory (Illinois). The large teams of scientists and engineers involved, the giant machines constructed, and the huge budgets required reflected a new style of research in peacetime, appropriately called Big Science. With these accelerators, American physicists were among the world's leaders in uncovering experimental data about elementary particles, one of the central fields of postwar physics. New particles were discovered by Emilio Segrè, Owen Chamberlain, Burton Richter, Samuel Ting, and Martin Perl, among others, while the necessary detection apparatus, such as bubble and spark chambers, were devised by Donald Glaser, Luis Alvarez, and others. Theoretical understanding included the work of Murray Gell-Mann, Steven Weinberg, and Sheldon Glashow in particle physics, Julian Schwinger, Richard P. Feynman, and Freeman Dyson in quantum electro dynamics, and Tsung Dao Lee and Chen Ning Yang in the nonconservation of parity. I. I. Rabi, Otto Stern, and others measured nuclear properties to unprecedented accuracy, while Maria Goeppert-Mayer advanced the shell model of the nucleus.
Charles H. Townes in the early 1960s played a major role in the development of the laser, an optical device useful both for research and several applications. These latter included bar-code readers in stores, compact disk players, and an X-ray laser, built in 1984 as a component of the now-defunct Strategic Defense Initiative, to destroy enemy missiles.
Meanwhile, physicists, notably at Princeton University, developed the tokamak, a donut-shaped magnetic enclosure in which ionized matter could be contained and heated to the very high temperatures necessary for nuclear fusion to take place. By 1991 they sustained fusion for two seconds, a step on the path to creating an energy machine similar to the fission reactor. Lasers were also being used in the attempt to achieve controlled nuclear fusion.
John Bardeen, Leon Cooper, and Robert Schrieffer in the early 1970s developed a theory of super conductivity to explain the phenomenon where, at very low temperatures, electrical resistance ceases. Physicists soon discovered that a combination of the elements niobium and germanium became superconducting at 22.3 K, about 2 degrees higher than the previous record, and in the late 1980s and 1990s scientists found yet other combinations with much higher (but still very cold) temperatures—above 120 K—that still lacked electrical resistance. Commercial applications, with great savings in electricity, were promising, but not near.
Other American physicists pursued such important fields as astrophysics and relativity, while in applied physics, William Shockley, John Bardeen, and Walter Brattain invented the transistor. This device, widely used in electronic products, made computers—and the information age—possible. It is an example of the way in which the products of physics research have helped to mold modern society. A measure of the quality of research in this country is the record that, from the time the Nobel Prize in Physics was initiated in 1901 to the year 2001, more than seventy American physicists won or shared this honor.
In the last half of the twentieth century physicists came out of their ivory towers to voice concerns about political issues with technical components. Veterans of the Manhattan Project in 1945–1946 created the influential Bulletin of the Atomic Scientists and formed the Federation of American Scientists to lobby for civilian control of atomic energy domestically and United Nations control of weapons internationally. During the intolerance of the McCarthy period of the 1950s, many physicists were held up to public scorn as communists or fellow travelers, or even feared as spies for the Kremlin. The President's Science Advisory Committee, formed in reaction to the Soviet Union's launch of Sputnik (1957), was initially dominated by physicists—whose understanding of the fundamentals of nature enabled them to advise knowingly on projects in other fields, such as missile technology.
The nation's principal organization of physicists, the American Physical Society, like many other professional groups, departed from its traditional role of publishing a journal and holding meetings. It began to lobby for financial support from a congress that contained few members with scientific credentials, and to issue reports on such controversial subjects as nuclear reactor safety, the Strategic Defense Initiative, and the alleged danger to health of electrical power lines. Some physicists participated in the long series of Pugwash Conferences on Science and World Affairs, meeting with foreign colleagues to help solve problems caused mostly by the arms race. Others created the Council for a Livable World, a political action committee whose goal was to help elect senators who supported arms control efforts. Still others joined the Union of Concerned Scientists, an organization that documented the danger of many nuclear reactors and the flaws of many weapons systems. The community of physicists had come of age, not only in producing world-class physics but in contributing to the economic and political health of society, often from a socially responsible perspective.
Bibliography
Childs, Herbert. An American Genius: The Life of Ernest Orlando Lawrence. New York: Dutton, 1968.
Coben, Stanley. "The Scientific Establishment and the Transmission of Quantum Mechanics to the United States, 1919–1932." American Historical Review 76 (1971): 442–466.
Kevles, Daniel J. "On the Flaws of American Physics: A Social and Institutional Analysis." In Nineteenth-Century American Science. Edited by George H. Daniels. Evanston, Ill.: Northwestern University Press, 1972.
———. The Physicists: The History of a Scientific Community in Modern America. Cambridge, Mass.: Harvard University Press, 1995.
National Research Council, Physics Survey Committee. Physics in Perspective. Washington, D.C.: National Academy of Sciences, 1973.
Reingold, Nathan. "Joseph Henry." In Dictionary of Scientific Biography. Volume 6. Edited by Charles C. Gillispie. New York: Scribners, 1972.
Tobey, Ronald. The American Ideology of National Science, 1919– 1930. Pittsburgh, Pa.: University of Pittsburgh Press, 1973.
High-Energy Physics
High-energy physics, also known as particle physics, studies the constitution, properties, and interactions of elementary particles—the basic units of matter and energy, such as electrons, protons, neutrons, still smaller particles, and photons—as revealed through experiments using particle accelerators, which impart high velocities to charged particles. This extension of nuclear physics to higher energies grew in the 1950s. Earlier generations of accelerators, or "atom smashers, " such as the cyclotron, reached the range of millions of electron volts (MeV), allowing fast-moving charged particles to crash into targeted particles and the ensuing nuclear reactions to be observed. (Particles must collide with the nucleus of the target matter in order to be observed.) Immediately after World War II, Vladimir I. Veksler in the Soviet Union and Edwin McMillan at Berkeley independently devised the synchrotron principle, which adjusts a magnetic field in step with the relativistic mass increase experienced by particles traveling near the velocity of light. In this way more energy could be imparted to the projectiles. Since the moving particle's wavelength decreases as its energy increases, at high energies it provides greater resolution to determine the shape and structure of the target particles. By the 1970s large accelerators could attain hundreds of millions or even several billion electron volts (GeV) and were used to produce numerous elementary particles for study. Cosmic rays provide another source of high-energy particles, but machines offer a greater concentration under controlled circumstances, and are generally preferred.
Theoretical physics kept pace in understanding these particles, which compose the atomic nucleus, and their interactions. By the early 1960s physicists knew that in addition to the protons, neutrons, and electrons that had been used to explain atomic nuclei for several decades, there was a confusing number of additional particles that had been found using electron and proton accelerators. A pattern in the structure of the nucleus was discerned by Murray Gell-Mann at the California Institute of Technology and by the Israeli Yuval Ne'eman. Gaps in the pattern were noticed, predictions of a new particle were made, and the particle (the so-called Omega-minus) was promptly discovered. To explain the pattern, Gell-Mann devised a theoretical scheme, called the eightfold way, that attempted to classify the relationship between strongly interacting particles in the nucleus. He postulated the existence of some underlying but unobserved elementary particles that he called "quarks."
Quarks carry electrical charges equal to either one-third or two-thirds of the charge of an electron or proton. Gell-Mann postulated several different kinds of quarks, giving them idiosyncratic names such as "up" (with a charge of plus two-thirds), "down" (with a charge of minus one-third), and "strange." Protons and neutrons are clusters of three quarks. Protons are made of two up quarks and a single down quark, so the total charge is plus one. Neutrons are made of one up quark and two down quarks, so the total charge is zero.
Another group of particles, the mesons, are made up of quarks and antiquarks (identical to quarks in mass, but opposite in electric and magnetic properties). These more massive particles, such as the ones found independently by Burton Richter at the Stanford Linear Accelerator and Samuel C. C. Ting at Brookhaven National Laboratory in 1974, fit into the picture as being made from charm quarks. The masses of these particles, like the spectrum of the hydrogen atom used by Niels Bohr many decades earlier to elucidate the quantum structure of the outer parts of atoms, now provided a numerical key for understanding the inner structure of the atom. Six different "flavors" of quarks are required to account for these heavy particles, and they come in pairs: up-down, charm-strange, and top-bottom. The first member of each pair has an electrical charge of two-thirds and the second of minus one-third.
Meanwhile, Sheldon Lee Glashow at Harvard University, Steven Weinberg at the Massachusetts Institute of Technology, and Abdus Salam at Imperial College in London in 1968 independently proposed a theory that linked two of the fundamental forces in nature, electromagnetism and the so-called weak nuclear force. Their proposal, known as quantum field theory, involved the notion of quarks and required the existence of three massive particles to "carry" the weak force: two charged particles (W+ and –) and one neutral particle (Z). These particles are short-lived, massive versions of the massless photons that carry ordinary light. All of these particles are called bosons, or more precisely, gauge bosons, because the theory explaining them is called a gauge theory. The name boson, which comes about for purely historical reasons, refers to a type of symmetry in which labels of the particles can be interchanged according to rules suggested by quantum mechanics, and the resulting forces (and gauge bosons) are found as a consequence of the symmetry requirements. By 1972 indirect evidence for the existence of the Z particle was found in Geneva at the European Organization for Nuclear Research (CERN). It was not until 1983 that the Z particle itself was found, also at CERN, and close on the heels of this discovery came the detection of the W particle.
In the United States, accelerator construction and use was supported primarily by the Atomic Energy Commission, by its successor, the Department of Energy, and by the National Science Foundation. One of the nation's principal machines, the Stanford Linear Accelerator fires particles down its two-mile length. Most other machines, such as those at CERN, Brookhaven (New York), KEK (Japan), and DESY (Germany) are circular or oval in shape. To increase energies still more, beams traveling in opposite directions are led to meet in "colliders, " thereby doubling the energy of collision. In early 1987 the Tevatron proton-antiproton accelerator at the Fermi National Accelerator Laboratory (Fermilab) in Illinois came into operation, a machine in the trillion electron volt range. Having narrowly missed out on some of the earlier discoveries, Fermilab scientists were particularly keen to find evidence for the postulated top quark, the only one of the quarks not yet measured and a particle so massive that only the most powerful accelerators could produce enough energy to find it. Their search at last succeeded in 1995.
The Standard Model
By the closing decades of the twentieth century, along with the quarks and bosons, a third type of particle completed the roster: the lepton, of which the electron, positron, and a group of neutrinos are the best known examples. The leptons and quarks provide the building blocks for atoms. The gauge bosons interact with the leptons and quarks, and in the act of being emitted or absorbed, some of the gauge bosons transform one kind of quark or lepton into another. In the standard model, a common mechanism underlies the electromagnetic, weak, and strong interactions. Each is mediated by the exchange of a gauge boson. The gauge bosons of the strong and weak interactions carry electrical charges, whereas the photon, which carries the electromagnetic interactions, is electrically neutral.
In its simplest formulation, the standard model of the strong, weak, and electromagnetic interactions, although aesthetically beautiful, does not agree with all the known characteristics of the weak interactions, nor can it account for the experimentally derived masses of the quarks. High-energy physicists hoped that the Superconducting Super Collider (SSC), a machine with a fifty-mile circumference that was under construction in Texas in the late 1980s, would provide data to extend and correct the standard model. They were greatly disappointed when Congress cut off funding for this expensive atom smasher.
The standard model is one of the great achievements of the human intellect. It will be remembered—together with general relativity, quantum mechanics, and the unraveling of the genetic code—as one of the outstanding intellectual advances of the twentieth century. It is not, however, the "final theory, " because too many constants still must be empirically determined. A particularly interesting development since the 1970s is the joining of particle physics with the astrophysics of the earliest stages of the universe. The "Big Bang" may provide the laboratory for exploration of the grand unified theories (GUTs) at temperatures and energies that are and will remain inaccessible in terrestrial laboratories. Also of profound significance will be an understanding of the so-called dark matter that comprises most of the mass of the universe.
In acknowledgment of the importance of the subject, experimental and theoretical high-energy physics research was recognized with a host of Nobel Prizes, many of them to American scientists. With the demise of the SSC, however, the field's future is likely to lie in machines built by associations of several nations.
Bibliography
Brown, Laurie M., and Lillian Hoddeson, eds. The Birth of Particle Physics. New York: Cambridge University Press, 1983.
Close, Frank, Michael Marten, and Christine Sutton. The Particle Explosion. New York: Oxford University Press, 1987.
Taubes, Gary. Nobel Dreams: Power, Deceit, and the Ultimate Experiment. New York: Random House, 1986.
Weinberg, Steven. Dreams of a Final Theory. New York: Pantheon Books, 1992.
Nuclear Physics
The age-old goal of physicists has been to understand the nature of matter and energy. Nowhere during the twentieth century were the boundaries of such knowledge further extended than in the field of nuclear physics. From an obscure corner of submicroscopic particle research, nuclear physics became the most prominent and fruitful area of physical investigation because of its fundamental insights and its applications.
Discovery of the Nucleus
In the first decade of the twentieth century J. J. Thomson's discovery of the electron at Cambridge University's Cavendish Laboratory changed the concept of the atom as a solid, homogeneous entity—a "billiard ball"—to one of a sphere of positive electrification studded throughout with negative electrons. This "plum pudding" atomic model, with a different number of electrons for each element, could not account for the large-angle scattering seen when alpha particles from naturally decaying radioactive sources were allowed to strike target materials. Thomson argued that the alpha particles suffered a series of small deflections in their encounters with the target atoms, resulting in some cases in a sizable deviation from their initial path. But between 1909 and 1911 in the Manchester laboratory of Thomson's former pupil Ernest Rutherford, Hans Geiger and Ernest Marsden produced scattering data that showed too many alpha particles were bent through angles too large for such an explanation to be valid.
Instead of a series of small deflections, Rutherford suggested early in 1911 that large-angle scattering could occur in a single encounter between an alpha particle and a target atom if the mass of the atom were concentrated in a tiny volume. While the atomic diameter was of the order of 10–8 centimeters, this atomic core (or nucleus), containing virtually the atom's entire mass, measured only about 10–12 centimeters. The atom, therefore, consisted largely of empty space, with electrons circulating about the central nucleus. When an alpha-particle projectile closely approached a target nucleus, it encountered concentrated electrostatic repulsion sufficient to deflect it more than just a few degrees from its path.
The Danish physicist Niels Bohr absorbed these concepts while visiting Rutherford's laboratory and in 1913 gave mathematical formulation to the rules by which the orbital electrons behaved. The order and arrangement of these electrons were seen to be responsible for the chemical properties exhibited by different elements. Pursuit of this field led to modern atomic physics, including its quantum mechanical explanation, and bore fruit earlier than did studies in nuclear physics. Radioactivity was recognized as a nuclear phenomenon, and the emission of alpha particles, known by then to be nuclei of helium atoms; beta particles, long recognized as electrons; and gamma rays, an electromagnetic radiation, reopened the question of whether atoms were constructed from fundamental building blocks. The work in 1913 of Henry G. J. Moseley, another former student of Rutherford's, showed that an element's position in the periodic table (its atomic number), and not its atomic weight, determined its characteristics. Moreover, he established that the number of positive charges on the nucleus (equal to its atomic number) was balanced by an equal number of orbital electrons. Since atomic weights (A) were (except for hydrogen) higher than atomic numbers (Z), the atom's nuclear mass was considered to be composed of A positively charged particles, called protons, and A–Z electrons to neutralize enough protons for a net nuclear charge of Z.
Early Nuclear Transmutations
In 1919 Rutherford announced another major discovery. Radioactivity had long been understood as a process of transmutation from one type of atom into another, occurring spontaneously. Neither temperature, nor pressure, nor chemical combination could alter the rate of decay of a given radio element or change the identity of its daughter product. Now, however, Rutherford showed that he could deliberately cause transmutations. His were not among the elements at the high end of the periodic table, where natural radioactivity is commonly found, but were among the lighter elements. By allowing energetic alpha particles (42He) from decaying radium C' to fall upon nitrogen molecules, he observed the production of hydrogen nuclei, or protons (11 H), and an oxygen isotope. The reaction may be written aswhere the superscript represents the atomic weight and the subscript the atomic number, or charge.
During the first half of the 1920s Rutherford, now at Cambridge, where he had succeeded Thomson, was able to effect transmutations in many of the lighter elements. (In this work he was assisted primarily by James Chadwick.) But elements heavier than potassium would not yield to the alpha particles from their strongest radioactive source. The greater nuclear charge on the heavier elements repelled the alpha particles, preventing an approach close enough for transmutation. This finding suggested that projectile particles of energies or velocities higher than those found in naturally decaying radio elements were required to overcome the potential barriers of target nuclei. Consequently, various means of accelerating particles were devised.
The Neutron
In 1920 William D. Harkins, a physical chemist at the University of Chicago, conceived that the existence of a neutron would simplify certain problems in the construction of nuclei. In the same year, Rutherford (on the basis of incorrectly interpreted experimental evidence) also postulated the existence of such a neutral particle, the mass of which was comparable to that of the proton. Throughout the 1920s he, and especially Chadwick, searched unsuccessfully for this particle. In 1931 in Germany Walther Bothe and H. Becker detected, when beryllium was bombarded by alpha particles, a penetrating radiation, which they concluded consisted of energetic gamma rays. In France, Irène Curie and her husband, Frédéric Joliot, placed paraffin in the path of this radiation and detected protons ejected from that hydrogenous compound. They, too, believed that gamma rays were being produced and that these somehow transferred sufficient energy to the hydrogen atoms to break their chemical bonds. Chadwick learned of this work early in 1932 and immediately recognized that beryllium was yielding not gamma rays but the longelusive neutron and that this particle was encountering protons of similar mass, transferring much of its kinetic energy and momentum to them at the time of collision. Since the neutron is uncharged, it is not repelled by atomic nuclei. Consequently, it can enter easily into reactions when it finds itself near a nucleus; otherwise it travels great distances through matter, suffering no electrostatic attractions or repulsions.
Quantum Mechanics Applied to the Nucleus
Werner Heisenberg, in Leipzig, renowned for his articulation of quantum mechanics and its application to atomic physics, in 1932 applied his mathematical techniques to nuclear physics, successfully explaining that atomic nuclei are composed not of protons and electrons but of protons and neutrons. For a given element, Z protons furnish the positive charge, while A–Z neutrons bring the total mass up to the atomic weight A. Radioactive beta decay, formerly a strong argument for the existence of electrons in the nucleus, was now interpreted differently: the beta particles were formed only at the instant of decay, as a neutron changed into a proton. The reverse reaction could occur also, with the emission of a positive electron, or positron, as a proton changed into a neutron. This reaction was predicted by the Cambridge University theoretician P. A. M. Dirac and was experimentally detected in 1932 by Carl D. Anderson of the California Institute of Technology in cloud-chamber track photographs of cosmic-ray inter-actions. Two years later the Joliot-Curies noted the same result in certain radioactive decay patterns. The "fundamental" particles now consisted of the proton and neutron—nucleons (nuclear particles) with atomic masses of about 1—and of the electron and positron, with masses of about 1/1,840 of a nucleon.
The existence of yet another particle, the neutrino, was first suggested in 1931 by Wolfgang Pauli of Zurich in an address before the American Physical Society. When a nucleus is transmuted and beta particles emitted, there are specific changes in energy. Yet, unlike the case of alpha decay, beta particles exhibited a continuous energy distribution, with only the maximum energy seen as that of the reaction. The difference between the energy of a given beta particle and the maximum was thought to be carried off by a neutrino, the properties of which—very small or zero mass and no charge—accounted for the difficulty of detecting it. In 1934 Enrico Fermi presented a quantitative theory of beta decay incorporating Pauli's hypothesis. Gamma radiation, following either alpha or beta decay, was interpreted as being emitted from the daughter nucleus as it went from an excited level to its ground state.
Further Understanding Provided by the Neutron
The neutron, the greatest of these keys to an understanding of the nucleus, helped to clarify other physical problems besides nuclear charge and weight. In 1913 Kasimir Fajans in Karlsruhe and Frederick Soddy in Glasgow had fit the numerous radio elements into the periodic table, showing that in several cases more than one radio element must be placed in the same box. Mesothorium I, thorium X, and actinium X, for example, all were chemically identical to radium; that is, they were isotopes. This finding meant they each had 88 protons but had, respectively, 140,136, and 135 neutrons. Also, in the pre–World War I period Thomson showed that nonradioactive elements exist in isotopic forms—neon, for example, has atomic weights of 20 and 22. His colleague F. W. Aston perfected the mass spectrograph, with which during the 1920s he accurately measured the masses of numerous atomic species. It was revealed that these masses were generally close to, but were not exactly, whole numbers. The difference was termed the "packing effect" by Harkins and E. D. Wilson as early as 1915, and the "packing fraction" by Aston in 1927. After 1932 it was also learned that atomic masses were not the sums of Z proton masses and A–Z neutron masses, and the difference was termed the "mass defect." The concept of nuclear building blocks (protons and neutrons) was retained; however, it was seen that a certain amount of mass was converted into a nuclear binding energy to overcome the mutual repulsion of the protons. This binding energy is of the order of a million times greater than the energies binding atoms in compounds or in stable crystals, which indicates why nuclear reactions involve so much more energy than chemical reactions.
The existence of deuterium, a hydrogen isotope of mass 2 (21 H), present in ordinary (mass 1) hydrogen to the extent of about 1 part in 4,500, was suggested in 1931 by Raymond T. Birge and Donald H. Menzel at the University of California at Berkeley and shortly there after was confirmed by Harold C. Urey and George M. Murphy at Columbia University, in collaboration with Ferdinand G. Brickwedde of the National Bureau of Standards. The heavy-hydrogen atom's nucleus, called the deuteron, proved to be exceptionally useful: it entered into some nuclear reactions more readily than did the proton.
Shortly after their discovery in 1932, neutrons were used as projectiles to effect nuclear transmutations by Norman Feather in England and Harkins, David Gans, and Henry W. Newson at Chicago. Two years later the Joliot-Curies reported the discovery of yet another process of transmutation: artificial radioactivity. A target not normally radioactive was bombarded with alpha particles and continued to exhibit nuclear changes even after the projectile beam was stopped. Such bombardment has permitted the production of about 1,100 nuclear species beyond the 320 or so found occurring in nature.
Nuclear Fission, Fusion, and Nuclear Weapons
During the mid-1930s, Fermi and his colleagues in Rome were most successful in causing transmutations with neutrons, particularly after they discovered the greater likelihood of the reactions occurring when the neutrons' velocities were reduced by prior collisions. When uranium, the heaviest known element, was bombarded with neutrons, several beta-particle (0–1 e)-emitting substances were produced, which Fermi reasoned must be artificial elements beyond uranium in the periodic table. The reaction may be expressed aswith a possible subsequent decay of But radiochemical analyses of the trace amounts of new substances placed them in unexpected groupings in the periodic table, and, even worse, Otto Hahn and Fritz Strassmann, in Berlin toward the end of 1938, were unable to separate them chemically from elements found in the middle part of the periodic table. It seemed that the so-called transuranium elements had chemical properties identical to barium, lanthanum, and cerium. Hahn's longtime colleague Lise Meitner, then a refugee in Sweden, and her nephew Otto R. Frisch, at that time in Bohr's Copenhagen laboratory, in 1938 saw that the neutrons were not adhering to the uranium nuclei, followed by beta decay, but were causing the uranium nuclei to split (fission) into two roughly equal particles. They recognized that these fission fragments suffered beta decay in their movement toward conditions of greater internal stability.
With the accurate atomic-mass values then available, it was apparent that in fission a considerable amount of mass is converted into energy; that is, the mass of the neutron plus uranium is greater than that of the fragments. The potential for utilizing such energy was widely recognized in 1939, assuming that additional neutrons were released in the fission process and that at least one of the neutrons would rupture another uranium nucleus in a chain reaction. The United States, Great Britain, Canada, France, the Soviet Union, Germany, and Japan all made efforts in this direction during World War II. A controlled chain reaction was first produced in Fermi's "pile, " or "reactor, " in 1942 at the University of Chicago, and an uncontrolled or explosive chain reaction was first tested under the direction of J. Robert Oppenheimer in 1945 in New Mexico. Among the scientific feats of the atomic-bomb project was the production at Berkeley in 1940–1941 of the first man-made transuranium elements, neptunium and plutonium, by teams under Edwin M. McMillan and Glenn Seaborg, respectively. A weapon involving the fission of the uranium isotope 235 was employed against Hiroshima, and another using plutonium (element 94, the "Y" above) nuclei destroyed Nagasaki.
Like the fission of heavy elements, the joining together (fusion) of light elements is also a process in which mass is converted into energy. This reaction, experimentally studied as early as 1934 by Rutherford and his colleagues and theoretically treated in 1938 by George Gamow and Edward Teller, both then at George Washington University, has not been controlled successfully for appreciable periods of time (preventing its use as a reactor); but its uncontrolled form is represented in the Hydrogen Bomb, first tested in 1952.
Particle Accelerators
The growth of "big science, " measured by its cost and influence, is manifest not only in weaponry and power-producing reactors but also in huge particle-accelerating machines. Alpha particles from naturally decaying radio-elements carry a kinetic energy of between about 4 and 10 million electron volts (MeV). But, as only one projectile in several hundred thousand is likely to come close enough to a target nucleus to affect it, reactions occur relatively infrequently, even with concentrated radioactive sources. Cosmic radiation, which possesses far greater energy, has an even lower probability of interacting with a target nucleus. Means were sought for furnishing a copious supply of charged particles that could be accelerated to energies sufficient to overcome the nuclear electro-static repulsion. This feat would both shorten the time of experiments and increase the number of reactions. Since electrical technology had little or no previous application in the range of hundreds of thousands or millions of volts, these were pioneering efforts in engineering as well as in physics. In the late 1920s Charles C. Lauritsen and H. R. Crane at the California Institute of Technology succeeded with a cascade transformer in putting 700,000 volts across an X-ray tube. Merle A. Tuve, at the Carnegie Institution of Washington, in 1930 produced protons in a vacuum tube with energies of more than a million volts. The next year, at Princeton, Robert J. Van de Graaff built the first of his electrostatic generators, with a maximum potential of about 1.5 million volts. In 1932 Ernest O. Lawrence and his associates at Berkeley constructed a magnetic resonance device, called a cyclotron because a magnetic field bent the charged particles in a circular path. The novelty of this machine lay in its ability to impart high energies to particles in a series of steps, during each revolution, thereby avoiding the need for great voltages across the terminals, as in other accelerators. The cyclotron soon exceeded the energies of other machines and became the most commonly used "atom smasher."
Although Americans excelled in the mechanical ability that could produce such a variety of machines, they were only beginning to develop theoretical and experimental research to use them. They also lacked the driving force of Rutherford. Since 1929 John D. Cockcroft and E. T. S. Walton had been building and rebuilding, testing and calibrating their voltage multiplier in the Cavendish Laboratory. Rutherford finally insisted that they perform a real experiment on it. The Russian George Gamow and, independently, Edward U. Condon at Princeton with R. W. Gurney of England, had applied quantum mechanics to consideration of the nucleus. Gamow concluded that particles need not surmount the potential energy barrier of about 25 MeV, for an element of high atomic number, to penetrate into or escape from the nucleus; instead these particles could "tunnel" through the barrier at far lower energies. The lower the energy, the less likely it was that tunneling would occur, yet an abundant supply of projectiles might produce enough reactions to be recorded. With protons accelerated to only 125,000 volts, Cock-croft and Walton, in 1932, found lithium disintegrated into two alpha particles in the reaction Not only was this the first completely artificial transmutation (Rutherford's transmutation in 1919 had used alpha-particle projectiles from naturally decaying radio-elements), but the two also measured the products' range, and therefore energy, combined with a precise value of the mass lost in the reaction, and verified for the first time Albert Einstein's famous E=mc2 equation.
The United States continued to pioneer machine construction, often with medical and biological financial support: Donald W. Kerst of the University of Illinois built a circular electron accelerator, called a betatron, in 1940, and Luis W. Alvarez of Berkeley designed a linear proton accelerator in 1946. D. W. Fry in England perfected a linear electron accelerator (1946), as did W. W. Hansen at Stanford. Since particles traveling at velocities near that of light experience a relativistic mass increase, the synchrotron principle, which uses a varying magnetic field or radio frequency to control the particle orbits, was developed independently in 1945 by Vladimir I. Veksler in the Soviet Union and by McMillan at Berkeley. By the 1970s, large accelerators could attain hundreds of millions, or even several billion, electron volts and were used to produce numerous elementary particles. Below this realm of high-energy or particle physics, recognized as a separate field since the early 1950s, nuclear physics research continued in the more modest MeV range.
Nuclear Structure
With these methods of inducing nuclear reactions and the measurements of the masses and energies involved, questions arose about what actually occurs during a transmutation. Traditional instruments—electroscopes, electrometers, scintillating screens, electrical counters—and even the more modern electronic devices were of limited value. Visual evidence was most desirable. At Chicago in 1923 Harkins attempted unsuccessfully to photograph cloud-chamber tracks of Rutherford's 1919 transmutation of nitrogen. In 1921 Rutherford's pupil P. M. S. Blackett examined 400,000 tracks and found that 8 exhibited a Y-shaped fork, indicating that the alpha-particle projectile was absorbed by the nitrogen target into a compound nucleus, which immediately became an isotope of oxygen by the emission of a proton. The three branches of the Y consisted of the incident alpha and the two products, the initially neutral and slow-moving nitrogen having no track. Had the now-discredited alternative explanation of the process been true, namely, that the alpha particle merely bounced off the nitrogen nucleus, which then decayed according to the reaction a track of four branches would have been seen.
Experimental work by Harkins and Gans in 1935 and theoretical contributions by Bohr the next year clearly established the compound nucleus as the intermediate stage in most medium-energy nuclear reactions. Alvarez designed a velocity selector for monoenergetic neutrons that allowed greater precision in reaction calculations, while Gregory Breit at the University of Wisconsin and the Hungarian refugee Eugene P. Wigner at Princeton in 1936 published a formula that explained the theory of preferential absorption of neutrons (their cross sections): If the neutrons have an energy such that a compound nucleus can be formed at or near one of its permitted energy levels, there is a high probability that these neutrons will be captured.
It was recognized that the forces holding nucleons together are stronger than electrostatic, gravitational, and weak interaction (beta particle–neutrino) forces and that they operate over shorter ranges, namely, the nuclear dimension of 10–12 centimeters. In 1936 Bohr made an analogy between nuclear forces and those within a drop of liquid. Both are short range, acting strongly on those nucleons/molecules in their immediate neighborhood but having no influence on those further away in the nucleus/drop. The total energy and volume of a nucleus/drop are directly proportional to the number of constituent nucleons/molecules, and any excess energy of a constituent is rapidly shared among the others. This liquid-drop model of the nucleus, which meshed well with Bohr's understanding of the compound-nucleus stage during reactions, treated the energy states of the nucleus as a whole. Its great success, discovered by Bohr in collaboration with John A. Wheeler of Princeton (1939), in explaining fission as a deformation of the spherical drop into a dumbbell shape that breaks apart at the narrow connection, assured its wide acceptance for a number of years.
The strongest opposition to this liquid-drop interpretation came from proponents of the nuclear-shell model, who felt that nucleons retain much of their individuality—that, for example, they move within their own well-defined orbits. In 1932 James H. Bartlett of the University of Illinois, by analogy to the grouping of orbital electrons, suggested that protons and neutrons in nuclei also form into shells. This idea was developed in France and Germany, where it was shown in 1937 that data on magnetic moments of nuclei conform to a shell-model interpretation.
To explain the very fine splitting (hyperfine structure) of lines in the optical spectra of some elements—spectra produced largely by the extra nuclear electrons—several European physicists in the 1920s had suggested that atomic nuclei possess mechanical and magnetic moments relating to their rotation and configuration. From the 1930s on, a number of techniques were developed for measuring such nuclear moments—including the radio-frequency resonance method of Columbia University's I. I. Rabi—and from the resulting data certain regularities appeared. For example, nuclei with an odd number of particles have half units of spin and nuclei with an even number of particles have integer units of spin, while nuclei with an even number of protons and an even number of neutrons have zero spin. Evidence such as this suggested some sort of organization of the nucleons.
With the shell model overshadowed by the success of the liquid-drop model, and with much basic research interrupted by World War II, it was not until 1949 that Maria Goeppert Mayer at the University of Chicago and O. Haxel, J. H. D. Jensen, and H. E. Suess in Germany showed the success of the shell model in explaining the so-called magic numbers of nucleons: 2, 8, 20, 28, 50, 82, and 126. Elements having these numbers of nucleons, known to be unusually stable, were assumed to have closed shells in the nucleus. Lead 208, for example, is "doubly magic, " having 82 protons and 126 neutrons. More recent interpretations, incorporating features of both liquid-drop and shell models, are called the "collective" and "unified" models.
Aside from the question of the structure of the nucleus, after it was recognized that similarly charged particles were confined in a tiny volume, the problem existed of explaining the nature of the short-range forces that overcame their electrical repulsion. In 1935 Hideki Yukawa in Japan reasoned that just as electrical force is transmitted between charged bodies in an electromagnetic field by a particle called a photon, there might be an analogous nuclear-field particle. Accordingly, the meson, as it was called (with a predicted mass about 200 times that of the electron), was soon found in cosmic rays by Carl D. Anderson and Seth H. Neddermeyer. The existence of this particle was confirmed by 1938. But in 1947 Fermi, Teller, and Victor F. Weisskopf in the United States concluded that this mumeson, or muon, did not interact with matter in the necessary way to serve as a field particle; and S. Sakata and T. Inoue in Japan, and independently Hans A. Bethe at Cornell and Robert E. Marshak at the University of Rochester, suggested that yet another meson existed. Within the same year, Cecil F. Powell and G. P. S. Occhialini in Bristol, England, found the pi meson, or pion—a particle slightly heavier than the muon into which it decays and one that meets field-particle requirements—in cosmic-ray tracks. Neutrons and protons were thought to interact through the continual transfer of positive, negative, and neutral pions between them.
Wider Significance of Nuclear Physics
In addition to the profound insights to nature revealed by basic research in nuclear physics, and the awesome applications to power sources and weapons, the subject also contributed to important questions in other fields. Early in the twentieth century, Bertram B. Boltwood, a radiochemist at Yale, devised a radioactive dating technique to measure the age of the earth's oldest rocks, at one time a subject considered the domain of geologists. These procedures were refined, largely by British geologist Arthur Holmes and later by geochemist Claire Patterson at the California Institute of Technology, as data on isotopic concentrations were better appreciated and better measured, leading to an estimation of the earth's antiquity at several billion years. Measuring a shorter time scale with unprecedented accuracy, chemist Willard Libby at the University of Chicago developed a method of dating artifacts of anthropological age using the carbon 14 isotope. Nuclear physics informed yet another subject of longstanding fascination to humanity: What keeps stars shining over enormous periods of time? Just before World War II, Hans Bethe of Cornell conceived the carbon cycle of nuclear reactions and calculated the energy output of each step. And shortly after the war, Gamow extended the range of nuclear physics to the entire universe, answering the cosmological question of origin with the "Big Bang, " and detailing the nuclear reactions that occurred over the next several hundred million years.
The Profession
Although nuclear physics is sometimes said to have been born during the early 1930s—a period of many remarkable discoveries—it can more appropriately be dated from 1911 or 1919. What is true of the 1930s is that by this time nuclear physics was clearly defined as a major field. The percentage of nuclear-physics papers published in Physical Review rose dramatically; other measures of the field's prominence included research funds directed to it, the number of doctoral degrees awarded, and the number of fellowships tendered by such patrons as the Rockefeller Foundation. Although they were by no means the only scientists fashioning the subject in the United States, Lawrence at Berkeley and Oppenheimer at the California Institute of Technology and Berkeley were dominating figures in building American schools of experimental and theoretical research, respectively. This domestic activity was immeasurably enriched in the 1930s by the stream of refugee physicists from totalitarian Europe—men such as Bethe, Fermi, Leo Szilard, Wigner, Teller, Weisskopf, James Franck, Gamow, Emilio Segrè, and, of course, Einstein. Prominent Europeans had earlier taught at the summer schools for theoretical physics held at several American universities; now many came permanently.
Much of this domestic and foreign talent was mobilized during World War II for the development of radar, the proximity fuse, and most notably the Manhattan Project, which produced the first atomic bombs. So stunning was the news of Hiroshima's and Nagasaki's obliteration that nuclear physicists were regarded with a measure of awe. In the opinion of most people nuclear physics was the most exciting, meaningful, and fearful area of science, and its usefulness brought considerable government support. American domination of nuclear physics in the postwar decades resulted, therefore, from a combination of the wartime concentration of research in the United States and the simultaneous disruptions in
Europe, and from another combination of rising domestic abilities and exceptional foreign talent, financed by a government that had seen (at least for a while) that basic research was applicable to national needs.
In the postwar period, the U.S. Atomic Energy Commission and then the Department of Energy supported most research in this field. It was conducted in universities and in several national laboratories, such as those at Los Alamos, Livermore, Berkeley, Brookhaven, Argonne, and Oak Ridge. With the most fashionable side of the subject now called high-energy or particle physics, ever more energetic particle accelerators were constructed, seeking to produce reactions at high energies that would reveal new particles and their interactions. Their size and cost, however, led to dwindling support. By the end of the twentieth century, the nation's two most significant machines were at the Stanford Linear Accelerator Center and the Fermi National Accelerator Laboratory. A larger machine of the next generation, the Super conducting Super Collider, was authorized by Congress and then cancelled when its fifty-mile-long tunnel was but a quarter excavated, because of its escalating, multi-billion-dollar price tag. Consequently, the research front will be at accelerator centers run by groups of nations for the foreseeable future.
Bibliography
Glasstone, Samuel. Sourcebook on Atomic Energy. 3d ed. Princeton, N.J.: Van Nostrand, 1967.
Livingston, M. Stanley. Particle Accelerators: A Brief History. Cambridge, Mass.: Harvard University Press, 1969.
Stuewer, Roger, ed. Nuclear Physics in Retrospect: Proceedings of a Symposium on the 1930s. Minneapolis: University of Minnesota Press, 1979.
Weiner, Charles, ed. Exploring the History of Nuclear Physics. New York: American Institute of Physics, 1972. Proceedings of the institute's conferences of 1967 and 1969.
Weisskopf, Victor F. Physics in the Twentieth Century: Selected Essays. Cambridge, Mass.: MIT Press, 1972.
Solid-State Physics
Solid-state is the branch of research that deals with properties of condensed matter—originally solids such as crystals and metals, later extended to liquids and more exotic forms of matter. The multitude of properties studied and the variety of materials that can be explored give this field enormous scope.
Modern solid-state physics relies on the concepts and techniques of twentieth-century atomic theory, in which a material substance is seen as an aggregate of atoms obeying the laws of quantum mechanics. Earlier concepts had failed to explain the most obvious characteristics of most materials. A few features of a metal could be explained by assuming that electrons moved freely within it, like a gas, but that did not lead far. Materials technology was built largely on age-old craft traditions.
The Rise of Solid-State Theory
Discoveries in the first quarter of the twentieth century opened the way to answers. The work began with a puzzle: experiments found that for most simple solids, as the temperature is lowered toward absolute zero, adding even an infinitesimally small amount of heat produces a large change in temperature. The classical model of a solid made up of vibrating atoms could not explain this. In 1907, Albert Einstein reworked the model using the radical new idea that energy comes in tiny, discrete "quantum" packets. The qualitative success of Einstein's theory, as refined by other physicists, helped confirm the new quantum theory and pointed to its uses for explaining solid-state phenomena.
In 1912, scientists in Munich discovered an experimental method of "seeing" the internal arrangement of atoms in solids. They sent X rays through crystals and produced patterns, which they interpreted as the result of the scattering of the X rays by atoms arranged in a lattice. By the late 1920s, X-ray studies had revealed most of the basic information about how atoms are arranged in simple crystals.
The theories that attempted to explain solids still contained crippling problems. Solutions became available only after a complete theory of quantum mechanics was invented, in 1925 and 1926, by the German physicist Werner Heisenberg and the Austrian physicist Erwin Schrödinger, building on work by the Danish physicist Niels Bohr. A quantum statistics that could be applied to the particles in condensed matter was invented in 1926 by the Italian physicist Enrico Fermi and the British physicist P. A. M. Dirac.
The next few years were a remarkably productive period as the new conceptual and mathematical tools were applied to the study of solids and liquids. Many leading physicists were involved in this work—Germans, Austrians, Russians, French, British, and a few Americans, notably John Van Vleck and John Slater. Between 1928 and 1931, Felix Bloch, Rudolf Peierls, Alan Wilson, and others developed a powerful concept of energy bands separated by gaps to describe the energy distribution of the swarm of electrons in a crystal. This concept explained why metals conduct electricity and heat while insulators do not, and why the electrical conductivity of a class of materials called semiconductors varies with temperature. Another breakthrough came in 1933 when Eugene Wigner and his student Frederick Seitz at Princeton University developed a simple approximate method for computing the energy bands of sodium and other real solids. By 1934, some of the most dramatic properties of solids, such as magnetism, had received qualitative (if not quantitative) explanation.
But the models of the new theory remained idealizations, applicable only to perfect materials. Physicists could not extend the results, for the available materials contained far too many impurities and physical imperfections. Most practically important characteristics (such as the strength of an alloy) were far beyond the theorists' reach. In the mid-1930s, many theorists turned their attention to fields such as nuclear physics, which offered greater opportunities for making exciting intellectual contributions.
Yet a broader base was being laid for future progress. Established scientists and engineers, particularly in the United States, were avidly studying the new quantum theory of solids. It also became a standard topic in the graduate studies of the next generation. Meanwhile, leaders in universities, industrial labs, and philanthropies were deliberately striving to upgrade American research in all fields of physics. Their efforts were reinforced by the talents of more than 100 European physicists who immigrated to the United States between 1933 and 1941 as a result of the political upheavals in Europe.
Dynamic Growth in World War II and After
Military-oriented research during World War II (1939–1945) created many new techniques that would be useful for the study of solids. For example, Manhattan Project scientists studied neutrons, and in the postwar period these neutral subatomic particles were found to be effective probes of solids, especially in exploring magnetic properties. The fervent wartime development of micro-wave radar also brought a variety of new techniques useful for studying solids, such as microwave spectroscopy, in which radiation is tuned to coincide with natural vibrational or rotational frequencies of atoms and molecules within a magnetic field. The Collins liquefier, developed just after the war at the Massachusetts Institute of Technology, made it possible for laboratories to get bulk liquid helium and study materials under the simplified conditions that prevail at extremely low temperatures. Methods were also developed during the war for producing single crystals in significant quantities. The production of pure crystals of the elements silicon and germanium, which found wartime use in microwave devices, became so highly developed that an enormous number of postwar studies used these semiconductors as prototypes for the study of solid-state phenomena in general.
Thus, by the late 1940s a seemingly mature field of solid-state physics was growing in scope and also in terms of the number of physicists attracted to the field. By 1947 solid-state physics had become a large enough field to justify establishing a separate division for it within the American Physical Society.
In the postwar period solid-state physics became even more closely tied to practical applications, which then stimulated new interest in the field and increased its funding. The development of the transistor offers a striking example. In January 1945, the Bell Telephone Laboratories in New Jersey officially authorized a group to do fundamental research on solids. William B. Shockley, one of the group's two leaders, believed that such research could lead to the invention of a solid-state amplifier. Members of a small semiconductor subgroup led by Shockley directed their attention to silicon and germanium, whose properties had been closely studied during the wartime radar program. In December 1947, two members of the group, the theorist John Bardeen and the experimentalist Walter Brattain, working closely together, invented the first transistor.
The transistor rectifies and amplifies electrical signals more rapidly and reliably than the more cumbersome, fragile, and costly vacuum tube. It rapidly found practical application. Among the first to take an interest were military agencies, driven by Cold War concerns to fund advanced research as well as development in all fields of physics. Commercial interests promptly followed; the first "transistorized" radio went on the market in 1954, and the term "solid-state" was soon popularized by advertisers' tags. Transistorized devices revolutionized communications, control apparatus, and data processing. The explosive growth of commercial and national security applications led to wide popular interest, and swift increases in funding for every kind of research. By 1960, there were roughly 2,000 solid-state physicists in the United States, making up one-fifth of all American physicists. Here, as in most fields of science since the war, more significant work had been done in the United States than in the rest of the world put together. As other countries recovered economically, they began to catch up.
Unlike most fields of physics at that time, in solid-state about half of the U.S. specialists worked in industry. Universities did not want to be left out, and starting in 1960 they established "materials science" centers with the aid of Department of Defense funding. As the name implied, the field was reaching past solid-state physicists to include chemists, engineers, and others in an interdisciplinary spirit.
Throughout the 1950s and 1960s, theory, technique, and applications of solid-state physics all advanced rapidly. The long list of achievements includes a theory for the details of atomic movements inside crystals, understanding of how impurities and imperfections cause optical properties and affect crystal growth, quantitative determination of such properties as electrical resistivity, and a more complete theory for the phase transitions between different states of matter. The biggest theoretical breakthrough of the period was an explanation of super conductivity in 1957 by Bardeen and two other American physicists, Leon N. Cooper and J. Robert Schrieffer. Their theory led the way to explanations of a whole series of so-called cooperative phenomena (which also include super fluidity, phase transitions, and tunneling) in which particles and sound-wave quanta move in unison, giving rise to strongly modified and sometimes astonishing properties. Theorists were further stimulated in 1972 when scientists at Cornell University, deploying ingenious new techniques to reach extremely low temperatures, discovered that Helium-3 could become a super fluid with remarkable properties.
Meanwhile, many other important techniques were developed, such as the Josephson effect. In 1962, the young British physicist Brian D. Josephson proposed that a super current can "tunnel" through a thin barrier separating two super conductors. This led to important devices such as the SQUID (Super conducting Quantum Interference Device), which can probe the surface structures of solids and can even map faint magnetic fields that reflect human brain activity. Still more versatile were techniques to create entirely new, artificially structured materials. With vapors or beams of molecules, physicists could build up a surface molecule by molecule like layers of paint.
The list of applications continued to grow rapidly. By the mid-1970s, in addition to countless varieties of electronic diodes and transistors, there were, for example, solid-state lasers employed in such diverse applications as weaponry, welding, and eye surgery; magnetic bubble memories used in computers to store information in thin crystals; and improved understanding of processes in areas ranging from photography to metallurgy. In 1955, Shockley had established a semiconductor firm in California near Stanford University, creating a nucleus for what was later dubbed Silicon Valley—a hive of entrepreneurial capital, technical expertise, and innovation, but only one of many locales from Europe to Japan that thrived on solid-state physics. The creation of entire industries in turn stimulated interest in the specialty, now virtually a field of its own. In 1970 when the American Physical Society split up its massive flagship journal, the Physical Review, into manageable sections, the largest was devoted entirely to solids. But it was the "B" section, for in terms of intellectual prestige, solid-state physics had always taken second place behind fields such as nuclear and particle physics, which were called more "fundamental."
Condensed Matter from Stars to Super Markets
To advance their status, and to emphasize their interest in ever more diverse materials, practitioners renamed the field; in 1978 the American Physical Society's division changed its name from "Solid State" to "Condensed Matter." The condensed-matter physicists were rapidly improving their understanding and control of the behavior of fluids and semidisordered materials like glasses. Theoretical studies of condensed matter began to range as far afield as the interiors of neutron stars, and even the entire universe in the moment following the Big Bang. Meanwhile, theory had become a real help to traditional solid-state technologies like metallurgy and inspired entire new classes of composite materials.
Experiment and theory, seemingly mature, continued to produce surprises. One spectacular advance, pointing to a future technology of submicroscopic machinery, was the development in the early 1980s of scanning micro-scopes. These could detect individual atoms on a surface, or nudge them into preferred configurations. Another discovery at that time was the Quantum Hall Effect: jumps of conductivity that allowed fundamental measurements with extraordinary precision. Later, a startling discovery at Bell Laboratories—using semiconductor crystals of unprecedented quality—revealed a new state of matter: a Quantum Hall Effect experiment showed highly correlated "quasiparticles" carrying only fractions of an electron's charge.
For research that could be considered fundamental, attention increasingly turned toward condensed matter systems with quantized entities such as cooperatively interacting swarms of electrons, seen especially at very low temperatures. The physics community was galvanized in 1986 when scientists at IBM's Zurich laboratory announced their discovery of super conductivity in a ceramic material, at temperatures higher than any previous super conductor. The established way of studying solids had been to pursue the simplest possible systems, but this showed that more complex structures could display startling new properties all their own. The study of "high-temperature" super conductivity has led to new concepts and techniques as well as hosts of new materials, including ones that super conduct at temperatures an order of magnitude higher than anything known before 1986. Many novel applications for microelectronics have grown from this field. Equally fascinating was the creation in the 1990s of microscopic clouds of "Bose-Einstein condensed" gases, in which the atoms behave collectively as a single quantum entity.
Most of this work depended on electronic computers: the field was advancing with the aid of its own applications. With new theoretical ideas and techniques developed in the 1960s, calculations of electronic structures became routine during the 1970s. In the 1980s, numerical simulations began to approach the power of experiment itself. This was most visible where the study of chaos and nonequilibrium phenomena, as in the phase transition of a melting solid, brought new understanding of many phenomena. There was steady progress in unraveling the old, great puzzle of fluids—turbulence—although here much remained unsolved. Studies of disorder also led to improved materials and new devices, such as the liquid crystal displays that turned up in items on super market shelves. Magnetism was studied with special intensity because of its importance in computer memories.
Physicists also cooperated with chemists to study polymers, and edged toward the study of proteins and other biological substances. Spider silk still beat anything a physicist could make. But the discovery that carbon atoms could be assembled in spheres (as in buckminsterfullerene) and tubes held hopes for fantastic new materials.
Some research problems now required big, expensive facilities. Ever since the 1950s, neutron beams from nuclear reactors had been useful to some research teams. A larger step toward "big science" came with the construction of machines resembling the accelerators of high-energy physics that emitted beams of high-intensity radiation to probe matter. The National Synchrotron Light Source, starting up in 1982 in Brookhaven, New York, was followed by a half-dozen more in the United States and abroad. Yet most condensed-matter research continued to be done by small, intimate groups in one or two rooms.
In the 1990s the steep long-term rise of funding for basic research in the field leveled off. Military support waned with the Cold War, while intensified commercial competition impelled industrial leaders like Bell Labs to emphasize research with near-term benefits. The community continued to grow gradually along with other fields of research, no longer among the fastest. By 2001 the American Physical Society division had some 5,000 members, largely from industry; as a fraction of the Society's membership, they had declined to one-eighth. This was still more than any other specialty, and represented much more high-level research in the field than any other country could muster.
The field's impact on day-to-day living continued to grow. The applications of condensed-matter physics were most conspicuous in information processing and communications, but had also become integral to warfare, health care, power generation, education, travel, finance, politics, and entertainment.
Bibliography
Hoddeson, Lillian, et al, eds. Out of the Crystal Maze: Chapters from the History of Solid-State Physics. New York: Oxford University Press, 1992. Extended essays by professional historians (some are highly technical).
Hoddeson, Lillian, and Vicki Daitch. True Genius: The Life and Science of John Bardeen. Washington, D.C.: The Joseph Henry Press, 2002. Includes an overview of solid-state physics for the general reader.
Kittel, Charles. Introduction to Solid-State Physics. New York: Wiley, 1953. In five editions to 1976, the classic graduate-student text book.
Mott, Sir Nevill, ed. The Beginnings of Solid-State Physics. A Symposium. London: The Royal Society; Great Neck, N.Y.: Scholium International, 1980. Reminiscences by pioneers of the 1930s–1960s.
National Research Council, Solid-State Sciences Panel. Research in Solid-State Sciences: Opportunities and Relevance to National Needs. Washington, D.C.: National Academy of Sciences, 1968. The state of U.S. physics fields has been reviewed at intervals by panels of leading physicists. Later reviews, by the National Academy of Sciences, are:
National Research Council, Physics Survey Committee. Physics in Perspective. Vol. II, part A, The Core Subfields of Physics. Washington, D.C.: National Academy of Sciences, 1973. See "Physics of Condensed Matter, " pp. 445–558.
National Research Council, Physics Survey Committee, Panel on Condensed-Matter Physics. Condensed-Matter Physics. In series, Physics Through the 1990s. Washington, D.C.: National Academy Press, 1986.
National Research Council, Committee on Condensed-Matter and Materials Physics. Condensed-Matter and Materials Physics: Basic Research for Tomorrow's Technology. In series, Physics in a New Era. Washington, D.C.: National Academy Press, 1999.
Riordan, Michael, and Lillian Hoddeson. Crystal Fire: The Birth of the Information Age. New York: Norton, 1997. For the general reader.
Weart, Spencer R., and Melba Phillips, eds. History of
