VI.

Developments in Physics Since 1930

The rapid expansion of physics in the last few decades was made possible by the fundamental developments during the first third of the century, coupled with recent technological advances, particularly in computer technology, electronics, nuclear-energy applications, and high-energy particle accelerators.

A.

Accelerators

Rutherford and other early investigators of nuclear properties were limited to the use of high-energy emissions from naturally radioactive substances to probe the atom. The first artificial high-energy emissions were produced in 1932 by the British physicist Sir John Douglas Cockcroft and the Irish physicist Ernest Thomas Sinton Walton, who used high-voltage generators to accelerate protons to about 700,000 eV and to bombard lithium with them, transmuting it into helium. One electron volt is the energy gained by an electron when the accelerating voltage is 1 V; it is equivalent to about 1.6 × 10^-19 joule (J). Modern accelerators produce energies measured in million electron volts (usually written mega-electron volts, or MeV), billion electron volts (giga-electron volts, or GeV), or trillion electron volts (tera-electron volts, or TeV). Higher-voltage sources were first made possible by the invention, also in 1932, of the Van de Graaff generator by the American physicist Robert van de Graaff.

This was followed almost immediately by the invention of the cyclotron by the American physicists Ernest Orlando Lawrence and Milton Stanley Livingston. The cyclotron uses a magnetic field to bend the trajectories of charged particles into circles, and during each half-revolution the particles are given a small electric “kick” until they accumulate the high energy level desired. Protons could be accelerated to about 10 MeV by a cyclotron, but higher energies had to await the development of the synchrotron after the end of World War II (1939-1945), based on the ideas of the American physicist Edwin Mattison McMillan and the Soviet physicist Vladimir I. Veksler. After World War II, accelerator design made rapid progress, and accelerators of many types were built, producing high-energy beams of electrons, protons, deuterons, heavier ions, and X rays. For example, the accelerator at the Stanford Linear Accelerator Center (SLAC) in Stanford, California, accelerates electrons down a straight “runway,” 3.2 km (2 mi) long, at the end of which they attain an energy of more than 20 GeV.

While lower-energy accelerators are used in various applications in industry and laboratories, the most powerful ones are used in studying the structure of elementary particles, the fundamental building blocks of nature. In such studies elementary particles are broken up by hitting them with beams of projectiles that are usually protons or electrons. The distribution of the fragments yields information on the structure of the elementary particles.

To obtain more detailed information in this manner, the use of more energetic projectiles is necessary. Since the acceleration of a projectile is achieved by “pushing” it from behind, to obtain more energetic projectiles it is necessary to keep pushing for a longer time. Thus, high-energy accelerators are generally larger in size. The highest beam energy reached at the end of World War II was less than 100 MeV. A bigger accelerator, reaching 3 GeV, was built in the early 1950s at the Brookhaven National Laboratory at Upton, New York. A breakthrough in accelerator design occurred with the introduction of the strong focusing principle in 1952 by the American physicists Ernest D. Courant, Livingston, and Hartland S. Snyder. Today the world's largest accelerators have been or are being built to produce beams of protons beyond 1 TeV. Two are located at the Fermi National Accelerator Laboratory, near Batavia, Illinois, and at the European Organization for Nuclear Research, known as CERN, in Geneva, Switzerland. See Particle Accelerators.

B.

Particle Detectors

Detection and analysis of elementary particles were first accomplished through the ability of these particles to affect photographic emulsions and to energize fluorescent materials. The actual paths of ionized particles were first observed by the British physicist Charles Thomson Rees Wilson in a cloud chamber, where water droplets condensed on the ions produced by the particles during their passage. Electric or magnetic fields can be used to bend the particle paths, yielding information about their momentum and electric charges. A significant advance on the cloud chamber was the construction of the bubble chamber by the American physicist Donald Arthur Glaser in 1952. It uses a liquid, usually hydrogen, instead of air, and the ions produced by a fast particle become centers of boiling, leaving an observable bubble track. Because the density of the liquid is much higher than that of air, more interactions take place in a bubble chamber than in a cloud chamber. Furthermore, the bubbles clear out faster than water droplets, allowing more frequent cycling of the bubble chamber. A third development, the spark chamber, evolved in the 1950s. In this device, many parallel plates are kept at a high voltage in a suitable gas atmosphere. An ionizing particle passing between the plates breaks down the gas, forming sparks that delineate its path.

A different type of detector, the discharge counter, was developed early during the 20th century, largely by the German physicist Hans Wilhelm Geiger, and later improved by the German American physicist Walther Müller. It is now commonly known as the Geiger-Müller counter, and although small and convenient, it has been largely replaced by faster and more convenient solid-state counting devices, such as the scintillation counter, developed about 1947 by the German American physicist Hartmut Paul Kallmann and others. It uses the ability of ionized particles to produce a flash of light as they pass through certain organic crystals and liquids. See Particle Detectors.

C.

Cosmic Rays

About 1911 the Austrian-American physicist Victor Franz Hess discovered that cosmic radiation, consisting of rays originating outside the earth's atmosphere, arrived in a pattern determined by the earth's magnetic field (see Cosmic Rays). The rays were found to be positively charged and to consist mostly of protons with energies ranging from about 1 GeV to 10¹¹ GeV (compared to about 30 GeV for the fastest particles produced by artificial accelerators). Cosmic rays trapped into orbits around the earth account for the Van Allen radiation belts discovered during an artificial-satellite flight in 1959 (see Radiation Belts).

When a very energetic primary proton smashes into the atmosphere and collides with the nitrogen and oxygen nuclei present, it produces large numbers of different secondary particles that spread toward the earth as a cosmic-ray shower. The origin of the cosmic-ray protons is not yet fully understood; some undoubtedly come from the sun and the other stars. Except for the slowest rays, however, no mechanism can be found to account for their high energies and the likelihood is that weak galactic fields operate over very long periods to accelerate interstellar protons (see Galaxy; Milky Way).

D.

Elementary Particles

To the electron, proton, neutron, and photon have been added a number of fundamental particles. In 1932 the American physicist Carl David Anderson discovered the antielectron, or positron, predicted in 1928 by Dirac. Anderson found that the stopping of an energetic cosmic gamma ray near a heavy nucleus yielded an electron-positron pair out of pure energy. When a positron subsequently meets an electron, they annihilate each other with a burst of photons of energy.

D.1.

Discovery of the Muon

In 1935 the Japanese physicist Yukawa Hideki developed a theory explaining how a nucleus is held together, despite the mutual repulsion of its protons, by postulating the existence of a particle intermediate in mass between the electron and the proton. In 1936 Anderson and his coworkers discovered a new particle of 207 electron masses in secondary cosmic radiation; now called the mu-meson or muon, it was first thought to be Yukawa's nuclear “glue.” Subsequent experiments by the British physicist Cecil Frank Powell and others led to the discovery of a somewhat heavier particle of 270 electron masses, the pi-meson or pion (also obtained from secondary cosmic radiation), which was eventually identified as the missing link in Yukawa's theory.

Many additional particles have since been found in secondary cosmic radiation and through the use of large accelerators. They include numerous massive particles, classed as hadrons (particles that take part in the “strong” interaction, which binds atomic nuclei together), including hyperons and various heavy mesons with masses ranging from about one to three proton masses; and intermediate vector bosons such as the W and Z⁰ particles, the carriers of the “weak” nuclear force. They may be electrically neutral, positive, or negative, but never have more than one elementary electric charge e. Enduring from 10^-8 to 10^-14 sec, they decay into a variety of lighter particles. Each particle has its antiparticle and carries some angular momentum. They all obey certain conservation laws involving quantum numbers, such as baryon number, strangeness, and isotopic spin.

In 1931 Pauli, in order to explain the apparent failure of some conservation laws in certain radioactive processes, postulated the existence of electrically neutral particles of zero-rest mass that nevertheless could carry energy and momentum. This idea was further developed by the Italian-born American physicist Enrico Fermi, who named the missing particle the neutrino. Uncharged and tiny, it is elusive, easily able to penetrate the entire earth with only a small likelihood of capture. Nevertheless, it was eventually discovered in a difficult experiment performed by the Americans Frederick Reines and Clyde Lorrain Cowan, Jr. Understanding of the internal structure of protons and neutrons has also been derived from the experiments of the American physicist Robert Hofstadter, using fast electrons from linear accelerators.

In the late 1940s a number of experiments with cosmic rays revealed new types of particles, the existence of which had not been anticipated. They were called strange particles, and their properties were studied intensively in the 1950s. Then, in the 1960s, many new particles were found in experiments with the large accelerators. The electron, proton, neutron, photon, and all the particles discovered since 1932 are collectively called elementary particles. But the term is actually a misnomer, for most of the particles, such as the proton, have been found to have very complicated internal structure.

Elementary particle physics is concerned with (1) the internal structure of these building blocks and (2) how they interact with one another to form nuclei. The physical principles that explain how atoms and molecules are built from nuclei and electrons are already known. At present, vigorous research is being conducted on both fronts in order to learn the physical principles upon which all matter is built.

One popular theory about the internal structure of elementary particles is that they are made of so-called quarks (see Quark), which are subparticles of fractional charge; a proton, for example, is made up of three quarks. This theory was first proposed in 1964 by the American physicists Murray Gell-Mann and George Zweig. The theory explains a number of phenomena, and physicists have collected a great deal of evidence of quarks in combinations with each other. No individual quarks have been observed, however, and current theory suggests that quarks may never be released as separate entities except under such extreme conditions as those found during the very creation of the universe. The theory postulated three kinds of quarks, but later experiments, especially the discovery of the J/psi particle in 1974 by the American physicists Samuel C. C. Ting and Burton Richter, called for the introduction of three additional kinds.

D.2.

Unified Field Theories

The interaction between elementary particles—and if quarks exist, between the quarks—is a more difficult area of research. The most successful theories, thus far, are called gauge theories. In these, the interaction between two kinds of particles is characterized by symmetry. The symmetry between neutrons and protons, for example, is such that if the identities of the particles are interchanged, nothing changes as far as the “strong” force is concerned. The first of the gauge theories applied to the electric and magnetic interactions between charged particles. Here, the symmetry consists in the fact that changes in the combination of electric and magnetic potentials have no effect on the results. A powerful gauge theory, which has since been verified, was that proposed independently by both the American physicist Steven Weinberg and the Pakistani physicist Abdus Salam in 1967 and 1968. Their model linked the intermediate vector boson with the photon, thus uniting the electromagnetic and weak interactions, although only for leptons. Later work by others (Sheldon Lee Glashow, J. Iliopolis, and L. Maiani) showed how the model could be applied to hadrons (the strongly interacting particles) as well.

Gauge theory, in principle, can be applied to any force field, holding out the possibility that all the interactions, or forces, can be brought together into a single unified field theory. Such efforts inevitably involve the concept of symmetry. Generalized symmetries extend to particle interchanges that vary from point to point in space and time. The difficulty for physicists is that such symmetries, while mathematically elegant, do not extend scientific understanding of the underlying nature of matter. For this reason, many physicists are exploring the possibilities of so-called supersymmetry theories, which would directly relate fermions and bosons to one another by postulating further particle “twins” to those now known, differing only in spin. Doubts have been expressed about such efforts, but another approach known as “superstring” theory is attracting a good deal of interest. In such theories, fundamental particles are considered not as dimensionless objects but as “strings” that extend one-dimensionally to lengths of no more than 10^-35 meters. Such theories solve a number of problems for the physicists who are working on unified field theories, but they are still only highly theoretical constructs.

E.

Nuclear Physics

In 1931 the American physicist Harold Clayton Urey discovered the hydrogen isotope deuterium and made heavy water from it. The deuterium nucleus, or deuteron (one proton plus one neutron), makes an excellent bombarding particle for inducing nuclear reactions. The French physicists Irène and Frédéric Joliot-Curie produced the first artificially radioactive nucleus in 1933 and 1934, leading to the production of radioisotopes for use in archaeology, biology, medicine, chemistry, and other sciences.

Fermi and many collaborators attempted a series of experiments to produce elements beyond uranium by bombarding uranium with neutrons. They succeeded, and now at least a dozen such transuranium elements have been made. As their work continued, an even more important discovery was made. Irène Joliot-Curie, the German physicists Otto Hahn and Fritz Strassmann, the Austrian physicist Lise Meitner, and the British physicist Otto Robert Frisch found that some uranium nuclei broke into two parts, a phenomenon called nuclear fission. At the same time, a huge amount of energy was released by mass conversion, as well as some neutrons. These results suggested the possibility of a self-sustained chain reaction, and this was achieved by Fermi and his group in 1942, when the first nuclear reactor went into operation. Technological developments followed rapidly; the first atomic bomb was produced in 1945 as a result of a massive program under the direction of the American physicist J. Robert Oppenheimer, and the first nuclear power reactor for the production of electricity went into operation in England in 1956, yielding 78 million watts. See Nuclear Weapons.

Further developments were based on the investigation of the energy source of the stars, which the German American physicist Hans Albrecht Bethe showed to be a series of nuclear reactions occurring at temperatures of millions of degrees. In these reactions, four hydrogen nuclei are converted into a helium nucleus, with two positrons and massive amounts of energy forming the by-products. This nuclear-fusion process was adopted in modified form, largely based on ideas developed by the Hungarian-American physicist Edward Teller, as the basis of the fusion or hydrogen bomb. First detonated in 1952, it is a weapon much more powerful than the fission bomb. A small fission bomb provides the high temperature necessary to trigger fusion of hydrogen.

Much current research is devoted to producing a controlled, rather than an explosive, fusion device, which would be less radioactive than a fission reactor and would provide an almost limitless source of energy. In December 1993 significant progress was made toward this goal when researchers at Princeton University used the Tokamak Fusion Test Reactor to produce a controlled fusion reaction that output 5.6 million watts of power. However, the tokamak consumed more power than it produced during its operation.

F.

Solid-State Physics

In solids, the atoms are closely packed, leading to strong interactive forces and numerous interrelated effects that are not observed in gases, where the molecules largely act independently. Interaction effects lead to the mechanical, thermal, electrical, magnetic, and optical properties of solids, which is an area that remains difficult to handle theoretically, although much progress has been made.

A principal characteristic of most solids is their crystalline structure, with the atoms arranged in regular and geometrically repeating arrays (see Crystal). The specific arrangement of the atoms may arise from a variety of forces; thus, some solids, such as sodium chloride, or common salt, are held together by ionic bonds originating in the electric attraction between the ions of which the materials are composed. In others, such as diamond, atoms share electrons, giving rise to covalent bonding. Inert substances, such as neon, exhibit neither of these bonds. Their existence is a result of the so-called van der Waals forces, named after the Dutch physicist Johannes Diderik van der Waals. These forces exist between neutral molecules or atoms as a result of electric polarization. Metals, on the other hand, are bonded by a so-called electron gas, or electrons that are freed from the outer atomic shell and shared by all atoms, and that define most properties of the metal (see Metallography; Metals).

The sharp, discrete energy levels permitted to the electrons in individual atoms become broadened into energy bands when the atoms become closely packed in a solid. The width and separation of these bands define many properties, and thus the separation by a so-called forbidden band, where no electrons may exist, restricts their motion and results in a good electric and thermal insulator. Overlapping energy bands and their associated ease of electron motion results in their being good conductors of electricity and heat. If the forbidden band is narrow, a few fast electrons may be able to jump across, yielding a semiconductor. In this case the energy-band spacing may be greatly affected by minute amounts of impurities, such as arsenic in silicon. The lowering of a high-energy band by the impurity results in a so-called donor of electrons, or an n-type semiconductor. The raising of a low-energy band by an impurity like gallium results in an acceptor, where the vacancies or “holes” in the electron structure act like movable positive charges and are characteristic of p-type semiconductors. A number of modern electronic devices, notably the transistor, developed by the American physicists John Bardeen, Walter Houser Brattain, and William Bradford Shockley, are based on these semiconductor properties.

Magnetic properties in a solid arise from the electrons' acting like tiny magnetic dipoles. Electron spin plays a big role in magnetism, leading to spin waves that have been observed in some solids. Almost all solid properties depend on temperature. Thus, ferromagnetic materials, including iron and nickel, lose their normal strong residual magnetism at a characteristic high temperature, called the Curie temperature. Electrical resistance usually decreases with decreasing temperature, and for certain materials, called superconductors, it becomes extremely low, near absolute zero. These and many other phenomena observed in solids depend on energy quantization and can best be described in terms of effective “particles” such as phonons, polarons, and magnons.

G.

Cryogenics

At very low temperatures (near absolute zero), many materials exhibit strikingly different characteristics (see Cryogenics). At the beginning of the 20th century the Dutch physicist Heike Kamerlingh Onnes developed techniques for producing these low temperatures and discovered the superconductivity of mercury: It loses all electrical resistance at about 4 K. Many other elements, alloys, and compounds do the same at their characteristic near-zero temperature, with originally magnetic materials becoming magnetic insulators. The theory of superconductivity, developed largely by the American physicists John Bardeen, Leon N. Cooper, and John Robert Schrieffer, is extremely complicated, involving the pairing of electrons in the crystal lattice.

Another fascinating discovery was that helium does not freeze but changes at about 2 K from an ordinary liquid, He I, to the superfluid He II, which has no viscosity and has a thermal conductivity about 1000 times greater than silver. Films of He II can creep up the walls of their containing vessels and He II can readily permeate some materials like platinum. No fully satisfactory theory is yet available for this behavior.

H.

Plasma Physics

A plasma is any substance (usually a gas) whose atoms have one or more electrons detached and therefore become ionized. The detached electrons remain, however, in the gas volume that in an overall sense remains electrically neutral. The ionization can be effected by the introduction of large concentrations of energy, such as bombardment with fast external electrons, irradiation with laser light, or by heating the gas to very high temperatures (see Laser). The individually charged plasma particles respond to electric and magnetic fields and can therefore be manipulated and contained.

Plasmas are found in gas-filled light sources, such as a neon lamp, in interstellar space where residual hydrogen is ionized by radiation, and in stars whose great interior temperatures produce a high degree of ionization, a process closely connected with the nuclear fusion that supplies the energy of stars. For the hydrogen nuclei to fuse into heavier nuclei, they must be fast enough to overcome their mutual electric repulsion. This implies high temperature (millions of degrees) when the hydrogen ionizes into a plasma. In order to produce a controlled fusion, or thermonuclear reaction, it is necessary to generate and contain plasmas magnetically; this is an important but difficult problem that falls in the field of magnetohydrodynamics.

I.

Lasers

An important recent development is that of the laser, an acronym for light amplification by stimulated emission of radiation. In lasers, which may have gases, liquids, or solids as the working substance, a large number of atoms are raised to a high energy level and caused to release this energy simultaneously, producing coherent light where all waves are in phase. Similar techniques are used for producing microwave emissions by the use of masers. The coherence of the light allows for very high intensity, sharp wavelength light beams that remain narrow over tremendous distances; they are far more intense than light from any other source. Continuous lasers can deliver hundreds of watts of power, and pulsed lasers can produce millions of watts of power for very short periods. Developed during the 1950s and 1960s, largely by the American engineer and inventor Gordon Gould and the American physicists Charles Hard Townes, T. H. Maiman, Arthur Leonard Schawlow, and Ali Javan, the laser today has become an extremely powerful tool in research and technology, with applications in communications, medicine, navigation, metallurgy, fusion, and material cutting.

J.

Astrophysics

The construction of large and specially designed optical telescopes has led to the discovery of new stellar objects, including a number of quasars, which are billions of light-years away, and has led to a better understanding of the structure of the universe. Radio astronomy has yielded other important discoveries, such as pulsars and the cosmic background radiation, which probably dates from the origin of the universe. The evolutionary history of the stars is now well understood in terms of nuclear reactions. As a result of recent observations and theoretical calculations, the belief is now widely held that all matter was originally in one dense location and that about 14 billion years ago it exploded in one titanic event often called the big bang. The aftereffects of the explosion have led to a universe that appears to be still expanding. A puzzling aspect of this universe, recently revealed, is that the galaxies are not uniformly distributed. Instead, vast voids are bordered by galactic clusters shaped like filaments. The pattern of these voids and filaments lends itself to nonlinear mathematical analysis of the sort used in chaos theory. See also Inflationary Theory.