Physics

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.

	More from Encarta
	•  7 tips for funding an online degree •  How to succeed in the fashion industry without being a top designer •  Presidential Myths Quiz •  Coffee break: Recharge your brain