Two major new developments during the first third of the 20th century, the quantum theory and the theory of relativity, explained these findings, yielded new discoveries, and changed the understanding of physics as it is known today.
Relativity
To extend the example of relative velocity introduced with the Michelson-Morley experiment, two situations can be compared. One consists of a person, A, walking forward with a velocity v in a train moving at velocity u. The velocity of A with regard to an observer B stationary on the ground is then simply V = u + v. If, however, the train were at rest in the station and A was moving forward with velocity v while observer B walked backward with velocity u, the relative speed between A and B would be exactly the same as in the first case. In more general terms, if two frames of reference are moving relative to each other at constant velocity, observations of any phenomena made by observers in either frame will be physically equivalent. As already mentioned, the Michelson-Morley experiment failed to confirm the concept of adding velocities, and two observers, one at rest and the other moving toward a light source with velocity u, both observe the same light velocity V, commonly denoted by the symbol c.
Einstein incorporated the invariance of c into his theory of relativity. He also demanded a very careful rethinking of the concepts of space and time, showing the imperfection of intuitive notions about them. As a consequence of his theory, it is known that two clocks that keep identical time when at rest relative to each other must run at different speeds when they are in relative motion, and two rods that are identical in length (at rest) will become different in length when they are in relative motion. Space and time must be closely linked in a four-dimensional continuum where the normal three-space dimensions must be augmented by an interrelated time dimension.
Two important consequences of Einstein's relativity theory are the equivalence of mass and energy and the limiting velocity of the speed of light for material objects. Relativistic mechanics describes the motion of objects with velocities that are appreciable fractions of the speed of light, while Newtonian mechanics remains useful for velocities typical of the macroscopic motion of objects on earth. No material object, however, can have a speed equal to or greater than the speed of light.
Even more important is the relation between the mass m and energy E. They are coupled by the relation E = mc2, and because c is very large, the energy equivalence of a given mass is enormous. The change of mass giving an energy change is significant in nuclear reactions, as in reactors or nuclear weapons, and in the stars, where a significant loss of mass accompanies the huge energy release.
Einstein's original theory, formulated in 1905 and known as the special theory of relativity, was limited to frames of reference moving at constant velocity relative to each other. In 1915, he generalized his hypothesis to formulate the general theory of relativity that applied to systems that accelerate with reference to each other. This extension showed gravitation to be a consequence of the geometry of space-time and predicted the bending of light in its passage close to a massive body like a star, an effect first observed in 1919. General relativity, although less firmly established than the special theory, has deep significance for an understanding of the structure of the universe and its evolution. . also Cosmology.
Quantum Theory
The quandary posed by the observed spectra emitted by solid bodies was first explained by the German physicist Max Planck. According to classical physics, all molecules in a solid can vibrate with the amplitude of the vibrations directly related to the temperature. All vibration frequencies should be possible and the thermal energy of the solid should be continuously convertible into electromagnetic radiation as long as energy is supplied. Planck made a radical assumption by postulating that the molecular oscillator could emit electromagnetic waves only in discrete bundles, now called quanta, or photons. . Photon; Quantum Theory. Each photon has a characteristic wavelength in the spectrum and an energy E given by E = hf, where f is the frequency of the wave. The wavelength l related to the frequency by l f = c, where c is the speed of light. With the frequency specified in hertz (Hz), or cycles per second, h, now known as Planck's constant, is extremely small (6.626 × 10-27 erg-sec). With his theory, Planck again introduced a partial duality into the theory of light, which for nearly a century had been considered to be wavelike only.
Photoelectricity
If electromagnetic radiation of appropriate wavelength falls upon suitable metals, negative electric charges, later identified as electrons, are ejected from the metal surface. The important aspects of this phenomenon are the following: (1) the energy of each photoelectron depends only on the frequency of the illumination and not on its intensity; (2) the rate of electron emission depends only on the illuminating intensity and not on the frequency (provided that the minimum frequency to cause emission is exceeded); and (3) the photoelectrons emerge as soon as the illumination hits the surface. These observations, which could not be explained by Maxwell's electromagnetic theory of light, led Einstein to assume in 1905 that light can be absorbed only in quanta or photons, and that the photon completely vanishes in the absorption process, with all of its energy E (=hf) going to one electron in the metal. With this simple assumption Einstein extended Planck's quantum theory to the absorption of electromagnetic radiation, giving additional importance to the wave-particle duality of light. It was for this work that Einstein was awarded the 1921 Nobel Prize in physics.
X Rays
These very penetrating rays, first discovered by Roentgen, were shown to be electromagnetic radiation of very short wavelength in 1912 by the German physicist Max Theodor Felix von Laue and his coworkers. The precise mechanism of X-ray production was shown to be a quantum effect, and in 1914 the British physicist Henry Gwyn-Jeffreys Moseley used his X-ray spectrograms to prove that the atomic number of an element, and hence the number of positive charges in an atom, is the same as its position in the periodic table (. Periodic Law). The photon theory of electromagnetic radiation was further strengthened and developed by the prediction and observation of the so-called Compton effect by the American physicist Arthur Holly Compton in 1923.
Electron Physics
That electric charges were carried by extremely small particles had already been suspected in the 19th century and, as indicated by electrochemical experiments, the charge of these elementary particles was a definite, invariant quantity. Experiments on the conduction of electricity through low-pressure gases led to the discovery of two kinds of rays: cathode rays, coming from the negative electrode in a gas discharge tube, and positive or canal rays from the positive electrode. Sir Joseph John Thomson's 1895 experiment measured the ratio of the charge q to the mass m of the cathode-ray particles. Lenard in 1899 confirmed that the ratio of q to m for photoelectric particles was identical to that of cathode rays. The American inventor Thomas Alva Edison had noted in 1883 that very hot wires emit electricity, called thermionic emission (now called the Edison effect), and in 1899 Thomson showed that this form of electricity also consisted of particles with the same q to m ratio as the others. About 1911 Millikan finally determined that electric charge always arises in multiples of a basic unit e, and measured the value of e, now known to be 1.602 × 10-19 coulombs. From the measured value of q to m ratio, with q set equal to e, the mass of the carrier, called electron, could now be determined as 9.110 × 10-31 kg.
Finally, Thomson and others showed that the positive rays also consisted of particles, each carrying a charge e, but of the positive variety. These particles, however, now recognized as positive ions resulting from the removal of an electron from a neutral atom, are much more massive than the electron. The smallest, the hydrogen ion, is a single proton with a mass of 1.673 × 10-27 kg, about 1837 times more massive than the electron (. Ion; Ionization). The "quantized" nature of electric charge was now firmly established and, at the same time, two of the fundamental subatomic particles identified.
Atomic Models
In 1913 the New Zealand-born British physicist Ernest Rutherford, making use of the newly discovered radiations from radioactive nuclei, found Thomson's earlier model of an atom with uniformly distributed positive and negative charged particles to be untenable. The very fast, massive, positively charged alpha particles he employed were found to deflect sharply in their passage through matter. This effect required an atomic model with a heavy positive scattering center. Rutherford then suggested that the positive charge of an atom was concentrated in a massive stationary nucleus, with the negative electron moving in orbits about it, and positioned by the electric attraction between opposite charges. This solar-system-like atomic model, however, could not persist according to Maxwell's theory, where the revolving electrons should emit electromagnetic radiation and force a total collapse of the system in a very short time.
Another sharp break with classical physics was required at this point. It was provided by the Danish physicist Niels Henrik David Bohr, who postulated the existence within atoms of certain specified orbits in which electrons could revolve without electromagnetic radiation emission. These allowed orbits, or so-called stationary states, are determined by the condition that the angular momentum J of the orbiting electron must be a positive multiple integral of Planck's constant, divided by 2 p, that is, J = nh/2p, where the quantum number n may have any positive integer value. This extended "quantization" to dynamics, fixed the possible orbits, and allowed Bohr to calculate their radii and the corresponding energy levels. Also in 1913 the model was confirmed experimentally by the German-born American physicist James Franck and the German physicist Gustav Hertz.
Bohr developed his model much further. He explained how atoms radiate light and other electromagnetic waves, and also proposed that an electron "lifted" by a sufficient disturbance of the atom from the orbit of smallest radius and least energy (the ground state) into another orbit, would soon "fall" back to the ground state. This falling back is accompanied by the emission of a single photon of energy E = hf, where E is the difference in energy between the higher and lower orbits. Each orbit shift emits a characteristic photon of sharply defined frequency and wavelength; thus one photon would be emitted in a direct shift from the n = 3 to the n = 1 orbit, which will be quite different from the two photons emitted in a sequential shift from the n = 3 to n = 2 orbit, and then from there to the n = 1 orbit. This model now allowed Bohr to account with great accuracy for the simplest atomic spectrum, that of hydrogen, which had defied classical physics.
Although Bohr's model was extended and refined, it could not explain observations for atoms with more than one electron. It could not even account for the intensity of the spectral colors of the simple hydrogen atom. Because it had no more than a limited ability to predict experimental results, it remained unsatisfactory for theoretical physicists.
Quantum Mechanics
Within a few years, roughly between 1924 and 1930, an entirely new theoretical approach to dynamics was developed to account for subatomic behavior. Named quantum mechanics or wave mechanics, it started with the suggestion in 1924 by the French physicist Louis Victor, Prince de Broglie, that not only electromagnetic radiation but matter could also have wave as well as particle aspects. The wavelength of the so-called matter waves associated with a particle is given by the equation l = h/mv, where m is the particle mass and v its velocity. Matter waves were conceived of as pilot waves guiding the particle motion, a property that should result in diffraction under suitable conditions. This was confirmed in 1927 by the experiments on electron-crystal interactions by the American physicists Clinton Joseph Davisson and Lester Halbert Germer and the British physicist George Paget Thomson. Subsequently, Werner Heisenberg, Max Born, and Ernst Pascual Jordan of Germany and the Austrian physicist Erwin Schrödinger developed Broglie's idea into a mathematical form capable of dealing with a number of physical phenomena and with problems that could not be handled by classical physics. In addition to confirming Bohr's postulate regarding the quantization of energy levels in atoms, quantum mechanics now provides an understanding of the most complex atoms, and has also been a guiding spirit in nuclear physics. Although quantum mechanics is usually needed only on the microscopic level (with Newtonian mechanics still satisfactory for macroscopic systems), certain macroscopic effects, such as the properties of crystalline solids, also exist that can only be satisfactorily explained by principles of quantum mechanics.
Going beyond Broglie's notion of the wave-particle duality of matter, additional important concepts have since been incorporated into the quantum-mechanical picture. These include the discovery that electrons must have some permanent magnetism and, with it, an intrinsic angular momentum, or spin, as a fundamental property. Spin was subsequently found in almost all other elementary particles. In 1925 the Austrian physicist Wolfgang Pauli expounded the exclusion principle, which states that in an atom no two electrons can have precisely the same set of quantum numbers. Four quantum numbers are needed to specify completely the state of an electron in an atom. The exclusion principle is vital for an understanding of the structure of the elements and of the periodic table. Heisenberg in 1927 put forth the uncertainty principle, which asserted the existence of a natural limit to the precision with which certain pairs of physical quantities can be known simultaneously.
Finally, a synthesis of quantum mechanics and relativity was made in 1928 by the British mathematical physicist Paul Adrien Maurice Dirac, leading to the prediction of the existence of the positron and bringing the development of quantum mechanics to a culmination.
Largely as a result of Bohr's ideas, a different and statistical approach developed in modern physics. The fully deterministic cause-effect relations produced by Newtonian mechanics were supplanted by predictions of future events in terms of statistical probability only. Thus, the wave property of matter also implies that, in accordance with the uncertainty principle, the motion of the particles can never be predicted with absolute certainty even if the forces are known completely. Although this statistical aspect plays no detectable role in macroscopic motions, it is dominant on the molecular, atomic, and subatomic scale.
Nuclear Physics
The understanding of atomic structure was also facilitated by Becquerel's discovery in 1896 of radioactivity in uranium ore (. Uranium). Within a few years radioactive radiation was found to consist of three types of emissions: alpha rays, later found by Rutherford to be the nuclei of helium atoms; beta rays, shown by Becquerel to be very fast electrons; and gamma rays, identified later as very short wavelength electromagnetic radiation. In 1898 the French physicists Marie and Pierre Curie separated two highly radioactive elements, radium and polonium, from uranium ore, thus showing that radiations could be identified with particular elements. By 1903 Rutherford and the British physical chemist Frederick Soddy had shown that the emission of alpha or beta rays resulted in the transmutation of the emitting element into a different one. Radioactive processes were shortly thereafter found to be completely statistical; no method exists that could indicate which atom in a radioactive material will decay at any one time. These developments, in addition to leading to Rutherford's and Bohr's model of the atom, also suggested that alpha, beta, and gamma rays could only come from the nuclei of very heavy atoms. In 1919 Rutherford bombarded nitrogen with alpha particles and converted it to hydrogen and oxygen, thus producing the first artificial transmutation of elements.
Meanwhile, a knowledge of the nature and abundance of isotopes was growing, largely through the development of the mass spectrograph. A model emerged in which the nucleus contained all the positive charge and almost all the mass of the atom. The nuclear-charge carriers were identified as protons, but except for hydrogen, the nuclear mass could be accounted for only if some additional uncharged particles were present. In 1932 the British physicist Sir James Chadwick discovered the neutron, an electrically neutral particle of mass 1.675 × 10-27 kg, slightly more than that of the proton. Now nuclei could be understood as consisting of protons and neutrons, collectively called nucleons, and the atomic number of the element was simply the number of protons in the nucleus. On the other hand, the isotope number, also called the atomic mass number, was the sum of the neutrons and protons present. Thus, all atoms of oxygen (atomic no. 8) have eight protons, but the three isotopes of oxygen, O16, O17, and O18, also contain within their respective nuclei eight, nine, or ten neutrons.
Positive electric charges repel each other, and because atomic nuclei (except for hydrogen) have more than one proton, they would fly apart except for a strong attractive force, called the nuclear force, or strong interaction that binds the nucleons to each other. The energy associated with this strong force is very great, millions of times greater than the energies characteristic of electrons in their orbits or chemical binding energies. An escaping alpha particle (consisting of two protons and two neutrons), therefore, will have to overcome this strong interaction force to escape from a radioactive nucleus such as uranium. This apparent paradox was explained by the American physicists Edward U. Condon, George Gamow, and Ronald Wilfred Gurney, who applied quantum mechanics to the problem of alpha emission in 1928 and showed that the statistical nature of nuclear processes allowed alpha particles to "leak" out of radioactive nuclei, even though their average energy was insufficient to overcome the nuclear force. Beta decay was explained as a result of a neutron disruption within the nucleus, the neutron changing into an electron (the beta particle), which is promptly ejected, and a residual proton. The proton left behind leaves the "daughter" nucleus with one more proton than its "parent" and thus increases the atomic number and the position in the periodic table. Alpha or beta emission usually leaves the nucleus with excess energy, which it unloads by emitting a gamma-ray photon.
In all these nuclear processes a large amount of energy, given by Einstein's E = mc2 equation, is released. After the process is over, the total mass of the product is less than that of the parent, with the mass difference appearing as energy. . Nuclear Energy.
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.
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 J. 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 Laboratory for Particle Physics, known as CERN, in Geneva, Switzerland. . Particle Accelerators.
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 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. . Particle Detectors.
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 (. 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 (. 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 (. Galaxy; Milky Way).
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.
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 (. 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. Despite the theory's ability to explain a number of phenomena, no quarks have yet been found, 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.
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.
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. . 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 providing the necessary high triggering temperature.
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.
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 (. 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 (. 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.
Cryogenics
At very low temperatures (near absolute zero), many materials exhibit strikingly different characteristics (. 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.
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 (. 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.
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.
Astrophysics
The construction of large and specially designed optical telescopes has led to the discovery of new stellar objects, including 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 interstellar 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 between 10 and 20 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. . also Inflationary Theory.
Relativity
To extend the example of relative velocity introduced with the Michelson-Morley experiment, two situations can be compared. One consists of a person, A, walking forward with a velocity v in a train moving at velocity u. The velocity of A with regard to an observer B stationary on the ground is then simply V = u + v. If, however, the train were at rest in the station and A was moving forward with velocity v while observer B walked backward with velocity u, the relative speed between A and B would be exactly the same as in the first case. In more general terms, if two frames of reference are moving relative to each other at constant velocity, observations of any phenomena made by observers in either frame will be physically equivalent. As already mentioned, the Michelson-Morley experiment failed to confirm the concept of adding velocities, and two observers, one at rest and the other moving toward a light source with velocity u, both observe the same light velocity V, commonly denoted by the symbol c.
Einstein incorporated the invariance of c into his theory of relativity. He also demanded a very careful rethinking of the concepts of space and time, showing the imperfection of intuitive notions about them. As a consequence of his theory, it is known that two clocks that keep identical time when at rest relative to each other must run at different speeds when they are in relative motion, and two rods that are identical in length (at rest) will become different in length when they are in relative motion. Space and time must be closely linked in a four-dimensional continuum where the normal three-space dimensions must be augmented by an interrelated time dimension.
Two important consequences of Einstein's relativity theory are the equivalence of mass and energy and the limiting velocity of the speed of light for material objects. Relativistic mechanics describes the motion of objects with velocities that are appreciable fractions of the speed of light, while Newtonian mechanics remains useful for velocities typical of the macroscopic motion of objects on earth. No material object, however, can have a speed equal to or greater than the speed of light.
Even more important is the relation between the mass m and energy E. They are coupled by the relation E = mc2, and because c is very large, the energy equivalence of a given mass is enormous. The change of mass giving an energy change is significant in nuclear reactions, as in reactors or nuclear weapons, and in the stars, where a significant loss of mass accompanies the huge energy release.
Einstein's original theory, formulated in 1905 and known as the special theory of relativity, was limited to frames of reference moving at constant velocity relative to each other. In 1915, he generalized his hypothesis to formulate the general theory of relativity that applied to systems that accelerate with reference to each other. This extension showed gravitation to be a consequence of the geometry of space-time and predicted the bending of light in its passage close to a massive body like a star, an effect first observed in 1919. General relativity, although less firmly established than the special theory, has deep significance for an understanding of the structure of the universe and its evolution. . also Cosmology.
Quantum Theory
The quandary posed by the observed spectra emitted by solid bodies was first explained by the German physicist Max Planck. According to classical physics, all molecules in a solid can vibrate with the amplitude of the vibrations directly related to the temperature. All vibration frequencies should be possible and the thermal energy of the solid should be continuously convertible into electromagnetic radiation as long as energy is supplied. Planck made a radical assumption by postulating that the molecular oscillator could emit electromagnetic waves only in discrete bundles, now called quanta, or photons. . Photon; Quantum Theory. Each photon has a characteristic wavelength in the spectrum and an energy E given by E = hf, where f is the frequency of the wave. The wavelength l related to the frequency by l f = c, where c is the speed of light. With the frequency specified in hertz (Hz), or cycles per second, h, now known as Planck's constant, is extremely small (6.626 × 10-27 erg-sec). With his theory, Planck again introduced a partial duality into the theory of light, which for nearly a century had been considered to be wavelike only.
Photoelectricity
If electromagnetic radiation of appropriate wavelength falls upon suitable metals, negative electric charges, later identified as electrons, are ejected from the metal surface. The important aspects of this phenomenon are the following: (1) the energy of each photoelectron depends only on the frequency of the illumination and not on its intensity; (2) the rate of electron emission depends only on the illuminating intensity and not on the frequency (provided that the minimum frequency to cause emission is exceeded); and (3) the photoelectrons emerge as soon as the illumination hits the surface. These observations, which could not be explained by Maxwell's electromagnetic theory of light, led Einstein to assume in 1905 that light can be absorbed only in quanta or photons, and that the photon completely vanishes in the absorption process, with all of its energy E (=hf) going to one electron in the metal. With this simple assumption Einstein extended Planck's quantum theory to the absorption of electromagnetic radiation, giving additional importance to the wave-particle duality of light. It was for this work that Einstein was awarded the 1921 Nobel Prize in physics.
X Rays
These very penetrating rays, first discovered by Roentgen, were shown to be electromagnetic radiation of very short wavelength in 1912 by the German physicist Max Theodor Felix von Laue and his coworkers. The precise mechanism of X-ray production was shown to be a quantum effect, and in 1914 the British physicist Henry Gwyn-Jeffreys Moseley used his X-ray spectrograms to prove that the atomic number of an element, and hence the number of positive charges in an atom, is the same as its position in the periodic table (. Periodic Law). The photon theory of electromagnetic radiation was further strengthened and developed by the prediction and observation of the so-called Compton effect by the American physicist Arthur Holly Compton in 1923.
Electron Physics
That electric charges were carried by extremely small particles had already been suspected in the 19th century and, as indicated by electrochemical experiments, the charge of these elementary particles was a definite, invariant quantity. Experiments on the conduction of electricity through low-pressure gases led to the discovery of two kinds of rays: cathode rays, coming from the negative electrode in a gas discharge tube, and positive or canal rays from the positive electrode. Sir Joseph John Thomson's 1895 experiment measured the ratio of the charge q to the mass m of the cathode-ray particles. Lenard in 1899 confirmed that the ratio of q to m for photoelectric particles was identical to that of cathode rays. The American inventor Thomas Alva Edison had noted in 1883 that very hot wires emit electricity, called thermionic emission (now called the Edison effect), and in 1899 Thomson showed that this form of electricity also consisted of particles with the same q to m ratio as the others. About 1911 Millikan finally determined that electric charge always arises in multiples of a basic unit e, and measured the value of e, now known to be 1.602 × 10-19 coulombs. From the measured value of q to m ratio, with q set equal to e, the mass of the carrier, called electron, could now be determined as 9.110 × 10-31 kg.
Finally, Thomson and others showed that the positive rays also consisted of particles, each carrying a charge e, but of the positive variety. These particles, however, now recognized as positive ions resulting from the removal of an electron from a neutral atom, are much more massive than the electron. The smallest, the hydrogen ion, is a single proton with a mass of 1.673 × 10-27 kg, about 1837 times more massive than the electron (. Ion; Ionization). The "quantized" nature of electric charge was now firmly established and, at the same time, two of the fundamental subatomic particles identified.
Atomic Models
In 1913 the New Zealand-born British physicist Ernest Rutherford, making use of the newly discovered radiations from radioactive nuclei, found Thomson's earlier model of an atom with uniformly distributed positive and negative charged particles to be untenable. The very fast, massive, positively charged alpha particles he employed were found to deflect sharply in their passage through matter. This effect required an atomic model with a heavy positive scattering center. Rutherford then suggested that the positive charge of an atom was concentrated in a massive stationary nucleus, with the negative electron moving in orbits about it, and positioned by the electric attraction between opposite charges. This solar-system-like atomic model, however, could not persist according to Maxwell's theory, where the revolving electrons should emit electromagnetic radiation and force a total collapse of the system in a very short time.
Another sharp break with classical physics was required at this point. It was provided by the Danish physicist Niels Henrik David Bohr, who postulated the existence within atoms of certain specified orbits in which electrons could revolve without electromagnetic radiation emission. These allowed orbits, or so-called stationary states, are determined by the condition that the angular momentum J of the orbiting electron must be a positive multiple integral of Planck's constant, divided by 2 p, that is, J = nh/2p, where the quantum number n may have any positive integer value. This extended "quantization" to dynamics, fixed the possible orbits, and allowed Bohr to calculate their radii and the corresponding energy levels. Also in 1913 the model was confirmed experimentally by the German-born American physicist James Franck and the German physicist Gustav Hertz.
Bohr developed his model much further. He explained how atoms radiate light and other electromagnetic waves, and also proposed that an electron "lifted" by a sufficient disturbance of the atom from the orbit of smallest radius and least energy (the ground state) into another orbit, would soon "fall" back to the ground state. This falling back is accompanied by the emission of a single photon of energy E = hf, where E is the difference in energy between the higher and lower orbits. Each orbit shift emits a characteristic photon of sharply defined frequency and wavelength; thus one photon would be emitted in a direct shift from the n = 3 to the n = 1 orbit, which will be quite different from the two photons emitted in a sequential shift from the n = 3 to n = 2 orbit, and then from there to the n = 1 orbit. This model now allowed Bohr to account with great accuracy for the simplest atomic spectrum, that of hydrogen, which had defied classical physics.
Although Bohr's model was extended and refined, it could not explain observations for atoms with more than one electron. It could not even account for the intensity of the spectral colors of the simple hydrogen atom. Because it had no more than a limited ability to predict experimental results, it remained unsatisfactory for theoretical physicists.
Quantum Mechanics
Within a few years, roughly between 1924 and 1930, an entirely new theoretical approach to dynamics was developed to account for subatomic behavior. Named quantum mechanics or wave mechanics, it started with the suggestion in 1924 by the French physicist Louis Victor, Prince de Broglie, that not only electromagnetic radiation but matter could also have wave as well as particle aspects. The wavelength of the so-called matter waves associated with a particle is given by the equation l = h/mv, where m is the particle mass and v its velocity. Matter waves were conceived of as pilot waves guiding the particle motion, a property that should result in diffraction under suitable conditions. This was confirmed in 1927 by the experiments on electron-crystal interactions by the American physicists Clinton Joseph Davisson and Lester Halbert Germer and the British physicist George Paget Thomson. Subsequently, Werner Heisenberg, Max Born, and Ernst Pascual Jordan of Germany and the Austrian physicist Erwin Schrödinger developed Broglie's idea into a mathematical form capable of dealing with a number of physical phenomena and with problems that could not be handled by classical physics. In addition to confirming Bohr's postulate regarding the quantization of energy levels in atoms, quantum mechanics now provides an understanding of the most complex atoms, and has also been a guiding spirit in nuclear physics. Although quantum mechanics is usually needed only on the microscopic level (with Newtonian mechanics still satisfactory for macroscopic systems), certain macroscopic effects, such as the properties of crystalline solids, also exist that can only be satisfactorily explained by principles of quantum mechanics.
Going beyond Broglie's notion of the wave-particle duality of matter, additional important concepts have since been incorporated into the quantum-mechanical picture. These include the discovery that electrons must have some permanent magnetism and, with it, an intrinsic angular momentum, or spin, as a fundamental property. Spin was subsequently found in almost all other elementary particles. In 1925 the Austrian physicist Wolfgang Pauli expounded the exclusion principle, which states that in an atom no two electrons can have precisely the same set of quantum numbers. Four quantum numbers are needed to specify completely the state of an electron in an atom. The exclusion principle is vital for an understanding of the structure of the elements and of the periodic table. Heisenberg in 1927 put forth the uncertainty principle, which asserted the existence of a natural limit to the precision with which certain pairs of physical quantities can be known simultaneously.
Finally, a synthesis of quantum mechanics and relativity was made in 1928 by the British mathematical physicist Paul Adrien Maurice Dirac, leading to the prediction of the existence of the positron and bringing the development of quantum mechanics to a culmination.
Largely as a result of Bohr's ideas, a different and statistical approach developed in modern physics. The fully deterministic cause-effect relations produced by Newtonian mechanics were supplanted by predictions of future events in terms of statistical probability only. Thus, the wave property of matter also implies that, in accordance with the uncertainty principle, the motion of the particles can never be predicted with absolute certainty even if the forces are known completely. Although this statistical aspect plays no detectable role in macroscopic motions, it is dominant on the molecular, atomic, and subatomic scale.
Nuclear Physics
The understanding of atomic structure was also facilitated by Becquerel's discovery in 1896 of radioactivity in uranium ore (. Uranium). Within a few years radioactive radiation was found to consist of three types of emissions: alpha rays, later found by Rutherford to be the nuclei of helium atoms; beta rays, shown by Becquerel to be very fast electrons; and gamma rays, identified later as very short wavelength electromagnetic radiation. In 1898 the French physicists Marie and Pierre Curie separated two highly radioactive elements, radium and polonium, from uranium ore, thus showing that radiations could be identified with particular elements. By 1903 Rutherford and the British physical chemist Frederick Soddy had shown that the emission of alpha or beta rays resulted in the transmutation of the emitting element into a different one. Radioactive processes were shortly thereafter found to be completely statistical; no method exists that could indicate which atom in a radioactive material will decay at any one time. These developments, in addition to leading to Rutherford's and Bohr's model of the atom, also suggested that alpha, beta, and gamma rays could only come from the nuclei of very heavy atoms. In 1919 Rutherford bombarded nitrogen with alpha particles and converted it to hydrogen and oxygen, thus producing the first artificial transmutation of elements.
Meanwhile, a knowledge of the nature and abundance of isotopes was growing, largely through the development of the mass spectrograph. A model emerged in which the nucleus contained all the positive charge and almost all the mass of the atom. The nuclear-charge carriers were identified as protons, but except for hydrogen, the nuclear mass could be accounted for only if some additional uncharged particles were present. In 1932 the British physicist Sir James Chadwick discovered the neutron, an electrically neutral particle of mass 1.675 × 10-27 kg, slightly more than that of the proton. Now nuclei could be understood as consisting of protons and neutrons, collectively called nucleons, and the atomic number of the element was simply the number of protons in the nucleus. On the other hand, the isotope number, also called the atomic mass number, was the sum of the neutrons and protons present. Thus, all atoms of oxygen (atomic no. 8) have eight protons, but the three isotopes of oxygen, O16, O17, and O18, also contain within their respective nuclei eight, nine, or ten neutrons.
Positive electric charges repel each other, and because atomic nuclei (except for hydrogen) have more than one proton, they would fly apart except for a strong attractive force, called the nuclear force, or strong interaction that binds the nucleons to each other. The energy associated with this strong force is very great, millions of times greater than the energies characteristic of electrons in their orbits or chemical binding energies. An escaping alpha particle (consisting of two protons and two neutrons), therefore, will have to overcome this strong interaction force to escape from a radioactive nucleus such as uranium. This apparent paradox was explained by the American physicists Edward U. Condon, George Gamow, and Ronald Wilfred Gurney, who applied quantum mechanics to the problem of alpha emission in 1928 and showed that the statistical nature of nuclear processes allowed alpha particles to "leak" out of radioactive nuclei, even though their average energy was insufficient to overcome the nuclear force. Beta decay was explained as a result of a neutron disruption within the nucleus, the neutron changing into an electron (the beta particle), which is promptly ejected, and a residual proton. The proton left behind leaves the "daughter" nucleus with one more proton than its "parent" and thus increases the atomic number and the position in the periodic table. Alpha or beta emission usually leaves the nucleus with excess energy, which it unloads by emitting a gamma-ray photon.
In all these nuclear processes a large amount of energy, given by Einstein's E = mc2 equation, is released. After the process is over, the total mass of the product is less than that of the parent, with the mass difference appearing as energy. . Nuclear Energy.
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.
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 J. 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 Laboratory for Particle Physics, known as CERN, in Geneva, Switzerland. . Particle Accelerators.
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 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. . Particle Detectors.
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 (. 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 (. 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 (. Galaxy; Milky Way).
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.
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 (. 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. Despite the theory's ability to explain a number of phenomena, no quarks have yet been found, 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.
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.
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. . 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 providing the necessary high triggering temperature.
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.
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 (. 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 (. 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.
Cryogenics
At very low temperatures (near absolute zero), many materials exhibit strikingly different characteristics (. 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.
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 (. 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.
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.
Astrophysics
The construction of large and specially designed optical telescopes has led to the discovery of new stellar objects, including 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 interstellar 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 between 10 and 20 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. . also Inflationary Theory.
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