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Glossary of Physical Terms Absolute zero. The temperature of -273.16C, or -459.69F, or zero kelvin (O K), thought to be the temperature at which molecular motion is at a minimum and a body has no heat energy. Alpha particle. A positively charged particle consisting of two pro- tons and two neutrons, identical with the nucleus of the helium atom; emitted by several radioactive substances. Angstrom. A unit of length equal to one ten-billionth of a meter (10-' m). Anisotropy. The characteristic of a substance for which a physical property varies in value with the direction in or along which the measurement is made. Antimatter. Material consisting of atoms that are composed of positrons, antiprotons, and antineutrons. Antineutron. The antiparticle to the neutron; a strongly interacting baryon that has no charge, mass of 939.6 MeV, spin 1/2, and mean life of almost 103 seconds. Antinucleon. An antineutron or antiproton, that is, a particle having the same mass as its nucleon counterpart but opposite charge or opposite magnetic moment. . ~ .. . . .... . Antiparticle. A counterpart to a particle, having mass, lifetime, and spin identical to the particle but with charge and magnetic moment . . reverser in sign. Antiproton. The antiparticle of the proton, a strongly interacting 142

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GLOSSAR Y OF PHYSICAL TERMS 143 baryon that is stable, carries unit negative charge, has the same mass as the proton (983.3 MeV), and has spin 1/2. Astrophysics. The study of such physical properties of celestial bodies as luminosity, size, mass, density, temperature, and chemical composition; the study of the origin and evolution of these bodies. Asymptotic freedom. In some particle-physics theories, the binding force between two quarks decreases as their relative momentum increases; equivalently, as two quarks approach, the force between them disappears. Atom. The individual structure that constitutes the basic unit of any chemical element. Atomic number. The number of protons in an atomic nucleus. Atomic physics. The science concerned with the structure of the atom. The characteristics of the elementary particles of which the atom is composed and the processes involved in the interactions of radiant energy with matter. Atomic spectrum. The spectrum of radiations due to transitions between energy levels in an atom, either absorption or emission. Aurora. The most intense of the several lights emitted by the Earth's upper atmosphere, seen most often along the outer realms of the Arctic and Antarctic, where it is called the aurora borealis and aurora australis, respectively; excited by charged particles from space. Baryon. A particle that can be transformed into a nucleon and some number of mesons and lighter particles; any of a group of hadrons (as nucleons) that undergo strong interactions and are held to be a combination of three quarks. Beta decay. Radioactive transformation of a nuclide in which the atomic number increases or decreases by unity with no change in mass number; the nucleus emits a beta particle. Beta particle. An electron or positron emitted from a nucleus during beta decay. Big bang. A theory in astronomy, according to which the universe originated billions of years ago from the explosion of a single mass of material, so that the pieces are still hying apart. Biophysics. The hybrid science involving the application of physical principles and methods to study and explain the structures of living organisms and the mechanics of life processes. Black hole. A star with radius just outside the Schwarzschild radius; it is invisible but can capture matter and light from the outside. Boson. A particle (as a photon, meson, or alpha particle) whose spin is zero or an integral number.

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144 GLOSSARY OF PHYSICAL TERMS "Breathing" mode. The vibrational state in which a nucleus under- goes spherically symmetric radial expansion and contraction. Charged particle. A particle whose charge is not zero; the charge of a particle is added to its designation as a superscript with particles of ~ 1 and -1 (in terms of the charge of the proton) denoted by ~ arid -, respectively. Coherent anti-Stokes Raman scattering. Nonlinear spectroscopy where laser light scattered from a sample undergoes an increase in frequency (anti-Stokes behavior), derived from vibrational states of the atoms or molecules in the sample. Condensed matter physics). The physics of the solid and liquid states. Cosmic rays. Electrons, muons, the nuclei of atoms, and photons that impinge upon the Earth from all directions of space with nearly the speed of light. Cosmology. The study of the overall structure of the physical uni- verse. Crystallography. The branch of science that deals with the geometric description of crystals and their internal arrangement. Cyclotron. An accelerator in which charged particles are succes- sively accelerated by a constant-frequency alternating electric field that is synchronized with movement of the particles on spiral paths in a constant magnetic field normal to their path. Deuteron. The nucleus of the deuterium atom consisting of one proton and one neutron. Diabatic. A change in which the environment of a system alters too rapidly for the system to read just continuously. Diamagnetic. Having a magnetic permeability less than that of a vacuum; slightly repelled by a magnet. Dirac electron theory. Theory that accounts for spin angular momen- tum of the electron and gives its magnetic moment and its behavior in an electromagnetic field. Doppler elect. The change in the observed frequency of a wave due to relative motion of source and observer. Electromagnetic theory. Theory according to which light is an elec- tromagnetic wave whose electric and magnetic fields obey Maxwell's equations. Electromagnetism. Magnetism produced by an electric current rather than by a permanent magnet. Electron. An elementary particle consisting of a charge of negative electricity equal to about 1.602 x 10-~9 coulomb and having a mass when at rest of about 9.109534 x 10-28 gram (or about 1/1836 that of a proton).

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GLOSSARY OF PHYSICAL TERMS 145 Electron accelerator. energies. Electron charge. The charge carried by an electron, equal to about -1.602 x 10-~9 coulomb, or -4.803 x 10-~ statcoulomb. Elementary particle. A particle that, in the present state of knowl- edge, cannot be described as compound and is thus one of the fundamental constituents of all matter. Entropy. The degradation of the matter and energy in the universe to an ultimate state of inert uniformity. Epitaxy. Growth of one crystal on the surface of another crystal, in which the growth of the deposited crystal is oriented by the lattice structure of the substrate. Equivalence Principle. In general relativity, the principle that the observable local effects of a gravitational field are indistinguishable from those arising from acceleration of the frame of reference. Far-ultraviolet radiation. Ultraviolet radiation in the wavelength of 200 to 300 nanometers; germicidal effects are greatest in this range. Femto-. Prefix meaning one quadrillionth (lo-~5) part of. Fermion. A particle (as an electron, proton, or neutron) whose spin quantum number is an odd multiple of l/2. Ferromagnet. A substance with an abnormally high magnetic perme- ability, a definite saturation point, and appreciable residual magne- tism and hysteresis. Fission. The division of an atomic nucleus into parts of comparable mass; usually restricted to heavier nuclei such as isotopes of uranium, plutonium, and thorium. Fluorescence. Emission of, or the property of emitting, electromag- netic radiation (usually as visible light) resulting from and occurring only during absorption of radiation from some other source. Free-electron laser. A laser in which beams of unbound electrons interact with a strong magnetic field to produce tunable laser light. Free radical. An atom or a diatomic or polyatomic molecule that possesses at least one unpaired electron. Fusion. Combination of two light nuclei to form a heavier nucleus (and perhaps other reaction products) with release of some binding A device that accelerates electrons to high energy. Galaxy. A large-scale aggregate of stars, gas, and dust. The aggregate is a separate system of stars covering a mass range from 107 to 10~2 solar masses and ranging in diameter from 1500 to 300,000 light- years. Gamma ray. A high-energy photon, especially as emitted by a nucleus in a transition between two energy levels. General relativity. The theory of Einstein that generalizes special

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146 GLOSSARY OF PHYSICAL TERMS relativity to noninertial frames of reference and incorporates gravi- tation and in which events take place in a curved space. Geophysics. The physics of the Earth and its environment, i.e., earth, air, and (by extension) space. Glass. A hard, amorphous, inorganic, usually transparent, brittle substance made by fusing silicates, sometimes berates and phos- phates, with certain basic oxides and then rapidly cooling to prevent crystallization (NB: not the sense in which the term is used in the term spin glass). Gluon. A hypothetical, neutral, massless particle believed to bind together quarks to form hadrons. Gluon string. A particle-physics theoretical model to account for the binding force between quarks that increases monotonically as they are separated. Gravitation. The mutual attraction among all masses in the universe. Gravitational force. The force on a particle due to its gravitational attraction to other particles. Gravitational radiation. A propagating gravitational field predicted by general relativity, which is produced by some change in the distribution of matter; it travels at the speed of light, exerting forces on masses in its path. Gravitational redshift. A displacement of spectral lines toward the red when the gravitational potential at the observer of the light is greater than at its source. Gyrotron. A device for producing microwave energy. Also called an electron cyclotron maser. Hadron. Any of the particles that take part in the strong interaction. Hall conductivity. The reciprocal of the electrical resistivity associ- ated with the Hall current. Hall current. When an electric current in a conductor is placed in a magnetic field that is perpendicular to the current a transverse electric field is created, which in turn can support a transverse current. Heavy-ion linear accelerator. A linear accelerator that produces a beam of heavy particles of high intensity and sharp energy; used to produce transuranic elements and short-lived isotopes and to study nuclear reactions, nuclear spectroscopy, and the absorption of heavy . . tons In matter. Helicity. The component of the spin of a particle along its momen- tum. Hydrogen maser. A maser in which hydrogen gas is the basis for providing an output signal with a high degree of stability and spectral purity.

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GLOSSAR Y OF PHYSICAL TERMS 147 Hypercharge. A quantum number conserved by strong interactions, equal to twice the average of the changes of the numbers of an isospin multiplet. Hypernucleus. A nucleus containing one or more hyperons in addi- tion to the nucleons. Hyperon. A hadron that has baryon number B = + 1, i.e., that can be transformed into a nucleon and some number of mesons or lighter particles and that has a nonzero strangeness number. Ionicity. The state of being characterized by, relating to, or existing as ions. Isospin multiplet. A collection of hadrons that have approximately the same mass and the same quantum numbers except for charge. Isotropy. The quality of a property that does not depend on the direction along which it is measured or of a medium or entity whose properties do not depend on the direction along which they are measured. Joule. Unit of energy, work, or quantity of heat equal to one newton-meter. JO particle. An unstable, neutral meson that has a mass about 3 times the mass of the photon. K meson. See kaon. Kaon. Collective name for four pseudoscalar mesons having masses of about 495 MeV and decaying via weak interactions; an unstable meson produced in high-energy particle collisions with its electri- cally charged forms being 966.3 times more massive than the electron; also known as K meson. Lamb shift. A small shift in the energy levels of a hydrogen atom, and of hydrogenlike ions, from those predicted by the Dirac electron theory, in accord with principles of quantum electrodynamics. Lambda hyperon (A). A quasi-stable baryon, forming an isotopic singlet, having zero charge and hypercharge, a spin of 1/2, positive parity, and mass of 1115.5 MeV. Laser. A device that uses the maser principle of amplification of electromagnetic waves by stimulated emission of radiation and operates in the optical or infrared region. Derived from light amplification by stimulated emission of radiation. Laser interferometer. An interferometer that uses a laser as a light source; because of the monochromaticity and high intrinsic brilliance of laser light, it can operate with path differences in the interfering beams of hundreds of meters, in contrast to a maximum of about 20 centimeters for classical interferometers. Laser light. Coherent, nearly single-frequency, highly directional

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148 GLOSSAR Y OF PHYSICAL TERMS electromagnetic radiation emitted in the range from infrared to ultraviolet and x-ray wavelengths. Laser optics. Optical systems utilizing the properties of laser light. Laser spectroscopy. A branch of spectroscopy in which a laser is used as an intense, monochromatic light source; in particular, it includes saturation spectroscopy, as well as the application of laser sources to Raman spectroscopy and other techniques. Lepton. Any of a family of particles (as electrons, muons, and neutrinos) that have spin quantum number 1/2 and that experience no strong interactions; a fermion having a mass smaller than the proton mass. Light. Electromagnetic radiation with wavelengths capable of caus- ing the sensation of vision, ranging approximately from 4000 (ex- treme violet) to 7700 angstroms (extreme red); more generally, electromagnetic radiation of any wavelength. Light-year. A unit of measurement of astronomical distance; it is the distance light travels in one sidereal year and is equivalent to 9.461 x 10~2 kilometers or 5.879 x 10~2 miles. Luminosity. In optics, a measure of the brightness of a light source; in colliding-beam accelerators, a measure of the rate of collisions of the particles in the colliding beams. Macromolecule. A large molecule in which there is a large number of -one or several relatively simple structural units, each consisting of several atoms bonded together. Magnetic hysteresis. Lagging of changes in the magnetization of a substance behind changes in the magnetic field as the magnetic field 1S varlec . Magnetic moment. A vector associated with a magnet, current loop, particle, or such, whose cross product with the magnetic induction (or alternatively, the magnetic-field strength) of a magnetic field is equal to the torque exerted on the system by the field. Magnetic monopole. A hypothetical particle carrying magnetic charge; it would be a source for magnetic field in the same way that a charged particle is a source for electric held. Magnetic permeability. A factor, characteristic of a material, that is proportional to the magnetic induction produced in a material divided by the magnetic-field strength; it is a tensor when these qualities are not parallel. Magnetic resonance. A phenomenon exhibited by the magnetic spin systems of certain atoms whereby the spin systems absorb energy at specific (resonant) frequencies when subjected to magnetic fields alternating at frequencies that are in synchronism with natural frequencies of the system.

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GLOSSAR Y OF PHYSICA ~ TERMS 149 Maser. A device for coherent amplification or generation of electro- magnetic waves in which an ensemble of atoms or molecules, raised to an unstable energy state, is stimulated by an electromagnetic wave to radiate excess energy at the same frequency and phase as the stimulating wave. Mass number. The sum of the numbers of protons and neutrons in the nucleus of an atom or nuclide. Matter. The substance composing bodies perceptible to the senses; includes any entity possessing mass when at rest. Meson. Any of a group of hadrons (as the pion and Loon) that are strongly interacting and have zero or an integer number of quantum units of spin. Metastability. The property of having only a slight margin of stabil- ity. Metrology. The science of weights and measures or of measurement. Mho. A unit of conductance, admittance, and susceptance equal to the conductance between two points of a conductor such that a potential difference of 1 volt between these points produces a current of 1 ampere; the conductance of a conductor in mhos is the reciprocal of its resistance in ohms. Microelectronics. The technology of constructing circuits and de- vices in extremely small packages by various techniques. Microemulsion. A homogenous, single-phase, thermodynamically stable mixture of oil, water, and surfactant. Million electron volts (Me VJ. A unit of energy commonly used in nuclear and particle physics, equal to the energy acquired by an electron in falling through a potential of 106 volts. Molecular biology. ~ . . . . . . That nart of biology which attempts to interpret o~o~og~ca~ events In terms of the physicochemical properties of molecules in a cell. Molecular ion. A molecule possessing nonzero net electric charge. Molecular physics. The study of the behavior and structure of molecules, including the quantum-mechanical explanation of several kinds of chemical binding between atoms in a molecule; directed valence; the polarizability of molecules; the quantization of vibra- tional' rotational, and electronic motions of molecules; and the phenomena arising from intermolecular forces. Molecule. A group of atoms held together by chemical forms; a molecule is the smallest unit of matter that can exist by itself and retain all its chemical properties. Monte Carlo method. A technique that obtains a probabilistic ap- proximation to the solution of a problem by using statistical sampling techniques.

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150 GLOSSAR Y OF PHYSICAL TERMS Multiphoton spectroscopy. Nonlinear spectroscopy usually involv- ing intense laser light enabling more than one photon to interact with an atom within a time frame shorter than the decay time of the atomic state. Muon. Collective name for two semistable elementary particles with positive and negative charge, which are leptons and have a spin of 1/2 and a mass of approximately 105.7 MeV. Muonium. An atom consisting of an electron bound to a positively charged muon by their mutual Coulomb attraction, just as an electron is bound to a proton in the hydrogen atom. Nano-. Prefix meaning one billionth (lo-9) part of. Neutrino. A neutral lepton having zero rest mass and spin 1/2. Neutron. An uncharged hadron that has a mass nearly equal to that of the proton and is present in all known atomic nuclei except the hydrogen nucleus. Newton. The unit of force equal to one kilogram-meter per second squared (I kg m/s2~. Nonlinear optics. The study of the interaction of radiation with matter in which certain variables describing the response of the matter (such as electric polarization of power absorption) are not proportional to variables describing the radiation (such as electric- field strength or energy flux). Nonlinear spectroscopy. Spectroscopy usually using laser light and the nonlinear optical behavior of certain properties of matter; e.g., coherent anti-Stokes Raman scattering. Nova. A star that suddenly becomes explosively bright (the term is a misnomer because it does not denote a new star but the brightening of an existing faint star). Nuclear physics. The study of the characteristics, behavior, and internal structures of the atomic nucleus. Nucleon. A collective name for a proton or a neutron; these particles are the main constituents of atomic nuclei, have approximately the same mass, have a spin of l/2, and can transform into each other through the process of beta decay. Nucleosynthesis. The production of a chemical element from hydro- gen nuclei (as in stellar evolution). Nucleus. The central, positively charged, dense portion of an atom. Nuclide. A species of atom characterized by the number of protons, number of neutrons, and energy content in the nucleus; to be regarded as a distinct nuclide, the atom must be capable of existing for a measurable lifetime, generally greater than 10-~2 second. Parity. A physical property of a wave function that specifies its three behavior under an inversion, i.e., under simultaneous reflection of all

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GLOSSAR Y OF PHYSICAL TERMS 151 three spatial coordinates through the origin; if the wave function is unchanged by inversion, its parity is 1 (or even); if the function is changed only in sign, its parity is -1 (or odd). Particle physics. The branch of physics concerned with understand- ing the properties and behavior of elementary particles, especially through study of collisions or decays involving energies of hundreds of MeV or more. Pauli Exclusion Principle. The principle that no two fermions of the same kind may simultaneously occupy the same quantum state. Phase. A portion of a physical system (liquid, gas, solid) that is homogeneous throughout, has definable boundaries, and can be separated physically from other phases; the type of state of a system, such as solid, liquid, or gas. Phase transition. A change of a substance from one phase (e.g., solid, liquid, or gas) to another. Phonon. A quantum of an acoustic mode of thermal vibration in a crystal lattice. Photoemission. The ejection of electrons from a solid (or less com- monly, a liquid) by incident electromagnetic radiation. Photoionization. The removal of one or more electrons from an atom or molecule by absorption of a photon of visible or ultraviolet light. Photon. A massless particle, the quantum of the electromagnetic field, carrying energy, momentum, and angular momentum. Photovoltaic. Of, relating to, or utilizing the generation of a voltage when radiant energy falls on the boundary between two dissimilar substances (as two different semiconductors). Pion. A short-lived meson that is primarily responsible for the nuclear force and that exists as a positive or negative particle with mass 273.2 times the electron mass or as a neutral particle with mass 264.2 times the electron mass. Planck's constant. A fundamental physical constant, the elementary quantum of action; the ratio of the energy of a photon to its frequency, it is equal to 6.62620 + 0.00005 x 10-34 joule-second; symbolized by h. Plasma. A collection of charged particles (as in the atmosphere of stars or in a metal) containing about equal numbers of positive ions and electrons, exhibiting some properties of a gas but differing from a gas in being a good conductor of electricity and in being affected by a magnetic field. Plasma physics. The study of highly ionized gases. Plate tectonics. Global tectonics based on a model of the Earth characterized by a small number (10-25) of semirigid plates that float on some viscous underlayer in the mantle; each plate moves more or

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152 GLOSSARY OF PHYSICAL TERMS less independently and grinds against the others, concentrating more deformation, volcanism, and seismic activity along the periphery. Positive energy theorem. A recent theory that shows that in general relativity theory, any isolated system must have a positive value for its total energy. Positron. An elementary particle having mass equal to that of the electron and having the same spin and statistics as the electron but a positive charge equal in magnitude to the electron's negative charge; the antiparticle of the electron. Positronium. The bound state of an electron and a positron. Proton. A hadron that is the positively charged constituent of ordi- nary matter and, together with the neutron, is a building stone of all atomic nuclei; its mass is approximately 938 MeV. Proton accelerator. A particle accelerator that accelerates protons to high energies, as opposed to one that accelerates heavier ions or electrons. Pulsar. A celestial radio source, emitting intense short bursts of radio emission; the periods of known pulsars range between 33 millisec- onds and 3.75 seconds, and pulse durations range from 2 to about 150 milliseconds, with longer-period pulsars generally having a longer pulse duration. Quantize. To restrict an observable quantity, such as energy or angular momentum, to a discrete set of values, to subdivide (as energy) into small but measureable increments. Quantized Hall effect. The appearance of quantum levels in the Hall conductivity for a two-dimensional conductor in a magnetic field. Quantum. For certain physical quantities, a unit such that the values of the quantity are restricted to integral multiples of this unit (e.g., the quantum of angular momentum is Planck's constant divided by 2~; an entity resulting from quantization of a field or wave, having particlelike properties such as energy' mass, momentum, and angu- lar momentum (e.~., the photon is the quantum of an electromagnetic field, and the phonon is the quantum of a lattice vibration). Quantum chromodynamics. The quantum theory that describes the strong interactions that bind quarks together to form hadrons. Quantum electrodynamics. The quantum theory of electromagnetic radiation, synthesizing the wave and corpuscular pictures, and of the interaction of radiation with electrically charged matter, in particular with atoms and their constituent electrons. Quantum mechanics. The modern theory of matter, of electromag- netic radiation, and of the interaction between matter and radiation; it differs from classical physics, which it generalizes and supersedes, mainly in the realm of atomic and subatomic phenomena.

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GLOSSARY OF PHYSICAL TERMS 153 Quark. Hypothetical elementary particles that have charges whose magnitudes are 1/3 or 2/3 of the electron charge; quarks are thought to come in several types (as up, down, strange, charmed, and bottom) and are held to be a constituent of hadrons. Quark-gluon model. The particle-physics model of the constituents of hadrons and the force that binds them; see quantum chromodynam- iCS. Radiation pressure. The pressure exerted by electromagnetic radia- tion on objects on which it impinges. Radio astronomy. The study of celestial objects by measurement and analysis of their emitted electromagnetic radiation in the wavelength range from roughly 1 millimeter to 30 meters. Raman spectroscopy. Nonlinear spectroscopy named for Sir Chandrasekhara Venkata Raman (1888-1970), Indian physicist. Relativistic magnetron. A device to produce microwaves that uses electrons moving at velocities near the speed of light in a magnetic field. Renormalization group approach or theory. A mathematical tech- nique to avoid infinities that occur in certain classes of physical theories. Resistivity. The electrical resistance offered by a material to the flow of current times the cross-sectional area of current flow and per unit length of current path; the reciprocal of the conductivity. Robotics. Technology dealing with the design, construction, and operation of robots in automation. Schwarzschild radius. For a given body of matter, a distance equal to the mass of the body times the gravitational constant divided by the square of the speed of light. Second-order Doppler elect. At velocities close to the speed of light, additional Doppler effects may be detected even in cases where the source and observer are moving only transversely. Semiconductor. A solid crystalline material whose electrical conduc- tivity is intermediate between that of a metal and an insulator, ranging from about 105 mhos to 10-7 mho per meter, and is usually strongly temperature dependent. Semiconductor laser. A laser in which the wavelength of the coher- ent light beam is determined by a semiconductor compound. Sidereal year. The time period relative to the stars of one revolution of the Earth about the Sun; it is about 365.2564 mean solar days. Sigma-zero (I) particle. An unstable elementary particle of the baryon family, of neutral charge, with a mass about 1.4 times the mass of the proton.

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154 GLOSSARY OF PHYSICAL TERMS Soft x ray. An x ray having a comparatively long wavelength and poor penetrating power. Solar mass. The mass of the Sun, 2 x 103 kg. Solar physics. The scientific study of all physical phenomena con- nected with the Sun; it overlaps with geophysics in the consideration of solar-terrestrial relationships such as the connection between solar activity and auroras. Solid. A substance that has a definite volume and shape and resists forces that tend to alter its volume or shape; a crystalline material, i.e., one in which the constituent atoms are arranged in a three-di- mensional lattice, periodic in three independent directions. SolitonfsJ. Solitary waves (as in a gaseous plasma) that retain their phase and speed after colliding with each other. Space charge. An electric charge distributed throughout a three-di- . . menslona~ . region. Spectroscopy. The branch of physics concerned with the production, measurement, and interpretation of electromagnetic spectra arising from either emission or absorption of radiant energy by various substances. Spin. The intrinsic angular momentum of a particle or nucleus, which exists even when the particle is at rest, as distinguished from orbital angular momentum. Spin glass. A state of matter in which the magnetic spins of randomly located atoms freeze in direction at low temperature; often general- ized to other systems. Strong force. See strong interaction. Strong interaction. One of the fundamental interactions of elemen- tary particles, primarily responsible for nuclear forces and other interactions among hadrons. Substrate. The physical material on which a microcircuit is fabri- cated. Superconducting magnet. An electromagnet whose coils are made of a superconductor with a high transition temperature and extremely high critical field; it is capable of generating magnetic fields of 100,000 oersteds and more with no steady power dissipation. Superconductivity. A property of many metals, alloys, and chemical compounds at temperatures near absolute zero by virtue of which their electrical resistivity vanishes and they become strongly dia- magnetic. Superdense. Densities that are greater than that for an ordinary nucleus, such as may exist in the core of novae.

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GLOSSAR Y OF PH YSICA ~ TERMS 1 5 5 Supernova. A star that suddenly bursts into very great brilliance as a result of its blowing up; it is orders of magnitude brighter than a nova. Supersymmetry. A particle-physics theory that attempts to unite two particle classes of fermions and bosons into a unified theory. Symmetry. The property of remaining invariant under certain changes (as of orientation in space, of the sign of the electric charge, of parity, or of the direction of time flow). Synchrotron. A device for accelerating electrons or protons in closed orbits in which the frequency of the accelerating voltage is varied (or held constant in the case of electrons) and the strength of the magnetic field is varied so as to keep the orbit radius constant. Tau particle Lor tau lepton (I. A short-lived elementary particle of the lep ton family that exists in positive and negative charge states and has a mass about 3500 times heavier than an electron. Tectonics. A branch of geology concerned with structure, especially with folding and faulting. Tera-. Prefix meaning one trillion (10~21. Tesla. Unit of magnetic flux intensity equal to one weber per square meter (1 Wb/m2), or one volt second per square meter (1 V s/m21. Three-degree radiation. The remnant radiation, at microwave fre- quencies, of the big bang. Tokamak. A device for confining plasma within a toroidal chamber, which produces plasma temperatures, densities, and confinement times greater than those produced by any other such device. Tomography. A diagnostic technique using x-ray photographs in which the shadows of structures before and behind the section under scrutiny do not show. Toroidal. Of, relating to, or shaped like a torus; doughnut-shaped. Upsilon particle (Y). Any of a group of unstable electrically neutral mesons that have a mass about 10 times that of a proton. Vacuum polarization. A process in which an electromagnetic field gives rise to virtual electron-positron pairs that effectively alter the distribution of charges and currents that generated the original electromagnetic field. Valley of stability. The region on a chart of the nuclides where the majority of stable nuclides are found. W+. A positively charged boson with a mass about 87 times that of the proton that mediates the weak force. W-. The negatively charged counterpart to the W+. Wave. A disturbance that propagates from one point in a medium to other points without giving the medium as a whole any permanent displacement.

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156 GLOSSARY OF PHYSICAL TERMS Weak coupling. The coupling of four fermion fields in the weak interaction, having a strength many orders of magnitude weaker than that of the strong or electromagnetic interactions. Weak force. See weak interaction. Weak interaction. One of the fundamental interactions among ele- mentary particles responsible for beta decay of nuclei and for the decay of particles with lifetimes greater than about 10-~ second, such as muons, K mesons, and lambda hyperons; it is several orders of magnitude weaker than the strong and electromagnetic interac- tions. X-ray astronomy. The study of x rays mainly from sources outside the solar system; it includes the study of novae and supernovae in the Milky Way Galaxy, together with extragalactic radio sources. X-ray tomography. See tomography. Z. A neutral boson with a mass about 100 times that of the proton that mediates the weak force.