Scientists of every period have believed that certain objects are fundamental and that others are derived, in the sense that the latter are composed of the former. In one version of this distinction, the fundamental objects are particles, or points of matter carrying such properties as mass. Although the view of which objects qualify as fundamental particles has changed several times, the notion that the world is ultimately made of such material points, moving through space, has endured in some form ever since the theory of \Tatomism\t was first proposed by the Greeks \TLeucippus\t and \TDemocritus\t in the 5th century BC. The atomic theory languished until the 18th and early 19th centuries, when physicists and chemists revived it to explain the properties of gases and some of the facts of chemistry. In these theories the fundamental particles, the \Latom\ls, remained indivisible points. The discovery in the late 19th and the 20th centuries that atoms were composite, rather than indivisible, set the stage for modern discoveries about fundamental particles. HISTORY OF MODERN PARTICLE PHYSICS The history of modern particle physics has gone through four stages. In the first stage, Joseph J. \TThomson\t discovered (1897), by studying electricity passing through gases, that all atoms contain certain particles, called \Lelectron\ls, that carry a negative electric charge. Because atoms are electrically neutral, there must be balancing positive charges somewhere in the atom. Ernest \TRutherford\t proposed (1911), based on a series of experiments by Hans \TGeiger\t and Ernest Marsden, that these positive charges are concentrated in a very small volume, called the \Tatomic nucleus\t, at the center of the atom. In the second stage, scientists recognized, through an analysis of isotopes of elements, that all atomic nuclei could be thought of as composed of two types of particles: the \TProton\t, which carries both mass and electric charge, and the \Tneutron\t, which has about the same mass as a proton but is electrically neutral. This model was confirmed through the discovery (1932) of free neutrons by James \TChadwick\t. The third stage of modern particle physics came with the recognition that protons, neutrons, and electrons--the constituents of ordinary matter--were but three of a vast number of similar particles, which differed only in a few properties, such as their mass, and in their stability against spontaneous decay. Experiments with particle \Laccelerator\ls indicated that these many subatomic particles could be readily produced from protons and neutrons, provided that enough energy was available to produce the additional mass of the new particles predicted by the rules of Albert Einstein's \Trelativity\t theory. These discoveries in the 1940s and '50s indicated that the proton and neutron were not really fundamental particles and that they would have to be understood as part of a much larger family of similar objects. In the fourth stage physicists found a successful explanation for the large number of particles. The prevailing theory is that many of these particles are combinations of several types of quarks (see \Tquark\t), and that quarks are one of three classes of fundamental particles. A second class comprises the leptons (see \Tlepton\t), an example being the electron, and a third class comprises gauge particles, an example being the \Tphoton\t. PROPERTIES OF PARTICLES The subatomic particles carry two kinds of properties: those which can vary for a given particle, such as total energy, and those which remain the same for one type of particle, such as mass, spin, electric charge, and color. The combination of the latter four properties serves to define each particle type and to distinguish the particles from each other. Mass The mass of an object, originally thought of as an independent property of matter, is now recognized as a measure of the energy content of the object when it is at rest, according to Einstein's equation E = m times square of c. The rest energy of a fundamental particle ranges from zero to many millions or billions of electron volts. Spin Many fundamental particles behave as if they are spinning on an internal axis, as the Earth does. According to the theories of \Tquantum mechanics\t, the angular momentum related to this spin can take on only certain values: either zero or an integer or half-integer multiple of the constant h = 1.04 X 10 to the power of minus 27 erg-seconds. Each particle has a specific and unchanging value of spin. Electric Charge All known electric and magnetic effects originate from the property of electric charge, which is carried by certain subatomic particles. All particles of one type, such as electrons, have the same charge. The charges of different particles are simply related. All observed particles have either charge zero or a positive or negative integer multiple of the proton's charge, symbolized by e. Quarks, which have not been directly observed, are thought to have charges of 2/3 e and -1/3 e. Color Quarks carry another property, known as color, which, like electric charge, remains constant in any particle reaction. Three forms of color exist, and each quark bears one of these. A quark of one color can convert into one of another color by emission or absorption of a type of gauge particle called a \Tgluon\t, which also must carry the color property in order that the total color remain constant. INTERACTIONS AND CLASSIFICATION OF PARTICLES The fundamental particles exhibit several characteristic forms of behavior that have enabled physicists to find patterns among the particles and to make successful theories of their internal structure. The most remarkable thing that particles do is to change into one another, either by the decay of a single particle into several others (see \Tradioactivity\t) or in a collision between two particles from which several new ones may emerge. Quantum mechanics allows only the probabilities of these transformations to be predicted. Fundamental Interactions Because the fundamental particles change into one another, the old notion of a force--an influence that produces physical change--is insufficient to describe their behavior. Instead physicists speak of a \Tfundamental interaction\t--any influence that causes a collection of particles to undergo some change. A measure of the strength of the interaction is the rate at which such changes take place. If other conditions are equal, then the stronger the interaction, the less time is needed for a change to occur. Using this measure, physicists have discovered four fundamental interactions, known, in decreasing order of strength, as the strong nuclear, the electromagnetic, the weak nuclear, and the gravitational forces (see \Telectricity\t; \Tgravitation\t; \Tmagnetism\t; \Tnuclear physics\t). Each type of fundamental particle participates in a specific subset of the four interactions. The \Lhadron\ls--which comprise \Lmeson\ls, whose spin is an integer multiple of h, and \Lbaryon\ls, whose spin is a half-integer multiple of h--can undergo all four types of interaction. Another type of particle, the charged \Llepton\ls, can undergo all of the interactions except the strong nuclear. Uncharged leptons, or \Lneutrino\ls, undergo only weak-nuclear and gravitational interactions and therefore are little affected by other matter. The four known interactions are generated by the exchange of gauge bosons, also called gauge particles. Photons (see \Tphoton\t) are the carriers of electromagnetic interactions, gluons carry strong interactions between quarks, and W and Z particles -first observed in the 1980s) generate weak interactions. All of these gauge bosons have one unit of spin. The still-hypothetical graviton, which carries the gravitational force, would be a gauge boson with two units of spin. Conservation Laws The four interactions differ not only in strength but also in their detailed behavior, especially regarding which quantities are conserved (see \Tconservation, laws of\t) when the interaction operates. A quantity is said to be conserved in a process when its value remains the same throughout the process. In reactions involving fundamental particles, such quantities as energy, linear momentum, angular momentum, and electric charge are always conserved. Until recently, scientists believed that the number of baryons in a reaction is also always conserved. This law would forbid a proton from decaying into lighter particles, and indeed such decays have never been seen. Some scientists, however, have suggested that protons do decay, if rarely, and experiments to test this hypothesis are in progress. There are also conservation laws that apply for some interactions but not others. Some of the hadrons, such as the sigma-plus particle, are produced by strong interactions but decay only by weak interactions. Because of these circumstances, these particles, termed metastable, exist between production and decay for enough time to leave an observable track in a detection device (see \Tdetector, particle\t). The fact that metastable hadrons do not decay by strong interactions suggests that some law respected by these interactions inhibits the decay. Because the decay does occur by weak interactions, the latter must not obey this law. Physicists assign a property called strangeness to several of the metastable hadrons. Strangeness is conserved in strong and electromagnetic interactions but not in weak interactions. Several other properties of particles that obey such partial conservation laws are also recognized, each with its own set of metastable particles. Antimatter Another important criterion for classification is the division into particles and antiparticles (see \Tantimatter\t). This distinction was first theorized for electrons by Paul \TDirac\t in 1930 and was later extended to all other particles. According to the principles of relativity and quantum mechanics, for every type of particle there exists a corresponding antiparticle with the same spin and mass but with opposite electric charge. Such particles as the \Tphoton\t, whose electric charge, baryon number, and strangeness are all zero, are identical to their antiparticles. Antiparticles for most of the known particles have been observed, beginning with the antielectron, or \Tpositron\t, detected by Carl \TAnderson\t in 1932. The preponderance of matter over antimatter is a result of little understood processes that occurred very early in the history of the universe (see \Tcosmology\t). Quarks and Gluons In addition to the metastable hadrons, there are hundreds of known hadrons that are unstable and will decay by strong interactions. These unstable hadrons typically have lifetimes of 10 to the power of minus 20 seconds or less. The properties of hadrons have been explained with some success by the quark-gluon theory of strong interactions. According to this theory, hadrons are composed of combinations of the various types of quarks. The quarks are held together by unbreakable bonds resulting from gluon exchange between the quarks. Baryons are each composed of three quarks; mesons are composed of a quark and an antiquark. Six different types of quarks are theorized to exist, each of which can be any of the three colors. The quark-gluon theory also explains why quarks and gluons are never observed alone, but rather, in combinations such as the three quarks that make up a baryon. Gluon exchange induces a force between quarks, and between the gluons themselves, that remains strong even when the particles involved are relatively far apart. Because of this, it is not possible to separate the quarks and gluons of a single hadron far enough from each other so that they can be observed in isolation. This phenomenon is referred to as confinement of quarks and gluons. Certain combinations of quarks and gluons are color neutral, just as some combinations of electric charges are charge neutral. These color-neutral systems are the observed hadrons. It follows from the mathematical theory of the quark-gluon interaction that color neutrality can be achieved with three quarks, or with a quark and an antiquark. ADVANCED THEORIES Many aspects of fundamental particles remain to be understood. Although the photon and the W and Z particles have similar properties, their masses are very different. Theories accounting for this in terms of a breakdown of an underlying \Tsymmetry\t have been partially successful, but it is unknown whether this breakdown arises from the interaction of W and Z with undiscovered spin-zero particles called Higgs bosons, or from some other mechanism (see \THiggs particle\t). Many physicists are uncomfortable with the large number of particles. Some have devised theories describing particles as tightly bound combinations of a small number of more fundamental particles. According to the so-called Standard Model only three foursomes, or "generations," of particles exist, each consisting of two quarks and two leptons. The first, consisting of up and down quarks, the electron, and the electron neutrino, constitutes ordinary matter. The second consists of charm and strange quarks plus the muon and muon neutrino, while the third consists of top and bottom quarks plus the tau and tau neutrino. Experiments have supported this theory. Another approach involves a mathematical description of particles in which close relations are found between particles of different spin. These so-called supersymmetry theories imply the existence of many yet-undiscovered particles, such as spin-zero quark analogues. Many physicists think that the rest energies of such particles would be about one trillion electron volts (TeV). To produce them, accelerators of much higher energy than existing ones now operating are needed. In order to create the hypothesized particles, accelerators would require thousands of superconducting magnets, as well as rings that are 100 km (62 mi) in circumference (see \Taccelerator, particle\t). It is proposed that the collisions of protons accelerated to such energies might yield the new fundamental particles (see \Tunified field theory\t). Gerald Feinberg Bibliography: Feinberg, Gerald, What is the World Made of? (1977); Feynmann, R.P. Elementary Particles and the Laws of Physics (1988); Fritzsche, Harald, Quarks (1983); Pagels, Heinz, The Cosmic Code (1982); Weinberg, Steven, Subatomic Particles (1983).