statistical mechanics
statistical mechanics, quantitative study of systems consisting of a large number of interacting elements, such as the atoms or molecules of a solid, liquid, or gas, or the individual quanta of light (see photon) making up electromagnetic radiation. Although the nature of each individual element of a system and the interactions between any pair of elements may both be well understood, the large number of elements and possible interactions can present an almost overwhelming challenge to the investigator who seeks to understand the behavior of the system. Statistical mechanics provides a mathematical framework upon which such an understanding may be built. Since many systems in nature contain large number of elements, the applicability of statistical mechanics is broad. In contrast to thermodynamics, which approaches such systems from a macroscopic, or largescale, point of view, statistical mechanics usually approaches systems from a microscopic, or atomicscale, point of view. The foundations of statistical mechanics can be traced to the 19thcentury work of Ludwig Boltzmann, and the theory was further developed in the early 20th cent. by J. W. Gibbs. In its modern form, statistical mechanics recognizes three broad types of systems: those that obey MaxwellBoltzmann statistics, those that obey BoseEinstein statistics, and those that obey FermiDirac statistics. MaxwellBoltzmann statistics apply to systems of classical particles, such as the atmosphere, in which considerations from the quantum theory are small enough that they may be ignored. The other two types of statistics concern quantum systems: systems in which quantummechanical properties cannot be ignored. BoseEinstein statistics apply to systems of bosons (particles that have integral values of the quantum mechanical property called spin); an unlimited number of bosons can be placed in the same state. Photons, for instance, are bosons, and so the study of electromagnetic radiation, such as the radiation of a blackbody involves the use of BoseEinstein statistics. FermiDirac statistics apply to systems of fermions (particles that have halfintegral values of spin); no two fermions can exist in the same state. Electrons are fermions, and so FermiDirac statistics must be employed for a full understanding of the conduction of electrons in metals. Statistical mechanics has also yielded deep insights in the understanding of magnetism, phase transitions, and superconductivity.
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statistical mechanics
statistical mechanics Branch of physics that studies largescale properties of matter based on the statistical laws of large numbers. The large number of molecules in such a system allows the use of statistics to predict the probability of finding the system in any state. The entropy (disorder or randomness) of the system relates to its number of possible states; a system left to itself will tend to approach the most probable distribution of energy states. See also thermodynamics
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