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In quantum mechanics and particle physics, spin is an intrinsic form of angular momentum carried by elementary particles, composite particles (hadrons), and atomic nuclei. Spin is one of two types of angular momentum in quantum mechanics, the other being ''orbital angular momentum''. The orbital angular momentum operator is the quantum-mechanical counterpart to the classical notion of angular momentum: it arises when a particle executes a rotating or twisting trajectory (such as when an electron orbits a nucleus).〔("Angular Momentum Operator Algebra", class notes by Michael Fowler )〕〔(''A modern approach to quantum mechanics'', by Townsend, p. 31 and p. 80 )〕 The existence of spin angular momentum is inferred from experiments, such as the Stern–Gerlach experiment, in which particles are observed to possess angular momentum that cannot be accounted for by orbital angular momentum alone. In some ways, spin is like a vector quantity; it has a definite magnitude, and it has a "direction" (but quantization makes this "direction" different from the direction of an ordinary vector). All elementary particles of a given kind have the same magnitude of spin angular momentum, which is indicated by assigning the particle a ''spin quantum number''.〔 The SI unit of spin is the joule-second, just as with classical angular momentum. In practice, however, it is written as a multiple of the reduced Planck constant ''ħ'', usually in natural units, where the ''ħ'' is omitted, resulting in a unitless number. Spin quantum numbers are unitless numbers by definition. When combined with the spin-statistics theorem, the spin of electrons results in the Pauli exclusion principle, which in turn underlies the periodic table of chemical elements. Wolfgang Pauli was the first to propose the concept of spin, but he did not name it. In 1925, Ralph Kronig, George Uhlenbeck and Samuel Goudsmit at Leiden University suggested a physical interpretation of particles spinning around their own axis. The mathematical theory was worked out in depth by Pauli in 1927. When Paul Dirac derived his relativistic quantum mechanics in 1928, electron spin was an essential part of it. ==Quantum number== (詳細はquantized angular momenta do. On the other hand, spin has some peculiar properties that distinguish it from orbital angular momenta: *Spin quantum numbers may take half-integer values. *Although the direction of its spin can be changed, an elementary particle cannot be made to spin faster or slower. *The spin of a charged particle is associated with a magnetic dipole moment with a g-factor differing from 1. This could only occur classically if the internal charge of the particle were distributed differently from its mass. The conventional definition of the spin quantum number, ''s'', is ''s'' = ''n''/2, where ''n'' can be any non-negative integer. Hence the allowed values of ''s'' are 0, 1/2, 1, 3/2, 2, etc. The value of ''s'' for an elementary particle depends only on the type of particle, and cannot be altered in any known way (in contrast to the ''spin direction'' described below). The spin angular momentum, ''S'', of any physical system is quantized. The allowed values of ''S'' are: : where ''h'' is the Planck constant. In contrast, orbital angular momentum can only take on integer values of ''s''; i.e., even-numbered values of ''n''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Spin (physics)」の詳細全文を読む スポンサード リンク
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