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In quantum mechanics, the angular momentum operator is one of several related operators analogous to classical angular momentum. The angular momentum operator plays a central role in the theory of atomic physics and other quantum problems involving rotational symmetry. In both classical and quantum mechanical systems, angular momentum (together with linear momentum and energy) is one of the three fundamental properties of motion.〔Introductory Quantum Mechanics, Richard L. Liboff, 2nd Edition, ISBN 0-201-54715-5〕 There are several angular momentum operators: total angular momentum (usually denoted J), orbital angular momentum (usually denoted L), and spin angular momentum (spin for short, usually denoted S). The term "angular momentum operator" can (confusingly) refer to either the total or the orbital angular momentum. Total angular momentum is always conserved, see Noether's theorem. ==Spin, orbital, and total angular momentum== (詳細はclassical definition of angular momentum is . This can be carried over to quantum mechanics, by reinterpreting r as the quantum position operator and p as the quantum momentum operator. L is then an operator, specifically called the ''orbital angular momentum operator''. Specifically, L is a ''vector operator'', meaning , where ''L''x, ''L''y, ''L''z are three different operators. However, there is another type of angular momentum, called ''spin angular momentum'' (more often shortened to ''spin''), represented by the spin operator S. Almost all elementary particles have spin. Spin is often depicted as a particle literally spinning around an axis, but this is only a metaphor: spin is an intrinsic property of a particle, unrelated to any sort of motion in space. All elementary particles have a characteristic spin, for example electrons always have "spin 1/2" while photons always have "spin 1". Finally, there is total angular momentum J, which combines both the spin and orbital angular momentum of a particle or system: : Conservation of angular momentum states that J for a closed system, or J for the whole universe, is conserved. However, L and S are ''not'' generally conserved. For example, the spin–orbit interaction allows angular momentum to transfer back and forth between L and S, with the total J remaining constant. 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「Angular momentum operator」の詳細全文を読む スポンサード リンク
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