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In physics, the principle of locality states that an object is only directly influenced by its immediate surroundings. A physical theory is said to be a local theory if it is consistent with the principle of locality. An alternative to the earlier concept of instantaneous "action at a distance", locality evolved as a property of the field theories of classical physics. The concept of locality is that, for an action at one point to have an influence at another point, something in the space between the points, such as a field, must mediate the action. To exert an influence, something, such as a wave or particle, must travel through the space between the two points, to carry the influence. The Special Theory of Relativity limits the speed at which all such influences can travel to the speed of light, . Therefore, the principle of locality implies that an event at one point cannot cause a simultaneous result at another point. An event at point ''A'' cannot cause a result at point ''B'' in a time less than , where is the distance between the points. In other words, information cannot travel faster than the speed of light. In 1935 Albert Einstein, Boris Podolsky and Nathan Rosen in the EPR paradox raised the possibility that quantum mechanics might not be a local theory, since a measurement made on one of a pair of separated entangled particles causes simultaneous collapse of the wavefunction of the remote particle. However because of the probabilistic nature of wavefunction collapse, this violation of locality cannot be used to transmit information faster than light. In 1964 John Stewart Bell derived the Bell inequality, which if confirmed showed that quantum mechanics must violate either locality or another principle, ''realism'', relating to the value of unmeasured quantities. The two principles are often referred to together as a single principle of local realism. Experimental tests of the Bell inequality, beginning with Alain Aspect's photon experiments in 1972, seem to show that quantum mechanics disobeys the inequality, and thus must violate either locality or realism. However critics have pointed out that most experiments contained "loopholes" which prevented a definitive answer to this important question. This situation seems to have been resolved in 2015, when Dr. Ronald Hanson's group at Delft University performed what has been called the first loophole-free experiment. == Pre-quantum mechanics == (詳細はNewton's law of universal gravitation was formulated in terms of "action at a distance", thereby violating the principle of locality. Coulomb's law of electric forces was initially also formulated as instantaneous action at a distance, but was later superseded by Maxwell's Equations of electromagnetism which obey locality. In 1905 Albert Einstein's Special Theory of Relativity postulated that no material or energy can travel faster than the speed of light, and Einstein thereby sought to reformulate physical laws in a way which obeyed the principle of locality. He later succeeded in producing an alternative theory of gravitation, General Relativity, which obeys the principle of locality. However, a different challenge to the principle of locality subsequently emerged from the theory of Quantum Mechanics, which Einstein himself had helped to create. 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「Principle of locality」の詳細全文を読む スポンサード リンク
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