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In physics, action at a distance is the concept that an object can be moved, changed, or otherwise affected without being physically touched (as in mechanical contact) by another object. That is, it is the nonlocal interaction of objects that are separated in space. This term was used most often in the context of early theories of gravity and electromagnetism to describe how an object responds to the influence of distant objects. For example, Coulomb's law and the law of universal gravitation are such early theories. More generally "action at a distance" describes the failure of early atomistic and mechanistic theories which sought to reduce all physical interaction to collision. The exploration and resolution of this problematic phenomenon led to significant developments in physics, from the concept of a field, to descriptions of quantum entanglement and the mediator particles of the Standard Model. ==Electricity and magnetism== Efforts to account for action at a distance in the theory of electromagnetism led to the development of the concept of a field which mediated interactions between currents and charges across empty space. According to field theory we account for the Coulomb (electrostatic) interaction between charged particles through the fact that charges produce around themselves an electric field, which can be felt by other charges as a force. Maxwell directly addressed the subject of action-at-a-distance in chapter 23 of his A Treatise on Electricity and Magnetism in 1873.〔Clerk Maxwell (1873) (A Treatise on Electricity and Magnetism ), pages 426 to 38, link from Internet Archive〕 He began by reviewing the explanation of Ampere's formula given by Gauss and Weber. On page 437 he indicates the physicists' disgust with action at a distance. In 1845 Gauss wrote to Weber desiring "action, not instantaneous, but propagated in time in a similar manner to that of light." This aspiration was developed by Maxwell with the theory of an electromagnetic field described by Maxwell's equations, which used the field to elegantly account for all electromagnetic interactions, as well as light (which, until then, had been seen as a completely unrelated phenomenon). In Maxwell's theory, the field is its own physical entity, carrying momenta and energy across space, and action-at-a-distance is only the apparent effect of local interactions of charges with their surrounding field. Electrodynamics was later described without fields (in Minkowski space) as the direct interaction of particles with lightlike separation vectors . This resulted in the Fokker-Tetrode-Schwarzschild action integral. This kind of electrodynamic theory is often called "direct interaction" to distinguish it from field theories where action at a distance is mediated by a localized field (localized in the sense that its dynamics are determined by the nearby field parameters).〔Barut, A. O. "Electrodynamics and Classical Theory of Fields and Particles"〕 This description of electrodynamics, in contrast with Maxwell's theory, explains apparent action at a distance not by postulating a mediating entity (the field) but by appealing to the natural geometry of special relativity. Direct interaction electrodynamics is explicitly symmetrical in time, and avoids the infinite energy predicted in the field immediately surrounding point particles. Feynman and Wheeler have shown that it can account for radiation and radiative damping (which had been considered strong evidence for the independent existence of the field). However various proofs, beginning with that of Dirac have shown that direct interaction theories (under reasonable assumptions) do not admit Lagrangian or Hamiltonian formulations (these are the so-called No Interaction Theorems). Also significant is the measurement and theoretical description of the Lamb shift which strongly suggests that charged particles interact with their own field. Fields, because of these and other difficulties, have been elevated to the fundamental operators in QFT and modern physics has thus largely abandoned direct interaction theory. 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「Action at a distance」の詳細全文を読む スポンサード リンク
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