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In physics, the principle of locality is that distant objects cannot have direct influence on one another: an object is influenced directly only by its immediate surroundings. The principle is sometimes called local realism. This was stated as follows by Albert Einstein in his article "Quantum Mechanics and Reality" ("Quanten-Mechanik und Wirklichkeit", Dialectica 2:320-324, 1948):
The following idea characterises the relative independence of objects far apart in space (A and B): external influence on A has no direct influence on B; this is known as the Principle of Local Action, which is used consistently only in field theory. If this axiom were to be completely abolished, the idea of the existence of quasienclosed systems, and thereby the postulation of laws which can be checked empirically in the accepted sense, would become impossible.
This principle is a significant feature of general relativity, but some interpretations of quantum mechanics challenge the principle. These interpretations of QM make predictions that violate Bell's inequalities, thus implying, by Bell's theorem, that they are incompatible with the principle of locality. See also EPR paradox, quantum entanglement.
The above is only about one sense of locality. In another use of the term, if we have two observables, each localized within two distinct spacetime regions which happen to be at a spacelike separation from each other, both observables would commute and we have locality.
See also : nonlocality.
In computer science, the principle of locality is sometimes used as an alternative name for locality of reference.
Physics Computer science
