Principle of locality

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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 respect a principle of realism but much like others, they make experimentally verified predictions that violate Bell's inequalities. This violation implies, by Bell's theorem, that they are incompatible with the principle of locality. See also EPR paradox, quantum entanglement, quantum mechanics. However, some other interpretations of quantum physics describe physical phenomena as completely local, albeit probabilistic events. Only the measurable action at a distance - a superluminal propagation of real, physical information - would be usually considered to be a violation of locality by the physicists. Such phenomena have never been seen, and they are not predicted by the current theories. This is why relativistic quantum field theory is usually considered to be a local theory.

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. This interpretation of the word "locality" is closely related to the relativistic version of causality in physics.