Principle of locality

Physical principle that only immediate surroundings can influence an object

In physics, the principle of locality states that an object is directly influenced only by its immediate surroundings. A theory that includes the principle of locality is said to be a "local theory". This is an alternative to the older concept of instantaneous "action at a distance". Locality evolved out of the field theories of classical physics. The concept is that for an action at one point to have an influence at another point, something in the space between those 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, carrying the influence.

In 1935 Albert Einstein, Boris Podolsky and Nathan Rosen in their EPR paradox theorised that quantum mechanics might not be a local theory, because a measurement made on one of a pair of separated but entangled particles causes a simultaneous effect, the collapse of the wave function, in the remote particle (i.e. an effect exceeding the speed of light). But because of the probabilistic nature of wave function collapse, this violation of locality cannot be used to transmit information faster than light. In 1964 John Stewart Bell formulated the "Bell inequality", which, if violated in actual experiments, implies that quantum mechanics violates either locality or realism, another principle, which relates to the value of unmeasured quantities. The two principles are commonly referred to as a single principle, local realism.

Experimental tests of the Bell inequality, beginning with Alain Aspect's 1982 experiments, show that quantum mechanics seems to violate the inequality, so it must violate either locality or realism. However, critics have noted these experiments contained "loopholes", which prevented a definitive answer to this question. This might now be resolved: in 2015 Dr Ronald Hanson at Delft University performed what has been called the first loophole-free experiment.[1] On the other hand, some loopholes might persist, and may continue to persist to the point of being fundamentally untestable.[2]

In the 17th century Newton's law of universal gravitation was formulated in terms of "action at a distance", thereby violating the principle of locality.

It is inconceivable that inanimate Matter should, without the Mediation of something else, which is not material, operate upon, and affect other matter without mutual Contact…That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro' a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it. Gravity must be caused by an Agent acting constantly according to certain laws; but whether this Agent be material or immaterial, I have left to the Consideration of my readers.[3]

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 that 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.

Locality is one of the axioms of relativistic quantum field theory, as required for causality. The formalization of locality in this case is as follows: if we have two observables, each localized within two distinct spacetime regions which happen to be at a spacelike separation from each other, the observables must commute. Alternatively, a solution to the field equations is local if the underlying equations are either Lorentz invariant or, more generally, generally covariant or locally Lorentz invariant.