Editing Principle of locality (section)

{{Short description|Physical principle that only immediate surroundings can influence an object}}
{{About|the principle of locality in physics||Locality (disambiguation){{!}}Locality}}
{{Use mdy dates|date = January 2019}}
{{Use American English|date = January 2019}}

In [[physics]], the '''principle of locality''' states that an object is influenced directly 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 concept of instantaneous, or "non-local" [[action at a distance]]. Locality evolved out of the [[Classical field theory|field theories]] of [[classical physics]]. The idea is that for a [[cause]] at one point to have an effect at another point, something in the space between those points 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.

The [[special relativity|special theory of relativity]] limits the maximum speed at which causal influence can travel to the [[speed of light]], <math>c</math>. Therefore, the principle of locality implies that an event at one point cannot cause a truly simultaneous result at another point. An event at point <math>A</math> cannot cause a result at point <math>B</math> in a time less than <math>T=D/c</math>, where <math>D</math> is the distance between the points and <math>c</math> is the speed of light in vacuum.

The principle of locality plays a critical role in one of the central results of quantum mechanics. In 1935, [[Albert Einstein]], [[Boris Podolsky]], and [[Nathan Rosen]], with their [[EPR paradox]] thought experiment, raised the possibility that [[quantum mechanics]] might not be a complete theory. They described two systems physically separated after interacting; this pair would be called ''[[quantum entanglement|entangled]]'' in modern terminology. They reasoned that without additions, now called [[hidden-variable theory|hidden variables]], quantum mechanics would predict illogical relationships between the physically separated measurements.

In 1964, [[John Stewart Bell]] formulated [[Bell's theorem]], an inequality which, if violated in actual experiments, implies that quantum mechanics violates ''local causality'' (referred to as [[Principle of locality#Local realism|local realism]] in later work), a result now considered equivalent to precluding [[local hidden variables]]. Progressive variations on those [[Bell test]] experiments have since shown that [[quantum mechanics]] broadly violates Bell's inequalities.<ref>{{cite news |last=Markoff |first=Jack |title=Sorry, Einstein. Quantum Study Suggests 'Spooky Action' Is Real. |url=https://www.nytimes.com/2015/10/22/science/quantum-theory-experiment-said-to-prove-spooky-interactions.html |date=21 October 2015 |work=[[New York Times]] |accessdate=21 October 2015 }}</ref> According to some [[interpretations of quantum mechanics]], this result implies that some quantum effects violate the principle of locality.