Principle of locality

In physics, the principle of locality states that an object is directly influenced only by its immediate surroundings. A theory which 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. The special theory of relativity limits the speed at which all such influences can travel to the speed of light, c {displaystyle c} . Therefore, the principle of locality implies that an event at one point cannot cause a simultaneous result at another point. An event at point A {displaystyle A} cannot cause a result at point B {displaystyle B} in a time less than T = D / c {displaystyle T=D/c} , where D {displaystyle D} is the distance between the points. 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 1972 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. On the other hand, some loopholes might persist, and may continue to persist to the point of being fundamentally untestable. 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. 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 which obeyed the principle of locality. He later succeeded in producing an alternative theory of gravitation, General Relativity, which obeys the principle of locality.