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Bell's theorem is a term encompassing a number of closely related results in physics, all of which determine that quantum mechanics is incompatible with local hidden-variable theories, given some basic assumptions about the nature of measurement.
Bell expanded on the theorem to provide what would become the conceptual foundation of the Bell test experiments. [ citation needed ] A typical experiment involves the observation of particles, often photons, in an apparatus designed to produce entangled pairs and allow for the measurement of some characteristic of each, such as their spin .
They derived the CHSH inequality, which, as with John Stewart Bell's original inequality, [2] is a constraint—on the statistical occurrence of "coincidences" in a Bell test—which is necessarily true if an underlying local hidden-variable theory exists. In practice, the inequality is routinely violated by modern experiments in quantum mechanics.
Assuming the validity of Bell's theorem, any deterministic hidden-variable theory that is consistent with quantum mechanics would have to be non-local, maintaining the existence of instantaneous or faster-than-light relations (correlations) between physically separated entities.
John Stewart Bell FRS [2] (28 July 1928 – 1 October 1990) [3] was a physicist from Northern Ireland and the originator of Bell's theorem, an important theorem in quantum physics regarding hidden-variable theories.
A collection of related theorems, beginning with Bell's proof in 1964, show that quantum mechanics is incompatible with local hidden variables.However, as Bell pointed out, restricted sets of quantum phenomena can be imitated using local hidden-variable models.
Calculating expectations based on Bell's work implies that for quantum physics the assumption of "local realism" must be abandoned. [14] Bell's theorem proves that every type of quantum theory must necessarily violate locality or reject the possibility of extending the mathematical description with outcomes of measurements which were not ...
Bell's theorem can thus be interpreted as a separation between the quantum and classical predictions in a type of causal structures with just one hidden node (,). Similar separations have been established in other types of causal structures. [ 28 ]