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Hidden variable theories
Quantum mechanics generally does not predict the outcome of any measurement with certainty. Instead, it merely tells us what the probabilities of the outcomes are. This leads to the strange situation where measurements of a certain property done on two identical systems can give different answers. The question naturally arises whether there might be some deeper reality hidden beneath quantum mechanics, to be described by a more fundamental theory that can always predict the outcome of each measurment with certainty. Such a theory is called a hidden variable theory.
Einstein, Podolsky[?] and Rosen[?] argued in 1935 that such a theory was not only possible, but in fact necessary, proposing the EPR Paradox as proof. In 1964, John Bell showed, through his famous Bell inequalities, that the kind of theory proposed by Einstein, Podolsky and Rosen made different experimental predictions than orthodox quantum mechanics. Experiment showed the orthodox account to be correct, and the hope for a so-called local hidden variable theory had to be abandoned.
However, non-local theories, which are theories that allow systems to interact over distances with speeds greater than the speed of light, were not ruled out. In fact, the hidden variable theory created in 1952 by David Bohm, the so-called Bohmian mechanics, is a non-local hidden variable theory that is thought to be empirically equivalent to orthodox quantum mechanics. It still enjoys a modest popularity among physicists.
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