important paper in which they claimed that the whole formalism of
quantum mechanics together with what they called a "Reality Criterion"
imply that quantum mechanics cannot be complete. That is, there must
exist some elements of reality that are not described by quantum
mechanics. They concluded that there must be a more complete
description of physical reality involving some hidden variables that
can characterize the state of affairs in the world in more detail than
the quantum mechanical state. This conclusion leads to paradoxical
results.As Bell proved in 1964, under some further but quite plausible
assumptions, this conclusion that there are hidden variables implies
that, in some spin-correlation experiments, the measured quantum
mechanical probabilities should satisfy particular inequalities
(Bell-type inequalities). The paradox consists in the fact that
quantum probabilities do not satisfy these inequalities. And this
paradoxical fact has been confirmed by several laboratory experiments
since the 1970s.
Some researchers have interpreted this result as showing that quantum
mechanics is telling us nature is non-local, that is, that particles
can affect each other across great distances in a time too brief for
the effect to have been due to ordinary causal interaction. Others
object to this interpretation, and the problem is still open and hotly
debated among both physicists and philosophers. It has motivated a
wide range of research from the most fundamental quantum mechanical
experiments through foundations of probability theory to the theory of
stochastic causality as well as the metaphysics of free will.
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