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In physics, a hidden variable theory is urged by a minority of
physicists who argue that the statistical nature of quantum mechanics implies that it is really applicable only to ensembles of particles.
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
measurement with certainty. An analogy exists with opinion polling: it is not that opinions are indefinite, but only if a
reasonable sample of the population has been polled does one expect the poll results, as statistics, to be in line with the trend
in the population at large.
In other words, quantum mechanics as it stands might be an incomplete description of reality. Some physicists
maintain that underlying this level of indeterminacy there is an objective foundation. Such a theory is called a hidden variable
theory.
In 1935, Einstein, Podolsky and Rosen wrote
a 4 page paper called Can quantum-mechanical description of physical reality be considered
complete? that argued 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.
Experiments have been interpreted as showing the orthodox account to be correct, but the hope for a so-called local
hidden variable theory is still very much alive [1] . The "loopholes" in experiments such as Aspect's are more serious than is
generally realised.
If experiments really had behaved as popular accounts tell us, with indisputable violation of Bell's inequality, then
hidden-variable theories, with their underlying determinism, would have to be
non-local, maintaining the existence of instantaneous causal
relations between physically separated entities. Non-local theories, i.e. theories that allow systems to interact over
distances with speeds greater than the speed of light, would not be
ruled out. The best-known hidden-variable theory, the Bohmian
mechanics, of the physicist and philosopher David Bohm, created in 1952, is a non-local hidden variable theory.
It is thought to be empirically equivalent to orthodox quantum mechanics. It still enjoys a modest popularity among
physicists. What Bohm did was to distinguish between the quantum particle, e.g. an electron, and a hidden 'guiding wave' that
governs its motion. Thus, in this theory electrons are quite clearly particles. When you perform a double-slit experiment (see wave-particle duality), they go through one slit rather than the
other. However, their choice of slit is not random but is governed by the guiding wave, resulting in the wave pattern that is
observed.
Such a view contradicts the simple location of events in both classical atomism and relativity theory. It points to a more holistic view of the
quantum world. Indeed Bohm himself stressed the holistic aspect of quantum theory in his later years, after his conversion from
Marxism to theosophy.
The main weakness of Bohm's theory is that it looks contrived - which it is. It was deliberately designed to give predictions
which are in all details identical to conventional quantum mechanics. His aim was not to make a serious counterproposal but
simply to demonstrate that hidden-variables theories are indeed possible.
See also the many-worlds interpretation for
another way of understanding the implications of quantum theory.
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