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Hidden variable theory

In physics, a hidden variable theory is urged by a handful of physicists who persist in arguing that the statistical nature of quantum mechanics implies that it is really applicable only to ensembles of particles (just as an opinion poll is only meaningful if a reasonable sample of the population has been polled). In other words, quantum mechanics is an incomplete description of reality. They maintain that underlying this level of indeterminacy there is an objective foundation.

The best-known hidden-variables theory is that of the physicist and philosopher David Bohm. 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.

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.

It is sometimes suggested that hidden-variables theories have been ruled out by the Aspect experiment (see the EPR paradox). This is a misunderstanding of the experiment. What it did was to show that attempts to explain quantum phenomena cannot retain both the principle of reality and the principle of locality. The usual interpretation discards the principle of reality. Hidden-variables theories, with their underlying determinism, must be non-local, maintaining the existence of instantaneous causal relations between physically separated entities. 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.

See also the many-worlds interpretation for another way of understanding the implications of quantum theory.

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