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:'''Hidden variable' redirects here. For hidden variables in economics, see latent variable.
In physics, 'hidden variable theories' are espoused by a minority of physicists who argue that the statistical nature of quantum mechanics indicates that quantum mechanics is incomplete. Albert Einstein, the most famous proponent of hidden variables, insisted that, "I am convinced God does not play dice"^[1]— meaning that he believed that physical theories must be deterministic to be complete.^[2] If hidden variables exist, new physical phenomena beyond quantum mechanics are needed to explain the universe as we know it.

Contents

Motivation

EPR Paradox & Bell's Theorem

Hidden-Variable Theory

References

See also

Motivation

Quantum mechanics is nondeterministic, meaning that it generally does not predict the outcome of any measurement with certainty. Instead, it tells us what the probabilities of the outcomes are. This leads to the situation where measurements of a certain property done on two ''identical'' systems can give different answers. The question 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.
In other words, quantum mechanics as it stands ''might be'' an incomplete description of reality. A minority of physicists maintain that underlying the probabilistic nature of the universe is an objective foundation/property — the hidden variable. Most believe, however, that there is no deeper reality in quantum mechanics — experiments have shown a vast class of hidden variable theories to be incompatible with observations.
Although determinism was initially a major motivation for physicists looking for hidden variable theories, non deterministic theories trying to explain what the supposed reality underlying quantum mechanics formalism looks like are also considered hidden variable theories; for example Edward Nelson's stochastic mechanics.

EPR Paradox & Bell's Theorem

In 1935, Einstein, Podolsky and Rosen wrote a four-page paper titled "Can quantum-mechanical description of physical reality be considered complete?" that argued that such a theory was in fact necessary, proposing the EPR Paradox as proof. In 1964, John Bell showed through his famous theorem that if hidden variables exist, certain configurations would satisfy Bell's inequalities. Another no-go theorem on hidden variable theories is the Kochen-Specker theorem.
Physicists such as Alain Aspect and Paul Kwiat have performed experiments that have found violations of these inequalities up 242 standard deviations^[3](excellent scientific certainty). This rules out local hidden variable theories, but does not rule out non local ones. In addition, recently, the Bell theorem itself is criticized because of the Bell test loopholes.

Hidden-Variable Theory

A hidden-variable theory which is consistent with quantum mechanics would have to be non-local, maintaining the existence of instantaneous or faster than light causal relations between physically separated entities. The first hidden-variable theory was the 'pilot wave theory' by Louis de Broglie from the late 1920s. The currently 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.
What Bohm did, based on an idea originally by Louis de Broglie, was to posit ''both'' 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 idea of local events that is used in both classical atomism and relativity theory. It points to a more holistic, mutually interpenetrating and interacting view of the world. Indeed Bohm himself stressed the holistic aspect of quantum theory in his later years, when he became interested in the ideas of Jiddu Krishnamurti. The Bohm interpretation (as well as others) has also been the basis of some books which attempt to connect physics with Eastern mysticism and consciousness.
The conflicts with relativity — not just in terms of nonlocality, but more importantly in terms of Lorentz invariance — are seen as a main weakness of Bohm's theory by many physicists.^[4] Another is that it looks contrived. 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. His hope was that this could lead to new insights and experiments that would lead beyond the current quantum theories.
Another type of deterministic theory^[5] was recently introduced by Gerard 't Hooft. This theory is motivated by the problems that are encountered when one tries to formulate a unified theory of quantum gravity.
Most physicists however are of the position that the true theory of the universe is not a hidden variable theory and that particles do not have any extra information which is not present in their quantum mechanics description. These interpretations of quantum mechanics have their own philosophical issues. A very small number of physicists believe that local realism is correct and that quantum mechanics is ultimately incorrect.

References

1. private letter to Max Born, 4 December, 1926, Albert Einstein Archives reel 8, item 180
2. Einstein, A., Podolsky, B. and Rosen, N. (1935) Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?, ''Phys. Rev.'' '47', 777-780
3. Kwiat, P. G., ''et al.'' (1999) Ultrabright source of polarization-entangled photons, ''Physical Review A'' '60', R773-R776
4. "There is a certain irony here associated with the fact
that most physicists (at least, among those who have even
heard of it) reject the de Broglie - Bohm theory because
it is explicitly non-local." ''Comment on Experimental realization of Wheeler’s delayed-choice GedankenExperiment'' - Travis Norsen
5. 't Hooft, G. (1999) Quantum Gravity as a Dissipative Deterministic System, ''Class. Quant. Grav.'' '16', 3263-3279

See also

★ Local hidden variable theory

★ Bell's theorem

★ Bell test experiments

★ Quantum mechanics

★ Bohm interpretation