Bell's theorem/Wikipedia Lede

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Bell's theorem is a ‘no-go theorem’ that draws an important distinction between quantum mechanics (QM) and the world as described by classical mechanics. This theorem is named after John Stewart Bell.

In its simplest form, Bell's theorem states:[1]

No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.

Cornell| solid-state physicist David Mermin has described the appraisals of the importance of Bell's theorem in the physics community as ranging from "indifference" to "wild extravagance".[2] Lawrence Berkeley particle physicist Henry Stapp declared: "Bell's theorem is the most profound discovery of science."[3]

Bell's theorem rules out local hidden variables as a viable explanation of quantum mechanics (though it still leaves the door open for non-local hidden variables). Bell concluded:

In a theory in which parameters are added to quantum mechanics to determine the results of individual measurements, without changing the statistical predictions, there must be a mechanism whereby the setting of one measuring device can influence the reading of another instrument, however remote. Moreover, the signal involved must propagate instantaneously, so that a theory could not be Lorentz invariant.

Bell summarized one of the more obscure ways to address the theorem, superdeterminism, in a 1985 BBC Radio interview:

“There is a way to escape the inference of superluminal speeds and spooky action at a distance. But it involves absolute determinism in the universe, the complete absence of free will. Suppose the world is super-deterministic, with not just inanimate nature running on behind-the-scenes clockwork, but with our behavior, including our belief that we are free to choose to do one experiment rather than another, absolutely predetermined, including the ‘decision’ by the experimenter to carry out one set of measurements rather than another, the difficulty disappears. There is no need for a faster-than-light signal to tell particle A what measurement has been carried out on particle B, because the universe, including particle A, already ‘knows’ what that measurement, and its outcome, will be.”[4]

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This is a slightly edited version that was lifted from

  1. C.B. Parker (1994). McGraw-Hill Encyclopaedia of Physics (2nd ed.). McGraw-Hill. p. 542. ISBN 0-07-051400-3.  Bell himself wrote: "If [a hidden variable theory] is local it will not agree with quantum mechanics, and if it agrees with quantum mechanics it will not be local. This is what the theorem says." John Bell, Speakable and Unspeakable in Quantum Mechanics, Cambridge University Press, 1987, p. 65.
  2. Mermin, David (April 1985). "Is the moon there when nobody looks? Reality and the quantum theory". Physics Today: 38–47. doi:10.1063/1.880968. 
  3. Stapp, Henry P. (1975). "Bell's Theorem and World Process". [[w:Nuovo Cimento|]] 29B (2): 270. doi:10.1007/BF02728310.  (Quote on p. 271)
  4. The quotation is an adaptation from the edited transcript of the radio interview with John Bell of 1985. See The Ghost in the Atom: A Discussion of the Mysteries of Quantum Physics, by Paul C. W. Davies and Julian R. Brown, 1986/1993, pp. 45-46