In the context of quantum mechanics, superdeterminism is a term that has been used to describe a hypothetical class of theories that evade Bell's theorem by virtue of being completely deterministic. Bell's theorem depends on the assumption of "free will", which does not apply to deterministic theories. It is conceivable, but arguably unlikely, that someone could exploit this loophole to construct a local hidden variable theory that reproduces the predictions of quantum mechanics. Superdeterminists do not recognize the existence of genuine chances or possibilities anywhere in the cosmos.
Bell's theorem assumes that the types of measurements performed at each detector can be chosen independently of each other and of the hidden variable being measured. In order for the argument for Bell's inequality to follow, it is necessary to be able to speak meaningfully of what the result of the experiment would have been, had different choices been made. This assumption is called counterfactual definiteness. But in a deterministic theory, the measurements the experimenters choose at each detector are predetermined by the laws of physics. It can therefore be argued that it is erroneous to speak of what would have happened had different measurements been chosen; no other measurement choices were physically possible. Since the chosen measurements can be determined in advance, the results at one detector can be affected by the type of measurement done at the other without any need for information to travel faster than the speed of light.
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.
Although he acknowledged the loophole, he also argued that it was implausible. Even if the measurements performed are chosen by deterministic random number generators, the choices can be assumed to be "effectively free for the purpose at hand," because the machine's choice is altered by a large number of very small effects. It is unlikely for the hidden variable to be sensitive to all of the same small influences that the random number generator was.
[W]e always implicitly assume the freedom of the experimentalist... This fundamental assumption is essential to doing science. If this were not true, then, I suggest, it would make no sense at all to ask nature questions in an experiment, since then nature could determine what our questions are, and that could guide our questions such that we arrive at a false picture of nature.
- Bell's theorem
- Chaos theory
- De Broglie–Bohm theory
- Pre-established harmony
- Static interpretation of time
- BBC Radio interview with Paul Davies, 1985
- 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
- J. S. Bell, Free variables and local causality, Epistemological Letters, Feb. 1977. Reprinted as Chapter 12 of J. S. Bell, Speakable and Unspeakable in Quantum Mechanics (Cambridge University Press 1987)
- A. Zeilinger, Dance of the Photons, Farrar, Straus and Giroux, New York, 2010, p. 266. Abner Shimony, Michael Horne and John Clauser made a similar comment in replying to John Bell in their discussions in the Epistemological Letters: "In any scientiﬁc experiment in which two or more variables are supposed to be randomly selected, one can always conjecture that some factor in the overlap of the backward light cones has controlled the presumably random choices. But, we maintain, skepticism of this sort will essentially dismiss all results of scientiﬁc experimentation. Unless we proceed under the assumption that hidden conspiracies of this sort do not occur, we have abandoned in advance the whole enterprise of discovering the laws of nature by experimentation." (Shimony A, Horne M A and Clauser J F, "Comment on the theory of local beables", Epistemological Letters, 13 1 (1976), as quoted in Jan-Åke Larsson, "Loopholes in Bell inequality tests of local realism", J. Phys. A: Math. Theor. 47 (2014))
- Reality, locality, and "free will". Quote: He [Michael J. W. Hall] shows that locality and reality can be retained with a 14% reduction of the experimenters’ "free will".