In quantum mechanics, superdeterminism is a loophole in Bell's theorem that allows one to evade it[clarification needed] by postulating that all systems being measured are causally correlated with the choices of which measurements to make on them. It is conceivable 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.
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Bell's theorem assumes that the measurements performed at each detector can be chosen independently of each other and of the hidden variable being measured. But in a superdeterministic theory this is not true; they are necessarily correlated. Since the choice of measurements and the hidden variable are predetermined, the results at one detector can depend on which measurement is done at the other without any need for information to travel faster than the speed of light.
Thus, it is conceivable that freedom of choice has been restricted since the beginning of the universe in the Big Bang, with every future measurement predetermined by correlations established at the Big Bang . This would make superdeterminism untestable, since experimenters would never be able to eliminate correlations that were created at the beginning of the universe: the freedom-of-choice loophole could never be completely eliminated.
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.
Nobel Prize winner Gerard 't Hooft discussed this loophole with John Bell in the early 1980s. "I raised the question: Suppose that also Alice's and Bob's decisions have to be seen as not coming out of free will, but being determined by everything in the theory. John said, well, you know, that I have to exclude. If it's possible, then what I said doesn't apply. I said, Alice and Bob are making a decision out of a cause. A cause lies in their past and has to be included in the picture".
[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.
Physicists Sabine Hossenfelder and Tim Palmer have argued that superdeterminism "is a promising approach not only to solve the measurement problem, but also to understand the apparent non-locality of quantum physics".