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In quantum mechanics, superdeterminism is a hypothetical class of theories that evade Bell's theorem by virtue of having fine-tuned initial conditions. Bell's theorem depends on the assumption of "free will", which does not apply to superdeterministic theories. It is conceivable that someone could exploit this loophole to construct a local hidden variable theory that reproduces the predictions of quantum mechanics.

All superdeterministic theories are deterministic, but the converse is not true, and determinism alone does not provide a loophole to Bell's theorem. In ordinary determinism, the experimenters' choices of measurement settings are predetermined from the start of the universe, but this does not interfere with Bell's theorem because it can still be made statistically independent of the hidden variable. Superdeterminism differs from ordinary determinism (which has arbitrary initial conditions) in that it requires the fine-tuning of the initial conditions at the beginning of the universe so that in Bell test experiments at a later point in time there exist correlations between the choices of measurement settings and the hidden variable being measured, in order to guarantee ensure/guarantee results. Under this explanation, the 'apparent' experimental verification of quantum mechanics in Bell test experiments is produced through fine-tuning.

Bell's theorem requires that the types of measurements performed at each detector can be chosen independently of each other and of the hidden variable being measured. In superdeterminism, however, the hidden variable may be a function of the measurement settings, and this dependence cannot be removed no matter what process one uses to determine them. As Bell wrote,

"An essential element in the reasoning here is that a and b are free variables. One can envisage then theories in which there just are no free variables for the polarizer angles to be coupled to. In such ‘superdeterministic’ theories the apparent free will of experimenters, and any other apparent randomness, would be illusory. Perhaps such a theory could be both locally causal and in agreement with quantum mechanical predictions. However I do not expect to see a serious theory of this kind. I would expect a serious theory to permit ‘deterministic chaos’ or ‘pseudorandomness’, for complicated subsystems (e.g. computers) which would provide variables sufficiently free for the purpose at hand. But I do not have a theorem about that."

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.

In the 1980s, John Bell discussed superdeterminism in a BBC 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."

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.

Superdeterminism has also been criticized because of its implications regarding the validity of science itself. For example, Anton Zeilinger has commented:

"[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."