A hidden-variables interpretation assigns definite locations or other properties to quantum objects before they are observed, even if those properties can’t be calculated from the theory itself. These properties go unseen in the mathematics of quantum physics, hence “hidden” variables. Bohm’s pilot-wave interpretation is a prime example of such a theory: in Bohm’s world, particles always have positions, even though those positions are largely hidden from view and can’t be calculated from Schrödinger’s equation. The proofs of von Neumann, Jauch, and Gleason all suggested that this kind of
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