Could the probabilities of quantum mechanics have a deterministic origin? The desire to give a nonprobabilistic explanation of quantum probabilities is one reaction to the infamous measurement problem: Why does a quantum system evolve continuously and deterministically when it’s not observed but discontinuously and randomly once it is? Probability appears in only one postulate of quantum theory, the Born rule. If the Born rule could be derived without introducing a probabilistic assumption in the first place, it would furnish an appealing path to solving the measurement problem—the stochastic nature of measurement would have a nonstochastic explanation. However, a new analysis by Jiaxuan Zhang [1], a graduate student at the University of Oxford in the UK, shows that a particular probabilistic assumption is indispensable in every major derivation of the Born rule, casting fundamental doubt on this hope.
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