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In 1964, Northern Irish physicist John Bell proved mathematically that certain quantum correlations, unlike all other correlations in the Universe, cannot arise from any local cause1. This theorem has become central to both metaphysics and quantum information science. But 50 years on, the experimental verifications of these quantum correlations still have ‘loopholes’, and scientists and philosophers still dispute exactly what the theorem states.

Quantum theory does not predict the outcomes of a single experiment, but rather the statistics of possible outcomes. For experiments on pairs of ‘entangled’ quantum particles, Bell realized that the predicted correlations between outcomes in two well-separated laboratories can be profoundly mysterious (see ‘How entanglement makes the impossible possible’). Correlations of this sort, called Bell correlations,were verified experimentally more than 30 years ago (see, for example, ref. 2). As Bell proved in 1964, this leaves two options for the nature of reality. The first is that reality is irreducibly random, meaning that there are no hidden variables that “determine the results of individual measurements”1. The second option is that reality is ‘non-local’, meaning that “the setting of one measuring device can influence the reading of another instrument, however remote”1.

Most physicists are localists: they recognize the two options but choose the first, because hidden variables are, by definition, empirically inaccessible. Quantum information scientists embrace irreducible randomness as a resource for secure cryptography3. Other physicists and philosophers (the ‘non-localist camp’) dispute that there are two options, and insist that Bell’s theorem mandates non-locality4.

Such views seem contradictory. But I believe that these two camps can be partially reconciled5 by delving into what ‘causation’ means. Doing so reveals the depth of the real principles at stake, the challenges facing each camp, and the future priorities for closing the loopholes in experiments to observe Bell correlations.

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