That quantum mechanics is a probabilistic theory was, by 1964, an old but still troubling story. The fact that identical measurements of identically prepared systems can yield different outcomes seems to challenge a basic tenet of science and philosophy. Frustration with the indeterminacy intrinsic to quantum mechanics was famously expressed in Albert Einstein’s assertion that “God doesn’t play dice.”

By 1964 most physicists had abandoned the struggle and taken a more pragmatic view. The theory seemed to answer all questions in the workaday world of calculating ground states, energy levels, and scattering cross sections. Asking what actually happens at a measurement played no role in understanding, say, the properties of condensed matter or nuclei. The wavefunction and its evolution seemed to be all that was needed. The puzzle of indeterminism hadn’t gone away, but it was safely marginalized.

But 1964 brought a reversal of fortune. Indeterminacy, until then an unpleasant...

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