Albert Einstein, Boris Podolsky, and Nathan Rosen argued in 1935 that quantum mechanics is not a complete theory, because a measurement on one system can influence the wavefunction of another in a way that’s incompatible with the light-speed limit on information propagation.1 If that “spooky action at a distance” is to be avoided, there must be more to the reality of each system than its wavefunction describes. Quantum mechanics might be supplemented, for example, by a theory of hidden variables, so that the outcome of each measurement depends only on the local degrees of freedom.

Nearly 30 years later, John Bell showed that the issue is not merely philosophical: An experiment can be devised to distinguish quantum mechanics from any local hidden-variable theory.2 (See the article by David Mermin in PHYSICS TODAY, April 1985, page 38.) Two widely separated measurements whose correlations violate a form of Bell’s...

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