The Meaning of Bell’s Theorem

Abstract

Einstein’s insistence that, in any “reasonable” theory, whatever can be measured must have physical reality led him to the conclusion not that quantum mechanics was wrong but that it was incomplete. This led to the idea of “hidden variable” theories, that is, theories that satisfied his criteria and could be used to “explain” quantum phenomena. John Bell set out to derive specific consequences of any such theory. He derived a theorem—an inequality—that would have to be valid for it. Experiments undertaken to test this prediction showed that quantum mechanics violated the inequality. Aside from showing that quantum theory was not consistent with Einstein’s criteria, it appeared to show that hidden variable theories could not be valid.