Bell's theorem

Mathematical result derived from the postulates of quantum theory by the Northern Irish physicist John Bell (1928-1990). It shows that quantum theory predicts that the properties of widely separated systems that have interacted in the past - such as subatomic particles flying apart from a nuclear disintegration - can be correlated in ways that are impossible according to classical and commonsense ideas. Some scientists had hoped that it would be possible to find a ‘complete state’ of such a system that would explain the results of measurements on the system of such properties as momentum and spin. Bell showed that the idea of such a complete state is at variance with quantum mechanics. His work inspired a series of experimental tests in which quantum mechanics has repeatedly been vindicated.