This entry contributed by Mike Martin

A term used with respect to the Einstein-Podolsky-Rosen Paradox, first described by Einstein and his co-authors Boris Podolsky and Nathan Rosen in 1935.

Quantum mechanics makes an unusual prediction that although two entangled particles may be light-years apart, they seem to have the uncanny ability to affect one another instantaneously. If the two particles have a total spin of zero, for instance, an observer measuring the first particle's spin will instantly cause a so-called "collapse of the wave function," yielding a precise measurement of the second particle's spin. This information about the second particle becomes available to the observer far faster than the speed of light should allow.

Einstein challenged this prediction, which seemed to violate his own strict limits on the speed of information travel. Undefined "hidden variables" must be at work, Einstein claimed, in order for information about the second particle to become available instantaneously to an observer light-years away. Einstein called the effect 'spooky action at a distance' and attributed it to hidden variables.

In 1964, British physicist John Bell later disproved the notion that hidden variables affect interactions between particles with his well-known Bell's inequalities.

Bell's Inequalities, Einstein-Podolsky-Rosen Paradox