Nonlocality

Nonlocality: In quantum mechanics the term “nonlocality” refers to an apparent failure of a certain relativity-theory-based ► locality assumption. This assumption is that no information about which experiment is freely chosen and performed in one space-time region can be present in a second space-time region unless a point traveling at the speed of light (or less) can reach the second region from the first. This assumption is valid in relativistic classical physics. Yet quantum theory permits the existence of certain experiments in which this locality assumption seems to fail. Einstein called the faster-than-light effect evidently entailed by conventional (Copenhagen) quantum theory “spooky action at a distance”. (For Copenhagen interpretation, see ► Born rule; Consistent Histories; Metaphysics in Quantum Mechanics; Orthodox Interpretation; Schrödinger's Cat; Transactional Interpretation.)

The simplest of the experiments pertinent to this issue involve two measurements performed in two space-time regions that lie so far apart that nothing traveling at the speed of light or less can pass from either of these two regions to the other. The experimental arrangements are such that an experimenter in each region — or perhaps some device that he has set up — is able to choose between two alternative possible measurements. The locality assumption then demands, for each region, that the truth of statements exclusively about the outcomes of the possible measurements performed in that region be independent of which experiment is “freely chosen” in the other (faraway) region.