A Physical theory is said to exhibit strict nonlocality if in that theory it is not possible to treat widely separated systems as independent. The term is most often reserved, however, just for interaction supposed to occure outside the past light cone. Nonlocality does not necessarily imply a lack of causality. For instance, Newtonian gravitation is nonlocal because it involves instantaneous action-at-a-distance but Newtonian mechanics is certainly causal. Effects that appear nonlocal in Quantum Mechanics, some physicists say, actually obey locality; in these cases, the nonlocal interaction affects correlations that are considered within the Copenhagen interpretation of quantum mechanics to pertain to states of matter that result from the wave collapse upon measurement of irreal states comprised of the sum of mutually exclsive possibilities, e.g., the singlet state. Einstein criticised this interpretation of quantum mechanics on the grounds that these effects employed "spooky instantaneous action at a distance". This issue is very closely related to Bell's theorem and the EPR paradox. Quantum field theory, on the other hand, which is the relativistic generalization of quantum mechanics, contains mathematical features that assure locality, so that nonrelativistic quantum mechanics should be local as well. Thus, the EPR paradox.

See also

• Principle of locality
• Nonlocal Lagrangian
• Bell's theorem

Quantum measurement | Quantum field theory | Theoretical physics