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Nonlocality in quantum mechanics, refers to
the property of entangled quantum states in which both the entangled states "collapse" simultaneously upon measurement
of one of their entangled components, regardless of the spatial separation of the two states. This "spooky action at a distance" is the content of Bell's theorem and the EPR
paradox.
See also : Principle of locality
In field theory, a nonlocal Lagrangian is a functional
which contains terms which are
nonlocal in the fields i.e. which
are not polynomials or functions of the fields or their derivatives evaluated at a single point in the space of dynamical
parameters (eg. space-time). Examples of such nonlocal Lagrangians might be
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Actions obtained from nonlocal Lagrangians are called
nonlocal actions. The actions appearing in the fundamental theories of physics, such as the Standard Model, are local actions - nonlocal actions play a part in theories
which attempt to go beyond the Standard Model, and also appear in some effective field theories.
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