Locality (physics)

Locality is the principle that physical influences and descriptions are tied to events in bounded spacetime regions, rather than acting instantaneously across arbitrary distances.

Light cone: The boundary in spacetime separating events that can be causally connected from those that cannot. Relativistic locality restricts causal influence to events inside or on light cones.
Spacetime: The four-dimensional structure combining space and time in which physical events occur. Locality concerns how physical relations are confined within regions of spacetime.
Nonlocality: Dependence or influence between spatially separated systems that cannot be explained by local causal relations. It names the apparent failure of local explanation exposed by quantum correlations.
Bell test: An experiment measuring correlations between separated systems to test a Bell inequality. Bell tests show that observed quantum correlations exceed local-hidden-variable bounds.
Microcausality: The quantum-field-theory condition that observables at spacelike-separated points commute or, for fermionic fields, anticommute. It formalizes the requirement that spacelike-separated measurements cannot influence one another.
Causality: The relation by which one event can produce or influence another. Locality is chiefly a restriction on where causal influence can extend.
Bell's theorem: A theorem showing that no theory satisfying Bell's local-causality conditions reproduces all quantum predictions. It turns the locality dispute into experimentally testable inequalities.
Aspect experiment: A series of 1980s experiments by Alain Aspect and collaborators testing Bell inequalities with entangled photons. These experiments strengthened evidence against Bell-local explanations using rapidly changed measurement settings.
Local quantum field theory: A quantum field theory whose interactions are defined locally in spacetime and respect relativistic causal structure. It builds interactions from fields at the same spacetime point while enforcing relativistic locality.
Special relativity: The theory of space and time in inertial frames, with invariant light speed and no preferred inertial frame. Its invariant light cones define the relativistic boundary between local causal contact and separation.

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