In quantum physics, a Bell test tests Bell's theorem using entangled particles. The experiment compares correlations between measurements made on separated members of an entangled pair. Quantum mechanics predicts correlations that cannot be reproduced by theories using local hidden variables.
John Stewart Bell published his theorem in 1964. Bell derived inequalities that any local hidden-variable theory must satisfy. Quantum predictions can violate those inequalities, turning a philosophical dispute about quantum completeness into an experimentally testable question.
Experiments by Alain Aspect, John Clauser, Anton Zeilinger, and other researchers observed Bell-inequality violations under increasingly strict conditions. Clauser, Aspect, and Zeilinger shared the 2022 Nobel Prize in Physics for experiments with entangled photons and related foundations of quantum information. A Bell-test violation does not allow faster-than-light messaging; it reveals nonclassical correlations while preserving no-signalling.