Bell's Theorem

Bell's theorem is a result in quantum physics, proved by Northern Irish physicist John Stewart Bell in a 1964 paper, showing that no physical theory based on local hidden variables can reproduce all of the statistical predictions of quantum mechanics. The theorem responded to the 1935 Einstein-Podolsky-Rosen argument, which had suggested quantum mechanics might be an incomplete description of reality and that a deeper, local hidden variable theory might restore both determinism and locality. Bell derived a mathematical inequality, now called a Bell inequality, that any local hidden variable theory must satisfy, and showed that quantum mechanics predicts correlations between measurements on entangled particles that can violate this inequality. Experimental tests beginning with work by John Clauser in the 1970s, refined by Alain Aspect's experiments in the early 1980s and later loophole-free experiments in the 2010s, have consistently found violations matching the predictions of quantum mechanics rather than local hidden variable theories, work recognized in the 2022 Nobel Prize in Physics awarded to Clauser, Aspect, and Anton Zeilinger.