BELL'S THEOREM

Bell's theorem: translation

Bell's Interconnectedness theorem, proved by the physicist John Bell in 1964, asserts that no local model of reality can do justice to the facts of quantum behaviour. A local model of reality is one in which all causal connections propagate by signals that travel at less than the speed of light. Bell showed that quantum mechanics describes correlations that cannot be explained by a local model. The theorem considers a set-up which is a variation on that of the Einstein–Podolsky–Rosen thought experiment, and proves that if reality is local, we would expect certain defined measurements to show a certain inequality (the Bell inequality). But in fact the experimental results are otherwise, suggesting a conflict with relativity theory, which appears to require locality. Interpreters disagree whether the conflict is real—for example, perhaps one should say that the correlations are unexplained brute facts.