nonlocality

Nonlocality refers to a feature of certain physical theories, most notably quantum mechanics, in which correlations between outcomes on spatially separated systems cannot be explained by any theory that relies solely on local properties and causal influences within the light cone (local realism). In quantum theory, entangled states exhibit correlations that persist regardless of distance, so the outcome probabilities for one subsystem depend on measurements performed on another distant subsystem in ways that defy local hidden-variable explanations. These nonlocal correlations, however, do not enable faster-than-light communication due to the no-signaling principle.

History and theory: The concept emerged from the Einstein-Podolsky-Rodol paradox of 1935, which argued that quantum mechanics might be incomplete. Bell’s theorem (1964) showed that local hidden-variable theories cannot reproduce all quantum predictions, via inequalities that quantum mechanics can violate. Experimental tests since the 1980s, including Aspect’s experiments in 1982, reported violations. By the mid-2010s, several loophole-free Bell tests closed major experimental gaps (locality and detection) and provided stronger evidence for quantum nonlocality.

Experimental and theoretical status: In practice, quantum nonlocality is tied to entanglement and to fundamental questions about the nature of reality and causality. While nonlocal correlations are striking, they do not permit superluminal signaling. In quantum field theory, locality is preserved at the level of field operators (microcausality), even as entangled systems exhibit nonlocal correlations. Outside physics, the term also appears in mathematics to describe nonlocal operators, but in physics it denotes the distinctive, experimentally tested aspect of quantum correlations.