| locality | |
|---|---|
| Name | Locality |
| Caption | Schematic of causal light cones in Special relativity and quantum correlations |
| Field | Quantum physics |
| Related | Causality (physics), Entanglement, Bell's theorem |
locality Locality is the principle that physical influences propagate through space with limited range or at finite speed, so that events at one location cannot instantaneously affect distant events. In quantum physics, locality constrains models of interactions and is central to debates over entanglement, causality, and the interpretation of quantum mechanics. Understanding locality informs experimental tests such as Bell tests and has implications for quantum information tasks.
Locality is used in several precise senses in physics and philosophy. In classical mechanics and field theory it often refers to local interactions expressed by differential equations or local Lagrangian densities, where fields at a spacetime point couple only to their derivatives and nearby points. In quantum field theory (QFT) locality is formalized by microcausality: local operators commute at spacelike separation, expressed as [A(x),B(y)]=0 for (x−y)^2<0. Operational locality or signal locality demands that no superluminal signaling is possible, linking the concept to Special relativity and causality. Other distinctions include parameter locality and outcome locality used in discussions of hidden-variable models like those of David Bohm and Albert Einstein.
In classical theories such as Maxwell's equations and General relativity interactions propagate continuously through fields at bounded speeds (the speed of light c), embodying local action. Quantum theories introduce nonclassical correlations: while Quantum field theory maintains microcausality and relativistic invariance, nonrelativistic quantum mechanics permits entangled states whose correlations do not factorize into local probabilities. The tension appears in attempts to reconcile quantum correlations with classical intuitions of separability promoted by figures like Albert Einstein and Erwin Schrödinger.
Bell's theorem rigorously shows that no local hidden-variable theory satisfying locality and statistical independence can reproduce all the predictions of quantum mechanics. John S. Bell derived inequalities (e.g., the CHSH inequality by Clauser, Horne, Shimony, and Holt) that are violated by measurements on entangled states such as the singlet state of two spin-1/2 particles. Experimental violations imply that any underlying model must abandon at least one assumption: locality, realism, or freedom of choice. Key contributors include John Bell, John Clauser, Alain Aspect, and Anton Zeilinger.
Relativistic locality ties locality to the light cone structure of Minkowski space. In Quantum field theory, locality is implemented through microcausality and the requirement of Lorentz covariance, with formal frameworks like the Wightman axioms and Haag–Kastler axioms (algebraic QFT) specifying locality conditions. Respect for causality underpins concepts such as cause-effect ordering, the prohibition of superluminal signals, and the compatibility of quantum predictions with Special relativity. Proposed modifications include relativistic collapse models (e.g., GRW theory adaptations) and approaches in quantum gravity where locality may be emergent, studied by groups at institutions like CERN and Perimeter Institute for Theoretical Physics.
Locality constraints shape protocols in quantum information science: entanglement enables nonlocal correlations used in quantum teleportation, quantum key distribution (e.g., BB84 and device-independent protocols), and superdense coding, yet does not permit faster-than-light communication. The resource theory of entanglement distinguishes local operations and classical communication (LOCC) from global operations, and concepts like monogamy of entanglement and Bell nonlocality directly affect quantum cryptography and quantum networks pursued by organizations such as IBM Quantum and Google AI Quantum.
Empirical investigations of locality include Bell tests starting with early experiments by Freedman and Clauser and landmark closure of loopholes by Alain Aspect in the 1980s and by later experiments closing detection and locality loopholes, notably those by Anton Zeilinger, Giacomo Mauro D'Ariano collaborations, and the 2015 "loophole-free" Bell tests led by teams at Delft University of Technology, NIST, and Hannover. Experiments employ entangled photons, ions, or superconducting qubits and test spacelike separation, random setting choice, and high-efficiency detection. Results consistently confirm quantum predictions of Bell inequality violation while preserving no-signaling, aligning with operational locality.
Different interpretations of quantum mechanics treat locality differently. The Copenhagen interpretation avoids ontological commitment to hidden variables, emphasizing operational predictions. De Broglie–Bohm theory (pilot-wave theory) is explicitly nonlocal, with the quantum potential mediating instantaneous influences. Many-worlds interpretation preserves local unitary evolution but denies single-outcome realism by positing branching worlds. Objective-collapse theories such as GRW theory modify dynamics to induce localization while confronting relativistic locality. Alternative frameworks explore superdeterminism, retrocausality (advocated by some like Huw Price), or emergent locality in approaches to quantum gravity such as AdS/CFT correspondence, tensor networks, and loop quantum gravity. Debates continue about whether nonlocal correlations necessitate fundamental nonlocality or merely reflect the failure of classical intuitions about separability.
Category:Quantum physics Category:Foundations of quantum mechanics