What is the principle of locality in physics?

The principle of locality states that an object can only be directly influenced by its immediate surroundings, rejecting instantaneous action at a distance. Grounded in classical field theories and special relativity, it asserts that causal influences cannot travel faster than the speed of light. However, modern quantum mechanics presents challenges to this principle through phenomena like entanglement and violations of Bell's inequalities.

What we know

• The principle of locality dictates that an object is directly influenced only by its immediate surroundings. [1, 2]
• Special relativity limits the maximum speed of causal influence to the speed of light, implying that events cannot cause simultaneous results across distance. [2, 4]
• John Stewart Bell formulated Bell's theorem in 1964, providing an inequality that tests whether quantum mechanics violates local causality. [2]
• Progressive Bell test experiments have demonstrated that quantum mechanics broadly violates Bell's inequalities. [2]

Still open

• Whether quantum mechanics truly violates the principle of locality depends on the chosen interpretation of quantum theory.

Where to go next

• how it workedHow does special relativity limit the speed of causality?
• how we knowWhat was the Einstein-Podolsky-Rosen paradox?
• a case in pointHow do Bell test experiments work?
• compared withWhat is action at a distance in classical physics?
• whoAlbert Einstein
• an unexpected connectionHow does gravity connect to the geometry of spacetime?Although locality governs how individual influences travel through space, general relativity describes gravity not as a force traveling across space, but as the geometric curvature of four-dimensional spacetime itself.

Sources

• [1]principle of locality (Wikidata Q629202) · CC0 1.0
• [2]Principle of locality · CC BY-SA 4.0
• [3]General relativity · CC BY-SA 4.0
• [4]Special relativity · CC BY-SA 4.0
• [5]Gravitational wave · CC BY-SA 4.0