relative locality

{{Short description|Hypothetical phenomenon where observers disagree about the coincidence of spacetime events}}

{{Use dmy dates|date=December 2023}}

Relative locality is a proposed physical phenomenon in which different observers would disagree on whether two space-time events are coincident.[http://link.aps.org/doi/10.1103/PhysRevD.84.084010 Principle of relative locality], Giovanni Amelino-Camelia, Laurent Freidel, Jerzy Kowalski-Glikman, Lee Smolin, 5 Jan 2011, Phys. Rev. D 84, 084010 (2011), {{doi|10.1103/PhysRevD.84.084010}}, {{arxiv|1101.0931}} This is in contrast to special relativity and general relativity in which different observers may disagree on whether two distant events occur at the same time but if an observer infers that two events are at the same spacetime position then all observers will agree.

When a light signal exchange procedure is used to infer spacetime coordinates of distant events from the travel time of photons, information about the photon's energy is discarded with the assumption that the frequency of light doesn't matter. It is also usually assumed that distant observers construct the same spacetime. This assumption of absolute locality implies that momentum space is flat. However research into quantum gravity has indicated that momentum space might be curvedMax Born, A Suggestion for Unifying Quantum Theory and Relativity, Proc. R. Soc. Lond. A 165 (1938) 291. which would imply relative locality.Relative locality: A deepening of the relativity principle, Giovanni Amelino-Camelia, Laurent Freidel, Jerzy Kowalski-Glikman, Lee Smolin, International Journal of Modern Physics D, Vol. 20, No. 14 (2011) 2867–2873, World Scientific Publishing Company, {{doi|10.1142/S0218271811020743}}, {{arxiv|1106.0313}} To regain an absolute arena for invariance one would combine spacetime and momentum space into a phase space.