Fig. 1
Fig. 1. An artist’s impression of space–time foam in some models of quantum gravity: tiny (average size 10–35 m) and short-lived (average lifetime 10–43 s) fluctuations of space–time, with non-trivial topology and/or singular geometry (such as black holes), give the ground state of quantum gravity the nature of a stochastic medium. Propagation of ordinary matter in such backgrounds may result in non-trivial “optical properties” such as a refractive index, modifications of dispersion relations, deviations from normal quantum-mechanical evolution, light-cone fluctuations and violations of Lorentz invariance.