A theory that attempts to combine the strong, weak, and electromagnetic interactions into a single gauge theory with a single symmetry group. There are a number of different theories, most of which postulate that the interactions merge at high energies into a single interaction (the standard model (see elementary particles) emerges from the GUT as a result of broken symmetry). The energy above which the interactions are the same is around 10¹⁵ GeV, which is much higher than those obtainable with existing accelerators.

One prediction of GUTs is the occurrence of proton decay. Some also predict that the neutrino has nonzero mass. There is no evidence for proton decay at the moment. There is evidence that neutrinos have nonzero masses. There has been a considerable amount of activity in the 21st century in relating GUTs to the experimentally observed parameters associated with neutrinos with nonzero masses. It seems likely that GUTs need to be combined with supersymmetry and also family symmetry. See also superstring theory.