Definition
A grand unified theory (GUT) is any proposed extension of the Standard Model in which the three non-gravitational forces — electromagnetic, weak, and strong — are merged into a single gauge interaction at energies far above those accessible in current accelerators. At lower energies the unified group breaks down to the familiar U(1)×SU(2)×SU(3) of the Standard Model.
Classic candidates include the SU(5) model of Georgi and Glashow (1974) and SO(10). None has yet been confirmed.
Why it matters
How it works
In the Standard Model, the three gauge couplings — g₁ for U(1), g₂ for SU(2), and g₃ for SU(3) — change with energy according to the renormalization group. At low energy g₃ is the largest (the strong force is strong); at very high energy the three converge. If they meet exactly at a single point, the Standard Model gauge group could be embedded in a larger simple group at that scale.
SU(5) is the smallest simple group that contains U(1)×SU(2)×SU(3). It places each generation's quarks and leptons into two multiplets — a 5̄ and a 10 — unifying matter as well as forces. The unified group is broken down to the Standard Model at the GUT scale by a heavy Higgs field, leaving 12 new gauge bosons (the X and Y) so massive that we see no trace of them at low energy.
Proton decay is the dramatic prediction. X and Y bosons can mediate processes like p → e⁺π⁰, giving the proton a finite, calculable lifetime. Minimal SU(5) predicted around 10³¹ years; experiments have now excluded that. More elaborate GUTs, especially supersymmetric ones, push the predicted lifetime higher and remain viable.
The other GUT footprints would be the relic magnetic monopoles formed when the unified symmetry broke in the early universe — Hawking notes this as one of the motivations for inflationary cosmology, which dilutes their density to undetectable levels.