grand unified theories (GUTs)
/ GUT, rhymes with 'gut' /
The Standard Model describes three forces — strong, weak and electromagnetic — but it really runs them as three separate businesses, each with its own rulebook and its own strength. A grand unified theory is the proposal that, if you zoom in to high enough energies, these three are revealed as a single force, the way ice, water and steam turn out to be one substance seen at different temperatures. 'Grand' here just means more sweeping than the already-successful merger of the weak and electromagnetic forces.
Technically, the three Standard Model forces each follow a symmetry of their own; a GUT embeds all three inside one larger symmetry that breaks apart, as the universe cooled, into the three we see today. A striking bonus is that in many GUTs quarks and leptons sit in the same family slot, so the theory can explain why the proton's electric charge is exactly three times the electron's in magnitude — a precise fact the Standard Model simply takes as given. The simplest historical example, called SU(5), packs each generation of matter neatly into just two bundles.
Grand unification is compelling but unconfirmed, and it makes one famously testable prediction: because it mixes quarks with leptons, it allows the proton to decay, very slowly, into lighter particles. Giant underground detectors have watched enormous tanks of water for years and have never seen a proton die, which has already ruled out the simplest version of SU(5). So GUTs remain a leading dream of unification, kept honest by experiments that have so far only set limits, not made a discovery.
In an SU(5) grand unified theory the up quark, down quark, electron and neutrino of one generation are members of a single mathematical family, which forces the proton charge and electron charge to balance exactly — and predicts the proton will eventually decay.
A GUT folds the three forces into one and quarks and leptons into one family.
A GUT unifies the strong, weak and electromagnetic forces but still leaves gravity out. Folding gravity in too is a separate, much harder problem — the goal of a 'theory of everything', not of a GUT.