A grand unified theory (GUT) is a class of theories that attempts to merge all three non-gravitational forces – the electromagnetic force, weak force, and strong force – into a single theoretical framework. The grand unification energy is the energy above which, it is believed, the electromagnetic force, weak force, and strong force become indistinguishable from each other.

Following the success of the electroweak theory, physicists were encouraged to try to corral the strong force with the electroweak in what became known as a grand unified theory, or GUT. The strong force operates between quarks – the fundamental building blocks of hadrons, or heavyweight particles, of which protons and neutrons are the most familiar examples. It turns out that a theory very much like QED, known as quantum chromodynamics (QCD), does a good job of describing strong interactions in terms of exchange particles called gluons. But attempts to wed QCD with the electroweak theory to yield a credible GUT haven't met with unqualified success. The trouble is that most of the GUTs that make sense also lead to some startling predictions, such as the decay of the proton, which had been thought utterly stable, solitary north and south poles known as magnetic monopoles, and a new class of particles called leptobosons. Unfortunately, despite determined efforts, not a scrap of evidence for these exotica has been found. Many researchers, therefore, are now focusing on a different and more ambitious approach. They are bypassing the GUT stage and trying to unify all the four basic forces – electromagnetism, the strong and weak forces, and gravity – in one fell swoop. For more about this effort at ultimate synthesis, see string theory.