A class of theories that attempts to merge all the forces – the electromagnetic force, weak force, strong force and gravitational force – into a single theoretical framework. The grand unification energy is the energy above which, it is believed, the four fundamental forces become indistinguishable from each other. This temperature would have existed early in the Universe. As the Universe cooled the forces separated to become distinct.
Following the unification of the electric and magnetic forces by Maxwell, and 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, of which protons and neutrons are the most familiar examples. A theory similar to QED – which describes the electromagnetic force in terms consistent with relativity - 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 as a step towards a GUT haven't been successful. The trouble is that most of the GUTs that make sense also lead to some startling predictions, such as the decay of the proton - thought to be stable, isolated and stable 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.