Grand Unified Theory
In physics, a Grand Unified Theory (GUT) is a field theory that unifies three of the four known basic physical forces, namely the strong interaction, the weak interaction and the electromagnetic interaction (but not gravity).
With the GUT, one assumes that these basic forces were a single force at the time of the hot big bang. After the cooling of the universe, this single force would have split into the three mentioned forces. According to the GUT, the strong interaction (also called the strong nuclear force) becomes weaker at high energy, whereas the electromagnetic and the weak interaction become stronger at high energy. In this context, it is said that the phenomenon of asymptotic freedom known from the theory of the strong interaction (quantum chromodynamics) does not occur in the electromagnetic and weak interactions. At a certain very high energy, according to the GUT, all three forces would then have the same strength and could turn out to be different aspects of a single force. The grand unified theory further predicts that at this energy all matter particles with spin ½ (fermions), for example electrons and quarks, would be essentially the same.
In a narrower sense, GUT is understood as the unification of the gauge theories of the Standard Model (SU(3)C of quantum chromodynamics, SU(2)I × U(1)Y of the electroweak interaction) in a higher-order gauge group. Many such models, especially SU(5) and SO(10), but also exotic Lie groups up to E(8) have been studied, starting with the enforcement of the Standard Model in the 1970s. Here SU stands for the special unitary group, U for the unitary group, O for the orthogonal group and SO for the special orthogonal group. If one includes supersymmetry, further possibilities arise. The fact that the sliding coupling constants of the gauge theories of the Standard Model meet relatively exactly only for supersymmetric GUTs at one energy scale is seen as an argument for supersymmetry.
The energy scale of the grand unification is 1025 eV equal to 1016 GeV, where 1 GeV is approximately equal to the proton mass. These energies are not achievable in terrestrial accelerator experiments (the Large Hadron Collider reaches 13 000 GeV equal to 13 TeV centre-of-mass energy in its highest expansion stages). One possibility to test the existence of a unified force nevertheless is proton decays, which are predicted by almost all unified theories. The failure so far to detect such decays in detectors such as super-Kamiokande has already ruled out the simplest GUT models (minimal SU(5) theory, SO(10) without supersymmetry). According to the 2017 Super-Kamiokande results, the lower limit for the lifetime is 1.6-1034 years (For comparison, age of the universe: 1.4-1010 years). The exclusion of the minimal SU(5) theory, the first proposed unified theory (by Howard Georgi, Sheldon Glashow), happened already in 1996, but there are still some GUTs that remain in question, such as flipped SU(5) (developed by Dimitri Nanopoulos, Stephen Barr and others in the 1980s) or minimal supersymmetric SU(5) by Howard Georgi and Savas Dimopoulos (1981).
For a complete description of all known physical phenomena, this unification would also have to include the fourth fundamental force, gravity, with the general theory of relativity. Such a theory, which unifies quantum physics and gravitation theory (quantum gravity), is called the "Theory of Everything". Candidates are, for example, string theory, or rather the unification of the five string theory approximations, the so-called M-theory, which also includes supergravity, and loop quantum gravity.
