Grand Unified Theory

A Grand Unified Theory (GUT) is any <a href="/facts/Mathematical_model/CGhyuxgz">model</a> in <a href="/facts/Particle_physics/YgIuyj2t">particle physics</a> that merges the <a href="/facts/Electromagnetism/oAkmALHT">electromagnetic</a>, <a href="/facts/Weak_interaction/f6upmKMs">weak</a>, and <a href="/facts/Strong_interaction/JjRsKcLg">strong</a> <a href="/facts/Fundamental_interaction/WrFEJaFl">forces</a> (the three <a href="/facts/Gauge_theory/fSW3gggv">gauge interactions</a> of the <a href="/facts/Standard_Model/LhVGf9ow">Standard Model</a>) into a single force at high <a href="/facts/Energy/wd8PrQM7">energies</a>. Although this <a href="/facts/Unification_(physics)/GV8h3FNJ">unified</a> force has not been directly observed, many GUT models theorize its existence. If the unification of these three interactions is possible, it raises the possibility that there was a <a href="/facts/Grand_unification_epoch/3ePE7yDS">grand unification epoch</a> in the <a href="/facts/Chronology_of_the_universe/ezBKAdjy">very early universe</a> in which these three <a href="/facts/Fundamental_interaction/WrFEJaFl">fundamental interactions</a> were not yet distinct.
Experiments have confirmed that at high energy, the electromagnetic interaction and weak interaction unify into a single combined <a href="/facts/Electroweak_interaction/QATz4dYF">electroweak interaction</a>. GUT models predict that at even <a href="/facts/Grand_unification_energy/NuhE9yVn">higher energy</a>, the strong and electroweak interactions will unify into one electronuclear interaction. This interaction is characterized by one larger <a href="/facts/Gauge_theory/fSW3gggv">gauge symmetry</a> and thus several <a href="/facts/Force_carrier/zpF4Tyaa">force carriers</a>, but one unified <a href="/facts/Coupling_constant/Queop1TR">coupling constant</a>. Unifying <a href="/facts/Gravity/bqAN7DdQ">gravity</a> with the electronuclear interaction would provide a more comprehensive <a href="/facts/Theory_of_everything/rpossh4t">theory of everything</a> (TOE) rather than a Grand Unified Theory. Thus, GUTs are often seen as an intermediate step towards a TOE.
The novel particles predicted by GUT models are expected to have extremely high masses—around the <a href="/facts/GUT_scale/NuhE9yVn">GUT scale</a> of 1016 GeV/c2 (only three orders of magnitude below the <a href="/facts/Planck_units/xDU6KlDM">Planck scale</a> of 1019 GeV/c2)—and so are well beyond the reach of any foreseen <a href="/facts/Collider/j6wTDohV">particle hadron collider</a> experiments. Therefore, the particles predicted by GUT models will be unable to be observed directly, and instead the effects of grand unification might be detected through indirect observations of the following:

<ul><li><a href="/facts/Proton_decay/Awg3sEzD">proton decay</a>,</li>
<li><a href="/facts/Electric_dipole_moment/ou2LDUVg">electric dipole moments</a> of <a href="/facts/Elementary_particle/CTFDM97U">elementary particles</a>,</li>
<li>or the properties of <a href="/facts/Neutrino/yGWVfSg2">neutrinos</a>.</li></ul>
Some GUTs, such as the <a href="/facts/Pati%25E2%2580%2593Salam_model/AJK7zffd">Pati–Salam model</a>, predict the existence of <a href="/facts/Magnetic_monopole/R1TSbfMY">magnetic monopoles</a>.
While GUTs might be expected to offer simplicity over the complications present in the <a href="/facts/Standard_Model/LhVGf9ow">Standard Model</a>, realistic models remain complicated because they need to introduce additional fields and interactions, or even additional dimensions of space, in order to reproduce observed <a href="/facts/Fermion/LEckdDsR">fermion</a> masses and mixing angles. This difficulty, in turn, may be related to the existence of <a href="/facts/Family_symmetries/0swLGX6R">family symmetries</a> beyond the conventional GUT models. Due to this and the lack of any observed effect of grand unification so far, there is no generally accepted GUT model.
Models that do not unify the three interactions using one <a href="/facts/Simple_group/e5hGF7nT">simple group</a> as the gauge symmetry but do so using <a href="/facts/Reductive_group/z1zyJvUh">semisimple groups</a> can exhibit similar properties and are sometimes referred to as Grand Unified Theories as well.

Unsolved problem in physics
Are the three forces of the Standard Model unified at high energies? By which symmetry is this unification governed? Can the Grand Unification Theory explain the number of fermion generations and their masses?
<a href="/facts/List_of_unsolved_problems_in_physics/vJ3d1A83">More unsolved problems in physics</a>

Grand Unified Theory open-in-new

Grand Unified Theory