Grand unification energy

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The grand unification energy Λ_GUT, or the GUT scale, is the energy level above which, it is believed, the electromagnetic force, weak force, and strong force become indistinguishable from each other. The electromagnetic and weak forces become indistinguishable at a lower energy level, forming the electroweak force, and gravitational force requires an even higher energy level to unify with the other three. Specific Grand unification theories (GUTs) can predict the grand unification energy but, usually, with large uncertainties (due to the choice of the gauge group, of the higgs content, of the matter content and also due to the arbitrary parameters of the specific theory). Furthermore, at the moment it seems fair to state that there is no agreed minimal GUT.

According to Stephen Hawking in A Brief History of Time, "The value of the grand unification energy is not very well known, but it would probably have to be at least a thousand million million (10¹⁵) GeV." The most powerful colliders ever designed can go up to a few thousand GeV. The scale 10¹⁵ GeV is only a few orders of magnitude below the Planck scale at which gravitational effects become dominant.