One extension of the Standard Model attempts to combine the electroweak interaction with the strong interaction into a single 'grand unified theory' (GUT). Such a force would be spontaneously broken into the three forces by a Higgs-like mechanism. This breakdown is theorized to occur at high energies, making it difficult to observe unification in a laboratory. The most dramatic prediction of grand unification is the existence of X and Y bosons, which cause proton decay. The non-observation of proton decay at the Super-Kamiokande neutrino observatory rules out the simplest GUTs, however, including SU(5) and SO(10).