|Frontiers | Pages 184-185 ||
one unified force in such extreme conditions. Such theories are called grand unified theories, or "GUTs."
A successful GUT still would fall short of a theory of everything because of one missing element: gravity. Gravity is the toughest nut to crack in theoretical models of the behaviors of matter and forces. It is so much weaker than the other three forces that it has little effect on the scale of atoms. And yet it is so far reaching that it determines the appearance and fate of the universe. In its current form the standard model cannot account for gravity at all. Unifying the cosmic realm of gravity with the microscopic realm of quantum mechanics would rank as the crowning achievement of modern physics. It's a quest that may take decades.
Today's best candidate for such a unifying description is called string theory. For a moment, suspend all preconceptions you have about matter to entertain what string theorists claim. In the standard model, we can think of particles as points of mass. No, say string theorists; particles actually are minuscule strings or membranes that vibrate in space. Each particle would represent a different mode of vibration of the strings, much as a single guitar string can create many notes. The forces of nature would arise from the harmonies of the interacting strings.
String theorists did not invent these notions for their symphonic analogies, although they are pleasing. Rather, the mathematical details of these notions come closest to the theory of everything that physicists so fondly envision. In particular, string theory requires gravity to exist--whereas the standard model says nothing about the origins of gravity. The problem is that we have never seen one of these strings, nor can we ever hope to. By all indications, they are at least a billion billion times smaller than the dimensions of a proton.
Not all is hopeless, however. Even stranger consequences of string theory offer some chance of testing it in our lifetimes. First, the theory predicts that all particles have symmetric partners. These partners are not the same as matter and antimatter. Rather, every particle of matter would have a symmetric force-carrying counterpart, and vice versa. Given their penchant for superlative prefixes, theorists have named this notion supersymmetry. No such shadow partner has yet emerged from our particle accelerators, but the search goes on.