Yet the grand unified theories deal explicitly and mathematically with a time so soon after the Big Bang that the entire universe would have been small enough to fit inside a teacup. Under the conditions of inconceivable heat and inconceivable density that then reigned, all that existed was a madly boiling chaos of subatomic particles. In that cauldron, complex structures like atoms and molecules could not exist. Instead, matter only gradually took on the variety of complicated structures it has today as the universe expanded and cooled. And the grand unified theories propose that, not only has matter evolved, but the interactions that today govern that matter may themselves have evolved from one or two forces that reigned in the infancy of the universe.
Physicists believe that four forces, or interactions, govern all matter.
The four are gravity, electromagnetism, and what are called the strong nuclear interaction and the weak nuclear interaction. Gravity and electromagnetism have been known, if not understood, for centuries. The strong interaction, which holds atomic nuclei together, and the weak interaction, which causes radioative decay in some atomic nuclei, were discovered only after scientists began to explore the atom. The new grand unified theories that Glashow and other physicists around the world are working on attempt to unite the electroweak and the strong forces. If the new theories are correct, Glashow explained, one of their consequences is that protons are not eternal but decay.