Albert Einstein spent the last 30 years of his life working on his unification theory. Meanwhile, much of the rest of the physics world had traded its interest in unifying the forces of nature for a better understanding of the particles that make up matter. The result: the new and ever-developing quantum theory.

In the years leading up to Einstein's death in 1955 and afterward, the gap widened between physicists who relied on general relativity to study large objects and physicists who used quantum mechanics to understand tiny particles.

Each group had its own theory that worked for that particular discipline. That was the case until the concept of black holes was introduced during World War I. If black holes exist -- and physicists now generally agree they do -- they present a problem for the two-theory physics world. After all, the very bottom of a black hole is both massive and tiny, which means that to understand a black hole, both general relativity and quantum mechanics must be applied. Yet these two theories are in direct conflict with each other.

String theorists believe that the key to unifying these two conflicting ideas is in tiny loops or strands of energy called strings. If string theory is right, then all matter and all forces, no matter how big, small, powerful, or weak, are unified, because they all come from different vibrations of the same string.