When physicists first began investigating the structure of atoms in the early 1900s, they uncovered a strange new world. The subatomic particles they found -- electrons, protons, and neutrons -- seemed to behave according to a completely different set of laws than those governing our everyday world.

Then, in the late 1920s, a team of young physicists led by Niels Bohr introduced a theory that explained the behavior of atoms and their particles. Not surprisingly, the theory, called quantum mechanics, was as bizarre as the world it attempted to explain.

Rather than identifying precisely where an electron should be, for example, quantum mechanics predicts only the probability of finding that electron in one place or another. This description of unpredictability at the atomic level -- indeed, at any level -- was completely unacceptable to Einstein; it flew in the face of everything he believed, and directly contradicted his orderly theories of the universe.

Despite Einstein's disapproval, quantum mechanics has only grown in acceptance as a theory. According to present-day physicists, it is the only theory that successfully explains what is happening at the atomic level.