Is there an equation of everything?

Since the ancient Greeks first speculated that everything they observed in reality was the result of the interaction of tiny particles they called atoms, great thinkers have tried to find a single mathematical formula that governs and explains the workings of the entire universe. So far, though, even minds as brilliant as physicists Albert Einstein and Stephen Hawking have been unable to come up with that single grand equation of everything, also known as the theory of everything, or the final theory. Nevertheless, they continue to try, because without that final piece of the puzzle that is reality, the sum total of what we know falls a bit short of making sense.

Perhaps the most illustrious searcher for the equation of everything was physicist Albert Einstein, who spent the last 35 years of his life trying to uncover such an overarching explanation. Einstein's own theory of general relativity, published in 1916, explained gravity, one of the strong forces in the universe, as the bending of space-time by matter. But general relativity didn't explain electromagnetism, another strong force that was even more powerful than gravity. Einstein wasn't willing to accept that these two forces were unrelated, and he searched for a single explanation — a "unified field theory" — that would show how electromagnetism, gravity and space-time interacted. "I see in nature a magnificent structure … that must fill a thinking person with a feeling of humility," he once explained. But although he published a number of papers on his ideas about unified field theory, he died in 1955 without solving the problem to the satisfaction of other theoreticians.

In the six decades that followed, physicists gradually amassed a great deal more knowledge and insight about the workings of the universe. With the help of massive machines called particle accelerators, they've studied the subatomic realm and made discoveries about the forces that operate at that level. But the search for a comprehensive explanation still continues. In particular, physicists have sought to find a way to reconcile Einstein's model of space-time, which seems to work best when applied to big objects like stars and galaxies, and the theory of quantum mechanics, which offers an explanation of electromagnetic and nuclear forces that makes sense of reality at the tiniest level. In recent years, some have proposed a novel solution called "string theory," in which tiny particles are loops that vibrate like violin strings in a multidimensional space. In the mathematical calculations that make up string theory, gravitation is a byproduct of the process. While string theory offers a possible solution to the ultimate question of the ages, there's a hitch: Scientists have not yet found a way to test the theory experimentally to prove that it's more than just an elegant idea.