Unfinished Symphony
Strings may do what Einstein finally failed to do: tie together the two great irreconcilable ideas of 20th century physics


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I am generally regarded as a sort of petrified object, rendered deaf and blind by the years," Albert Einstein confided near the end of his life. He was, alas, correct. During the last three decades of his remarkable career, Einstein had become obsessed by the dream of producing a unified field theory, a series of equations that would establish an underlying link between the seemingly unrelated forces of gravity and electromagnetism.

In so doing, Einstein hoped also to resolve the conflict between two competing visions of the universe: the smooth continuum of space-time, where stars and planets reign, as described by his general theory of relativity, and the unseemly jitteriness of the submicroscopic quantum world, where particles hold sway.

Einstein worked hard on the problem, but success eluded him. That was no surprise to his contemporaries, who saw his quest as a quixotic indulgence. They were sure that the greatest of all their colleagues was simply wasting his time, relying on a conceptual approach that was precisely backward. In contrast to just about all other physicists, Einstein was convinced that in the conflict between quantum mechanics and general relativity, it was the former that constituted the crux of the problem. "I must seem like an ostrich who forever buries its head in the relativistic sand in order not to face the evil quanta," Einstein reflected in 1954.

We know now, however, that it is Einstein's theory that ultimately fails. On extremely fine scales, space-time, and thus reality itself, becomes grainy and discontinuous, like a badly overmagnified newspaper photograph. The equations of general relativity simply can't handle such a situation, where the laws of cause and effect break down and particles jump from point A to point B without going through the space in between. In such a world, you can only calculate what will probably happen next--which is just what quantum theory is designed to do.

Einstein could never accept that the universe was at its heart a cosmic crapshoot, so that today his papers on unified field theory seem hopelessly archaic. But the puzzle they tried to solve is utterly fundamental. In simply recognizing the problem, Einstein was so daringly far-sighted that only now has the rest of physics begun to catch up. A new generation of physicists has at last taken on the challenge of creating a complete theory--one capable of explaining, in Einstein's words, "every element of the physical reality." And judging from the progress they have made, the next century could usher in an intellectual revolution even more exciting than the one Einstein helped launch in the early 1900s.

Already, in fact, theoretical physicists have succeeded in constructing a framework that offers the best hope yet of integrating gravity with nature's other fundamental forces. This framework is popularly known as string theory because it postulates that the smallest, indivisible components of the universe are not point-like particles but infinitesimal loops that resemble tiny vibrating strings. "String theory," pioneering theorist Edward Witten of Einstein's own Institute for Advanced Study has observed, "is a piece of 21st century physics that fell by chance into the 20th century."

The trouble is, neither Witten nor anyone else knows how many other pieces must fall into place before scientists succeed in solving this greatest of all puzzles. One major reason, observes Columbia University physicist Brian Greene, is that string theory developed backward. "In most theories, physicists first see an overarching idea and then put equations to it." In string theory, says Greene, "we're still trying to figure out the central nugget of truth."

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