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.”

Over the years, enthusiasm for string theory has waxed and waned. It enjoyed a brief vogue in the early 1970s, but then most physicists stopped working on it. Theorist John Schwarz of Caltech and his colleague Joel Scherk of the Ecole Normale Superieure, however, persevered, and in 1974 their patience was rewarded. For some time they had noticed that some of the vibrating strings spilling out of their equations didn’t correspond to the particles they had expected. At first they viewed these mathematical apparitions as nuisances. Then they looked at them more closely; the ghosts that haunted their equations, they decided, were gravitons, the still hypothetical particles that are believed to carry the gravitational force.

Replacing particles with strings eliminated at least one problem that had bedeviled scientists trying to meld general relativity and quantum mechanics. This difficulty arose because space lacks smoothness below subatomic scales. When distances become unimaginably small, space bubbles and churns frenetically, an effect sometimes referred to as quantum foam. Pointlike particles, including the graviton, are likely to be tossed about by quantum foam, like Lilliputian boats to which ripples in the ocean loom as large waves. Strings, by contrast, are miniature ocean liners whose greater size lets them span many waves at once, making them impervious to such disturbances.

Nature rarely bestows gifts on scientists, however, without exacting a price, and the price, in this case, takes the form of additional complications. Among other things, string theory requires the existence of up to seven dimensions in addition to the by now familiar four (height, width, length and time). It also requires the existence of an entirely new class of subatomic particles, known as supersymmetric particles, or “sparticles.” Moreover, there isn’t just one string theory but five. Although scientists could rule out none of them, it seemed impossible that all of them could be right.

But that, in fact, has turned out to be the case. In 1995, Witten, perhaps the most brilliant theorist working in physics today, declared that all five supersymmetric string theories represented different approximations of a deeper, underlying theory. He called it M theory. The insight electrified his colleagues and inspired a flurry of productive activity that has now convinced many that string theory is, in fact, on the right track. “It smells right and it feels right,” declares Caltech’s Kip Thorne, an expert on black holes and general relativity. “At this early stage in the development of a theory, you have to go on smell and feel.”

The M in M theory stands for many things, says Witten, including matrix, mystery and magic. But now he has added murky to the list. Why? Not even Witten, it turns out, has been able to write down the full set of mathematical equations that describe exactly what M theory is, for it has added still more layers of complexity to an already enormous problem. Witten appears reconciled to the possibility that decades may pass before M matures into a theory with real predictive power. “It’s like when you’re hiking in the mountains,” he muses, “and occasionally you reach the top of a pass and get a completely new view. You enjoy the view for a bit, until eventually the truth sinks in. You’re still a long way from your destination.”

Einstein was brilliant, of course, but he was also lucky. When he developed the general theory of relativity, he dealt with a world that had just three spatial dimensions plus time. As a result, he could use off-the-shelf mathematics to develop and solve his equations. M theorists can’t: their science resides in an 11-dimensional world that is filled with weird objects called branes. Strings, in this nomenclature, are one-dimensional branes; membranes are two-dimensional branes. But there are also higher-dimensional branes that no one, including Witten, quite knows how to deal with. For these branes can fold and curl into any number of bewildering shapes.

Which shapes represent the fundamental structures in our universe? On this point, string theorists are currently clueless. For the world conjured into existence by M theory is so exotic that scientists are being forced to work not just at the frontier of physics but at the frontier of mathematics as well. Indeed, it may be that they lack some absolutely essential tool and will have to develop it, just as Isaac Newton was pushed by his investigations of the laws of motion to develop the calculus. As if that weren’t hard enough, there is yet another major impediment to progress: unlike quantum mechanics, string theory and its offshoots have developed in the virtual absence of experimental evidence that could help steer theorists in productive directions.

Over the next decade, this situation could change. Hopes are running high that upcoming experiments at giant particle colliders in the U.S. and Europe will provide the first tantalizing glimpses of supersymmetry. More speculatively, these experiments could also detect the first subtle signs of additional dimensions.

What would Einstein have made of such wild imaginings? Columbia’s Greene, for one, thinks he would have loved them. After all, Greene notes in his recently published book, The Elegant Universe, Einstein played around with the idea of extra dimensions as a strategy for producing a unified field theory.

In fact, Greene believes a young Einstein, starting his professional career now rather than at the turn of the past century, would have overcome his deep distrust of quantum mechanics and enthusiastically embraced branes and sparticles and superstrings. And given his almost superhuman ability to transcend conventional thinking and visualize the world in unprecedented ways, he might have been the one to crack the ultimate theory. It may in the end take an Einstein to complete Einstein’s unfinished intellectual symphony.

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