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*** This review may contain spoilers ***
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 Supérieure, 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.
In all, Twilight (Physics in the Twilight) (1995) was adequate.
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