Many people have heard of string theory, but few can really describe it. Simply put, it is one of the candidates to produce an all-encompassing "theory of everything" - a single theory that can explain the observed behaviour of the physical universe, from quarks to quasars.

Presently, physical science rests on two tremendously successful but separate theories. The first is Einstein's theory of gravity, known as general relativity, which describes the behaviour of matter over large distances. The other is quantum mechanics, which governs the sub-atomic world. So far, like an unwilling bride and groom resisting a forced marriage, the two have defied all attempts at unification.

But in the past two years more and more scientists have begun to take string theory seriously, thinking it has begun to offer a glimpse of what that fiery marriage might be like. The theory itself is a mathematical framework, in which matter is represented by tiny, one-dimensional loops of string rather than particles. It is, fundamentally, a quantum theory. But at the same time, supporters say that gravity emerges as a natural feature of the string representation. Proponents believe it's the best bet for a unified theory of physics.

The strings at the heart of the theory are unimaginably small - some hundred million million million (1020) times smaller than an atomic nucleus. According to the theory, as these strings vibrate they give rise to the properties of the various observed particles - just as a violin string, depending on the way it vibrates, can produce a range of musical notes. A string vibrating a certain way, for example, could appear as an electron; another mode of vibration might give rise to a photon of light.

The roots of string theory go back several decades; in fact, it had somewhat of a heyday in the early Eighties, when supporters first hailed it as a possible "theory of everything". About 10 years ago, however, string theory seemed to have gone limp. It had become tangled in a mess of mathematics, and made few predictions that could be independently tested. To compound the difficulty, it seemed to split up into five theories rather than one.

But a series of advances over the past 18 months has put strings back in the spotlight. Supporters are calling it the "second revolution" in string theory.

The most remarkable find came last June, in a discovery that brought string theorists face to face with a problem from the front lines of modern cosmology. At centre stage are the ultra-dense remnants of collapsed stars, known as black holes. For the first time, researchers showed how to count what are called the quantum states of a black hole - a measure of how these esoteric objects are structured on a microscopic scale. When the calculation was carried out and the result agreed with a prediction made by Cambridge physicist Stephen Hawking, using a different method in the mid-Seventies, it was hailed as a triumph for string theory.

A second advance has been the discovery that the five versions of string theory that once prevailed are, in fact, equivalent. They are now recognised as different formulations of the same mathematical structure.

With these developments, many of those who were once ready to bury string theory have now come to praise it - and that includes Hawking himself. Just two years ago, he dismissed the theory as "pretty pathetic", but at a conference in Chicago in December, Hawking devoted a substantial portion of his talk to the intricacies of string theory. He stopped short of an all-out endorsement, but he was impressed with the new quantum-state result, describing it as the first "credible mechanism" for explaining the microscopic structure of black holes. The quantum-state measurement has made a deep impression on many scientists. "That is such a remarkable confirmation, emerging from a very different concept ... that there has to be something right about it," says Stuart Shapiro, an astronomer and physicist at the University of Illinois. "String theory, if it's not the final quantum theory of gravity, is on the right track - it has the seeds of something true. And I say that as a long-time sceptic of exotic theories."

Martin Rees, an astrophysicist at Cambridge and Britain's current Astronomer Royal, concurs: "It certainly seems clear that it's the most fruitful approach to the idea of understanding the relationship between the different forces - to relate gravity, electric forces, and nuclear forces, and lead toward a unified theory. And if it does succeed, it will be of course a fantastic development in understanding the world."

While the potential of string theory seems immense, it has a number of hurdles to clear before it can reach widespread acceptance. Most importantly, it has yet to be tested against direct observation. In fact, the energy required for such a test is far beyond the capability of even the largest particle accelerators. Critics of the theory say it is not "falsifiable" - that is, it cannot be shown to be wrong - and therefore is not a genuine scientific theory.

"If you can't show it's wrong, it's not science - it's merely philosophy or mathematics," says Glenn Starkman, a particle physicist and cosmologist at Case Western Reserve University in Ohio. String theory, he says, is beginning to solve theoretical problems, but is still a long way from experimental verification. And that, Starkman says, is "what holds many people back from being true believers".

For Roger Penrose, a mathematical physicist at Oxford, even the black hole calculation should be taken with a grain of salt. "I think it's a common phenomenon in physics that you're working away at something, and something works better than you expect, and you say, `gosh, it's a miracle - there must be some real truth hiding behind there.' And there may well be, but that doesn't mean it's a theory, as it stands."

One of the string theorists' more pressing tasks is the job of simplifying their work, which remains so mathematically complex that few outside the field can comprehend it. Edward Witten, a leading string theorist and physicist at Princeton's Institute for Advanced Study, says he looks forward to the day when string theory will be taught to undergraduates, as the basics of quantum mechanics and relativity are today.

For now, the theoreticians will continue to take their calculations where no mathematics has gone before. Their great challenge is to convince outsiders that their equations actually describe the real world. Only then will it be possible to decide if a "theory of everything" is truly on the horizon - or if it's just another phase of scientific fashionn

Dan Falk is a science journalist based in Toronto, Canada.

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