The Unraveling of String Theory

By MICHAEL D. LEMONICK

By now, just about everyone has heard of string theory. Even those who don't really understand it--which is to say, just about everyone--know that it's the hottest thing in theoretical physics. Any university that doesn't have at least one string theorist on the payroll is considered a scientific backwater. The public, meanwhile, has been regaled for years with magazine articles breathlessly touting it as "the theory of everything." Brian Greene's 1999 book on the topic, The Elegant Universe, has sold more than a million copies, and his Nova series of the same name has captivated millions of TV viewers.

But despite its extraordinary popularity among some of the smartest people on the planet, string theory hasn't been embraced by everyone--and now, nearly 30 years after it made its initial splash, some of the doubters are becoming more vocal. Skeptical bloggers have become increasingly critical of the theory, and next month two books will be hitting the shelves to make the point in greater detail. Not Even Wrong, by Columbia University mathematician Peter Woit, and The Trouble with Physics, by Lee Smolin at the Perimeter Institute for Theoretical Physics in Waterloo, Ont., both argue that string theory (or superstring theory, as it is also known) is largely a fad propped up by practitioners who tend to be arrogantly dismissive of anyone who dare suggest that the emperor has no clothes.

There were good reasons for the theory's appeal when it first emerged in the late 1970s and early '80s. At the time, physicists found themselves facing a crisis: the two most important ideas of 20th century physics, relativity and quantum theory, were known to be fundamentally incompatible. Quantum theory describes the universe as intrinsically discontinuous: energy, for example, can come in bits just so small, but no smaller. Relativity treats time and space and gravity as a smooth, unbroken continuum. Each theory has its purposes, and they usually don't overlap. But when dealing with very large masses or time periods that are infinitesimally small, like the core of a black hole or the first moments after the Big Bang, neither quite works.

The answer, argued theorists John Schwartz of Caltech and Michael Green of Cambridge University, was to think of the basic units of matter and energy not as particles but as minuscule, vibrating loops and snippets of stuff resembling string, which turn out to exist not just in our familiar four dimensions of space and time but in 10 or more dimensions. Bizarre as it seemed, this scheme appeared on first blush to explain why particles have the characteristics they do. As a side benefit, it also included a quantum version of gravity and thus of relativity. Just as important, nobody had a better idea. So lots of physicists, including Woit and Smolin, began working on it.

Since then, however, superstrings have proved a lot more complex than anyone expected. The mathematics is excruciatingly tough, and when problems arise, the solutions often introduce yet another layer of complexity. Indeed, one of the theory's proponents calls the latest of many string-theory refinements "a Rube Goldberg contraption." Complexity isn't necessarily the kiss of death in physics, but in this case the new, improved theory posits a nearly infinite number of different possible universes, with no way of showing that ours is more likely than any of the others.

That lack of specificity hasn't slowed down the string folks. Maybe, they've argued, there really are an infinite number of universes--an idea that's currently in vogue among some astronomers as well--and some version of the theory describes each of them. That means any prediction, however outlandish, has a chance of being valid for at least one universe, and no prediction, however sensible, might be valid for all of them.

That sort of reasoning drives critics up the wall. It was bad enough, they say, when string theorists treated nonbelievers as though they were a little slow-witted. Now, it seems, at least some superstring advocates are ready to abandon the essential definition of science itself on the basis that string theory is too important to be hampered by old-fashioned notions of experimental proof.

And it is that absence of proof that is perhaps most damning. Physicists have a tolerance for theory; indeed, unless you were there to witness a phenomenon yourself--the Big Bang, say--it will always be, at some level, hypothetical. But the slow accretion of data and evidence eventually eliminates reasonable doubt. Not so--or at least not yet--with strings.

"It's fine to propose speculative ideas," says Woit, "but if they can't be tested, they're not science." To borrow the withering dismissal coined by the great physicist Wolfgang Pauli, they don't even rise to the level of being wrong. That, says Sean Carroll of the University of Chicago, who has worked on strings, is unfortunate. "I wish string theorists would take the goal of connecting to experiment more seriously," he says. "It's true that nobody has any good idea of how to test string theory, but who's to say someone won't wake up tomorrow morning and think of one? The reason so many people keep working on it is that, whatever its flaws, the theory is still more promising than any other approach we have."