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.
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. (1)Mathematician Peter Woit and Professor Lee Smolin both argue that string theory (or superstring theory, as it is also know) 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 1980s. 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 purpose, and the usually don’t overlap.(2)But when dealing with 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. So lots of physicists began working on string theory.
Since then, however, superstrings have proved a lot more complex than anyone expected. The mathematics is extremely tough, and when problems arise, the solutions often introduce yet another layer of complexity. (3)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 fashion among some astronomers as well-and some version of the theory describes each of them. That means any prediction, however outlandish, has chance of being valid for at least one universe, and no prediction, however sensible, might be valid for all of them.
However, it is that absence of proof that is perhaps most damning. (4)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 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? (5)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.