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Depending on whom you ask, it’s either the holy grail of physics¯the prophesied Theory of Everything¯or a Theory of Nothing, a monumental waste of time and money. What it can’t be is half right.

String theory, as the only viable candidate for a unified field theory, promises to reconcile the previously incompatible parents of science: quantum mechanics, observable at the atomic and subatomic levels, and Einstein’s general relativity, applicable everywhere else. That scientists are divided on string theory isn’t nearly as intriguing as the reasons they give. Setting aside those who insist that a grand theory of physics is and always will be a pipedream, its skeptics are most likely to accuse it of being un scientific. New in 2006 and reviewed by Jim Holt in this week’s New Yorker are two books calling string theory a faith-driven mistake from the get-go: The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next by Lee Smolin, and the cleverly titled Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law by Peter Woit.

Because of the unimaginably small scale at which these strings are said to exist, there are, as of yet and for the foreseeable future, no means of observing strings, no experiment to determine if string theory is bogus. Scientific claims, as distinct from religious beliefs, are by definition falsifiable, or so we are told. It is understandable, then, for physicist David Lindley to ask, "What is the use of a theory [string theory] that looks attractive but contains no additional power of prediction, and makes no statement that can be tested? Does physics then become a branch of aesthetics?" A better question might be whether aesthetics is a branch of science.

So how do string theorists defend their ideas, if not with laboratory data? They point to the capacity of the theory to incorporate and synthesize so much of what science already holds to be true. And yet, when talking about it, string theorists have a habit of lapsing into an aesthetic vocabulary (and frame of mind?). String theory becomes "attractive," "elegant," "too beautiful not to be true." Steven Weinberg tries to clear things up:

It may seem wacky that a physicist looking at a theory says, "That’s a beautiful theory," and therefore takes it seriously as a possible theory of nature. What does beauty have to do with it? I like to make an analogy with a horse breeder who looks at a horse and says, "That’s a beautiful horse." While he or she may be expressing a purely aesthetic emotion, I think there’s more to it than that. The horse breeder has seen lots of horses and from experience with horses knows that that’s the kind of horse that wins races. . . .

String theories in particular have gotten much more rigid as time has passed, which is good. You don’t want a theory that accounts for any conceivable set of [mathematical] data; you want a theory that predicts that the data must be just so, because then you will have explained why the world is the way it is. That’s a kind of beauty that you also see in works of art, perhaps in a sonata of Chopin, for example. You have the sense that a note has been struck wrong even if you’ve never heard the piece before. The kind of beauty that we search for in physics really does work as a guide, and it is a large part of what attracts people to string theory. And I’m betting that they’re right. .

Weinberg’s racehorse and Chopin analogies are, in fact, of two kinds. In the case of racehorses, a breeder has looked at previous examples, just as Weinberg says. Physics, quite differently, has only a single example of an observable universe, and there are no universe breeders. For this reason, string theory cannot be scored like a thoroughbred; doing so would amount to judging a hypothetical universe by the standards of a sensible, plausible universe. But what is a sensible, plausible universe, except the very universe (namely, ours) the physicist is trying to describe? To say of string theory, "I’ve seen theories like it before, and they generally turn out to be true" (as would a string theorist thinking like a horse breeder), is akin to reasoning that, on the basis of previous unified field theories, string theory is convincing.

There is, thankfully, a way out of this circle, and Weinberg gestures toward it with his second, aesthetic analogy. His musical metaphor of a Chopin sonata is apt: Even if string theory’s score is still in the process of being written and rewritten, somehow its melody is already known; scientists sense when a "verse" is out of tune, or a "note" is incorrect. The "kind of beauty" Weinberg feels is not a symptom of dry reason (string theory being scientifically detached, isolated) but real, self-attesting beauty¯the only "kind of beauty" there is. Simply put, one knows beautiful music when it falls on one’s ears as surely and objectively as one knows gravity exists. And it is the same beauty that tells us " this note should follow that note" that gives physicists confidence and bearings in something as abstract as string theory.

John Rose is an assistant editor at First Things .

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