SCOTT SIMON, host:

Everything is made up of tiny strings that are a millionth of a billionth of a billionth of a billionth of a centimeter. And if you believe that, there are some people who have a bridge in Brooklyn they'd like to sell you.

Two new books argue that string theory, an idea that's united everything in the universe - also called the theory of everything - is just a big ball of yarn. The books are called The Trouble with Physics: The Rise of String Theory, the Fall of Science and What Comes Next by Lee Smolin; and Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law by Peter Woit.

Both books are provoking a bit of a row in the physics world. And to explain what that row is all about is our math guy, Keith Devlin, who joins us from Stanford University.

Thanks very much for being us.

Professor KEITH DEVLIN (Stanford University): Hi, Scott, nice to be here.

SIMON: Now, Keith, first of all, of course what we need is a short, clear, concise, yet all-embracing and comprehensive definition of string theory. Let me get my watch here. I'm going to give you 20 seconds, okay? Three, two, one.

(Soundbite of laughter)

Prof. DEVLIN: Actually, Scott, you just gave it. String theorists believe that everything in the universe is ultimately made up of tiny little vibrating strings. The idea began in the 1960s, really took off in the '80s, and it was a reaction to some problems that had arisen with the particle view of matter. It wasn't particularly elegant. There were some mathematical difficulties. And it was that unease that really led to string theory being put forward as an alternative.

SIMON: Oh, I think we still have a little time left. That's why I was (unintelligible). Why is it so hard to prove or disprove string theory?

Prof. DEVLIN: We're at a scale here that's so small that no existing equipment, and no equipment we can ever imagine with our foreseeable future, could see that degree of granularity. We just couldn't see things that small. We've got no way of telling whether matter is ultimately made of particles or strings.

SIMON: I guess I can understand why you can't test it well enough to prove it, because the resources involved would just be so great. But why can't you come up with a test that would disprove it?

Prof. DEVLIN: Well, we don't know you can't do that, but certainly, even after 30 years of trying to think of one, no physicist has yet come up with a test that could conceivably tell the difference. I mean, you have to find - since the strings are too small to conceivably be identified in any way, you have to find some effects that would show the difference. You'd need some experiment which would show whether or not matter was made of strings or particles. And we have no such experiment. No one has been able to come up with anything.

String theory certainly makes predictions, but it doesn't make any predictions which are different from the predictions that come from classical physics. And so we don't have any possible experiments on the horizon that could tell one theory from the other theory. And since the particle view of physics has been tested by experiment and is accepted, the critics of string theory say string theory still remains outside the realms of science because it has yet come up with a means of justifying itself.

SIMON: I've been fortunate enough to interview a couple of string theorists, if I might call them that, over the years. They say that just the intellectual exploration to find out whether it's true or not, or what it is, or the intellectual adventure, in a sense, is worth it.

Prof. DEVLIN: Yeah. This is great stuff. This is a great intellectual activity. It's the kind of things universities should be doing. I'm a mathematician, for heaven's sake. I do a lot of pure mathematics, which have never even claimed is necessarily related to things in the real world.

The issue here, however, is whether what they're doing classifies as science. Is it great mathematics? You bet your life it's great mathematics. Does it justify as being mentioned in the history books of mathematics? You bet your life it does. Will string theory eventually make it into the history books of science rather the history books of mathematics? That's a more interesting question. It's not clear it will survive as a science. What is clear is it will survive as a great piece of mathematical exploration.

SIMON: Keith, thanks very much.

Prof. DEVLIN: Okay, my pleasure, Scott.

SIMON: Keith Devlin, executive director of the Center for the Study of Language and Information at Stanford University. His most recent book is called The Math Instinct: Why You're a Mathematical Genius Along with Lobsters, Birds, Cats and Dogs.

Dogs believe in string theory? Or is that a generalization?

(Soundbite of laughter)

Prof. DEVLIN: Well, we know that cats believe in string theory.

SIMON: Oh yes. They like to play with them.

Prof. DEVLIN: They play with balls of string all the time, indeed.

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