Or, Why I Don't Believe in String Theory.
String theory has a large body of ardent fans, both physicists and laypeople. I am not one of them.
"But string theory is so beautiful!"
a) Beauty does not mean truth. I can think of half-a-dozen symmetries at least as beautiful as Gell-Mann's Eightfold Way. So why is he a world-famous physicist and I'm an insignificant blogger? Because his version is not only beautiful, it works. Yes, most good theories are beautiful, but not every beautiful theory is a good theory.
b) Beauty is in the eye of the beholder. I actually don't think string theory is that beautiful! People complain about the ugliness of the Standard Model's eighteen adjustable constants; well, I find string theory's six (or seven, in M-theory) extra dimensions just as ugly and wasteful.
"It's going to be a unique theory of nature."
"Going to" is the operative word here. Perhaps someday, if we can ever get M-theory written down (see next point), it will be a unique theory of nature. But then it won't be string theory, it will be some other theory. There are several thousand, possibly several million, different string theories, and our universe could have been any one of those. (Of course, I believe that God could definitely control the string theory our universe is described by; however, I don't think he would have designed our universe so messily.)
"But M-theory is a unique theory of nature!"
And so it may be, but it doesn't do anyone any good. We can't even write down all of M-theory's solutions (string theories), and we can barely prove that M-theory exists! Even regular string theory is pretty useless: the theorists can't calculate with it and the experimentalists can't test it, so what are we to do with it?
"Well, anyway, it's the only game in town for quantum gravity."
No way. Loop quantum gravity! Noncommutative geometry! Doubly special relativity (in which lightspeed depends on the photon's energy)! Maybe none of these are any good. In that case, let's invent some more!
So my words to the string theorists: By all means, keep working on string theory; it's a fascinating structure with interesting symmetries. Just don't call it physics. Call it pure math.
P.S. If you enjoyed this post, check out Lee Smolin's books The Trouble with Physics and Three Roads to Quantum Gravity. He's a skeptic, but a very refreshing kind: he's as skeptical of established science as he is of religion.