As I understand it, neither any version of string theory nor M-theory has yet produced an experimentally verifiable prediction. So what are the reasons that so many physicists seem to believe the theory is correct (or at least, on the right track)?
While there is no experimental proof, string theory explains a number of already established facts better than any other existing theory.
Can you be any more specific?
I think the main reason is that with string theory, everything seems to fall into place. It’s been able to unify all the main forces with simple, elegant math. Or at least, so I’ve heard in the books and TV series I’ve seen on it. The reason there’s been no proof supporting it, is because it’s really hard to do experiments on that level. There was one facility under construction with massive supercollider that would finally try and put the string theory to the test, but the funding got cut early on, and as far as I know there’s been no further attempts to build it. Though I’m sure someone will be along shortly to tell me I’m dead wrong and have obviously been out of the loop for some time 
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More specifically, M-Theory and its variants offer the possibility of tying together General Relativity, including gravitation, with quantum mechanics. This alone justifies the intellectual investment in the field. OTOH, M-Theory has so many parameters that it can be fitted to just about any physically-realizable situation, which makes it effectively non-falsifiable, at least as far as our current experimental capablilities extend.
It’s a neat toy, and just maybe it offers a clue to the true nature to the Universe, but right now it is just speculation.
Stranger
You’re talking about the Superconducting Supercolider. But I don’t think it could have tested string theory. Most of the talk about it that I’d heard related to the search for the Higgs Boson. But hopefully that can be done at the Large Hadron Collider when it comes online. And maybe they can confirm supersymmetry, too. I still don’t think they’ll be able to test string theory, though – at least, not so far as I’ve heard.
Correct, it couldn’t have.
One prediction of string theory is a massless spin 2 particle which would have the effect of warping spacetime exactly as proposed by general relativity. So, to stretch the strings a little, one could say that string theory predicts gravity, which has been experimentally verified!
OK, I know, that sounds rather weak and even dishonest: scientific predictions should be of things we haven’t observed yet. But the point of string theory is that it is not just arbitrarily cobbled together so as to produce the things we see in the universe (gravity, photons etc.) but that these things logically follow from the beautifully consistent first principles of string theory.
Its predictions in the ‘proper’ sense might just be experimentally verified in the next few decades: strings of observable proportions might just exist, but the probabilities are remote. A more likely (but less conclusive) bet would be the detection in the LHC (2007) of superpartner particles. This does not support string theory solely, but would be important support nonetheless. (I also understand that black holes or radiation from the ultracompressed universe 15 Bn years away might provide empirical evidence, but I’m not sure how.)
I just read the The Elegant Universe which discusses all this stuff, very neat book. I’m normally not that knowledgeable in this field. That said…
Quantum Mechanics already had a good handle on subatomic particle properties but it could not explain why they have those properties. QM is also very much at odds with General Relativity. You must use one or the other, depending on the situation.
String theory (of which there are 5) and M theory (which joins the 5 string theories and 11-Dimensional Supergravity into one unified framework) provide an explanation into WHY particles have the properties they do. It also does not conflict with GR, so you can use a single theory to describe the universe, regardless of what particular part you’re looking at.
As for experimentation, the biggest problem is that many of the predictions string theory could make have already been observed by other means. For instance, string theory “predicts” gravity, as in it flows naturally from the theory, you don’t have to force it to exist. This is not exactly a breakthrough, so it carries very little weight.
To probe a string directly with current technology, you would need a rather large collider. Estimates range from the size of the galaxy to the size of the known universe. As Strainger said, though, it’s all still speculative, but promising.
Beautiful 
I don’t think very many physicists actually believe that String Theory is correct, or that it’s on the right track. There are many physicists who won’t believe it until they see experimental results, and will ignore it until then. Most physicists can get along perfectly fine without String Theory, or any “Grand Unified Theory”, for that matter. But since the only physicists who ever talk about String Theory are the ones who believe in it (who are mostly the ones who work on it), you get an inflated sense of how many physicists actually do believe in it.
I would say that the biggest reason the String Model (it does not justify being called a Theory) gets so much attention is because it’s the best we’ve got. Loop Quantum Gravity is at best as well-supported as the String Model, and has less explanatory power. And all of the rest of the candidates for quantum gravity have less support yet. So despite its abundant deficiencies (which I don’t think any physicists deny), we work on it because we want to work on some model of quantum gravity, and that’s the best we can do.
I was watching an interview with David J. Gross (who I saw on campus once) and Frank Wilczek here. At one point Gross says something about how promising string theory is for unifying all the forces to which Wilczek’s responds by saying he doesn’t even know what string theory is :eek:. Gross then said that “we” (by this he means physicists) know what string theory is but he did concede that String Theory is missing something fundamental. Wilczek seemed to remain skeptical about String Theory.
So String Theory seems to be lacking something very important but still a very promising idea as of current.
Although I read that somewhere that thanks to string and m theory physicist now have theory of the big splat which preceded and caused the big bang.