Well, M-string theory is one of many other string-theory conceptions, RightWhale. So i wonder why the respective theorists are running around describing their work as a "theory" in the first place. Wouldn't "hypothesis" be a more apt, more descriptive word?
Don't get me wrong. I don't "rule out string theory in principle." And it seems very clear to me that the theorists are right to propose additional dimensions beyond the 3+1D block in order to account for empirically observable phenomena. But there are also other string theory hypotheses that do not require the compactification of the newly imagined dimensions. Plus I've seen different proposals that stipulate different numbers of dimensions. Eleven seems to be the currently fashionable concept; but maybe you only need some X more than four of 'em.
To me (a non-expert), it looks like the jury's still out on string theory. Though I definitely wish the researchers well, and hope they will come up with the telling insights we need to advance the physical sciences.
Which of course means that I entirely agree with your statement, "Speculate away, but we will require laboratory data at some point."
Thanks for writing, RW!
String theory could successfully account for gravity and predict super-symmetric particles. But until a couple of years ago it had little connection with puzzles in physics. There were no results or concrete predictions to show off. It could have been nothing more than a beautiful mathematical construction.
Things changed in 1996. Andrew Strominger, then at the Institute for Theoretical Physics in Santa Barbara, and Cumrun Vafa from Harvard University, used string theory to "construct" a certain type of black hole, much the same way one can "construct" a hydrogen atom by jotting down the equations, derived from quantum mechanics, that describe an electron bound to a proton.
Strominger and Vafa confirmed a result derived by Jacob Bekenstein and Stephen Hawking back in the late 1970's. Bekenstein and Hawking found that the amount of disorder (or "entropy") in a special kind of black hole was very large. This was a surprising result, since no one could understand (and nor did the computations give any insight) how an object as simple as a black hole (which can be characterized simply by its mass and its spin) could have such a large amount of disorder within it.
As a result of building this special black hole using string theory, Strominger and Vafa were able to obtain the correct value for the disorder predicted by Bekenstein and Hawking. This result electrified the physics community! For the first time, a result derived with "classical physics" could be obtained from string theory. Even though the black holes for which the result was derived have very little in common with the black holes which are believed to sit in the middle of galaxies, this new computation illustrated the connection between strings and gravity. In addition, the computation provides insight into the physical reasons for the answer.