What are M-Theory and String theory etc. and are they valid scientific theories?

I'm not Cavediver, but I'm going to give this a shot anyway. I'll cast caution to the wind under the assumption that Cavediver will correct my mistakes.I think M-theory derives from or at least has close links to string theory. I don't think all string theorists buy into M-theory, but I think all M-theorists are string theorists.String theory and M-theory are scientific in the sense that they attempt to explain observed natural phenomena, and they are unscientific in the sense that they have too many free variables and so can be infinitely tweaked to describe any sort of universe you desire, including our own. The hope of string theorists is that they'll eventually find an approach where some of the fundamental constants of the universe fall out of the math, and the criticism is that there has been more than enough time to accomplish this, that it's time to devote attention elsewhere.Probably the strongest criticism of string theory is that it doesn't make any testable predictions. This is likely a consequence of the theory's flexibility. It describes so many possible universes that it is not an easy matter to narrow down the potential consequences to specific predictions. String theorists hope to be able to make testable predictions in the near future.Whether or not string theory eventually pans out, criticism is growing that it is receiving too large a proportion of the effort in the search for unified theories of physics.--Percy

Hi Cavediver, thanks for the info. Should I read you as saying that pencil and paper exercises have demonstrated that ST is consistent with observed phenomena? If so, I agree. Probably the strongest criticism of string theory is that it doesn't make any testable predictions that differentiate it from the standard model, though string theorists expect this to change in the next few years as new instruments come on line. I agree that practitioners of ST in general are not losing faith, it's more of a public relations issue. The lack of any new groundbreaking discoveries unexplained by the standard model in the past 20 or 30 years has kind of left string theory saying in effect, "I know I only explain the same things the standard model already explains, but I have the potential to explain much more!"I think string theory's primary problem is an extended period of unfilled promise. If new colliders only further confirm the standard model and reveal no new physics, then Ockham's Razor neatly clips away the necessity for string theory. Or saying it another way, string theory's advantage would then lie only in its explanatory power of things that can't be verified.Does this bring our views closer together?--Percy

I said that if the new colliders provide results that reveal no new physics and only confirm the SM that the need for ST goes away, but you pointed out ST is a unifying theory while SM is a collection of theories. I think your point neatly trumps mine.That theory has outdistanced experiment is a point we both made, but you noted that our lack of technical ability to perform experiments that can differentiate between SM and ST should not be construed as a fault of ST. I'm again convinced.About public opinion on string theory and why it should count, we agree that it shouldn't count in any assessment of a theory's validity. But it does count in funding, and public opinion is in part responsible for the cancellation of the superconducting supercollider project back in 1993. The public wants to see results, and 30 years of no positive confirmations is a long time to wait. There is, of course, a catch-22 here, since it can be pointed out that had that project gone forward we might already have such positive confirmations. But anyway, that's how I meant public opinion plays a role.Thanks very much for the response and information - it helps a lot!--Percy