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

The strongest criticism in my view is that there are several string theories. Many of its tests aren't unique to string theory. M-theory is a way of unifying the different string theories.In order that the unique predictions it makes to be tested we need some sort of Large Hadron Collider.

This is not really valid anymore. By 1996 we knew we were dealing with a single theory - thus crushing one of the big complaints. M-theory began as this single theory but I think there is now a possible direction to specifically titled "M-theory" research such that it is possible to be a string theorist and not an M-theorist.

wrong recipientEdited by cavediver, : f'ing laptops and wireless connections

duplicateEdited by cavediver, : No reason given.

Sorry, just lost a huge reply to this and too f'ing annoyed and ill to re-write it just yet... arrrggghhh

Ok, let me try again Yes, M-theory is a 'grand unification' of the various string-theories, although it isn't a string theoy itself! Strings are just one aspect of M-theory and potentially lose their fundemental role. This can be quite upsetting, as the geometric view of a 2d universe giving birth to a virtual 'real world' was quite compelling and is what drew me into the subject. Viewing 1d loops of string as an extention of usual 0d particles is not an elegant, nor compelling entry point into string theory IMO, although it is the traditional view and the one most touted in layman guides. I would alter this to say that ST attempts to expalin low-energy physics. Much like GR, ST is a 'super' theory (bad choice of words - not why superstring theory is so called!!) as in it is a theory of theories. GR is not a theory of our Universe, it is a theory of universes - of which one is an approximation to our own. Similarly Faraday/Maxwell electromagnetic theory is not the theory of one particlar e/m configuration, but rather the theory of all e/m configurations. ST attempts to explain the laws we perceive at low energey: GR, e/m, weak, stong, etc.The completely amazing thing about ST is that it does just this - it gives GR as a low energy theory - totally mindblowing. But it also gives e/m, strong and weak like throies as well. Unfortunately there is a slight over abundance of riches, and the precise make-up of fields is determined by how the extra dimensions are wrapped up.That is where multiple possibilities come in. It is not multiple theories, but one theory having numerous possibilities. Just like a single protien having numerous potential ways it could fold - finding the correct folding is immensely difficult. And this isn't true - it makes many predictions but most of these are tested not with particle colliders but with a pencil and paper. We already know what ST has to predict - everything we know! So we check to see if ST does indeed predict what we know. It passes on GR at low energy, and this is an enormous hurdle in itself. I would stick my neck out and say that this is probably false as well. The majority of the practitioners in ST are theoretical/mathematical particle physicists. If not ST they would probably be working on CFT, sigma-models, Liouville theory, Matrix theory, and all that other stuff that such people enjoy. Admittedly, some would also be back in classic GUT work. But QG and ToE - that is really more the realm of the relativity-inspired STists, and they are not such a great number. Even if ST was abandoned as a prime ToE candidate, it would still have many many followers as a wonderful piece of mathematical physics.

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

Sorry Percy, didn't mean to keep you waiting... Yes, that is a good way of putting it. Mostly true although it does make some suggestions that provide observed bounds on the CMB... but that's scraping the barrel a bit The main reason is that the Standard Model is still being used to try to tune String Theory - providing differentiation from the SM is definitely a job for the future. And this is where it gets bizarre... what have the public to do with it? No-one outside the field has any clue as to the subject - I don't care that Smollin and Motl (anti-string and string-nazi ) are trying to take this outside the confines of the departments with blogs and books - it is a discussion/argument for the departments and no-one else. c.f Dr. Guillermo Gonzalez... But the SM doesn't explain the results so much as it is the collective name for the results - it is the individual theories that are succesful: GR, electro-weak, SU(3) colour (QCD)... the SM is simply the statement that, yes, these theories all seem to work in their domains.The trouble is theory has been outpacing experiment for so long - all of the results of the past thirty years have effectively been confirming what we alreay suspected from theory. We haven't been surprised for a long long time - we desperately need new experimental results and very very high energies - but no-one wants to pay. The LHC will be nice, but it's only scratching the energies we need... so all we have is the top-down approach that we employ in string theory (and loop-gravity in a limited sense) True, but that's idiots from within showing off to those without - how many times has this happened? It should not reflect upon the subject itself. No, not at all. The SM is not a ToE, it is just a collection of what we have got. There are huge holes in the SM, and it is more a statement of how little we understand. The answers may be in reach of the LHC but more likely they will be far far above in energy. If the new colliders reveal no new physics, it just confirms what we already know: that when it comes to energy-scales, human existence is at the arse-end of this fascinating Universe.Edited by cavediver, : typos

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

This is where the public comes into it: paying the bills. Agreed. Can tell me why string theory is not regarded as equal to the standard model when they produce the same results from the data? Seems to me (IMHysaO)* you have two strong contenders.Enjoy.* or should I make that IMHNMSI(ysa)O**?** in my humble narrow minded somewhat ignorant (yet sometimes arrogant) opinion?Edited by RAZD, : * **

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But I (heart) physics.Unless I'm mistaken, the LHC should be operational in early 2008, yes?
Leastaways that's what The New Yorker said:
Crash Course | The New Yorker

That's what I'm here for Absolutely, but this is experimental physics don't forget. You can buy quite a few pencils and pads of paper for the price of the SSC, not to mention tenured professorships Us theorists are a bargain...You're right, I think that is the real problem - too much talk of theory and not enough experimental results... but you don't get experiments without big $$$

As I mentioned to Percy, it's all about unification. We observe Gravity x SU(3) x SU(2) x U(1) - this is the standard model, essentially 3-4 separate theories. If String Theory works out, it gives us, say, Gravity x SU(5) with a nice break down to Gravity x SU(3) x SU(2) x U(1), all from one overarching theory. It is the explanation of why we have the standard model. This is what a ToE does. And this is much more on-topic for this thread Hmmm, sounds like someone is feeling better

Glad to hear it The LHC is suffering a few setbacks - some testing planned for November has been put back to early next year - but if you think middle of 2008 you're probably about right. That said, it's not a case of switch on and results are printed out... probably looking at late 08, early 09 for that I guess.Of course, it could be switch on and watch* the entire universe tunnel to a new vacuum state *if you are God, otherwise you are dead