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

I guess I asked for this, but you could have timed this better... I'm off on holiday in 2 days Oh well, if it's a whole thread, we may as well take our time and do it right Some background:We like things simple, and it appears that the Universe does too. How anyone at the end of the 19C thought we could be coming to an end of physics, I just don't know. What kind of decent universe is going to be made up of around fifty to one hundred basic buliding blocks (atoms) plus some mysterious ultra-light electron thing?Splitting the atom into protons, neutons and electrons was the simplification we were looking for - rather than over one hundred different atoms, we have just three particles. Unfortunately, two of these particles (neutron and proton) look so similar that they are obviously slight variations of the same thing - a hint of some deeper structure. Now we understand that the neutron and proton are actually each a package of three quarks - which is ok, becasue there are only two flavours of quark: up and down. So we still just have three particles: up, down and electron. Nice BUT - we now discover that there are actually six flavours of quark in total, two extra pairs which are just like heavier copies of the up and down. Even worse, there are also two heavier versions of the electron: the muon and tauon - and associated with each electron type there is a neutrino! So we are now up to twelve different particles, many of which are extremely similar and so point to a deeper structure.It's worse than this as we also have all of the force particles: the photon, W+/-, Z0, and eight types of gluon! Finally, we have gravitation (and the graviton) to somehow squeeze into all of this.This is the current state of known physics - a complete mess!

Don't worry, I wasn't planning to I shall be playing with sharks, mantas and turtles for the week But don't worry, it seems like Rob is an expert - probably knows more than me - and he'll keep you informed until I can get back

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.

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

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

Just about... it has come to mean different things to different physicists: a particle physicist thinks the SM is SU(3) x SU(2) x SU(1). Anyone with any relativity background will add in gravity. Then there is the cosmological SM, now with added lambda and CDM SO yes, I guess it does evolve. And as it's not actually a theory, per se, you shouldn't be too worried about it not being falsifiable