Question over dimensions, something that's been bothering me

[KartikAddanki]

Maybe someone could inform me a little on what I'm talking about, because, honestly, I have no idea. If 11 dimensions=1 string theory -> 10 dimensions=5 string theories, is it possible to derive(right word? maybe) more string theories using lesser dimensions? Thanks. Even a yes or no answer would help, with a link to the appropriate wikipedia article. Or if you had time I'd be more than happy to read over whatever you have to say.

 

[SimonA]

A while back there were five string theories in ten dimensions that were all stable in that they didn't predict infinities, then Witten among others noticed that you could have one 11 dimensional theory in which the other five were like different ways of looking at the same thing (=M thory). Now however M Theory is not one theory but countless theories - leading to otherwise smart people believing that there are countless universes despite the fact we don't have a single bit of evidence for anything other than the one universe we observe.

Of course there are many other theories using less dimensions, in fact those are what string theories came from, but there are all kinds of issues with them and if you're interested you really need to get yourself one of the good books on the subject.

 

[Coin]

If you're looking for stuff on wikipedia I suggest

http://en.wikipedia.org/wiki/String_Theory#Number_of_dimensions
http://en.wikipedia.org/wiki/Non-critical_string_theory

Basically the idea as I understand it is this:

You can formulate string theory in any number of dimensions you like.

However, the background space where the strings live must be "critical", or else the theory is unstable. With supersymmetric strings, the only way anyone knows to do this is if the space is 10-dimensional.

The M-Theory people have a trick that let's them bump that up to 11 dimensions while keeping the strings critical. But nobody knows how to turn it down to lower than 10 dimensions. If anyone knew of such a trick then surely they would use it, because the more dimensions you have past 4 the more dimensions you have to explain away.

You can again just go ahead and create "non-critical" string models with fewer dimensions, and sometimes these models have mathematical uses. But because of the instability you cannot use these models for a force-unification "theory of everything", which is what people would really like to use string theory for, so people are not as interested in the non-critical string theories.