String Theory in the Low Energy Realm

maverick280857

While going through the first chapter of "A First Course in String Theory", I read that String Theory being a unified theory should include the Standard Model and in the low energy realm, String Theory should boil down to the Standard Model.

To quote the book,

String Theory is said to be a promising candidate for a unified theory. Then why is it not so straightforward to get the low-energy theories (semiclassical or non-relativistic quantum) out of it? In particular the standard model?

Can string theory be shown to boil down to classical electrodynamics or quantum mechanics?

mathman

The sentence you quoted is your answer. It is not easy to derive the standard model as the low energy limit of string theory. As far as I can gather it hasn't been done yet.

maverick280857

Yes but why not? How did they incorporate the Standard Model in the first place? Wouldn't that give a hint on how to "recover it"?

EDIT: Evidently I am not even a beginner for this book...reading it for the heck of it.

Parlyne

As I understand it, the biggest problem is that the standard model is formally a quantum field theory, while every current formulation of string theory is not. To actually have the full formalism of the standard model fall out of string theory, there would need to be a string field theory to start from.

maverick280857

I believe the author of the book is one of the authorities on string field theory. So if he says its not been done it must be a shortcoming of the construction of the theory (right now or at least at the time of writing of the book).

selfAdjoint

I don't think Parlyne's suggestion is correct. The gauge symmetry of the standard model is a low energy phenomenon which string theory solutions ought to reproduce if they could be found. There are theorems that say the standard model group is of frequent occurrence in very general circumstances.

Nor is the lack of low energy predictions necessarily due to a faulty construction of the theory; that overlooks one of the leading properties of string physics; its immense, subtle technical complexity. Later on in Zee's book he has a chapter describing just one of the attempts to reproduce standard model physics using strings and branes. You can look into that chapter and see the hoops they have to jump though. It's very unnatural, and clearly something better is needed. But there is no reason (except maybe the landscape problem) to suppose that something better is impossible to find. In addition to being complex and difficult, string theory is undeniably powerful.

BTW there is a string field theory, due to Witten and others, but it is even harder than first quantized string theory and only the most basic things have been worked out in spite of decades of work by brilliant people. They're still working on the vacuum!

maverick280857

I think that for a self-consistent toe, it is necessary that it should boil down to theories and equations that have been experimentally verified in different realms.

My question really is: string theory is said to include mechanics, electrodynamics, statmech, gravity, etc. so why is it 'difficult' to recover these? Isn't this apparent downside a disturbing feature of a theory that is in contention to encompass everything else that is well established?

I think, perhaps naively, that even if string theory correctly boils down to lower energy theories which are verifiable by experiment or have already been verified, it is as good as being a real TOE and then the "correctness" of string theory will not hinge on the success of experiments to see 'strings' and verify supersymmetry.