Is String Theory Still a Viable Path to a Theory of Everything?

[Fra]

Why is starting with a string any more arbitrary than starting with a particle?

I have objections to a particle in a continuous space as well. So some of the critique, certainly applies to QFT as well. I guess I think ST is not radical enough. I appreciate the "framework" idea, I have not problem wit that, I just don't think it's the right CHOICE of framwork.

But the obvious argument is that if we for a second ignore the embedding, a "point" represents in my view a single distinguishability index, a string represents a continuum of them - it's simple more complex.

I want to point out that I have been attempting to do exactly this in this thread. As you see: tom.stoer asked for a construction of the standard model coming from string theory, and I showed him one, to which he abruptly changed his criteria. Can I ask: what level of proof do you need? If you can't see for yourself that string theory gives you physically interesting quantum field theories which have a fully consistent embedding into quantum gravity, what other evidence do you want?

I'm not asking for a formal proof of course, that would be unreasonable. I just find the non-formal arguments given very weak. But what's weak or not is certainly a biased opinion. The only reasons I insist putting forward these objections is that I am convinced (right or wrong) that there is a MORE general AND more selective framwork, where it's even possible that the "string" can be seen as a special case of a primordal observer. I know most string theorist just doesn't think like this, but still.

My projection of the landscape problem onto my view, is that choosing the background is simply related to the problem of "choosing observer". And the selection problem is related to predicting the population of observers in the actual universe.

But in this view, I just can't make sense of thining of the continuum string as the simpelst possible observer, as it's a continuum of distinguishable events. A continuum of distinguishable events isn't inferrable in my view. I think the continuum is also responsible for large part of the "redundancy" that has made he landscape so large.

My alternative is certainly not more clear at this point, but then again ST is a big program that has been around for years. I think if this is to be seen as a framework for generating physical theories then and inference perspective is reasonable.

/Fredrik

 

[tom.stoer]

"It is by no means clear that it is uniquely specified. "

So long as you insist that the anomalies cancel, you have specified the theory exactly and uniquely, all the couplings are then fixed. You can parse some legalese language with regards to M theory and dualities (are they or are they not separate perse), but even if you take the extreme view still held by a few people, you end up with a maximum of 5 theories.

Let me explain why I think that ST is not uniquely defined.

First of all there are the well-known 5 theories; their dualities are highly impressive, but afaik not always strictly prooven. Instead they partially rely on strong-weak coupling and/or large-N approximations. Therefore one does not know if these theories are dual, or if only their "asymptotically domains" are dual.

Second there is the perturbative "definition" of one theory. Perturbative treatment is not defined beyond 2 loops (afaik there are attempts to construct the superspace measure at 3 and 4 loops, but no results for higher genus are known). Then we know that perturbative treatment is not sufficient as a) it missing non-perturbative effects and b) the perturbation series itself may be ill-defined (as long as there is no measure for higher genus it does not even exist); but even if it exists one has to prove that the series is i) finite order by order and ii) convergent. Both proofs do not exist.

Third we have to look at non-perturbative approaches. Most results we know today are purely classical. Most D-brane caclulations are classical identifications of vacua w/o a full quantized theory on top of it. Full non-peturbative approaches are still missing.

Forth most calculations are low-energy effective theories (e.g. SUGRA, SUSY / gauge theories) derived from the zero-mass string modes. The higher modes are "integrated out" or just neglected. So many results from ST are not strictly ST but due to effective theories.

Fifth afaik there are still obstacles to define the theory fully background independent. That means it is not clear how the theory can "connect" different backgrounds dynamically. And there are backgrounds for which we do not know how to quantize the theory on top of it (the last thing I remember was that one has to restrict to "stationary" spacetimes with timelike Killing vector)

Last but not least afaik a strict definition of M-theory is still missing, even if there are some promising approaches like 3-algebras and matrix theory. But there is no established procedure how to solve the theory, not even in principle. It is not that we know what to do in principle but are not able to do the calculations in practice (as they are too complex, time-consuming etc.).

All this seems to be unfair as these requirements do not apply to quantum field / gauge theories. This is correct as one can always refer to a (unique) UV-completion beyond quantum field / gauge theory. But in string theory this is no longer possible; there is no theory beyond ST which could solve all these ST issues.

 

[BenTheMan]

It seems that most of your objections can be answered with "Sure, it's a hard problem, people are working on it". So, you can throw up your hands and give up, or you can actually try to calculate things.

Specifically, though, you even criticize the instances when people CAN calculate things:

Forth most calculations are low-energy effective theories (e.g. SUGRA, SUSY / gauge theories) derived from the zero-mass string modes. The higher modes are "integrated out" or just neglected. So many results from ST are not strictly ST but due to effective theories.

I can't claim any expertise with the other issues that you've brought up, but I DO know something about this.

Let me ask: why do you list this as a problem? The (Wilson) effective field theory can be defined, and the KK modes can be summed over, and integrated out (which is a strictly defined process, which you seem to neglect?---see Fermi Theory, for example). One can then calculate threshold corrections to the gauge couplings (for example). This was done in 1987 by Kaplunovsky, and since has been applied to most of the approaches where people actually get a complete EFT. In fact, constructing the EFT and calculating threshold corrections is a textbook exercize in, for example, the hetorotic orbifold compactifications.

 

[tom.stoer]

It seems that most of your objections can be answered with "Sure, it's a hard problem, people are working on it". So, you can throw up your hands and give up, or you can actually try to calculate things.

I only want to make clear that ST is not yet "defined" - whatever it means - but that this "definition" is still work in progress.

Let me ask: why do you list this as a problem? ... This was done in 1987 by Kaplunovsky, and since has been applied to most of the approaches where people actually get a complete EFT.

I listed this as a problem since I was not aware of the fact that a full Wilsonian approach has been carried through in ST and that this is standard for deriving SUGRA, gauge theories, etc. This is due to my ignorance, I apologize for it and delete it from my list.