Why I am REALLY disappointed about string theory

[suprised]

Now that we have consensus (OK, not really, Suprised will not agree) the interesting question is how to identify the underlying theory from which all these effective string models do emerge.

Well I am playing advocatus diaboli here, in raising awareness that certain views that are taken for granted by most, may potentially be wrong or at least based on misleading preconceptions.

I had expressed my personal views on "the underlying theory" already here:

https://www.physicsforums.com/showpost.php?p=2867470&postcount=251

https://www.physicsforums.com/showpost.php?p=2386391&postcount=9

 

[tom.stoer]

Well I am playing advocatus diaboli here, in raising awareness that certain views that are taken for granted by most, may potentially be wrong or at least based on misleading preconceptions.

I fully understand.

My question is this: dropping uniqueness as guiding principle, do you have a something new?

 

[Calrid]

I fully understand.

My question is this: dropping uniqueness as guiding principle, do you have a something new?

Wouldn't you have to establish uniqueness as being more coherent as an axiom than not being unique first at least inductively. Philosophically it would be difficult to do I think. I don't understand the meat and potatoes of the math, but that at least is logically sound. Something new or something better in terms of an overarching meta theory. I think differences that converge are ok, but differences that don't are forbidden. So with appropriate terms you could say that the same way of describing the same thing is better than something different, but I'm not sure how you would justify that beyond an axiom without something else to weight the consequences.

Sometimes agreeing is probably not the best way to explore things perhaps is putting it too simply.

Perhaps I am misreading your point?

 

[tom.stoer]

I don't really understand what you are saying.

My idea is rather simple: up to know physics collected experimental phenomena and theoretical descriptions and tried to "unify" them via a few axioms, principles, formulas etc. QED is based on a single Lagrangian and a common understanding how to quantize it and how to extract physics. So in some way we agree on a "unique fundamental formulation" of QED. It's not one single formula, but a few formulas plus a few principes how to use them. The same applies to QCD, etc.

All what I am saying is that this was always one guiding principle in physics. If one drops this guiding principle (there is no unique formulation, there are no fundamental degrees of freedom, ... could be 5-branes, could be E8 heterotic strings, ...) there should be some replacement, a new guideline for a research program.

My question to suprised is whether he has something to offer.

 

[suprised]

My question is this: dropping uniqueness as guiding principle, do you have a something new?

Well that was a loaded remark... let's not open another Pandora's box. Just a brief comment. Think about a conformal field theory. One and the same CFT can arise in the IR limit of many different microscopic theories; a CFT can be viewed in this sense as a universality class of theories, with the defining property that they lead to the same IR physics.

In perturbative string theory, CFTs appear as part of the world-sheet theory, and each choice corresponds to a classical background, and defines some on-shell physics (because the equations of motion are equivalent to requiring conformal invariance). One may speculate that going away from conformality is like going off-shell, and a priori it is unclear whether doing this is unique or not. In fact, it is known that a given on-shell theory may have different off-shell completions. So it may be that there is a bunch of "different" underlying theories that lead all lead to the same on-shell physics.

Essentially, this boils down to semantics and what one means by "unique" underlying theory. Eg., is lattice QCD a "different" theory as compared to the usual perturbative lagrangian formulation of QCD? No, because when performing the proper limits it lies in the same universality class. A similar phenomenon could happen eg for LQG and strings, etc.

 

[atyy]

Well that was a loaded remark... let's not open another Pandora's box. Just a brief comment. Think about a conformal field theory. One and the same CFT can arise in the IR limit of many different microscopic theories; a CFT can be viewed in this sense as a universality class of theories, with the defining property that they lead to the same IR physics.

In perturbative string theory, CFTs appear as part of the world-sheet theory, and each choice corresponds to a classical background, and defines some on-shell physics (because the equations of motion are equivalent to requiring conformal invariance). One may speculate that going away from conformality is like going off-shell, and a priori it is unclear whether doing this is unique or not. In fact, it is known that a given on-shell theory may have different off-shell completions. So it may be that there is a bunch of "different" underlying theories that lead all lead to the same on-shell physics.

That would be really nice. Is there any construction that does give the off shell theory from a QFT, or something in that direction? (I assume this is in a completely different direction from AdS/CFT?)

 

[marcus]

... "different" underlying theories that lead all lead to the same on-shell physics.

Essentially, this boils down to semantics and what one means by "unique" underlying theory. Eg., is lattice QCD a "different" theory as compared to the usual perturbative lagrangian formulation of QCD? No, because when performing the proper limits it lies in the same universality class. A similar phenomenon could happen eg for LQG and strings, etc.

The new formulation of LQG has analogies with both lattice QCD and diagram QED. The way the combinatorial structures (spin networks, spinfoams) are defined makes them analogous to both generalized lattice and Feynman diagram. 1102.3660 talks about this if I remember right.

The main difference from lattice QCD, I think, is that the lattice is irregular and has no "metric" content, no fixed edgelengths etc. The main difference from Feynman diagrams is that you have a 2-cell complex instead of a 1-cell (graph). The vertices are still places where an interaction occurs.

"...phenomenon could happen eg for LQG and strings,.." I take to suggest that some type of LQG spinfoam model could turn out to be the combinatorial cousin of some type of String model. I'm not sure what I mean by that. Perhaps there is no definite meaning at this point. Just a vague notion.

Do you watch NCG? Are you familiar with the "almost commutative space" C(M) x F where F is the finite noncommutative piece? Any ideas about this mysterious F entity?

 

[tom.stoer]

@suprised: very good point, the "off-shell d.o.f." do not matter as long as the on-shell results are correct. [but in QCD there seem to be a very good reasons to identify quarks and gluons as the fundamental d.o.f. - even if they cannot be identified as real on-shell particles]

 

[suprised]

there any construction that does give the off shell theory from a QFT, or something in that direction?...

Roughly, in QFT with perturbative lagrangian formulation this is built in. The issue arises for string theory, which is a priori formulated as an on-shell, S-matrix theory.

Constructing an off-shell theory is what string field theory aims for. One hopes that this would shed light on issues like how to "compare" different vacua, how to describe transitions between different vacua, vacuum selection, etc.

 

[suprised]

Do you watch NCG? Are you familiar with the "almost commutative space" C(M) x F where F is the finite noncommutative piece? Any ideas about this mysterious F entity?

Not really...it's still another kind of generalized geometry and to me it is unclear why NCG would be able to answer any of the open questions that we already have in other approaches.

 

[Fra]

Constructing an off-shell theory is what string field theory aims for. One hopes that this would shed light on issues like how to "compare" different vacua, how to describe transitions between different vacua, vacuum selection, etc.

This is interesting.

I know this is an open question, but if I ask for your expectation here: Do you think that the "description" which allows comparasion/(define a measure?) between diffferent vacua

1. requires an external context, that in some mathematical/realist realm allows this? If so, what happens with time, does these transition take place in some time?

2. or do you think the solution context will attach holographically to anothre vacua? Ie. that two vacuas can be compared, only with respect to a third vacua? So that rather than some external description of the landscape, you have only intrisic descirptions where the effective landscape can instead be reduced, and thus allow selection?

If 2 - is there any chance that in order to do this, the landscape will be yet bigger?

/Fredrik

 

[Calrid]

I don't really understand what you are saying.

My idea is rather simple: up to know physics collected experimental phenomena and theoretical descriptions and tried to "unify" them via a few axioms, principles, formulas etc. QED is based on a single Lagrangian and a common understanding how to quantize it and how to extract physics. So in some way we agree on a "unique fundamental formulation" of QED. It's not one single formula, but a few formulas plus a few principes how to use them. The same applies to QCD, etc.

All what I am saying is that this was always one guiding principle in physics. If one drops this guiding principle (there is no unique formulation, there are no fundamental degrees of freedom, ... could be 5-branes, could be E8 heterotic strings, ...) there should be some replacement, a new guideline for a research program.

My question to suprised is whether he has something to offer.

Yes and I was pointing out the dangers of assuming axioms without proofs. But ok.

Is it better to search for common ground or better ground is a loaded question true its also something that cannot be answered without some means to distinguish something other than maths. I would therefore say that its a moot point and that any formulation that works whether it agrees with any other or not is equal, and you don't need to agree on anything, after all hypothesis is about imagination. It's only the scientific method that distinguishes ideas from reality. Hard to say is it better to have infinite ideas all things being equal that are all different or infinite ideas that are all the same? I'd say that was a contentious question. If I had to make a judgement call I'd say meh all ideas are good congruent or not.

 

[suprised]

Fra, I guess it's better to close the Pandora's box again ;-) I can't give reasonable answers to your questions. These matters are very speculative and potentially just create more confusion than necessary.

 

[atyy]

Constructing an off-shell theory is what string field theory aims for. One hopes that this would shed light on issues like how to "compare" different vacua, how to describe transitions between different vacua, vacuum selection, etc.

Are these the same or different as trying to find non-perturbative formulations of string theory (ie. the S-matrix on a fixed background only gives perturbative string theory)?

 

[tom.stoer]

Could it be that the missing off-shell formulation was another wrong turn? (I do not know much about string field theory but it seems that it's not an active research topic).

 

[atyy]

Could it be that the missing off-shell formulation was another wrong turn? (I do not know much about string field theory but it seems that it's not an active research topic).

Taylor's http://arxiv.org/abs/hep-th/0605202 seems to indicate not so much a lack of interest, in fact, it was even hoped that this would provide manifest background independence - but more that they got really stuck in mathematical difficulties.

 

[negru]

Note that the work on ads4/cft3 is in large part motivated by applications of the high spin side to closed string field theory. The usual string theory with ever increasing masses could be coming from something else...where some symmetry is unbroken and all states are massless.

 

[suprised]

Could it be that the missing off-shell formulation was another wrong turn? (I do not know much about string field theory but it seems that it's not an active research topic).

This is a good question - many people believe that string field theory should be developed in order to ever get a handle at background indepence, vacuum selection, etc. but the program didn't really get far, so that's why not many work on this today. Perhaps it's a red herring, perhaps it is an essential missing point - who knows ;)

 

[Fra]

Taylor's http://arxiv.org/abs/hep-th/0605202 seems to indicate not so much a lack of interest, in fact, it was even hoped that this would provide manifest background independence

The paper Atyy linked to touches upong the question I asked surprised about.

2. or do you think the solution context will attach holographically to anothre vacua? Ie. that two vacuas can be compared, only with respect to a third vacua?

"Any quantum theory of gravity which attempts to deal with the landscape
of string vacua by constructing different vacua as solutions of a
single theory in terms of a single set of degrees of freedom will face
this field-redefinition problem in the worst possible way. Generally, the
degrees of freedom of one vacuum (or metastable vacuum) will be defined
in terms of the degrees of freedom natural to another vacuum
(or metastable vacuum) through an extremely complicated, generically
quantum, field redefinition of this type. This presents a huge obstacle
to achieving a full understanding of quantum cosmology. This obstacle
is very concrete in the case of string field theory, where it will make it
difficult to describe the landscape of string vacua in the language of a
common theory. It is also, however a major obstacle for any other attempt
to construct a background-independent formulation of quantum
gravity (such as loop quantum gravity or other approaches reviewed in
this book). Only the future will tell what the best means of grappling
with this problem may be, or if in fact this is the right problem to pose.
Perhaps there is some radical insight not yet articulated which will make
it clear that we are asking the wrong questions, or posing these questions
in the wrong way."
-- http://arxiv.org/abs/hep-th/0605202

I think a clue here is to see that the core of this problem is not specific to a particular program. It just tends to "show up" in different ways. And maybe it alone suggest that we need a new way of thinking of what a theory is. Should we think of theories as descriptions of reality, that are either wrong or corroborated, or should we think of theories as interaction tools?

ie

but more that they got really stuck in mathematical difficulties.

isn't the situation here suggesting something much worse than just mathematical difficulties? If it is that we simply are conceptually confused about certain things here, it's not a mathematical problem. One problem seems to shave out and separate conceptuallt unclear questions from technical problems of conceptually clear questions.

/Fredrik

 

[atyy]

The paper Atyy linked to touches upong the question I asked surprised about.

"Any quantum theory of gravity which attempts to deal with the landscape
of string vacua by constructing different vacua as solutions of a
single theory in terms of a single set of degrees of freedom will face
this field-redefinition problem in the worst possible way. Generally, the
degrees of freedom of one vacuum (or metastable vacuum) will be defined
in terms of the degrees of freedom natural to another vacuum
(or metastable vacuum) through an extremely complicated, generically
quantum, field redefinition of this type. This presents a huge obstacle
to achieving a full understanding of quantum cosmology. This obstacle
is very concrete in the case of string field theory, where it will make it
difficult to describe the landscape of string vacua in the language of a
common theory. It is also, however a major obstacle for any other attempt
to construct a background-independent formulation of quantum
gravity (such as loop quantum gravity or other approaches reviewed in
this book). Only the future will tell what the best means of grappling
with this problem may be, or if in fact this is the right problem to pose.
Perhaps there is some radical insight not yet articulated which will make
it clear that we are asking the wrong questions, or posing these questions
in the wrong way."
-- http://arxiv.org/abs/hep-th/0605202

I think a clue here is to see that the core of this problem is not specific to a particular program. It just tends to "show up" in different ways. And maybe it alone suggest that we need a new way of thinking of what a theory is. Should we think of theories as descriptions of reality, that are either wrong or corroborated, or should we think of theories as interaction tools?

ie

isn't the situation here suggesting something much worse than just mathematical difficulties? If it is that we simply are conceptually confused about certain things here, it's not a mathematical problem. One problem seems to shave out and separate conceptuallt unclear questions from technical problems of conceptually clear questions.

/Fredrik

Yes, that's what Taylor himself says. OTOH, aren't the important mathematical difficulties always conceptual ones (unless you are Euler or Ramanujan)?

 

[Fra]

OTOH, aren't the important mathematical difficulties always conceptual ones (unless you are Euler or Ramanujan)?

I'm not Euler but I'm not sure I would agree. Certainly the development of new mathematics, is a creative process, so in that sense yes they are related.

But sometimes I have a distinct feeling that people does not SEE the conceptual issues (as they are often dismissed as philosophy) and instead get occupied with technical excercises. But I don't think it's due to ignorance, it seems to be more pragmatic reasons. Some questions are simply too diffucult, and therefore it's easier to try to find an easier question.

This is what I mean with making the difference.

Rovelli ones said in a talk or some book that, even if diversions are enligtening, one should never loose focus of the real problem. (context was: don't focus on 2+1 quantum gravity just because it's easier than 3+1)

/Fredrik

 

[tom.stoer]

There is an important distinction between technical and conceptual issues.

Let's make some examples:

We can ask "why is space four-dimensional". Then it's of course conceptual question whether this should be an input to our theory or whether it can somehow be derived. One has to find a framework whioch allows one to ask these questions, based on which we are able to formulate a theory which is dimension-agnostic at all, or which allows for a different number of dimensions. The question why 6 out of 10 dimensions shall be compactified in certain string models is not conceptual. It's over, the interesting things already happened when chosing a certain background. It's no longer possible to ask the interesting questions. One can go through all CY compactifications and study their properties but one will never find out the answer to the original question.

In that sense string theory seems to ask too many technical questions instead of conceptual ones. Finding new vacua in the landscape may be interesting, but it does not help to understand deeper problems. Looking at the shape of the Earth and some atlases does not explain why the Earth is nearly spherical.

In that sense most string reserach programs do not address the fundamental issues. CY compactification, D-branes, nearly MSSM models, ... are so to speak phenomenological models. How close can one get to the real world? Arbitrary clsose I would say. What would happen if at some day the the standard model is reproduced by such a string vacuum exactly? What would we learn? Nearly nothing.

My feeling is that there is some kind of huge duality, but string theory is exactly the wrong point to start with. Whenever one wants to make any calculation one immediately leaves string theory, derives some low-energy limit and studies ordinary SUSY or SUGRA gauge theory. That's not what I am expecting from a fundamenal theory. In QCD there are w/o doubt problems that cannot be derived from QCD by first principles. I think low-energy pion-nucleon scattering is still best understood in chiral-effective theories, not in QCD. But there are many topics which can be addressed in QCD (DIS and scaling violations, lattice calculations, QGP). In string theory (this is my perception) one immediately leaves string theory and steps to some effective theory which allows in a certain limit to ask (and answer) rather specific questions. But if string theory is the fundamental theory, then we should ask (and answer) these questions in string theory, otherwise something is fundamentally wrong.

That's the reason why I guess that string theory is exactly the wrong place to start. My expectation is that string theory is correct in some sense, but that it has to be replaced by a fundamental theory from which strings, branes, backgrounds and compactifications, perhaps even dimensions can emerge.

 

[mitchell porter]

In that sense most string reserach programs do not address the fundamental issues. CY compactification, D-branes, nearly MSSM models, ... are so to speak phenomenological models. How close can one get to the real world? Arbitrary clsose I would say. What would happen if at some day the the standard model is reproduced by such a string vacuum exactly? What would we learn? Nearly nothing.

Just to be specific, let me refer to http://arxiv.org/abs/1103.4800" , which is presented as a candidate for the real world. What would it mean if that became our standard model? It would have some "particle physics" implications - in this case, the existence of a fourth generation - and it is normal for such models to imply something extra (e.g. supersymmetry). Also, quantities like the particle masses would now be understood as expectation values of geometric moduli; in this case, the resting point of a stack of branes in a six-dimensional hypertorus. We would have passed from thinking abstractly about the possibility of braneworlds, to dealing with the fact that a particular braneworld model exactly matched the data.

That's all quite a lot. It's not the same as an explanation for why that configuration of braneworlds, or what the fundamental physical picture is, but it would be an extremely revolutionary development.

 

[tom.stoer]

Good point. The question is why do we need string theory to address these questions and to study this specific model? That's what I meant with "Whenever one wants to make any calculation one immediately leaves string theory, derives some low-energy limit and studies ordinary SUSY or SUGRA gauge theory. That's not what I am expecting from a fundamental theory."

In which sense is string theory required to study this specific model? Would we learn anything beyond this specific SUSY gauge theory in the sense that it tells us something regarding string theory itself?

The benefit I see is that string theory constrains and guides model building; that's a great achievement. But once one studies a single model I have the feeling that one does not learn anything regarding string theory anymore - just something reagarding a specific model (I agree that this model seems to be interesting and that one might expect some experimental hints in similar directions).

 

[suprised]

Just to be specific, let me refer to http://arxiv.org/abs/1103.4800" , which is presented as a candidate for the real world. What would it mean if that became our standard model? It would have some "particle physics" implications - in this case, the existence of a fourth generation - and it is normal for such models to imply something extra (e.g. supersymmetry). Also, quantities like the particle masses would now be understood as expectation values of geometric moduli; in this case, the resting point of a stack of branes in a six-dimensional hypertorus. We would have passed from thinking abstractly about the possibility of braneworlds, to dealing with the fact that a particular braneworld model exactly matched the data.

That's all quite a lot. It's not the same as an explanation for why that configuration of braneworlds, or what the fundamental physical picture is, but it would be an extremely revolutionary development.

The problem with that is that there is most likely a huge variety of very similar models and there is absolutely no reason why the model one picks would describe nature exactly. Most likely one would need to readjust the model all the time, while experimental data trickle in. If this would converge in a reasonable amount time, well then OK, but I don't think this to be likely.

I guess the last 20 years have shown that there is little hope to get substantially beyond toy model status. And what's wrong about that? Many other things in nature cannot be explained/computed to high detail. And it is highly non-trivial that many qualitative features of the standard model work out well in string theory, and even important conceptual points like "explaining" the smallness of the cosmological constant in terms of a landscape can be captured in terms of phenomenogical toy models. My attitude would be not to try to ask too much.

 

[tom.stoer]

My attitude would be not to try to ask too much.

Surrender?

 

[suprised]

There is an important distinction between technical and conceptual issues.

True. But one shouldn't forget that things must work out technically, at least to some convincing degree, otherwise conceptual issues are pure speculation/philosophy. What fascinates many string physicists is that the theory works technically so well. I guess these computational results are much more non-trivial than non-experts can appreciate.

We can ask "why is space four-dimensional". Then it's of course conceptual question whether this should be an input to our theory or whether it can somehow be derived. One has to find a framework whioch allows one to ask these questions, based on which we are able to formulate a theory which is dimension-agnostic at all, or which allows for a different number of dimensions. The question why 6 out of 10 dimensions shall be compactified in certain string models is not conceptual. It's over, the interesting things already happened when chosing a certain background. It's no longer possible to ask the interesting questions. One can go through all CY compactifications and study their properties but one will never find out the answer to the original question.

In that sense string theory seems to ask too many technical questions instead of conceptual ones. Finding new vacua in the landscape may be interesting, but it does not help to understand deeper problems. Looking at the shape of the Earth and some atlases does not explain why the Earth is nearly spherical.

In that sense most string reserach programs do not address the fundamental issues. CY compactification, D-branes, nearly MSSM models, ... are so to speak phenomenological models. How close can one get to the real world? Arbitrary clsose I would say.

Fully agreed.

What would happen if at some day the the standard model is reproduced by such a string vacuum exactly? What would we learn? Nearly nothing.

Well if the string model would be fully correct and if one could do computations with arbitrary precision, one could generate infinitely many predictions.

My feeling is that there is some kind of huge duality, but string theory is exactly the wrong point to start with. Whenever one wants to make any calculation one immediately leaves string theory, derives some low-energy limit and studies ordinary SUSY or SUGRA gauge theory.

That's not quite correct, although many poeple do it that way, and that's what I criticised above. This concerns phenomenological investigations. But many investigations deal precisely with conceptual stringy features, see for example microstates in black holes, high energy scattering, etc. The AdS/CFT duality is another example of conceptional works that did not come out of a phenomenological direction. There IS a considerable effort into formal/conceptional directions, perhaps this is generally not so visible because not so many people do it and because these things may be hard to understand by non-experts.

That's the reason why I guess that string theory is exactly the wrong place to start. My expectation is that string theory is correct in some sense, but that it has to be replaced by a fundamental theory from which strings, branes, backgrounds and compactifications, perhaps even dimensions can emerge.

I would say: string theory, as formulated as an on-shell theory, is useful to describe certain features (like particle spectrum, black holes microstates, etc), but this formulation has its limitations when it comes to dynamical issues like vacuum selection.

Surrender?

Why surrender? This is research in progress, and one shouldn't try to make too large steps at once, nor generate too high expections for quick success.

 

[tom.stoer]

That's not quite correct, although many poeple do it that way, and that's what I criticised above. ... There IS a considerable effort into formal/conceptional directions, perhaps this is generally not so visible because ...

I would say: string theory, as formulated as an on-shell theory, is useful to describe certain features (like particle spectrum, black holes microstates, etc), but this formulation has its limitations when it comes to dynamical issues like vacuum selection.

OK, thanks for the explanation. I think I have to study the relevant papers more carefully. But I think we should really add background-dependence and focus on on-shell formulation to your list ...

 

[smoit]

There is an important distinction between technical and conceptual issues.

Let's make some examples:

We can ask "why is space four-dimensional". Then it's of course conceptual question whether this should be an input to our theory or whether it can somehow be derived. One has to find a framework whioch allows one to ask these questions, based on which we are able to formulate a theory which is dimension-agnostic at all, or which allows for a different number of dimensions. The question why 6 out of 10 dimensions shall be compactified in certain string models is not conceptual. It's over, the interesting things already happened when chosing a certain background. It's no longer possible to ask the interesting questions. One can go through all CY compactifications and study their properties but one will never find out the answer to the original question.

The question above might as well be technical in its nature. Let's say we want to obtain 3+1 large space-time dimensions while having the rest of them compactified on a CY 3-fold.
Let us restrict to the cases where the large dimensions are either Minkowski or nearly de Sitter to avoid the cosmological solutions with a big crunch.

Apriory, even if we assume such a compactification, we don't know if the compactified dimensions can actually remain compact until we find a reliable mechanism to stabilize all the moduli that parameterize the deformations of the internal metric. A canonical example is the Type IIB flux compactifications, where fluxes only stabilize the complex structure moduli and the axio-dilaton while the Kahler moduli remain unfixed. Stabilizing the remaining moduli is paramount for keeping the internal manifold compact. However, this task is highly non-trivial. In order to fix the Kahler moduli one must satisfy certain topological conditions that determine the number of fermionic zero modes in the corresponding non-perturbative contributions to the superpotential, which is possible in principle but extremely hard to achieve in practice, especially when charged chiral matter is present at various intersections. In addition, there is something called the overshoot problem, which in the case of multiple Kahler moduli may become a very severe problem. So, the bottom line is that in the vast majority of cases one cannot stabilize all the moduli by currently known mechanisms because one cannot generate the potential due to the topological constraints.

So, the next question would be, is it possible to stabilize all the moduli assuming a compactification down to 2+1 or 1+1 or even 0+1 dimensions?

This is not a conceptual but rather a technical question, which would require some new ideas. I personally don't know if it would be possible to have a stable compactification of, say, M-theory on a CY 5-fold or some toroidal orbifold so that all 10 spatial dimensions are compact but the vacuum energy is nearly zero. My guess is that it would be a really tough problem and it is quite possible that there are just not enough ingredients to generate a potential to stabilize all the moduli, in which case, some internal cycles will never be stabilized and will get as large as the corresponding dynamics allows them to get.

 

[smoit]

The problem with that is that there is most likely a huge variety of very similar models and there is absolutely no reason why the model one picks would describe nature exactly. Most likely one would need to readjust the model all the time, while experimental data trickle in. If this would converge in a reasonable amount time, well then OK, but I don't think this to be likely.

I guess the last 20 years have shown that there is little hope to get substantially beyond toy model status. And what's wrong about that? Many other things in nature cannot be explained/computed to high detail. And it is highly non-trivial that many qualitative features of the standard model work out well in string theory, and even important conceptual points like "explaining" the smallness of the cosmological constant in terms of a landscape can be captured in terms of phenomenogical toy models. My attitude would be not to try to ask too much.

To me, the number one problem with this Type IIA model is that the moduli are assumed to be stabilized somehow, and they go from there and compute various parameters. But the truth is that it's not at all clear if it's even possible to stabilize all the moduli in such toroidal Type IIA compactifications. IMHO, this and similar models are dead from the start until one demonstrates explicitly how all the moduli can be stabilized. Just to remind everyone, it has been shown that it is possible to fix all the moduli in massive Type IIA supergravity, which is NOT what one really wants, and the vacua one obtains are AdS with or without SUSY. The only examples of dS vacua in Type IIA that I'm aware of are the ones constructed by Eva Silverstein for compactifications on nil manifolds.