What are unified theories trying to match at lower energies?

[tom.stoer]

QCD is not in the same category as ST and many of the qft methods used in arriving at this clarity of QCD were already developed (unlike ST).

I serves as an example only; but if you look at my list most topics were completely unknown prior to QCD. But ...

And didn't you say in #42 that you do expect the complexity to increase since the aim of ST is very ambitious?

... you are right, my reasoning sounds inconsistent. I would like to stress that talking about growing complexity I tried to be unbiased, whereas my last remark is certainly a personal belief only, and everybody is free to disagree.

My impression is that sooner or later during the development of a theory there should be a turn where some (hidden) rather simple structures, principles or axioms are discovered. This did not happen in ST, even though thousands of highly talented people spent decades of research. This is one reason why I believe that ST is wrong.

 

[atyy]

This is one reason why I believe that ST is wrong.

Do you mean wrong as in "not a consistent theory, and hence not capable of making predictions in principle" or "a consistent theory whose predictions will not be borne out experimentally"?

 

[Fra]

@ Fra
I think you are taking the wrong direction again. My question is not "why should we strike off ST?" - rather, it is "why should we strike off ST while not striking off SUSY/GUT?". To say ST does not relate to what we know is wrong because we don't even know anything certain at SUSY/GUT..

You are right that I didn't quite get that you made that comparasion, sorry.

But while ST may require some form of SUSY, but the opposite isn't true, so I don't understand your comparasion. Also the ambition of ST is higher, therefore the two things you compare aren't even fair competitors?

To say ST does not relate to what we know is wrong because we don't even know anything certain at SUSY/GUT..

You probably missed the level of where I meant this comment. Yes SUSY/GUT is not something we know, which means ST must connect directly to the more firm ground, which to me means agreement with the subset of predictions of the SM that is currently tested against experiment, nothing else.

I would ask the same out of any other approach. Experimental contact is the only thing that counts as firm ground. Other guides are bound to be subjective, that's I guess why we have different research programs.

The conceptual questions need to be answered, but that's again not a good criteria for writing off - there are still a lot of unanswered questions with good old quantum mechanics too (like the main quantization process itself).

My ambition that the unaswered questions with good old QM SHOULD be accounted for in a plausible program. Anything less, and I would certainly find the "minimal extension" philosophy working from the set of experimentally confirmed domains more rational.

The latter is I think the natural choice for a experimentalist, but for a theorist seeking deeper understanding I see no excuse for halfhearted programs, that without good arguments ignores several deep issues.

/Fredrik

 

[tom.stoer]

Let me first remark that I am not an insider, therefore there my be new developments which I am not aware of. I'll appreciate hints where some progress regarding these topics can be found.

To me ST seems to be "wrong" in the following sense:
1) nobody knows how to quantize the entire theory; what is known are some classical ground states (a glimpse of the infamous landscape) + perturbative quantization up to a two loops (*); that means that we do not know the full theory, not even formally. Therefore in strict sense the theory does not even exist. (**)
2a) Regarding predictions: ST does make predictions, but most of them are wrong (most of the derived low energy effective theories) or incomplete (there is e.g. no mechanism which explains the tiny but non-zero masses of the well-known elementary particles). So ST is wrong in the sense that its known predictions are wrong.
2b) ST does not make unique predictions that cannot be derived from other theories. In the current status there is no reason to derive one low-energy theory from ST. Writing it down w/o any reference to ST and saying "that's it, unfortunately it does not fit to our world" is neither worse nor better. To my knowledge all predictions that could eventually be derived are always due to the specific form of a low energy effective theory. So as long as string theory is not able to tell my which theory I should select, ST is wrong in the sense that it creates a huge landscape which is totally unobservable w/o telling me where I am in this landscape. Therefore the theory is as wrong as the aether theories prior to SR.

(*) I am not sure about the number two; but definately not to arbitrary loops;
(**) this is not a problem if you deal with a non-fundamental theory; chiral perturbation theory does not exist either (as it is non-renormalizable), but it does not claim to be the fundamental theory

Now string theorists will reply that there are many deeply hidden connections between different areas of ST, dualities, hints regarding a full M-theory, hope for UV finiteness, ... I agree and this is why there is some hope that not the entire idea but only the way it is treated is wrong. Perhaps its like the theories Einstein tried during the years where he developed GR. He tried several approaches and some of them where physically wrong whereas the whole idea was of course correct.

 

[tom.stoer]

I made some notes based on slides David Gross presented at a string conference; unfortunately I cannot find the slides, so here are the notes:

-----------------------------------------------

WHAT IS THE NATURE OF STRING PERTURBATION THEORY?
Our present understanding of string theory has been restricted to perturbative treatments. Does this perturbation theory converge? Most likely it does not. In that case when does it give a reliable asympototic expansion of physical quantities? How can one go beyond perturbation theory and what is the nature of nonperturbative string dynamics? This question is particularly difficult since we currently lack a useful nonperturbative formulation of the theory.

STRING PHENOMENOLOGY
Here there are many questions that can all be summarized by asking whether one can construct a totally realistic four-dimensional model which is consistent with string theory and agrees with observation?
Great progress, but still not constructed.

WHAT PICKS THE CORRECT VACUUM?
This is one of the great mysteries of the theory which appears, at least when treated perturbatively, to possesses an enormous number of acceptable (stable) vacuum states. Why, for example, don’t we live in ten dimensions? Does the theory possesses a unique vacuum, in which case all dimensionless physical parameters would be calculable or is the vacuum truly degenerate, in which case we would have free parameters? How does the value of the dilaton field get fixed, thereby giving the dilaton a mass? Does the vanishing of the cosmological constant survive the mechanism that lifts the vacuum degeneracy?

The crucial issue is still unresolved: The cosmological Constant

IS THERE A MEASURABLE, QUALITATIVE, DISTINCTIVE PREDICTION OF STRING THEORY?
String theories can, in principle, make many “postdictions” (such as the calculation of the mass ratios of quarks and leptons, Higgs masses and couplings, gauge couplings, etc.). They can also make many new predictions (such as the masses of the supersymmetric partners of the observed particles, new gauge interactions, etc.) These would be sufficient to establish the validity of the theory, however in each case one can imagine (although with some difficulty) conventional field theories coming up with similar pre or post dictions. It would be nice to predict a phenomenon, which would be accessible at observable energies and is uniquely characteristic of string theory.

WHAT IS STRING THEORY?
This is a strange question since we clearly know what string theory is to the extent that we can construct the theory and calculate some of its properties. However our construction of the theory has proceeded in an ad hoc fashion, often producing, for apparently mysterious reasons, structures that appear miraculous. It is evident that we are far from fully understanding the deep symmetries and physical principles that must underlie these theories. It is hoped that the recent efforts to construct covariant second quantized string field theories will shed light on this crucial question.

What is the fundamental formulation of string theory?
Quantum Space of all 2-d field theories
Second Quantized Functionals of loops (SFT)
M-theory . . .
Is string theory a framework, not a theory?
What is missing?

 

[Fra]

I pretty much share Tom's view of ST.

I would like to add one thing about this:

Now string theorists will reply that there are many deeply hidden connections between different areas of ST, dualities, hints regarding a full M-theory, hope for UV finiteness, ... I agree and this is why there is some hope that not the entire idea but only the way it is treated is wrong. Perhaps its like the theories Einstein tried during the years where he developed GR. He tried several approaches and some of them where physically wrong whereas the whole idea was of course correct.

I symphatise with some of the possible ideas of ST defendeers here, but here is the additional objection I have: When I really try my best, to make sense out of string theory, and then in particular the problem of how to select the physically viable domains of these landscapes, then I am led to things that suggests that the only solution is that strings and branes themselves must be emergent, and that this may provide the selection principle needed, but then the point is that I am then lead to a kind of reasoning that is at face with the original starting points of string theory. ie. it's no longer string theory. And I think there is in this case probably a much better path, than going first through the string jungle.

So even if I try my best and make sense and see hope, my conclusion suggest that the founding principles of ST as it's define can't be fundamental.

/Fredrik

 

[tom.stoer]

I can agree to Fra's idea; I would like to add that it is in prinicple not unusual in science, but only in this particular case it takes hundreds of thousands of man-years to uncover the fundamental structures. Compare it with loop quantum gravity: you start with GR, change variables to Ashtekar's, change to loops, change again to cyclindric functions and end up with a uniquily determined Hilbert space of spin networks (LOST theorem).
(LQG is certainly not as ambitious as ST and does not provide the basis for a unified theory as far as we know; and I don't want to promote LQG as the correct theory to describe QG, it simply serves as an example)

I want to make clear that sometimes there are some detours in order to derive the fundamental structure. I don't mistrust the detour by itself, I mistrust the unmanageable complexity and length of the detour.

 

[tom.stoer]

It would be interesting to hear more about these issues from the leading string theorists ...

 

[Fra]

It would be interesting to hear more about these issues from the leading string theorists ...

Same here.

/Fredrik

 

[atyy]

Let me first remark that I am not an insider, therefore there my be new developments which I am not aware of. I'll appreciate hints where some progress regarding these topics can be found.

To me ST seems to be "wrong" in the following sense:
1) nobody knows how to quantize the entire theory; what is known are some classical ground states (a glimpse of the infamous landscape) + perturbative quantization up to a two loops (*); that means that we do not know the full theory, not even formally. Therefore in strict sense the theory does not even exist. (**)
2a) Regarding predictions: ST does make predictions, but most of them are wrong (most of the derived low energy effective theories) or incomplete (there is e.g. no mechanism which explains the tiny but non-zero masses of the well-known elementary particles). So ST is wrong in the sense that its known predictions are wrong.
2b) ST does not make unique predictions that cannot be derived from other theories. In the current status there is no reason to derive one low-energy theory from ST. Writing it down w/o any reference to ST and saying "that's it, unfortunately it does not fit to our world" is neither worse nor better. To my knowledge all predictions that could eventually be derived are always due to the specific form of a low energy effective theory. So as long as string theory is not able to tell my which theory I should select, ST is wrong in the sense that it creates a huge landscape which is totally unobservable w/o telling me where I am in this landscape. Therefore the theory is as wrong as the aether theories prior to SR.

(*) I am not sure about the number two; but definately not to arbitrary loops;
(**) this is not a problem if you deal with a non-fundamental theory; chiral perturbation theory does not exist either (as it is non-renormalizable), but it does not claim to be the fundamental theory

Now string theorists will reply that there are many deeply hidden connections between different areas of ST, dualities, hints regarding a full M-theory, hope for UV finiteness, ... I agree and this is why there is some hope that not the entire idea but only the way it is treated is wrong. Perhaps its like the theories Einstein tried during the years where he developed GR. He tried several approaches and some of them where physically wrong whereas the whole idea was of course correct.

I think 2a and 2b are widely accepted, even by string theorists. Zwiebach's text for undergraduates explicitly states so, and Jed Distler made a similar comment on his blog a couple of years back. Regarding 1 it's true that the full theory is still unknown, but that's quite different from the theory being inconsistent. In fact if it's inconsistent, 2a and 2b cannot be true, since the theory would not even make predictions (one should be able to get any prediction from an inconsistent theory, which means no prediction). Also, the true meaning of renormalization was only known many years after renormalization had been achieved in QED by Dyson and colleagues, and till then many suspected the theory was inconsistent. Now we know it is inconsistent at the highest energies, but it doesn't matter for predictions at low energies. Given AdS/CFT I would say that string theory does have a proposed complete formal theory of quantum gravity in some universes (not ours).

Tong's comments at the close of his lecture notes http://arxiv.org/abs/0908.0333 "The problems that we alluded to in Section 6.4.5 are real. Non-perturbative completions of string theory are only known in spacetimes which are asymptotically anti-de Sitter, but cosmological observations suggest that our home is not among these. In attempts to make contact with the standard models of particle physics and cosmology, we typically return to the old idea of Kaluza-Klein compactifications. Is this the right approach? Or are we missing some important and subtle conceptual ingredient? Or is the existence of this remarkable mathematical structure called string theory merely a red-herring that has nothing to do with the real world?"

 

[crackjack]

Also the ambition of ST is higher, therefore the two things you compare aren't even fair competitors?

All the more reason to not strike off ST without striking off SUSY/GUT first.

Yes SUSY/GUT is not something we know, which means ST must connect directly to the more firm ground, which to me means agreement with the subset of predictions of the SM that is currently tested against experiment, nothing else.

Like I said ST can relay the baton only to SUSY/GUT which should then relay it further to SM.

 

[crackjack]

@ tom.stoer

What you say is all true, but such problems abound all theories under development - especially the very ambitious ones. In fact, if such issues (what I would call open problems) were not present, the theory would have been considered finished. They are all valid reasons for rejecting the theory (even partially) only when there are no-go theorems (with reasonable assumptions) which prevent these objectives from being achieved.

On the other hand, for each of your point, I can give you similar issues with SUSY/GUT. And yet, most of those who oppose ST do not oppose SUSY/GUT as much.

I am not defending ST, but just trying to understand if there are valid reasons (and what they might be) for writing off ST (without striking off SUSY/GUT).

 

[arivero]

SUSY/GUT is a minimal, from experiment upwards, program, and while you can argue that it does not predicts, its group theory helps to explain the structure of charges in the standard model. SUSY/GUT is not a program to pick a random lie group, in this sense its "landscape" is as big as the one of string theory, or biggest (at least string theory is limited to D<26 and G < E8xE8) and of course such approach would be absurd.

The weak points of S/GUT is, to me, the "U". It could be that the group theory chain is not really related to a traversal of a high energy desert of, how many, 20 orders of magnitude?

On the other hand, it is true that the big picture is that ST relays the baton down to SGUT and then down to SM. But perhaps the alternative way is that ST relays to KaluzaKlein and then ir to SM, without the "U" of GuT.

 

[Physics Monkey]

In my opinion, it would be totally unprecedented if the physics we observe at ordinary energy scales (even up to a TeV) were to somehow uniquely determine the structure of the world at arbitrarily high energies. I think nothing like this has ever happened before, and we don't really have a good reason to believe it will happen now. Nevertheless, this uniqueness myth is a part of string culture and has in the past motivated much string research (and perhaps still does). Now I don't want to take this cartoon characterization too seriously, but the influence is there. And we need look no further than the first chapter of Polchinski to find this uniqueness myth in all its glory (not that Polchinski really believes it). The amusing thing is that string theory destroyed its own myth with the stringy landscape.

Of course, we could in principle see Regge trajectories of strings at the LHC or at whatever comes after the LHC, but this seems unlikely. We may see SUSY or extra dimensions or who knows what, but except for actually seeing the strings, I predict that we won't need string theory to understand what's going on for a long time (except perhaps indirectly via ads/cft for example). And even if we assume strings, the presence of the landscape means we probably still won't be able to say much about what comes after the next generation of experiments.

It seems to me that string theory is in the process of changing form. String theory now appears less like a specific theory of the world, like the standard model, and more like a framework for understanding a large class of worlds, like the framework of quantum field theory. I think our reasons for studying string theory are changing as well, and similarly, our old expectation that string theory is simply right or wrong is also outdated.

 

[arivero]

Just a point, even without disputing the validity of your argument.

I predict that we won't need string theory to understand what's going on for a long time

I think that the orthodoxy, or at least Witten et alikes, do not expect string theory to be the main piece.

Lets take Weinberg's volume three. He explains that Supergravity forces the existence of a tensor A_{MNR} in 11 dimensions, and that this tensor has two roles:

- it stabilizes the compactification to 4+7 dimensions.
- its sources, via the "generalisation" of maxwell equations, are not charged point particles, not charged strings, but charged membranes.

Then Witten approach comes to tell us that when descending from 11, where sugra lives, to 10, where we can formulate chirality, this membrane, ie this source of the 11D tensor implied by supergravity, wraps to build strings, and that this is the object which is being classifiyed in 10D.

And then the usual lore.

But strings as fundamental objects are not there. What is fundamental is n-dimensional supersymmetry. In turn, you can look for mathematical uniqueness of this structure, n-supersymmetry, via Evans interpretation (as division algebras) or Duff "Brane scan"

 

[Fra]

All the more reason to not strike off ST without striking off SUSY/GUT first.

It sounds to me like you see only two ways here, this is what confuses me. I certainly see more than these two ways. Relative to my preferred way, ST is stil not "sufficiently" ambitious as it still relies on the old QFT framework.

I think there are two natural two fronts of research: the front starting from a minimally speculative experimental basis; and a front more radical that aims to adress the deeper conceptual issues.

Like I said ST can relay the baton only to SUSY/GUT which should then relay it further to SM.

I guess I didn't get your arguments for this. If ST can't connect to actual experiments, what does that have to do with SUSY based GUTs? (which we agreed are also so far just a set of theories; and there is no unique connection point)

Also, my personal opinon is that a new program does not need to match the exact mathematical structure of the SM, since this is not proved, it's merely consistent with observations - so far, but as we know there are domains where we lack experimental data. One only needs to mathc the experimentally verified parts of the SM. It's quite possible that the symmetries in the SM, as well as the spacetime structure, can be explained as emergent in particular limits (this is my personal guess).

/Fredrik

 

[tom.stoer]

Let's look at other theories with similar problems as in string theory. In Loop Quantum Gravity there is a unique description of "spacetime" - unfortunately it's a spin network to which we can neither associate dimension, nor space and time as we know it. Therefore we expect that all these low-energy entities will emerge in some appropriate limit. Then we have similar problems as with string: We must show that these entities do really emerge - at least their experimentally known signatures. The key difference is that Loop Quantum Gravity does not rely on the emergence of "intermediate theories" like SUGRA, SUSY, etc. but that it tries to derive the low-energy physics directly from its fundamental entities. So if this works then it's a success for LQG, if it does not work, it's a failure of LQG.

In string theory you always have these "intermediate theories". The question is: what is string theory good for if it only generates these theories? What is the benefit?

 

[Fra]

No matter how this is turned around, there are I think no excuses for producing not something constructive that connects to measureable things. But I some problems might be impossible to answer in one ago, and then I see a sense in that the answer is another, but refined question, and in such a question the measure of success should be one of relative progress, or evolution.

Maybe we still don't understand the nature of the relation between a high energy view and a low energy view? I am at least very much in doubt about that.

I think of the so called high energy TOE picture, as a low complexity limit if you talk about the inside observer (ie matter). But this picture itself, has IMO no objective status, since this low complexity limit refers to another observer of higher complexity.

I think that part of the puzzle might be that there is no objective absolute meaning of energy scales, no more than there is an objective meaning of "complexity scales". Instead all we can do is try to understand the relation between the lowest and higher distinguishable scale in each case, and understand how this picture again scale as this "case" changes.

After all, the context of all our theories, are Earth based. This is the constraining factor for both particle experiments as well as cosmological observations.

I think we might need to start asking some of these "open questions" in a different way. We probably don't even quite understand what we ask, even though the questions seems simple to phrase, at least that's the case if I speak for myself :)

An implication of this thinking is that the idea of an objective universal super-symmetry that exists in a realist sense may simple be the wrong picture. The nature and meaning of the symmetries of the laws of physics may be more subtle that the conventional picture. I am increasingly more convinced that this is the case, and this is why several of the existing programs to me, are not just infruitful, they might even be considered to be asking the questions in a wrong (the old) way.

I think the revolution we need, is not an invention of another mysterious supergroup of some kind, it's rather a deeper understanding of the nature of symmetry.

/Fredrik