Why I am REALLY disappointed about string theory

[tom.stoer]

I think it's time to write a short essay why I am really disappointed about string theory.

It is not because ST is wrong (we don't know if it is wrong; and we have no proof that other theories such as the SM are right).
It is not because it is complicated (the electroweak model is complicated as well), and it is not because I do not understand it (there are not many people who should claim that a theory is wrong simply b/c they do not understand)

It has something to do with its potential and with keeping (or neglecting) promises.

As far as I can see string theory (whatever this means - ST, F-, M-, ...) is the only candidate with the potential to unify all interactions including gravity. Other approaches may seem more straightforward, less complicated or exotic, closer to the well-known realm of the SM, ... but none is able to unify all interactions.

• Loop Quantum Gravity (which I really like and which I understand quite well) is a very promising candidate for QG - but as far as I can see only for QG. Topological braiding of q-deformed spin-networks from which all elementary particles can emerge is a brilliant idea - but up to know wishful thinking.
• Non-commutative geometry does not expalin why there is a unique nc structure (or I have overlooked this as I am not an expert in this subject)
• Asymptotic safety is fine for QG, but it does not explain the symmetry structure, masses, coupling constants, mixing angles etc. of the SM.
• SUGRA is neither unique nor finite (perhaps finite order by order, but not for the whole series) - nor elegant if restricted to four dimensions

What I like about string theory.

I think the greatest achievement of string theory (provided that it's true; afaik we do not have a sound proof, whether it's true) is that string theory turns most (all?) possible theories including gravity from theories into solutions. If the ideas regarding SUGRA, swampland etc. are correct, then there is a huge class of theories (in the classical sense) that can be "derived" from string theory. And perhaps even the opposite is true: theories which cannot be "derived" from string theory will turn out to be inconsistent.

From that point of view I should be a fan of string theory, shouldn't I?

So here's what I don't like about string theory.

I don't like that string theory comes with an enormous mathematical and physical apparatus, w/o being able to give us a hint why we should believe in this apparatus (10/11 dim., SUSY, CY, ...). For me there is only one good reason to become a believer, namely to follow my argument from above - that string theory defines a unique framework from which all candidate theories can emerge. So it's about a promises!

So essentially I like string theory b/c it makes these promises - and I don't like string theory b/c it only makes these promises!

Here are some questions for which I would like to see progress [I only list problems that are not inherent to string theory; I don't care how to count CY spaces b/c this is not a physical question; I don't care about the definition of the higher genus measure for the amplitudes in superspace as this is not relevant physically - it is a problem not created by nature but by the string theory formalism; I don't care about AdS/CFT b/c this is not our universe; so I will list questions asked by nature]

• Why do we live in a 3+1 dimensional space-time?
• How is the big bang singularity resolved?
• Why do we see the matter content and interactions we see? (why three fermion generations, why the symmetry group of the standard model, why the Higgs (or not?), ...?
  do we live in a 3+1 dimensional space-time?
• What is the mechanism breaking symmetries and selecting the true vacuum? (which according to string theory defines the above mentioned interactions and structures)
• What is the microscopically picture regarding dynamical spacetime including black holes, entropy of the gravitational field etc.?

Of course I am prepared for answers regarding landscapes (Susskind) and mathematical universes (Tegmark). But frankly: I will never accept these arguments. This is regarding string theory, therefore I expect answers in the context of string theory (if my daughter has to go upstairs in order to shampoo I don't accept discussions regarding justice; that does not mean that I am unintersted in justice - I am - but not in the context of telling a six-year old girls to go upstairs in order to shampoo!)

So my expectation is that string theory does something very natural: be aware of the true problems of nature, provide ideas how to address them, provide a status or summary regarding progress and obstacles.

Instead of listing obstacles (which may sound biased or even rude) I would like to ask the string theory audience here in this forum for their assessment.

• What are the major achievements of string theory?
• Are there predictions subject to (accessable to) experimental verification / falsification both in principle and in practice? Are there physical phenoma which (once observed) would kill string theory?
• Are there predictions specific for the string theory context (nothing that may follow from SUSY as SUSY could be true even w/o string theory)
• What are the short-term / long-term research programs?
• What are the major obstacles inherent to string theory preventing the theory from delivering on its promises?
• What will be the final theory in terms of strings - a theory, or a framework to create theories?

A last remark for all those who are still with me and did not stop reading: this is all about progress in physics and waiting for illumination. It is not about fighting against a theory - doing that would require less thinking and writing ...

Regards & Thanks
Tom

 

[negru]

The big problem with answering these deep questions is that we don't yet have a working definition for string theory. What we have so far are only bits and pieces, HINTS that something must be going on. Just because ads/cft makes some correct calculations, doesn't mean that ads/cft is THE string theory. Similarly just because some CY compactifications lead to SM (or MSSM) type physics, doesn't mean that must be the way to do it. There are also of course the twistor strings which are a whole different beast.

All of these different frameworks have to come together somehow, they can't all be describing different (but sometimes even the same) parts of reality without being connected. That's what we need to understand before even attempting to answer physical questions.

As for some major achievements, how about the KLT relations? If strings "don't exist", why did that work?

Think about what it means for something to "exist" anyway. How do we know particles "exist"? We don't, in fact we have no idea what these particles "really" are. Maybe these particles are actually tiny vibrating apples made of energy - we don't care. We just come up with a model that attempts to make correct calculations, up to a particular accuracy. If the model works, its constituents are said to "exist". This is what we're doing with string theory in ads/cft. If those calculations lead to something correct, there you have it, strings "exist". Whether or not you'll see them in an experiment, and whether or not our universe is AdS.

 

[MTd2]

The attitude of string theorists towards twistor strings is either as a superficial mathematical, not good beyond 1 loop calculations or just as a flawed physical theory. This is not an opinion of mine, I actually asked important string theorists about this. And that goes along what Witten said about it being incomplete.

As KLT relations, they might be or not imply the need for strings. Apparently, it works for SUGRA N8 in relation to SYM N4, in 4d, up to 4 loops, beyond that no one is mathematically sure because there is no proof of renormalization. Some say that counter terms of 7 loops or not may rely on terms that are not due supersymmetry.

 

[negru]

It looks like you're trying to bend over backwards to deny the use of strings. Next thing you know, even if string theory makes a prediction about new particles or something which are later found, you'll say ah that doesn't mean strings exists, we can have those particles by just putting them in the SM by hand.

The KLT relations, at tree level (which doesn't care much about susy), predicted a relationship between gravitons and gluons. There's no field theory understanding (as far as I know) of why this should be true. So you use string theory to derive a non-trivial relationship, and then you say we don't actually need strings for that to work?

So let me get this straight:
-if you use string theory to explain stuff about known theories, that doesn't mean strings exists - since the theory as well as the particular fact can exist independently from strings?
-if you use string theory to describe possible new things - that's silly because we haven't seen any of the new things?

As for twistor strings - yes it doesn't work beyond loop level yet. But at tree level it works pretty nicely. Is that a coincidence, a fluke, or what? Just this particular fluke has been more useful than everything eg LQG has done so far, so I don't see why we should disregard it so quickly.

 

[tom.stoer]

The big problem with answering these deep questions is that we don't yet have a working definition for string theory. What we have so far are only bits and pieces, HINTS that something must be going on. Just because ads/cft makes some correct calculations, doesn't mean that ads/cft is THE string theory. Similarly just because some CY compactifications lead to SM (or MSSM) type physics, doesn't mean that must be the way to do it.

I appreciate very much what you say; this is my impression as well, but I would like to learn something like that from string theorists.

All of these different frameworks have to come together somehow, they can't all be describing different (but sometimes even the same) parts of reality without being connected. That's what we need to understand before even attempting to answer physical questions.

If thus is true than it's still too early to ask physical questions. So perhaps string theory is not wrong, but just too difficult to understand. The problem is again about promises. If you propose such complex theory, the results must WOW you in order to justify this complexity.

As for some major achievements, how about the KLT relations? If strings "don't exist", why did that work?

Do you really need strings; I thought SUGRA was enough.

Think about what it means for something to "exist" anyway. How do we know particles "exist"? We don't, in fact we have no idea what these particles "really" are. Maybe these particles are actually tiny vibrating apples made of energy - we don't care. We just come up with a model that attempts to make correct calculations, up to a particular accuracy. If the model works, its constituents are said to "exist".

I agree.

This is what we're doing with string theory in ads/cft. If those calculations lead to something correct, there you have it, strings "exist". Whether or not you'll see them in an experiment, and whether or not our universe is AdS.

I don't agree.You can't start with a wrong AdS universe, do some calculations and hope that this proves anything. If you start from "the moon is made of green cheese" and if you are able to derive the standard model, your proposition remains wrong!

I agree that AdS/CFT is a major achievement, but more a mathematical than a physical one.

 

[negru]

I don't agree.You can't start with a wrong AdS universe, do some calculations and hope that this proves anything. If you start from "the moon is made of green cheese" and if you are able to derive the standard model, your proposition remains wrong!

I agree that AdS/CFT is a major achievement, but more a mathematical than a physical one.

No one is saying that AdS/CFT means that the universe is AdS. It's just a toy model. There are other variants of gauge/string dualities, ads/cft is just one example. You can think of it as just a framework used for solving SYM. Another example would be building a sphere to solve a matrix problem. It doesn't mean that our universe has anything to do with either the sphere or the matrix. You can still use this construction though for practical reasons. All this construction says is that there is probably a deep connection between spheres and matrices, or something like that. Just like apparently there is a deep connection between particles and gauge groups.

 

[atyy]

No one is saying that AdS/CFT means that the universe is AdS. It's just a toy model. There are other variants of gauge/string dualities, ads/cft is just one example. You can think of it as just a framework used for solving SYM. Another example would be building a sphere to solve a matrix problem. It doesn't mean that our universe has anything to do with either the sphere or the matrix. You can still use this construction though for practical reasons. All this construction says is that there is probably a deep connection between spheres and matrices, or something like that. Just like apparently there is a deep connection between particles and gauge groups.

These authors speculate on getting de Sitter solutions from AdS/CFT - is there a handwavy way that a layman could understand this? http://arxiv.org/abs/0908.0756 "We then generalize these to AdS4/CFT3 duals, and suggest extensions of the method to obtain de Sitter solutions."

 

[MTd2]

Next thing you know, even if string theory makes a prediction about new particles or something which are later found, you'll say ah that doesn't mean strings exists, we can have those particles by just putting them in the SM by hand.

You are making this up. But you put up pretty much things very well straight. That's the usual scientific skepticism.

So you use string theory to derive a non-trivial relationship, and then you say we don't actually need strings for that to work?

Yes. But you do know that just like contour integrals exist in real life twisotrs are held as a mathematical device (which also works by simplifying contour integrals...). Twistor strings are not strings, at best U(1) instatons according to Berkovits. By Witten's twistor string used to build it are not really strings, but topological strings.

You have to defend String Theory by means of the F-Theory derived GUTs.

 

[negru]

F theory is just one part of string theory. I don't see any reason why you would consider it more string theory than the other parts. Anything that resembles strings and has applications (whether it's gluon amplitudes, quantum gravity, unification, or condensed matter) is good string theory to me.

 

[MTd2]

I don't see any reason why you would consider it more string theory than the other parts.

Because that`s the one that actually is making advances where it was supposed to be making, including the mass relations between generations, quantum corrections that are not supposed to be not existent without the theory, that is, among all of the other uses, things that leads falsifiable predictions.

 

[negru]

Well, no one really cares what it was supposed to be doing. It was actually only supposed to explain mesons..

We don't know enough string theory to predict masses and stuff like that, that's the short story. And that's why everyone is trying to understand it from different perspectives. I would find it very surprising if some sort of holography wasn't needed in the full theory which gives the SM. I think there's deeper reason why we have both ads/cft and f theory type of formulations. The chances of both of them existing independently are very slim, I'd say. F theory and ads/cft are just the tip of the iceberg.

By the way, I forgot to remind you about the dual conformal invariance of YM amplitudes which lead to the discovery of the T fermionic duality for strings. Like I said before, since gauge theories and string theory are the same thing, whatever you find in one you'll find in the other. So the whole discussion of who is working on exactly what is pretty silly to me. It seems to me like most anti-string people care more about who is working on string theory than the string theorists.

Also ads/cft can lead to falsifiable predictions, about SYM (or maybe quark gluon plasma). Why doesn't that count?

 

[Haelfix]

I don't speak with any authority on this, not being a string theorist, so take the following for what its worth.

"Why do we live in a 3+1 dimensional space-time? "

Unknown, but a good attempt at a guess might be string gas cosmology.

"How is the big bang singularity resolved?"

I don't know. Some more benign singularities seem to be resolved or smoothed out by perturbative string theory, but others (like apparently the bb singularity) really require the full nonperturbative analysis (see AdS/CFT or Matrix theory) to even begin to talk about.

"Why do we see the matter content and interactions we see? (why three fermion generations, why the symmetry group of the standard model, why the Higgs (or not?), ...?"

Certain promising vacua give very specific answers to those questions. Typically in the form of a statement about the geometry of the underlying space. Of course you can always ask, well why that particular geometry and you go right back to the vacuum selection problem which as you know is currently unsolved. Nevertheless, that any vacua possesses this sort of capability and there are by now hundreds of papers that have identified more or less mssm or close to msssm physics; is highly nontrivial and suggestive.

"What is the mechanism breaking symmetries and selecting the true vacuum? "

Again, unknown depending on what you mean. For instance cosmology likely has something to say about the vacuum selection method, and it might simply be more field theory phenomenology that has to do with say SuSY breaking.

"What is the microscopically picture regarding dynamical spacetime including black holes, entropy of the gravitational field etc."

Entropy is a macroscopic phenomenon, given by the usual laws of thermodynamics. Black holes satisfy the usual Hawking-Beckenstein entropy bound (which you can actually rigorously show with st). AdS/CFT is probably the framework where the most well defined answers come from regarding what to expect form exact black hole microphysics, but of course this game is still highly incomplete (sometimes you only know answers from one side of the duality).

 

[tom.stoer]

I don't speak with any authority on this, not being a string theorist, so take the following for what its worth. ...

Thanks a lot. This is exactly what we should do. Try be honest both on progress and open issues.

What about the assessment? Second list of bullet points?

 

[suprised]

Ok this seems to start out as a fair discussion and the setup is perfectly reasonable. Let me get through the numerous points over time, there's too much to say in a single shot.

I don't like that string theory comes with an enormous mathematical and physical apparatus, w/o being able to give us a hint why we should believe in this apparatus (10/11 dim., SUSY, CY, ...). For me there is only one good reason to become a believer, namely to follow my argument from above - that string theory defines a unique framework from which all candidate theories can emerge. So it's about a promises!

Comment on mathematics: Even ordinary quantum mechanics appears to the outsider as "too mathemaical" (Hilbert Spaces, Matrices) since he/she can't explain it in simple terms to Mom. But that's not the issue, isn't it? It's the proper language to use.

So why do we need mathemathical structures like Susy, CY, higher dimensions? I guess here a severe misunderstanding is taking place, even many string theorists are making it. The simple point is that SUSY makes things tractable. One can use the powerful methods of algebraic geometry, holomorphy and so on. Most of the progress in recent years in strings and gauge theory relies on that. Since this is a highly developed branch in mathematics it is natural that much of this has been made use of.

But IMHO all this geometrization is a toy model; a very powerful and fruitful one which has allowed to get many important insights in different problems like non-perturbative gauge dyamics, black holes, string dualities, AdS/CFT. But IMHO the confusion is that many people take it too literal and for the real thing. The real thing, the non-susy standard model incl gravity, may not be ever tractable to high accuracy. It is simply because non-susy dynamics is infinitely more complicated, esp when we deal with gravity; only with great luck we may find an approximate (broken) susy at low energies that makes things to some extent tractable.

I think that this SUSY-Toy machinery has be proven invaluable, much more than outsiders can possibly recognize. The string physicists are very excited about the progress that was achieved, like state counting in black holes (which eg shows that the theory is consistent and nothing is missing). So they say: great we begin to understand black holes! Now other people sit in armchairs and complain that only SUSY black holes were investigated, and no experimental predicitions were made, so all of this is worth nothing; a failure, a dead horse, a waste of ressources. But that's not the point - at this stage it is about to learn how things work! And it is as exciting as non-trivial to see a glimpse of it!

So, summarizing, the "heavy mathematics" mostly concerns algebraic geometry, but this is just a powerful toolset to deal with the supersymmertic toy models. One needs to use it for doing actual computations, which are important for understand how, or if, things work. Whether this whole susy framework has anything directly to do with nature, is a different question, and some people try to cook up susy constructions to describe the standard model. It's too early to tell whether this real-world application with bear out or not, I'd be skeptical, but this is just one part of the string theory program and definitely worth a try.

Comment on extra dimensions: this is another great piece of confusion. One may loosely say that extra dimensions are one way to parametrize the extra matter, non-gravitational fields. Only a small subset of 4d string constructions have a direct interpretation as geometrical compactifications, and one may formulate everything in a language where extra dimensions don't even appear… better just call this sector of the theory "internal" degrees of freedom. And this purported disadvantage of string theory is in fact the opposite, namely a blessing: strings need those internal degrees of freedom for consistency, and in a sense "predict" extra matter. This is what we actually want to have.

In "alternative" approaches, such as LQG, AFAIK there exists no consistency requirement that would demand extra matter; so this is put in by hand without any guiding principle - I see this as a desaster for achieving a unification of matter with gravity. No conceptual solution in sight here!

Also poeple often do not realize, that due to the dualities, one and the same theory can have different interpretations in terms of geometry and dimensions; in fact there is no absolute meaning of this! One and the same theory may be described as a 4d gauge theory, or 10d string theory compactified on an AdS space; if fact it seems to be a general phenomenon for "ordinary" gauge theories that their strong coupling limit has a higher dimensional interpretation. If one does not like it, one is cordially invited to close the eyes; but others will go on and try to see what can be learned from this extra-dimensional perspective. And a lot can, probably the holographic property of QFT's was one if the most important discoveries in many decades.

So in short, extra dimensional physics is automatically built in ordinary gauge theories, this is a computational fact and not an ideology, so there is no way to avoid it for any serious researcher.

 

[tom.stoer]

Thanks for the long and elaborated answer.

Two comments are in order:

1) I can agree with extra dimensions interpreted as internal degrees of freedom. Let's compare it with qm (Hilbert spaces). Assume for a moment that qm would not be able to make experimentally falsifiable predictions. Then we would wonder what these wave functions and Hilbert spaces should be, where they are and how we can measure them. I guess we would come to the conclusion that they are unphysical.

2) It's different with SUSY (already with SUSY in MSSM) and with superstrings. Here the problem is that these entities are not "hidden" mathematical entities but physical ones: directly shows up SUSY in the particle spectrum.

And here I see the following problems:

• SUSY is elegant - as long as you do not try to break it (OK, this is not a very good point :-)
• SUSY / MSSM does not need string theory (OK, refer to my argument from above: it may be a progress to have MSSM as one solution of string theory instead of just another theory introduced by hand)
• SUSY has to be verified experimentally sooner or later

I think the last bullet point is a serious issue: as long as there is no experimental support, string theory (and even SUSY) is somehow a solution hunting for a problem; and if one does not find SUSY at the LHS one can again say "that one will find it at higher energies"; so in the end it's about a promise again.

I studied SU(N) gauge theories for some time, even large-N limit. What about the following idea: is it possible that string theory is nothing else but a large-N approximation of certain supersymmetric gauge theories? If yes, would this kill string theory as a fundamental theory?

Taking all your ideas into consideration my conclusion is that string theory is still in an early state of its development; it makes progress, but many people are uncomfortable with its velocity.

 

[mln326]

The problem with the theory is that it thinks the universe does calculus every time. In genetics there are only four base units and that creates all living things. What if there is something simple that creates all of matter from energy?

 

[tom.stoer]

The problem with the theory is that it thinks the universe does calculus every time. In genetics there are only four base units and that creates all living things. What if there is something simple that creates all of matter from energy?

Interesting idea, but that's not about strings, it about physics :-)

 

[Theorem.]

The problem with the theory is that it thinks the universe does calculus every time. In genetics there are only four base units and that creates all living things. What if there is something simple that creates all of matter from energy?

It is important to add that the four base units themselves are not so simple on a molecular level. We use abstract theories to visualize them.

 

[negru]

I studied SU(N) gauge theories for some time, even large-N limit. What about the following idea: is it possible that string theory is nothing else but a large-N approximation of certain supersymmetric gauge theories? If yes, would this kill string theory as a fundamental theory?

(I assume you are referring to ads/cft - f theory for example couldn't care less - in principle - if ads/cft was just plain wrong)

There are many reasons to believe that ads/cft can be extended to non-susy, non large N. For example because you can break some pieces of susy, and still get right answers. Polchinski here gives a discussion and a number of references on this sort of stuff
http://arxiv.org/PS_cache/gr-qc/pdf/0602/0602037v3.pdf
(might want to skip to page 8 to "Lessons, generalizations, and open questions")

 

[tom.stoer]

No, I am referring to ordinary QCD in the large-N limit.

Thanks for the link; I've studied this paper several times but it does not answer my question if it's possible that instead of having some SUSY as low-energy limit of a fundamental string theory, string theory itself is only a (large-N) approximation to a certain fundamental SUSY. If this would be the case string theory would become of less interest.

I just want to entrtain this possibility as it eventually dates back to the origin of string theory were one tried to understand hadrons in terms of strings. Assume for a moment that it's true and that somebody finds a proof that some ST is nothing else but a large-N approx to the MSSM.

 

[atyy]

I studied SU(N) gauge theories for some time, even large-N limit. What about the following idea: is it possible that string theory is nothing else but a large-N approximation of certain supersymmetric gauge theories? If yes, would this kill string theory as a fundamental theory?

I don't know if this will turn out to be true, but if it is, surely an achievement of string theory - this is a clear case of emergent space and gravity.

On the LQG side, what if LQG is nothing but some limit of a GFT - would that kill LQG as a fundamental theory of gravity? I hope so, but that would be an achievement of LQG, I think!

 

[suprised]

Two comments are in order:

1) I can agree with extra dimensions interpreted as internal degrees of freedom. Let's compare it with qm (Hilbert spaces). Assume for a moment that qm would not be able to make experimentally falsifiable predictions. Then we would wonder what these wave functions and Hilbert spaces should be, where they are and how we can measure them. I guess we would come to the conclusion that they are unphysical.

Well extra dimensions can be physical in the sense that if we go up in energy, the theory miraculously gains higher dimensional Lorentz invariance. This can also happen if we go to a strong coupling limit. One question is whether this is necessarily so, but I don’t think so; there are non-geometric compactifications for which there is no energy scale above which the theory looks higher dimensional. But actually the problem is more complicated than it can be explained here in brief. At any rate, it is totally irrelevant whether one calls the internal degrees of freedom extra-dimensional or not, it just seems to bother a few laymen.

2) And here I see the following problems:

• SUSY is elegant - as long as you do not try to break it (OK, this is not a very good point :-)
• SUSY / MSSM does not need string theory (OK, refer to my argument from above: it may be a progress to have MSSM as one solution of string theory instead of just another theory introduced by hand)
• SUSY has to be verified experimentally sooner or later

I think the last bullet point is a serious issue: as long as there is no experimental support, string theory (and even SUSY) is somehow a solution hunting for a problem; and if one does not find SUSY at the LHS one can again say "that one will find it at higher energies"; so in the end it's about a promise again.

Agreed.

Taking all your ideas into consideration my conclusion is that string theory is still in an early state of its development; it makes progress, but many people are uncomfortable with its velocity.

First part: agreed.

Seond part: if people are uncomfortable with the velocity of progress on these very tough problems, they just should get out of their armchairs and do better than the hundreds of smart people who work in the field and who dedicate their life to it; or should those drop their reserach simply because the armchair experts are not content with the progress?

I just want to entrtain this possibility as it eventually dates back to the origin of string theory were one tried to understand hadrons in terms of strings. Assume for a moment that it's true and that somebody finds a proof that some ST is nothing else but a large-N approx to the MSSM.

How could that work? The MSSN has gauge group SU(3)xSU(2)xU(1), so where is an SU(N) for which N->infty...

But more seriously, indeed particle phenomenogists have been playing since long with string-inspired ideas, like strong coupling limits of gauge theories behaving like higher dimensional theories, the renormalization group flows of coupling constants were given a geometric interpretation in terms of trajectories in higher dimensions, etc.

But note that is certainly not so that gravity is the same as gauge theory, they are more like two different facets of the same thing. The duality works by relating different (weak/strong coupling) regimes and depending where you are, one formulation is better than the other; so when you have a gauge theory at strong coupling, the dominant degrees of freedom are string like and one should then use the proper formalism to describe this regime, which is string theory. Calling it "nothing else but a large-N gauge theory" would throw away just the important key feature of this regime...

Asking whether this would "kill" string theory... why do I always sense that this would be somehow desirable? Why are people here so obsessed in playing down the role of string theory? Can't they see what has been achieved... so much has been learned for gauge theory from the string perspective, first on the non-perturbative level, now more recently even the whole perturbative QFT based on Feynman diagrams is about to be rewritten as a consequence of insights originating from string theory ("gravity = Yang-Mills-squared", ie KLT relations etc).

 

[MTd2]

Why are people here so obsessed in playing down the role of string theory?

Because nothing experimental came from it, human ambition is big and scarcity of funding is even bigger. I am not talking about of the scope of the theory, but human ambition. For example, string theory accomplishments inspired many mathematical insights, but you do know they were mostly from topological strings or other unrealistic models. So, people think that why not trying something else, like "my pet model"? Who's to say that not even something non realist from "my pet model" could also yield very deep results too.

You could easily dismiss claims if there were experimental evidence for the theory. But how will you do without any? So people will start fighting against string theory with more and more strength due ambition and seek of funding.

Conclusion: there isn't even a snowball chance in hell that eventually, without experimental results, people won't start to reject string theory. Science pervades everywhere, so skepticism and so ambition.

 

[suprised]

Well string theory is work in progress and what will utimately come out will be seen, so far it has been the driving engine for many ideas in particle physics and many important conceptual insights. If the research program would have been stopped like 10,15 years ago, as many self-declared armchair "experts" have demanded, there would have been a huge damage.

Why do you complain about what other people choose for their research work, based on insight gained by decade-long hard work and experience? So why don't you go just ahead and develop your own pet theory? There is funding for all sorts of crappy stuff, eg Lisi theory, so what prevents you from doing it?

 

[tom.stoer]

How could that work? The MSSN has gauge group SU(3)xSU(2)xU(1), so where is an SU(N) for which N->infty...

In SU(3) = QCD you simply set 1/3 = 0; this is not so bad as it seems :-)

Asking whether this would "kill" string theory... why do I always sense that this would be somehow desirable?

As I said in the very beginning; this discussion is not about killing string theory but it's about focussing at the weak points = learning what COULD kill string theory. I bet you learn most if you focus on the critical issues - reagrdless what happens (regardless if you have a positive or a negative results; think about Bell' theorem, Kochen-Specker, ...)

Why are people here so obsessed in playing down the role of string theory? Can't they see what has been achieved... so much has been learned for gauge theory from the string perspective, first on the non-perturbative level, now more recently even the whole perturbative QFT based on Feynman diagrams is about to be rewritten as a consequence of insights originating from string theory ("gravity = Yang-Mills-squared", ie KLT relations etc).

It is not about playing down string theory. It's about classifying and ranking it.

So I repeat my questions from post #1

• What are the major achievements of string theory?
• Are there predictions subject to (accessable to) experimental verification / falsification both in principle and in practice? Are there physical phenoma which (once observed) would kill string theory?
• Are there predictions specific for the string theory context (nothing that may follow from SUSY as SUSY could be true even w/o string theory)
• What are the short-term / long-term research programs?
• What are the major obstacles inherent to string theory preventing the theory from delivering on its promises?
• What will be the final theory in terms of strings - a theory, or a framework to create theories?

 

[ensabah6]

Well string theory is work in progress and what will utimately come out will be seen, so far it has been the driving engine for many ideas in particle physics and many important conceptual insights. If the research program would have been stopped like 10,15 years ago, as many self-declared armchair "experts" have demanded, there would have been a huge damage.

Why do you complain about what other people choose for their research work, based on insight gained by decade-long hard work and experience? So why don't you go just ahead and develop your own pet theory? There is funding for all sorts of crappy stuff, eg Lisi theory, so what prevents you from doing it?

String theory is crowding out other promising research programs, i.e LQG, and may not be physically correct (i.e 4D, non-SUSY)

 

[atyy]

There are certainly people who work on stuff related to LQG who value string theory.

http://arxiv.org/abs/0705.0705
Non-commutative Renormalization
Vincent Rivasseau:

"In view of these difficulties some physicists have started to openly criticize what they consider a disproportionate amount of intellectual resources devoted to the study of string theory compared to other alternatives [32]. I do not share these critics. I think in particular that string theory has been very successful as a brain storming tool. It has lead already to many spectacular insights into pure mathematics and geometry. But my personal bet would be that if somewhere in the mountains near the Planck scale string theory might be useful, or even correct, we should also search for other complementary and more reliable principles to guide us in the maze of waterways at the entrance of terra incognita. ...

It is a rather natural remark that since gravity alters the very geometry of ordinary space, any quantum theory of gravity should quantize ordinary space, not just the phase space of mechanics, as quantum mechanics does. Hence at some point at or before the Planck scale we should expect the algebra of ordinary coordinates or observables to be generalized to a non commutative algebra. Alain Connes, Michel Dubois-Violette, Ali Chamseddine and others have forcefully advocated that the classical Lagrangian of the current standard model arises much more naturally on simple non-commutative geometries ...

A second line of argument ends at the same conclusion. String theorists realized in the late 90's that NCQFT is an effective theory of strings [34, 35]. ...

These two lines of arguments, starting at both ends of terra incognita converge to the same conclusion: there should be an intermediate regime between QFT and string theory where NCQFT is the right formalism. ..."

 

[suprised]

In SU(3) = QCD you simply set 1/3 = 0; this is not so bad as it seems :-)

That is the case of QCD strings, the original motivation; nothing wrong with that per se, but what does this have to do with unification and gravity?

As for the list of good questions, I will try to answer later, it's too much for the little time I have right now.

 

[MTd2]

Why do you complain about what other people choose for their research work, based on insight gained by decade-long hard work and experience? So why don't you go just ahead and develop your own pet theory? There is funding for all sorts of crappy stuff, eg Lisi theory, so what prevents you from doing it?

Don't say your pet theory, but "my pet theory". I don't have any of my own. I am just curious about things. Maybe one day, not now.

Experience from experiments is own thing, without experiment is another thing and it is something that does not earn respect from people that like science no more than experience with sports or astrology. It is extremely hard to not overlook the tremendous experimental results that brings, for example, absurdly powerful and seamless ever growing progress in computing that give me the possibility to communicate with you and access papers that would otherwise be impossible without being on a university. Compare to the experience in realizing hep theories and that is shameful. Even more without experimental results.

I don't know about the details of funding, but if I were in a committee, I'd rather fund something fashionable and clearly imply that professors should to call crank everyone that thought otherwise. No conspiracy here, just the usual human tendency to bandwagon and protect the back, from all parts.

The problem here is the corrosive effect of skepticism over professional status. There is a status for the rebel, the outcast, and well, what happen to the funding of Lisi is this exception. People exploiting the status of a rebel. But overtime, this will grow and fuel string theory skepticism.

 

[suprised]

String theory is crowding out other promising research programs, i.e LQG, and may not be physically correct (i.e 4D, non-SUSY)

.. simply not true, a lot of people work in this field.

And as I said otherplace, this program remains a smaller blip on the radar screen due to a lack of convincing progress for many years, conceptional foundation, and scope, so that's why it didn't convince the majority of researchers. Take my word, the moment a theory would look really promising, and this not for you but to people who understand things at a deeper level, many people would start working on it. That this didn't happen is not due to sociological reasons, as Smolin & Co try to fabricate, but due to scientific reasons.

 

[ensabah6]

.. simply not true, a lot of people work in this field.

And as I said otherplace, this program remains a smaller blip on the radar screen due to a lack of convincing progress for many years, conceptional foundation, and scope, so that's why it didn't convince the majority of researchers. Take my word, the moment a theory would look really promising, and this not for you but to people who understand things at a deeper level, many people would start working on it. That this didn't happen is not due to sociological reasons, as Smolin & Co try to fabricate, but due to scientific reasons.

Consider how much hype string theory has received from the likes of Kaku, Greene, Hawking, etc., and there's no current evidence for SUSY and higher dimensions. HEP has bet the farm on a highly speculative program. The best universities all have string research groups.

Let's say hypothetically speaking SUSY and extra dimensions and GUT's are unrealized in nature. Nature is 4D without SUSY or GUT. Would it make sense for physicists to continue to pour research effort into strings?

 

[yossell]

That this didn't happen is not due to sociological reasons, as Smolin & Co try to fabricate, but due to scientific reasons.

But these scientific reasons are not empirical reasons, right? I'm not taking sides, but it seems to represent some kind of shift in physics that, after a long period when `shut up and calculate' was the official philosophy, such ``philosophical'' virtues as elegance, simplicity and generality should suddenly count for so much.

 

[atyy]

But these scientific reasons are not empirical reasons, right? I'm not taking sides, but it seems to represent some kind of shift in physics that, after a long period when `shut up and calculate' was the official philosophy, such ``philosophical'' virtues as elegance, simplicity and generality should suddenly count for so much.

If you look at say Smolin's http://arxiv.org/abs/gr-qc/9508064 , where he argues that gravity is not a conventional field theory, this is exactly what AdS/CFT provides - it is a CFT in one dimension less!

Of course, this does not model our universe, but it is the closest anyone has come to quantum gravity.

Also, he says:

"It is then very impressive that there is one context in which this problem has been definitely solved, which is perturbative string theory."

"It is then very interesting that, as was shown by Klebanov and Susskind, continuum string theory can emerge from a lattice field theory in which there is a cutoff in the transverse directions by means of a limit in which the lengths of the strings diverge while the transverse cutoff remains fixed."

"it seems that any acceptable quantum theory of gravity, whatever its ultimate formulation, is likely to reduce to a perturbative string theory in the appropriate limit."

 

[negru]

Why is it so important for the features of string theory (susy, extra dim) to be realized independently in nature? Maybe they could just be internal machinery of the theory? Quantum mechanics uses Hilbert spaces. Have any of you guys ever seen a Hilbert space? Or maybe you could argue that string theory is wrong since it uses the identity 1+2+3+...=-/12, which we all know is false. Leave the possibly internal stuff out of our universe.

If string theory can compute things correctly, that's all we need. If it uses susy, apples, or sand, why does it matter if we don't see those things as we would naively expect? The only question which we need to ask is : can it be used for anything?. And the answer seems yes.And about funding. Who exactly should decide who or what project should get funding (government funding, that is)? This is pretty big problem and it's not limited to string theory/high energy/physics/research in general.

 

[MTd2]

That sum is not wrong:

http://en.wikipedia.org/wiki/Ramanujan_summation

It works because you assume an ordering of operators. Notice that it appears on dimensional regularization after you assume that operators are time ordered.