# Why I am REALLY disappointed about string theory

by tom.stoer
Tags: disappointed, string, theory
P: 308
 Quote by tom.stoer 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")
 Sci Advisor P: 5,366 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.
P: 8,393
 Quote by tom.stoer 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!
P: 407
 Quote by tom.stoer 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.

 Quote by tom.stoer 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.

 Quote by tom.stoer 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?

 Quote by tom.stoer 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).
PF Gold
P: 1,960
 Quote by suprised 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.
 P: 407 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?
P: 5,366
 Quote by suprised 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 :-)

 Quote by suprised 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, ...)

 Quote by suprised 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?
P: 716
 Quote by 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?
String theory is crowding out other promising research programs, i.e LQG, and may not be physically correct (i.e 4D, non-SUSY)
 Sci Advisor P: 8,393 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. ..."
P: 407
 Quote by tom.stoer 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.
PF Gold
P: 1,960
 Quote by suprised 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.
P: 407
 Quote by ensabah6 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.
P: 716
 Quote by suprised .. 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?
P: 341
 Quote by suprised 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.
P: 8,393
 Quote by yossell 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."
 P: 308 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.
 PF Gold P: 1,960 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.
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