
#19
Aug210, 01:37 AM

P: 308

There are many reasons to believe that ads/cft can be extended to nonsusy, 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/grqc/pdf/0602/0602037v3.pdf (might want to skip to page 8 to "Lessons, generalizations, and open questions") 



#20
Aug210, 02:05 AM

Sci Advisor
P: 5,307

No, I am referring to ordinary QCD in the largeN 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 lowenergy limit of a fundamental string theory, string theory itself is only a (largeN) 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 largeN approx to the MSSM. 



#21
Aug210, 07:17 AM

Sci Advisor
P: 8,005

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! 



#22
Aug210, 08:28 AM

P: 407

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? But more seriously, indeed particle phenomenogists have been playing since long with stringinspired 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 largeN 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 nonperturbative 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 = YangMillssquared", ie KLT relations etc). 



#23
Aug210, 09:55 AM

P: 1,923

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. 



#24
Aug210, 10:06 AM

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 selfdeclared 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 decadelong 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? 



#25
Aug210, 10:08 AM

Sci Advisor
P: 5,307

So I repeat my questions from post #1




#26
Aug210, 10:11 AM

P: 716





#27
Aug210, 10:16 AM

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P: 8,005

There are certainly people who work on stuff related to LQG who value string theory.
http://arxiv.org/abs/0705.0705 Noncommutative 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 DuboisViolette, Ali Chamseddine and others have forcefully advocated that the classical Lagrangian of the current standard model arises much more naturally on simple noncommutative 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. ..." 



#28
Aug210, 10:25 AM

P: 407

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. 



#29
Aug210, 10:28 AM

P: 1,923

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. 



#30
Aug210, 10:33 AM

P: 407

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. 



#31
Aug210, 10:42 AM

P: 716

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? 



#32
Aug210, 11:00 AM

P: 341





#33
Aug210, 11:20 AM

Sci Advisor
P: 8,005

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." 



#34
Aug210, 12:02 PM

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. 



#35
Aug210, 12:12 PM

P: 1,923

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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