Strings 2007

Strings 2007 is starting today, and already there seem to be a large number of different laptops connecting to this blog from wlan.uam.es. I’m hoping that some of their owners will write in here with news of how the conference is going. I’ll also try and add links here to any blog and press coverage of the conference that I see or hear about.

Witten’s new 83 page paper entitled Three-Dimensional Gravity Revisited appeared on the arXiv last night and presumably this is the detailed version of what he’ll be talking about in the first of tomorrow’s talks at the conference. It corresponds pretty much to what he talked about here in New York a few weeks ago, which was described here. I assume many of those laptops at the conference are being used to download and read copies of Witten’s paper. At his blog, Clifford Johnson notes that he won’t be at Strings 2007, and hopes that this will ensure that there will be an exciting breakthrough announced there, just like what happened when he decided not to attend Strings 1995.

As I write this, I see that string theorist Jacques Distler is there live-blogging. Here’s the first of his reports.

Update: Some of the slides of the talks are already on-line. The slides for Witten’s talk are available here.

Update: There’s an interesting posting here by Jacques Distler about the Witten talk, and, for those who enjoy such things, quite a rant from Lubos here, prompted by my comment that in this case Witten is investigating quantum gravity using non-perturbative QFT, not strings.

I’ve been reading Witten’s paper a bit more carefully, and it raises all sorts of interesting issues. He makes the point that even in 3d, we really don’t know exactly what “non-perturbative pure quantum gravity” is. He uses the Chern-Simons formulation of 3d gravity in terms of gauge theory to motivate his guess at the correct boundary CFT, and then once he has that he has something much more well-defined to study.

This is a bit reminiscent of the compact, non-gravitational situation. There Chern-Simons theory works fine perturbatively, but to understand the non-perturbative theory one connects it to a CFT on the boundary, in this case the Wess-Zumino model (this is the story that got Witten a Fields medal).

Update: B. Yen has set up a video-blog for Strings 2007, where there will be iTunes podcasts of the conference available.

Update: Latest report from Jacques Distler is that, since Witten’s one, which he got to write about:

there have been some very cool talks

(emphasis in the original), but he can’t tell us even which ones they were since his laptop is malfunctioning. Slides of the talks are available here. I’ve looked through them and, besides Witten’s, don’t see anything I would describe as “very cool”, but maybe that’s just me.

Update: All the talks are on-line now and I just looked through the last of them, and watched the summary talk by Gross. Lisa Randall discussed recent calculations of black-hole production at colliders, with the bottom line being that even in the unlikely event the gravity scale is within reach of the LHC, existing bounds already pretty much rule out the possibility of seeing the kind of dramatic effects from black hole production that have been widely advertised as something that might be seen at the LHC.

Gross noted that the conference was much less mathematical than last year’s, possibly because Yau was not involved in organizing it. He was most enthusiastic about describing the many talks on AdS/CFT, especially the Beisert talk which told about recent progress in getting an exact solution of N-4 SYM. Some talks referred to possible applications of AdS/CFT not just in QCD and heavy-ion physics, but in condensed matter physics (using the duality to get info about relevant CFTs). He told about Polchinski’s speculation that “maybe AdS/CFT will solve high Tc superconductivity”, but dismissed it with “sounds great, but seems unlikely to me.” He dealt with the landscape talks by flashing them by quickly, in a lower and less enthusiastic voice, noting that they made up at least a quarter of the talks at the conference. He dealt similarly with the cosmology/anthropic talks, describing Bousso’s as “an attempt to make the anthropic principle precise if not respectable.”

After the summary, he gave his own take on the state of string theory, saying that one had to be honest about the lack of falsifiable predictions and that now he had a slide headed “The Failures of String Theory”. He continues to feel that the main failure is because we “don’t know what string theory is”, that something is missing, some principle that would pick out not a “vacuum” but a “cosmology”, one perhaps using new ideas about what space and time are. He said he was not too upset by the landscape, because “we don’t know what the rules are” in string theory, so one can’t argue that string theory implies the landscape. He appeared to feel that he is losing the debate, complaining that this used to also be the opinion of his colleagues, but that they were going over to the other side because of the cosmological constant, saying that if another explanation of the CC was found 90% of the anthropicists would come back to his side. He tried to minimize the size of the CC problem, measuring it with respect to a supposed 1 TeV SSYM breaking scale and working in energy, not energy density units, so it is only too small by a factor 1016. He compared this to Dirac’s famous large number problem (which Dirac tried to solve not anthropically, but by time-varying constants, leading to a prediction that was falsified), which was finally “explained” by asymptotic freedom. His message to the anthropocists was “just because you don’t know an explanation doesn’t mean it doesn’t exist”.

Finally he mentioned that Strings 08 will be at CERN, Strings 09 in Rome, and no one has yet agreed to host Strings 10. He argued that the series of Strings conferences “must go on”, because they are “like the canary in the coal mine”, and if they stop that would be a very bad sign for string theory.

I’m curious to know what those in attendance thought of this; it wasn’t exactly a rousingly optimistic portrayal of the state of the subject…

Update: There’s an article in El Pais about the conference and about the state of string theory. My Spanish isn’t perfect, but as far as I can tell the piece was pretty much pure unadulterated hype, of the sort that it is one goal of the Strings XX bashes to generate.

Update: Jacques Distler finally got his computer fixed, and posted about one example of what he considered a “very cool talk” with exciting new ideas. Unfortunately, it seems the ideas are not that new, since a commenter wrote in to his blog to point to papers from four and a half years ago that do pretty much the same thing.

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80 Responses to Strings 2007

  1. ori says:

    I will study the paper, perhaps tomorrow. I don’t wanna give sloppy answers.

  2. amuseds says:

    Meanwhile at the pinnacle of the ‘Superstringy Universe’ edifying discourse on the subject can be found, such as


    …. So PW is basing his monster conjecture on a single footnote! Well, I’ve always been leery of a man with a foot fetish. Such a man reminds me too much of a gay shoe saleman several years back who kept marveling at my feet as though I was Cinderella in the flesh trying on glass slippers. I swear, I recall feeling both deeply annoyed and genuinely embarrassed by the whole ordeal!

    And since then I’ve come to realize that it takes a real man to become captivated by the more noteworthy parts of my body. In a way, I suppose, PW is nothing more than that faggot shoe salesman at Nordstrom’s.

    ….PW’s site would have already petered-out (no pun intended) if it weren’t for a few top contenders, such as Aaron. That being said, I must make mention that Lubos was (and always will be) Peter’s toughest challenger – without having a single close second coming from behind him!

    Now I’m totally clueless when it comes to military matters, but it seems pretty obvious to me that the only sure-fire way to stamp out PW’s site is to battle it out of existence, not to simply let it fizzle out – like an open can of coke. Moreover, I’ll go out on a limb and say that because his site is essentially limited to a single narrow issue, this severe handicap will work to hasten its demise.

    Now back to the discussion of the relevance of gauge theory to quantum gravity…

  3. hmmm says:

    …. So PW is basing his monster conjecture on a single footnote! Well, I’ve always been leery of a man with a foot fetish. Such a man reminds me too much of a gay shoe saleman several years back who kept marveling at my feet as though I was Cinderella in the flesh trying on glass slippers. I swear, I recall feeling both deeply annoyed and genuinely embarrassed by the whole ordeal!

    And since then I’ve come to realize that it takes a real man to become captivated by the more noteworthy parts of my body. In a way, I suppose, PW is nothing more than that faggot shoe salesman at Nordstrom’s.

    ….PW’s site would have already petered-out (no pun intended) if it weren’t for a few top contenders, such as Aaron. That being said, I must make mention that Lubos was (and always will be) Peter’s toughest challenger – without having a single close second coming from behind him!

    Now I’m totally clueless when it comes to military matters, but it seems pretty obvious to me that the only sure-fire way to stamp out PW’s site is to battle it out of existence, not to simply let it fizzle out – like an open can of coke. Moreover, I’ll go out on a limb and say that because his site is essentially limited to a single narrow issue, this severe handicap will work to hasten its demise.

    Whoa!!! who said what now.

  4. Peter Woit says:

    Please, Lubos’s craziness is bad enough, but if you start discussing here the commenters on his blog, that way lies madness…

  5. gunpowder&noodles says:

    Ori declaims:

    “If you had read the paper, ”

    No need to remind us that you’re a string theorist…..

    “you would have understood that the arguments against the gauge theory description are completely general, and can be extended to 4d as well.”

    Wow. Do tell us, in mathematical detail, how to perform that feat. Or at least refer us to EW’s paper. Chapter and verse please. And no “clearly a similar argument” stuff please.

  6. ori says:

    Dear “gunpowder&noodles”,

    Gauge theory decriptions contain in their path integral solutions which correspond to degenerate metrics in the gravitational description. These are not measure zero, as was previously thought (the absence of degenerate metrics gives rise to a different non perturbative complition).

    You may or may not be convinced, it is not important. There will certainly be people who will keep thinking in the gauge theory variables direction although it was proved incorrect for d=2 and d=3. I presume that sensible people understand this point very well and will take it into account.

    The situation has changed, since previously one could argue that the change of vairables is regular apart from a measure zero set of configurations. The same argument was presented for both 3d and 4d (in the 3d case even Witten himself was actually incorrect). Now that this argument is known to be incorrect for 3d and 2d it is most conceivable that the 4d case, which has the same singular locus , and essentially the same ambiguities, should not be described by gauge theory variables

  7. Peter Woit says:

    ori,

    Can you point me to somewhere where anyone has actually done the Chern-Simons integral non-perturbatively (in a non-abelian theory)? As far as I’ve ever been able to tell, this integral is just inherently ill-defined, you’re integrating a phase over an infinite-dimensional space, and there is no “measure” here with any reasonable properties that would allow you to talk about some configurations being “measure zero”. The problem is very fundamental, it exists even in simple QM analogs of the CS theory. If you actually try and discretize the Chern-Simons functional integral and do the the (finite) integral, it appears that there is no way to get an answer which is not completely dependent on how you do the cutoff.

  8. urs says:

    somewhere where anyone has actually done the Chern-Simons integral non-perturbatively

    I this lecture, Mike Hopkins hinted at a way how to do it. But didn’t really explain it.

  9. Peter Woit says:

    urs,

    That’s not at all what I’m talking about, since that kind of very abstract “integral” has no notion of “measure” or “measure zero”. What I don’t believe exists is the kind of standard integral used to define path integral quantization: an honest measure in the cut-off theory that, when you integrate appropriate quantities against it gives well-defined answers when you remove the cut-off.

  10. urs says:

    that kind of very abstract “integral” has no notion of “measure” or “measure zero”.

    Yes, as I said. He hinted at a way how to do it, but didn’t explain it.

  11. Arun says:

    Very brash out of ignorance, I’d say that in 2+1 pure gravity black holes have no way to evaporate (or to form) perturbatively, there being no radiation. So perturbation theory cannot take you from a non-black hole sector of states to a blackhole sector of states. I would guess that in 3+1 pure gravity neither condition holds.

  12. A122505 Arises from energy spectrum of three dimensional gravity with negative cosmological constant, in analysis by Edward Witten.

    n……………a(n)
    –……………—–
    1……………24
    2……………24
    3……………95
    4……………..1
    5………….143
    6……………..1
    7………….262
    8…………-213
    9………….453
    10……….-261
    11……………?
    12……………?
    –……………—–

    Can anyone extend this integer sequence, to be properly credited for said extension in the Online Encyclopedia of Integer Sequences? If so, please have at it…

  13. gunpowder&noodles says:

    PW said: “don’t see anything I would describe as “very cool”, but maybe that’s just me.”

    No doubt you are willing to make an exception for Bousso’s talk. His theory makes “thousands” of predictions. That is so cool that it suffers from frostbite. It’s almost as cool as the funereal garb and manner he affects in his classes. In fact I would put it right up there with Paris Hilton.

  14. anonym says:

    To Ori,

    Really basic question.
    Witten’s work is for AdS background only.
    I think LQG can handle dS case. (I am not an expert on LQG but just curious about your logic.)

    Then how can you say EW’s work rules out LQG?

    I think you can not say anything definite like this one especially when you rely on very incomplete arguments.

  15. ori says:

    Well, dS is argued not to exist alone, but only when there is also some AdS around as well. So, Witten’s work applies. You should think of dS as an unstable particle. Such a system can not be described completely without referring to the correct ground state (which is AdS).

  16. ori says:

    and btw, I don’t know which of the arguments you call “incomplete”. An argument is not a proof certainly. However, it is complete as an argument since it addresses the correct question, and suggests the correct answer convincingly or not, this depends on you (If you only “argue” you can not convince everybody, for this you need a proof. However, wise people often don’t need a proof at all).

  17. hmmm says:

    However, wise people often don’t need a proof at all

    In which case wise people are often wrong and usually less wise than they think.

    Well, dS is argued not to exist alone, but only when there is also some AdS around as well. So, Witten’s work applies. You should think of dS as an unstable particle. Such a system can not be described completely without referring to the correct ground state (which is AdS).

    A wise person should also add that the above statement may have nothing what-so-ever to do with the universe in which we live.

    However, in Witten’s opinion, it could be relevent to all those 2D beings living in rubber sheet world.

  18. Thomas Larsson says:

    Well, dS is argued not to exist alone, but only when there is also some AdS around as well.

    IOW, if observations don’t agree with your pet theory, observations are irrelevant. It might be true in this case, but it is nevertheless a classical crackpot argument.

    If we take the positive cc seriously, it inevitably leads to the conclusion that QG must have local observables, since there are no global observables in dS. The lession from CFT, viewed as diff-invariant field theory on the circle, is that diffeomorphism symmetry is only compabitible with locality in the presence of diff anomalies (multi-dimensional Virasoro algebra).

  19. GH says:

    Jonathan Vos Post asked:
    —————————
    n……………a(n)
    –……………—–
    1……………24
    2……………24
    3……………95
    4……………..1
    5………….143
    6……………..1
    7………….262
    8…………-213
    9………….453
    10……….-261

    Can anyone extend this integer sequence, …?
    —————

    Here:

    739
    -833
    1169
    -1168
    2172
    -2505
    3104
    -3581
    5255
    -6449

  20. anonym says:

    Orid said

    “You should think of dS as an unstable particle. Such a system can not be described completely without referring to the correct ground state (which is AdS).”

    Maybe I don’t understand correctly the meaning that dS corresponds to metastable state. Anyway let me ask it to you. Do you think our universe is metastable? If observation tells me this “according to our present knowledge” then I would think that there is some problem in our understanding rather than think our universe is metastable. Also this is one of what I am saying as “many” incomplete arguments. You seem to overly trust a theory which is at best a consistent approximation to the reality. You should think your present understanding could be very wrong although you may think it is your best at this point.

  21. I do not understand in which sense it can be true that “gauge variables for gravity are bad”, as ori said here:

    http://www.math.columbia.edu/~woit/wordpress/?p=571#comment-26430

    I can imaging that there is some true statement sounding vaguely similar, but as such I am having a hard time making sense of that.

    We are talking about two different sets of coordinates for one and the same configuration space: that of metrics.

    It is true that there are ISO(2,1)-connections which correspond to degenerate pseudo-Riemannian structures.

    But there are also metrics that correspod to degenerate Riemannian structure, clearly.

    I am willing to consider the question whether or not to include degenerate metrics in the path integral. But formulating a metric in any of the many equivalent ways there are can hardly change anything about the physics.

    Unless, of course, it suggests some different procedure for how to go beyond perturbation theory. But all this is shrouded in mystery right at the moment.

    Also, it would seem that those interested in string theory should be fond of the idea of encoding Riemannian structures in terms of ISO(n,1)-connections. As far as I am aware, that is the only sensible way to handle supergravity.

  22. Haelfix says:

    Its definitely extremely puzzling, and also whats so damn interesting about the paper, at least to me.

    Assume for a second you solve both methods exactly. The difference (if indeed there is one after careful analysis) should then sum up to the contribution of degenerate metrics in the path integral *I think*.

    This suggests a strategy for 4D. Use gauge variables, then ansatz the form of the contribution from 2d, 3d and see what we get.

  23. Haelfix says:

    I should say ‘ansatz the form of the degenerate metrics from 2d, 3d intuition’ and then sum them together.

  24. Chris W. says:

    Speaking of taking things seriously, recall this remark of Steven Weinberg’s, in The First Three Minutes (1977):

    “This is often the way it is in physics. Our mistake is not that we take our theories too seriously, but that we do not take them seriously enough. It is always hard to realize that these numbers and equations we play with at our desks have something to do with the real world.”

  25. M says:

    Actually, the article in El Pais mentions the problems of strings: here are the key sentences:

    “… la verdad es que la complejidad de la teoría está resultando enorme. Los mismos especialistas creen que esto se debe a que no es aún una teoría terminada. Por ejemplo, de las diferentes soluciones que encuentran a sus ecuaciones no saben cómo elegir la que realmente corresponde a nuestro universo.

    …no ha podido despegarse de la principal crítica: la falta de experimentos en perspectiva que verifiquen si es correcta. …

    … Para otros muchos, la ausencia de predicciones experimentales de una teoría física después de 20 años de desarrollo, es un inconveniente que no se puede pasar por alto en modo alguno.”

  26. Indrajeet Patil says:

    Is it true that Witten is no longer working in string theory?

  27. Peter Woit says:

    Indrajeet,

    During the last year Witten has written two long papers, neither of them about string theory. I don’t know what he spends his time working on. Besides the topics of the two recent papers (3d gravity and TQGT/geometric Langlands), quite likely he continues to spend some time thinking about string theory.

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