Strings 2008

Strings 2008 starts tomorrow at CERN, with about 400 physicists in attendance. CERN will be providing a live webcast for the rest of us. The timetable of the talks is here. The first afternoon will be devoted not to string theory, but to the LHC.

Those in attendance without their own blogs are encouraged to report on the goings-on by writing comments here when they get bored by the talks. I’ll try and watch some of the talks (or at least look at the slides), and use this posting to write about them.

Update: I’m not likely to be up early enough to catch the morning talks on the webcast, but Lubos is, so you can follow his virtual live-blogging.

Update: Live, the conference seems to be suffering from not always being up to the technology of displaying slides on a Mac to the audience. But slides of the previous talks are now beginning to be available here.

Update: After looking through the slides of the talks and hearing a few of them, the thing that strikes me most about Strings 2008 is how little there has been about strings. Particle theory may be moving to a model where the big annual conference is labeled “STRINGS”, and speakers make nods of respect toward string theory, but actually talk about something else.

The three big hot topics of the conference are

  • the LHC (talks by Evans, Engelen and Buchmuller), which has nothing at all to do with string theory
  • New 3d superconformal quantum field theories (talks by Lambert, Maldacena and Mukhi). One motivation for these is that they can be fit into a pattern of dualities, much like the famous 4d superconformal theories that have dominated particle theory research since Maldacena.
  • Scattering amplitudes, especially those of N=4 SYM and N=8 supergravity (talks by Veneziano, Kallosh, Dixon, Cachazo, Green, Sokatchev and, to come, Alday). Some of this looks a lot like particle physics from the mid-60s, based on the study of the analytic S-matrix, including the presence of Veneziano. While there has been a lot of progress in studying certain kinds of QFT S-matrix amplitudes in recent years, some of it coming out of string theory, the most dramatic news is that about the possible finiteness of perturbative N=8 supergravity. Remember all those talks you’ve heard where someone draws a Feynman diagram and a string diagram, then explains how this shows that perturbative QFT has deadly divergence problems due to point-like interactions, while perturbative string theory doesn’t? Well, it appears that you can forget about all that now. In a rear-guard action, some speakers point out that you need to understand non-perturbative N=8 supergravity, and maybe this can’t be done in a QFT context. Unclear why non-perturbative string theory is supposed to help here, since the only viable non-perturbative version of it is, by duality, a QFT itself…
  • Looking at the talks that actually are about string theory unification, you quickly see why most people are talking about something else. Ibanez starts off by asking whether string theory makes physical predictions, then claims that it does, with one of them being exactly the reason it doesn’t make predictions about physics: “There is a large landscape of string vacuum solutions…”, which he then goes on to describe. Donagi’s talk, about Heterotic Standard Models, was remarkable in how much the situation there hasn’t changed since 1985. You can come up with such models with the right quantum numbers (and actually, just about any quantum numbers you want…), but to get anything else, you have to address how to stabilize moduli and break supersymmetry, and Donagi just mentions these problems at the end as tasks to address in the future. For more about the F-theory-motivated models reported on, see the comments at this posting, where “anonymous” has an informed discussion with him (or her)self.

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    24 Responses to Strings 2008

    1. Thomas R Love says:

      Clicking the link to Motl’s site, one is subject to a rant which includes
      this:

      “Virtually everything written on the blog you visited a minute ago is nonsense and virtually all contributors are crackpots. The owner of the blog, Peter Woit, a computer administrator and a lecturer in discipline, is trying to revenge to the high-energy physics community for his inability to become a physicist himself.

      You can check that during the last two decades, Peter Woit only wrote 3 rants against string theory and one unpublishable, 0-citation demonization of spinors in quantum field theory, earning less than 10 suspicious citations in total. For comparison, Edward Witten wrote nearly 200 articles during the same period, earning almost 40,000 citations from them. Peter Woit is not a scientist in any sense; he is just an activist. ”

      First, your Columbia webpage does say you are a lecturer in discipline, it should be Lecturer in Mathematics, shouldn’t it?

      Secondly, Motl has confused quantity with quality. The number of articles written and the number of citations has nothing to do with the quality of the work.

      Third, why do you provide a link to a site which says you (and I) are crackpots?

    2. Peter Woit says:

      Thomas,

      I should update my web-page. For one thing, I’ve been promoted, and my title is now “Senior Lecturer” (the “in Discipline” part isn’t very meaningful, the discipline in question is mathematics).

      The links to Lubos’s blog are there when there’s something interesting on his blog. The ad hominem attacks he puts out on anyone who criticizes string theory I think speak for themselves. He’s done (and continues to do) more damage to the public perception of string theory than I ever will.

      Following his stream of consciousness about the talks is actually pretty fascinating. On the one hand, he’s giving an extremely competent and expert commentary of exactly what the conventional wisdom is at the top levels of the string theory establishment about currently fashionable research. On the other, he’s clearly a complete fanatic and out of his mind. Quite an amazing combination to watch…

    3. PAMELA says:

      An interesting result was presented at another conference, about Dark Matter in Stockholm. Preliminary data from the PAMELA experiment show an excess in the energy spectrum of astrophysical positrons. If this will turn out to be the first observation of Dark Matter annihilations, the authors will have to come back to Stockholm for the Nobel prize.

    4. Peter Woit says:

      Thanks PAMELA,

      I saw a mysterious rumor about this last week. If anyone knows of anywhere there’s slides or some details of this, please let us know.

    5. DB says:

      I imagine this is the talk that has got the rumour mill running:

      http://agenda.albanova.se/contributionDisplay.py?contribId=389&sessionId=257&confId=355

      By the way, congrats on the promotion Peter.

    6. anon. says:

      The PAMELA slide was shown at ICHEP, but they’re publishing in Nature (*exasperated sigh*) and consequently there’s an embargo on actually releasing the plot in advance of publication. So as far as I know it’s not available anywhere online; you have to talk to someone who saw it. There is a Nature News story about it, complete with crackpot comments.

    7. Shantanu says:

      There have been many such claims for “excess” in such
      indirect dark matter detection experiments, but none of them
      are smoking gun. I guess we will have to wait until the paper is out.
      Peter : congrats from me too.

    8. Click says:

      “After looking through the slides of the talks and hearing a few of them, the thing that strikes me most about Strings 2008 is how little there has been about strings.”

      Peter,

      This is not surprising at all as a lot of people are focused as the most expensive experiments in the history of science is about to be conducted. As far as I see the underlying motivation has come from string theory. I am constantly surprised why you hate the word “string theory” so much.

      However, I would like to thank you for putting good informative links on math and physics.

    9. Peter Woit says:

      Click,

      Despite what you have heard, the LHC has nothing to do with string theory. String theory is not the underlying motivation for the experiments at the LHC.

    10. PAMELA says:

      indeed there is an embargo: the result of PAMELA was flashed during conferences but the slides cannot be published on-line. I don’t know if a photo of the result taken during the talk would violate the embargo.

    11. nbutsomebody says:

      Point about N=8 super-gravity is indeed very important. If we can have a finite theory of gravity, then who needs string theory ?

    12. Per says:

      Matthias Staudacher just gave a talk about the spectral problem of AdS / CFT. With the help of integrability, it is quite amazing how close this problem is to a complete solution.

      This is actually something which is quite outstanding, and I sometimes get the feeling that you neglect this in your critique (which, in my opinion is often well based) against string theory. If you compare to any research in theoretical particle physics at the moment, would you say anything is as interesting as this? My vote would be that it ties with the N=8 stuff that’s going on at the moment (however, if I were to make a bet it would be against N=8 working out and for AdS / CFT being solved (in the planar limit)).

      Best wishes, P

    13. Daniel de França MTd2 says:

      Any positive reaction to Carlo Rovelli talk? According to Lubos, he was almost ignored.

    14. Peter Woit says:

      Per,

      There certainly has been real progress in the AdS/CFT calculations you mention, and I wouldn’t be surprised if sooner or later a complete understanding of the planar limit was achieved, which would be great.

      The problem though is that this doesn’t really change much, since everyone has been assuming that this works for 10 years now. The N=8 stuff is much more exciting, partly because it is much newer and much more poorly understood. If N=8 finiteness is true, it opens up all sorts of possibilities that had been thought to have been closed off, and these are possibilities relevant not just to solving QCD, but to unifying physics.

      Daniel,

      Lubos was just watching the webcast, I doubt that from that he could tell much about what the 400 people in the audience thought. Presumably some had the same juvenile thoughts as him, others may have been more interested. As string theorists are careful to point out, there’s a wide range of attitudes within their community, with Lubos just one extreme point.

    15. Daniel de França MTd2 says:

      Peter,

      I know that. But did you, or anyone else too, see the webcast? His coverage of the string subjects was excelent, but not about this one… So, it’s like I missed his talk…

    16. Peter Woit says:

      Daniel,

      I just caught the first few minutes of Rovelli’s talk, then had to leave. Maybe if one saw the question section one might have gotten more of an impression of the reaction of the audience, but I kind of doubt one could really tell much from the webcast.

    17. Per says:

      The talk by Rovelli was one of the talks that received most questions after wards. My impression was that it was very well received and the audience was genuinely interested. After the talk a rather big group assembled around Rovelli and asked him questions privately. This went on for quite some time if I am not mistaken. As part of the audience, it was very pleasing to see that none of the string / anti-string aggressiveness was there.

    18. Daniel de França MTd2 says:

      Peter,

      Renata Kallosh provided a proof for N=8 renormalizabily, given that there is no anomaly in the expansion. She published it in the same day of her talk:

      http://arxiv.org/PS_cache/arxiv/pdf/0808/0808.2310v1.pdf

      Do you agree with her? Also, do you agree with the argument that for d>3, it must be tied to strings? Why(not)?

    19. I just caught the first few minutes of Rovelli’s talk, then had to leave. Maybe if one saw the question section one might have gotten more of an impression of the reaction of the audience, but I kind of doubt one could really tell much from the webcast.

      One question was if the LQG methods had been applied to 3d gravity and results compared with the information one has about that, including Witten’s latest connection to 2d CFT.

      (Answer: unfortunately not.)

      One question was how one can talk in LQG about black hole entropy without being able to talk about black holes, due to a lack of semi-classical limit.

      (Answer: there are two different ways to define a horizon, globally and locally.)

      One question was if the advertized “loop quantum cosmology” is really be derived from LQG.

      (Answer: no, it is just quantization of cosmological models with some LQG inspired modifications.)

      One question was if the problem with the Immirzi-parameter has been resolved, since two different arguments seem to require two different values.

      (I think as a reply Rovelli reviewed what the Immirzi-parameter is.)

      My impression from watching the webcast of talk and question session: the audience was not ignorant about LQG and might have apprectiated a less introductory talk addressing more of the technical issues. It remains a bit frustrating to see Rovelli using up so much time to explain the bare idea of a “spin network” to an audience that is familiar with the concept of Wilson line and non-perturbative gauge theory on the lattice.

      Generally I think: everybody would be happy if a non-perturbative description of gravity using a parameterization of configuration space by Wilson loop observables were available. Lots of stringy work in supergravity is routinely using first-order Lagrangians that encode the Einstein-Hilbert action in terms of a connection. If there is disagreement about LQG, it seems it is not so much about this premise but about technical issues that follow.

    20. Thomas Larsson says:

      “including Witten’s latest connection to 2d CFT. ”

      But that was wrong, wasn’t it?

    21. Chris Austin says:

      Hi Daniel,

      “Renata Kallosh provided a proof for N=8 renormalizability … Do you agree with her?”

      The claim was actually of UV finiteness, i.e. the cancellation of all UV divergences, rather than renormalizability, which means that UV divergences can be absorbed in infinite redefinitions of coupling constants.

      It seems to me that there might be a problem with Kallosh’s argument, which was based on finding an inconsistency, at the linearized level, between counterterms in Lorentz-covariant gauges and light cone gauge, because the corresponding counterterms appear to be consistent between Lorentz-covariant gauges and light-cone gauge in d = 11, and we would therefore expect the same to be true after dimensional reduction to d = 4.

      Specifically we are considering local quartic vertices that are super-Poincare covariant at the linearized level, and in 11 dimensions these were constructed in Lorentz-covariant gauges by Deser and Seminara, in http://arxiv.org/abs/hep-th/9812136 and http://arxiv.org/abs/hep-th/0002241, and in light-cone gauge by Metsaev, in http://arxiv.org/abs/hep-th/0410239.

      Metsaev uses a formalism in eleven dimensions where only so(7) invariance is manifest, so as to have a completely unconstrained d = 11 superfield based on an 8 component so(7) spinor, but on dimensional reduction to d = 4, choosing the R and L transverse dimensions to be among the compact dimensions, this should reduce to the Brink, Kim, and Ramond formalism of Kallosh’s eqn. (1.5). In fact the superfields in Metsaev’s eqn. (2.16) and Kallosh’s eqn. (1.5) exactly match term by term, including the powers of p^+, if we reverse the order of the terms in comparing the two papers, and note that Metsaev uses the notation \beta for p^+, see eqn. (2.17).

      Metsaev explicitly constructs all the Poincare generators to verify Poincare invariance in d = 11, which should imply Poincare invariance in d = 4, and also finds, in eqn. (5.29) on page 23, the most general local quartic vertex that is super-Poincare covariant at the linearized level, and thus should contain terms built from four Riemann tensors, with possible covariant derivatives acting on them. This should reduce in four dimensions to counterterms such as Kallosh’s eqns. (3.3) and (3.6).

      Metsaev’s light-cone gauge result for the most general local quartic vertex that is super-Poincare covariant at the linearized level, namely that it is obtained from the leading one, that contains the t_8 t_8 R^4 term built from four Riemann tensors, by applying derivatives corresponding to the most general non-vanishing symmetric polynomial built from the Mandelstam invariants, see e.g. Metsaev’s formula (5.34), on page 24, is consistent with Deser and Seminara’s manifestly Lorentz-covariant construction, and shows that Deser and Seminara obtained the most general such vertex.

      To try to pin down further why Kallosh found a Lorentz-non-covariant numerator factor (p^+)^4 from her counterterm eqn. (3.6), we note that Metsaev’s local formula (5.29) involves, in effect, four powers of p^+ in the denominator, since \beta_{13} means \beta_1 + \beta_3, from (5.4) on page 21, and \beta_a means p^+ for the a’th leg of the vertex, see e.g. the footnote on page 3, or (2.17) on page 8, or (2.26) on page 9, and the top of page 12. Furthermore, there is nothing hidden in the other factors in (5.29) to cancel these overall denominator powers of p^+, and when we combine with the superfields and the measure factors as in (5.1), on page 20, taking the superfield and measure factors from (3.6) to (3.9) on page 11, to obtain the formula which on dimensional reduction to d = 4 should contain Kallosh’s eqn. (3.6), we see that Metsaev’s formula contains four overall powers of p^+ in the denominator, which it would appear could cancel the four numerator powers of p^+ found by Kallosh.

      Best regards,
      Chris

    22. Jeff McGowan says:

      OK, no doubt silly question, but I’m a mathematician, last physics classes I took were GR and relativistic QM around 1984. Is the excitement about the finiteness of N=8 supergravity simply because there is then ONE known finite theory? Assumption then being that if one theory is finite, others which might actually be relevant to the real world (whatever that might be :-) might also be finite?

      Don’t hold it against me that I study Riemann surfaces, really, I don’t have anything to do with strings (although I do remember a talk at a conference maybe 15 years ago where the number 26 kept coming up, and one of the questions at the end was “is that the 26 dimensions from string theory?”)

    23. Peter Woit says:

      Jeff,

      One reason for the excitement is that this indicates there is some new symmetry or important unexplained feature of this theory. If one can figure out what it is, the implications could be very important. Whatever it is, it seems to get around what was always considered a fatal disease of the theory.

      The fact that this destroys the main argument always used to justify the idea that to do quantum gravity one must abandon QFT and do string theory is one that string theorists are doing their best to ignore.

    24. Jeff McGowan says:

      Peter,

      Thanks. I have some sense of the issues in QFT with infinities, and that this finiteness might give physicists some hope that there might be a way around that. I have the mathematicians caution about generalizing from cases (and about renormalization, I know it seemingly works quite well thank you), but appreciate that it is always nice when you can find a case that works with the hope that it gives you an idea about how to handle the bad case. My joke (lame as it no doubt was) about Riemann surfaces and 26 dimensional string theory, was meant to indicate that I have always had issues with physical theories that are too “flexible” (26 dimensions, no 10, no 12, no 11….).

      Interestingly (to me at least), the issues between physics and math which somehow seem embodied in the string theory mess are just silly. Physics and math have always been good for each other, in both directions. I’m not so sure how much really interesting math there is in string theory, although there is clearly some. I owe a very useful part of my doctoral education to a physicist, my orals were explaining Witten’s beautiful JDG paper on Morse theory.

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