Recent Hot Topics in Hep-th

SLAC’s SPIRES database has a link you can use to search for articles heavily cited during 2009 and 2010. Just looking at the hep-th papers, three are review articles of older work on applying AdS/CFT to condensed matter physics, three are about Erik Verlinde’s claims that gravity is an “entropic force”, and the rest are about Petr Horava’s non-relativistic theory of quantum gravity.

The story of hep-th in 2009/10 seems to be that the only new ideas getting attention are ones coming from prominent string theorists who have become apostates advocating non-string theory approaches to quantum gravity. The idea of getting gravity out of simple thermodynamics didn’t get much attention back in 1995 when Ted Jacobson was discussing it, partly because it didn’t seem to go anywhere, partly because the conventional wisdom was that the spin-two massless mode of a string was the reason for gravity. Now that, fifteen years later, a prominent string theorist is promoting the idea (see his recent Harvard colloquium on the topic here), it is getting a lot of attention.

Those abandoning string theory as an explanation for quantum gravity do need to be careful in how they describe what they are doing; see for example the first part of the most heavily cited hep-th paper of 2009 (Horava’s), which begins as follows:

In recent decades, string theory has become the dominant paradigm for addressing questions of quantum gravity. There are many indications suggesting that string theory is sufficiently rich to contain the answers to many puzzles, such as the information paradox or the statistical interpretation of black hole entropy. Yet, string theory is also a rather large theory, possibly with a huge landscape of vacua, each of which leads to a scenario for the history of the universe which may or may not resemble ours. Given this richness of string theory, it might even be logical to adopt the perspective in which string theory is not a candidate for a unique theory of the universe, but represents instead a natural extension and logical completion of quantum field theory. In this picture, string theory would be viewed—just as quantum field theory—as a powerful technological framework, and not as a single theory.

If string theory is such an apparently vast structure, it seems natural to ask whether quantum gravitational phenomena in 3 +1 spacetime dimensions can be studied in a self-contained manner in a ‘‘smaller’’ framework. A useful example of such a phenomenon is given by Yang-Mills gauge theories in 3 + 1 dimensions. While string theory is clearly a powerful technique for studying properties of Yang-Mills theories, their embedding into string theory is not required for their completeness: In 3 + 1 dimensions, they are UV complete in the framework of quantum field theory.

In analogy with Yang-Mills, we are motivated to look for a ‘‘small’’ theory of quantum gravity in 3 + 1 dimensions, decoupled from strings.

Update: A commenter points out that Verlinde has just received a 2 million euro grant to support this kind of research, more info available here.

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18 Responses to Recent Hot Topics in Hep-th

  1. Anonymous says:

    What a great analogy of Yang-Mills! 🙂

    Entropic gravity and Lifshitz gravity indicate that the subfield known as quantum gravity may enter a “model-building” era, as opposed to “formal” approaches like superstring theory. Yet another possibility is that with continued frustration, this subfield will shrink and become non-mainstream, much like how so few people are working on the million-dollar Yang-Mills existence problem because they don’t see any hope of success.

  2. anon. says:

    It’s a little odd to call these “hot topics”. I think it’s more that they’re relatively non-technical ideas that are easy to work on, so they’ve been latched onto by not-very-competent people mostly in countries where publishing a large number of papers is required for employment but publishing quality papers is not. Horava-Lifshitz gravity, in particular, seems to be looked askance at by most good people. This isn’t because of prejudice against non-stringy ideas but because the field has already had a few iterations of clever modified gravity ideas and, through painful trial and error, learned the lesson that they almost all fail in a universal way. So few people were surprised when it turned out that H-L gravity has the usual problem. More recently people (including those who successfully found the expected problem) have claimed to modify it in ways that make it sensible, at least at a linearized level. I would bet that the problems are still lurking in the background, waiting for someone to do a more careful analysis. If not, then I think if anyone really convincingly shows they are absent, the field would pay attention.

    My assessment would be that there are no hot topics at the moment. The only recent big advances of high quality are being made in technical areas with a high barrier to entry, so they’re not picking up huge citation counts. In the hep-ph world, driven by experimental results, light (5 to 10 GeV-ish) dark matter is trending toward being a hot topic.

  3. Anonymous says:

    The only recent big advances of high quality are being made in technical areas with a high barrier to entry, so they’re not picking up huge citation counts.

    What’s your list, or a few examples, of these big advances?

  4. a says:

    What about and ?

    Somehow you don’t mention these two recent developments, quite convenient indeed…

  5. Peter Woit says:


    The reason is simply that they don’t appear in the SLAC recent topcites list I linked to. The Horava paper, of similar vintage, has about four times as many citations.

  6. Marcus says:

    There could be some problem with the Spires search because you asked for 100+ topicites with date = 2009. And the Alday et al paper mentioned by “a” has 111 cites (according to Spires.)

    Since Spires knows that got 111 cites, why would it not have included it on the list? Looks like a bug in the search engine to me. (Certainly not an intentional omission on anyone’s part.)

  7. Marcus says:

    The other 2009 paper that “a” mentioned is this:
    and it did not meet the search requirements, so there was no error.

    You asked that 2009 papers have 100+ cites and this one has 97.

    What puzzles me is
    with 111 cites.
    It fits the terms you defined but did not appear on the list.

  8. Marcus says:

    There appears to be something wrong with the record for that paper.
    When I tell Spires
    then it finds the paper. (but says it is 50+ cites when it should say 100+)

    On the other hand when I tell Spires
    it does not find the paper at all.

    Somebody should tell Alday about this so that he can write to Spires and get the record of his paper corrected.

    Everybody benefits from the reliable operation of a database like that so we should thank “a” despite his sarcastic and suspicious tone of comment, for revealing the flaw 😀

  9. Bee says:

    I don’t think the reason Verlinde’s paper got so much attention is that he’s a string theorist. It’s just that he’s formulated his claim by using only a handful of simple equations. Or possibly because people had nothing better to do.

  10. Peter Woit says:


    There are other examples easily found of papers with a bit more than 100 citations that are topcite 50, not topcite 100. I suspect that this is because they only periodically go through the whole database to update the topcite status. If so, this just means that for a 2009 paper to make the SLAC recent topcites list, it needs not just more than 100 citations now, but this had to be true a few weeks ago.

  11. anon. says:

    Anonymous asked:

    What’s your list, or a few examples, of these big advances?

    Definitely the things “a” linked to: Alday/Gaiotto/Tachikawa (which I’m a little surprised has 100+ citations, but good for them!) and integrability. Plus other work of Gaiotto on N=2 supersymmetric field theories. Also progress in relating scattering amplitudes with Wilson loops, Grassmannian representations of N=4 amplitudes, continued advances in using twistors and momentum twistors, improved understanding of consistency conditions in supergravity… Probably there’s a lot more that isn’t coming to mind, since I’m not actively involved in any of these things. I would say there’s been steady and significant progress in technical aspects of understanding quantum field theories, especially highly supersymmetric ones, for the last few years.

  12. BS says:

    Hi Peter, have you heard of the ERC grant for Erik Verlinde? This is quite a big success for him and the University of Amsterdam. Although I was very surprised to see that the research will be about emergent gravity.

  13. Peter Woit says:

    Thanks BS. I had seen from Verlinde’s twitter feed that he got the grant, but had assumed it was one or two orders of magnitude smaller in size. $3 million or so is a huge amount for a grant of this sort. So, “entropic gravity” is now not only a hot-topic, but an extremely well-funded one….

  14. Vince says:

    Has anyone read this paper:

    If there’s nothing wrong with it, why is entropic gravity being funded?

  15. somebody says:

    Entropic gravity is really nonsense. I have tried to read the paper giving Verlinde the maximum leverage in what he *could have* meant, and I still can’t come up with anything useful. Everything that was worth saying there was already said by Jacobson long ago. (In my opinion, of course!)

    On the other hand, Horava’s theory, you have to give him credit for trying something new… even if it is also likely to be unsalvageably wrong.

  16. Kea says:

    And will Verlinde give some of the money to all the starving people who have been working on related ideas for decades?

  17. Mitchell Porter says:

    In Erik Verlinde’s recent talk at KITP, starting at 31 minutes, he switches to a non-entropic perspective involving individual pure states rather than ensembles of them. Listen especially to the start of the 37th minute, where he says that under the right conditions, ergodic motion in a phase space would be macroscopically indistinguishable from a genuine thermal state. So it looks as if he’s moved to a deeper perspective that might evade the Motl-Kobakhidze argument.

  18. MathPhys says:

    “And will Verlinde give some of the money to all the starving people who have been working on related ideas for decades?”

    When I read what Padmanabhan writes on the topic, I feel that there is probably a point to it.

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