String Theory Visions

Strings 2014 ended today, with five separate “vision” talks, giving a good picture of where the leaders of the string theory community see the subject going. I saw the talks on streaming video, presumably they should appear on the conference website in the next few days.

  • Michael Green led off by noting that string theory had at one point been intended as a unified theory, but now has “blossomed into something much more significant”, a framework covering all sorts of things. He went on to say that he would avoid discussing the grand questions, and instead just give a summary of what he found interesting about work reported at the conference. The main area that he covered in the talk were the many different kinds of results about scattering amplitudes that are under study. He seemed to be somewhat of a skeptic about Arkani-Hamed’s widely publicized “Amplitudehedron”, saying it had some ways to go before it was useful for computations (unlike other methods).
  • Juan Maldacena gave a talk that had nothing to do with string theory, mainly about the conceptual issues of black holes and quantum theory. He claimed that the BICEP2 results showed that quantum gravity was now an experimentally-based subject, answering those who were skeptical about studying an untestable subject.

    At the end of the talk he gave an answer to the question “What is String Theory?”:

    Solid Theoretical Research In Natural Geometric Structures

  • Andy Strominger gave his own answer to the “What is String Theory?” question:

    anything that anybody in this room or any of their friends has ever worked on.

    He did note that there were hardly any strings anywhere to be seen at recent string theory conference talks.

    About the LHC and any conceivable follow-on higher energy accelerator, his comment was that it was now highly unlikely that string theory could make predictions relevant to them, and that he didn’t want this to be a defining goal of the field. Clearly, the failure to find SUSY at the LHC has now pretty much killed off most hopes that string theory unification is relevant to particle physics in any testable way. Like Maldacena, he pointed to BICEP2 as showing that quantum gravity was now an experimental subject.

    He ended by explaining that he had sent around emails to a hundred people asking for their suggestions about what problems there would be progress on over the next 5-10 years. He got 80 responses, and quickly put up some slides with them. No time to really read these, but he says that they’ll be posted online soon, and that should be a quite interesting document

  • The last talk was by David Gross, who pointed out that he has given many of these things before. He then went on to discuss Paul Steinhardt’s impassioned talk earlier in the week, which included a video of Richard Feynman discussing “Cargo Cult Science” and theories that were too vague to be testable. Steinhardt had been arguing that inflation was so vague and flexible a theory that it could not be tested and so was not science. Gross like many others realized that you could replace “inflation” by “string theory” in Steinhardt’s argument. He then gave a long and very defensive discussion of why string theory might still be science, invoking the recent book by Richard Dawid and telling the audience they needed to read it so they could defend their subject against the accusations it is facing.

    I wrote here last year about the Dawid book, including an explanation of Dawid’s three main arguments for string theory. Gross went through these in detail, and I think what I wrote last year also responds to what Gross has to say. He did include in the “Meta-Inductive Argument” the argument that SUSY is a related research program to string theory and that it will be vindicated at the LHC in the next few years. It will be interesting to hear what he has to say at a future Strings 20XX after this hasn’t worked out. He announced that Strings 2015 will be in Bangalore, Strings 2016 in Tsinghua, Strings 2017 in Israel, Strings 2018 in Japan and Strings 2019 in Belgium.

    He explicitly addressed the fact that many in the field were experiencing depression and anxiety due to things not working out, pointing out that even if the first derivative of progress in a field is negative, there can be jump discontinuities. So, although things don’t look good, maybe a big piece of progress will come along out of nowhere.

There was one more vision talk, but I’ll discuss that in a separate posting.

Update: Strominger’s slides are available here, and include the 80 responses he got from others about the open problems of the field.

Update: Videos of the talks are now available.

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18 Responses to String Theory Visions

  1. srp says:

    The discussion here about the LHC, SUSY, and strings reminds me of the argument about BICEP2, gravitational waves, and inflation. Absence of SUSY at LHC is taken here as a blow to strings, but finding SUSY wouldn’t have been seen here as supportive of string theory unification. So the asymmetry is similar to Linde’s attitude that it would have been a “smoking gun” for inflation had gravitational waves been found but no crushing blow had it not.

  2. Peter Woit says:

    I’m not sure what you mean. The prospect of SUSY at the LHC was always advertised as the main prospect of experimental vindication of the string theory picture, and the failure of it to show up is behind both the “depression and anxiety” Gross describes, as well as the disappearance of mention of the LHC from the conference.

    Gross and some others are still holding out for vindication from LHC Run II, but I think most string theorists are now agreeing with Strominger. Once you give up on experimental HEP as having any relevance to what you do though, you have to face the fact that other physicists are going to now be skeptical about whether you’re still doing science. Both Strominger and Maldacena tried to address that by invoking BICEP2, but that comes with its own problems. This concern was also clear in Gross’s talk.

  3. srp says:

    Please allow me to clarify. I’m not concerned with who advertised what in advance, but with the logical structure of the inferences being made. In this case I agree with your position, but it is inconsistent with how you approached the BICEP2 affair.

    In the “Smoking Gun” thread, a bunch of people (including me) argued that it was not inconsistent to see the BICEP2 finding of gravitational waves as a “smoking gun” for inflation if true but not a showstopper for inflation if false. In other words, we believe that this kind of asymmetry of inference based on a particular piece of evidence may be completely rational. One commenter drew an analogy with discovering life on another planet–that would be a smoking gun for the non-uniqueness of terrestrial life, but failing to find life on that particular planet wouldn’t measurably change your prior belief in extra-terrestrial life existing somewhere.

    Then this post made me realize that you were arguing (implicitly) for the same kind of asymmetry in the case of SUSY and the LHC. A positive finding of SUSY would NOT have led you to accept string theory unification as true, but the negative findings to date DO lead you to significantly decrement your probability that it is the right theory. I agree with this stance and simply point out that it is logically the same kind of asymmetry that you criticized in the case of inflation. Of course you may disagree with this assessment, but perhaps my contention is now clear.

  4. Peter Woit says:

    Actually a discovery of SUSY would have led me to dramatically revise my views, whereas what happened didn’t because it was what I expected. But this is really off topic, enough.

  5. Claudine Gérard says:

    Dear Peter,

    looking through the slides of the conference and your summary of the vision talks I am doubly puzzled. First, as you said, only few talks are about strings and membranes. But on top of that, no talk is really about unification itself! Even in the vision talks, the speakers avoid the issue. I am puzzled that string researchers seem not to pursue unification any more. Not giving a public talk is surely a natural consequence of this change. But what then is the reason for this conference?

    It sure looks as if a group of smart people heard, about 30 years ago, some voice shouting “Unification is in that green pasture over there!” They all run there, ploughed the field, found nothing, and now are saying to each other: “But we are in a beautiful field, aren’t we?” This might well be, but why are they there in the first place?

    Isn’t this avoidance to even talk about of unification – see how openly Green and Maldacena avoid it – the real proof that string and M theory has failed? Peter, we do not need you any more to point out the failure of string theory: the field’s elders do it all by themselves.

    If the conference invites five people to give a vision talk, and nobody of these five people talks about prospects for unification, then something is wrong. Deeply wrong.

    Peter, allow me a conjecture. In theoretical particle physics, it has become common to label people who work on unification as foolhardy or presumptuous. The conjecture is that string theorists now do it among themselves. It seem that they do not dare to even think about unification any more. Is there any hope for change?

  6. Peter Woit says:

    Strominger did address the unification issue, basically saying that he thought it didn’t work: he thinks there is no way string theory is going to say anything about particle physics (as opposed to quantum gravity).

    While some string theorists still work on unification, they generally weren’t invited to talk at this conference. I think that’s not because they’re perceived as foolhardy by the organizers, but because the organizers are aware those ideas are not working. Why invite someone to talk about a very old research program that has been shown not to work, unless they have found a way to get around the problems?

  7. SUSY says:

    LHC Run II – how long after 2015 will results be announced on SUSY? other than SUSy what other physics do they hope to see at 13TEV?

  8. Peter Woit says:

    There might be some early results on SUSY summer 2015, I’d guess results based on a data set of similar size to Run I would start winter 2016. I’m sure there will be searches for all the same things as Run I: extra dimensions, Zprime, black holes, you name it.

  9. Maurice Carid says:

    Hi Peter,
    when Strominger calls the progress
    of the last 30 years “incredible” and
    likens the present with the situation
    of physics in the early 20th century:
    is this whistling in the dark, propaganda
    or does he really believe this?
    cheers maurice

  10. Peter Woit says:

    Calling the last thirty years a period of “incredible” progress is just hype, and I doubt he or anyone else believes this in a serious way.

    Likening the present to the early 1900s is a common way of expressing hope for some imminent arrival of new, successful revolutionary insights into physics analogous to relativity and quantum theory. I don’t see any argument there, just an expression of wishful thinking for major progress to come soon (unlike the “incredible” progress of the last 30 years…)

  11. Elbi Gilgen says:

    The progress of the last 30 years *has* been incredible, strominger is absolutely right. The only problem is the sign.

  12. Christian Müller says:

    The numerous answers that Strominger collects miss the following:
    – The origin of the mass of the electron
    – The origin of the fine structure constant
    – The origin of the other standard model parameters
    So not it seems that not only Strominger, but also all his colleagues have given up on these issues.

    It remains amazing how a whole field is digging its own grave in public. A big change is needed: both in the approach and in the required optimism.

  13. Shantanu says:

    Peter or others: can someone point me to the videos of these talks.
    I couldn’t find them from
    but maybe they are elsewhere

  14. Peter Woit says:

    As far as I know videos have not been posted yet, but the intention is to do so at some point at the page you link to. My comments based on watching the talks were based on watching some of them in real time (they were being streamed live).

  15. Christian Marks says:

    Richard Dawid was very recently interviewed by Richard Marshall of 3AM Magazine about his book, “String Theory and the Scientific Method.”

  16. Peter Woit says:

    Thanks! Will write something about this interview soon.

  17. peter hickman says:

    With reference to the interview:
    Richard Dawid was very recently interviewed by Richard Marshall of 3AM Magazine about his book, “String Theory and the Scientific Method.”

    The message from string theorists “there is no alternate theory, supports their view that string theory is the so called final theory” is naïve.
    There was no alternative to Newtonian physics for c.200 years, until the advent of Special Relativity and Quantum mechanics.

    Feynman quote summaries what the scientific process is:
    In general we look for a new law by the following process. First we guess it. Then we compute the consequences of the guess to see what would be implied if this law that we guessed is right. Then we compare the result of the computation to nature, with experiment or experience, compare it directly with observation, to see if it works. If it disagrees with experiment it is wrong. In that simple statement is the key to science. It does not make any difference how beautiful your guess is. It does not make any difference how smart you are, who made the guess, or what his name is – if it disagrees with experiment it is wrong. That is all there is to it.
    chapter 7, “Seeking New Laws,” p. 156 [as presented in edited book]

    This is the salient point for any theory: “It does not make any difference how beautiful your guess is”.


  18. Chris W. says:

    A PS on Peter Hickman’s comment: The book in question is The Character of Physical Law (1965).

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