Strings 2021

Strings 2021 started today, program is available here. Since it’s online only, talks are much more accessible than usual (and since it’s free, over 2000 people have registered to in principle participate via Zoom). Talks are available for watching every day via Youtube, links are on the main page.

As has been the case for many years, it doesn’t look like there will be anything significantly new on the age-old problems of getting fundamental physics out of a string theory. But, as has also been the case for many years, the conference features many talks that have nothing to do with string theory and may be quite interesting. I notice that Roger Penrose, a well-know string theory skeptic, will be giving a talk on the last day of the conference next week.

Another series of talks that I took a look at and that I can recommend is Nima Arkani-Hamed’s lectures on Physics at Future Colliders at the ICTP summer school on particle physics. He never actually gets anywhere near discussing the topic of the title for the talks, but does give a very nice leisurely introduction to computing amplitudes for zero-mass particles. What he’s doing is emphasizing ideas that are often not taught in conventional QFT courses (although they should be). His second talk explains how to think of things in terms of classifying representations of the Poincare group, an old topic that unfortunately is often no longer taught (see chapter 42 of my QM textbook). His third talk emphasizes thinking of space-time vectors as two by two matrices (see section 40.4 of my QM book). This is a truly fundamental idea about space time geometry that gets too little attention in most physics courses.

Update: At String 2021, yesterday Nima Arkani-Hamed gave a talk on “Connecting String Theory to the Real World We See Outside Our Windows”, where he sometimes sounds like me, contrasting the pre-LHC claims of string theorists:

1. LHC will discover SUSY
2. String Theory Loves SUSY + Unification

to what they are saying now that the LHC has found no SUSY

CICADAS [i.e. crickets]. (Anyway, String Theory is mainly about Quantum Gravity).

He goes on to explain the “landscape philosophy”, which he sees string theorists (and himself) as now adopting. According to this philosophy, “connection to particle physics appear[s] hopeless/”parochial”/unimportant”. As a result, he sees the current situation as

  • String theorists are for the most part no longer actively pursuing connecting to particle physics of the real world.
  • Understandable as a short-term strategy
  • But in my view a real mistake in the long run…

One reason for this being a real mistake is that, divorced from input from the real world, theory becomes sterile:

Questions Posed by Nature are Vastly Deeper and more fruitful than ones we humans tend to pose for ourselves.

Unfortunately I don’t think Arkani-Hamed has any compelling argument against “string theory implies landscape implies nothing to say about particle physics”. He discusses the “swampland philosophy”, but gives as a challenge to theorists just making more precise the sort of empty question that this philosophy deals in (he asks whether D=9 SU(2021) to the power 2021 is in the swampland).

Update: In the final discussion section, Witten emphasizes that “What is string theory?” still has no answer, that we have “little idea what it really is”. He lists two main things we know about the supposed theory:

1. General string perturbation theory using 2d conformal field theory. He mentions that one basic problem with this is that there is no understanding of what happens in time-dependent backgrounds, so, in particular, this is useless for addressing the big bang, which is the one place people now point to as a possible connection to real world data.

2. AdS/CFT

He notes that to get at non-perturbative string theory we seem to need some more general understanding of quantum theories, going beyond the usual quantization starting with an action, and ends by saying maybe quantum information theory can help. In the discussion section, Vafa challenges him on this, saying he sees no indication that quantum information theory gives any insight into dualities he sees as the central aspect of the non-perturbative theory. Witten’s answer is that this was just a vague hope, that the duality ideas are now 25 years old, we haven’t progressed beyond them, need something new.

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34 Responses to Strings 2021

  1. Do_not_want_to_register says:

    Another talk of Penrose (and a conference in his honour):
    (videos of the talks at the bottom)

  2. Suomynona says:

    How is the idea that particles are irreducible representations of the Poincaré group work in the context of quasiparticles in condensed matter physics, where the underlying system background is generally not symmetric under Poincaré transformations (e.g. rotations and translations are typically discretized due to the presence of a lattice)?

  3. Peter Woit says:

    In other physical contexts you have different symmetries than Poincare, often approximate symmetries. The general principle is that whenever you have a group acting on a physical quantum system, the state space will be a unitary representation of that group, and unitary representations break up into irreducible representations. It will often be a good idea to analyze states in terms of these irreducible representations. How useful this will be will depend on the symmetries you have available. What’s remarkable is how powerful Poincare space-time symmetry is as a determinant of how elementary particles behave. Condensed matter physics is a quite different story.

    Much as I love this general topic, it’s getting far from the topic of the posting, so I’ll just advise: read my book…

  4. ay says:

    I am currently reading Weinberg’s book to fill some holes in my knowledge. It’s very good but he often leaves the reader a lot to figure out. I watched the entire video on the Poincare group and, although the path was the same as Weinberg, there was a lot of new information – I thought it was excellent. It also gives one a feeling for how Arkani-Hamed works through problems. The video answered some questions that I still had (e.g. how sensitive is the procedure to choice of k_mu – he did the case where k_mu included a boost).

  5. lun says:

    Starting from 1h14m of this “meet the public” vide
    you see Ed Witten, and then Shiraz Minwalla, basically agreeing with you.

  6. Peter Woit says:

    I just took a look at that session and thought one remarkable point was where people were asked “what would cause you to give up on string theory?” The only answers forthcoming were from Igor Klebanov, who said he had been working on it his whole life, so couldn’t see himself giving up on it, and Ashoke Sen, who said he would only give up on it if it were shown to be mathematically inconsistent. Of course, a few minutes earlier Witten had explained that no one knows what the theory actually is, which means there is no way to show that it is mathematically inconsistent.

    The whole tone of the discussion was pretty depressing. In response to the obvious question about relation to experiment and testability, people have clearly completely given up on this, with nothing to say (other than a bogus claim that string theory makes some prediction about B-mode polarization), going on about how it took thousands of years for the atomic hypothesis to be vindicated. There’s a lot of repetition of tired old arguments from decades ago, zero acknowledgement that things have not worked out as hoped. It seems that no one in the string theory community dares to publicly breathe a word of skepticism. Skeptical challenging of arguments is supposed to be what science is all about, but the speakers in this panel made it seem that this is absolutely not something that is part of their field or what they do.

  7. Jackiw–Teitelboim says:

    This goes without saying all technically challenging fundamentally important problems required for superstring theory unification be at least mathematically self-consistent have been left unanswered. (With exception of a group of very few dedicated researches, to whom I sincerely keep my deep respects.) While the community just spread out to other problems.

    Paraphrasing Fermat, “I have discovered the ugly truth of this, which this margin is too narrow to contain.” Nevertheless, it may be worthy registering on this blog some of those problems which aren’t enough emphasized. Nothing will be said about “multiverses” or “swampland” on what follows.

    (i) No one convincingly proved the mathematical equivalence between the RNS superstrings and GS formalism.

    (ii) Every-time I receive a grant report asking for resources, penned by a senior string theorist, I’m already able to guess the 1st lines: “ST is the only mathematically consistent candidate for understanding quantum gravity while providing a framework potentially unifying all know fundamental interactions.”
    I’m sorry if I cannot comment on other approaches to quantum gravity or non-stringy grand unification programs. But regarding superstrings, from such a strong statement one would expect at least proof (not rigorously by mathematical standards of course!) that perturbation series following from superstring S-matrix, already known to diverge as one sums over all moduli parameters (which isn’t really the issue, since this happens even in QED, see Dyson, F.J., 1952. Divergence of perturbation theory in quantum electrodynamics. Physical Review, 85(4), p.631.), one would at least expect that a well-behaved asymptotically perturbation series follows as one takes into account contributions from all the moduli parameters of Teichmüller spaces of Riemann surfaces with higher genus and market points.
    At the time I drop those lines, more than 50 years from dual resonance models?, I haven’t found any convincingly argument.

    (iii) Most superstring calculations are performed under euclidean worldsheet for many practical reasons, as we all know and love from Wick’s rotation in standard q.f.t.’s. Nevertheless, from a theory claiming to be the only mathematically self-consistent unification program, something regarded as just a mere technicality now requires an answer. The spin-bundle required to defined fermion content from SUSY multiplet depends on the metric signature. I would be eager to read a real formal discussion if first-quantised RNS superstring in euclidean signature is quantum mechanically equivalent (unitary Boguliubov transformation) to original version starting from Minkowski signature.

    I could go on the whole night, but I believe, as most other string theorists do, there are more “urgent” or at least doable problems to solve now. In my case, I will give a run, certainly much more healthy, no?

  8. PeterH says:

    Just viewed the last question and answer session.
    The only message I hear repeatedly is that ‘String theory is the only game in town’.
    Well, that would be the case if people follow the hype for last 20-30 years or more.
    For a so called well defined mathematical theory, cannot be tweaked etc, the number of unsolved issues with it and the claim not understanding what string theory is – is inconsistent.

  9. Peter Woit says:

    There’s no reason to get into the technical argument over whether the perturbative expansion of various versions of the superstring in 10d is mathematically well-defined. That looks nothing like the real world, and as Minwalla points out, no one at the Strings conference is even talking about that sort of “string theory” anymore. There’s a long list of things they are talking about under the name “string theory”, with the generic problem that the ones that are reasonably well-defined have nothing to do with the real world (e.g., wrong dimension).

    Sen is saying he’ll give up on string theory if people prove that “string theory” is inconsistent, but Minwalla and Witten are saying there is no definition of the term “string theory”.

    What’s really bizarre about the current situation is that “string theory” has completely decoupled from any well-defined proposal for an actual 3+1 d unified theory of gravity and the SM, while at the same time its proponents claim it is the “only game in town”. The “only game in town” in fundamental physics now is to give up on a theory of the real world.

  10. Jackiw–Teitelboim says:

    Hi Peter,

    Thanks for allowing my last comment. As you probably guess from my former comments under the present pseudonym, I’m one of those students whom, maybe for lacking courage or ,who knows? , self-confidence, went through a Faustian-like monologue, if you allow me a little of prose:

    I’ve studied, Schwarz, alas, Polchinski,
    Green and Witten, recto and verso
    And how I regret it, D-branes also
    Oh, God, how hard I’ve slaved away,
    With what result? Poor fool that I am,
    I’m no whit wiser than when I began

    You are possibly right it’s now pointless to debate those issues. But I went through the hard road, being told just to accept all fundamental questions I raised above as unanswered and move on to the “cutting-edge” problems whatever and whonever decides what these are. So I wrote that list as both an act of rebellion nevertheless also of relive: I don’t think one needs to wait for Planck’s law to clean up our field through funerals in this century anymore & here’s the right point for me to close: It’s time for new ideas and we are going to find them.

  11. DKepler says:

    I just had a look at that session, and more than anything, felt sorry seeing such eminent physicists giving such vacuous reasons just for the sake of defending a theory they have worked on for a long time. Even beginning PhD students will no longer believe that ST is the only framework that reproduces BH entropy. Even without LQG, there are many ideas on/in QG which are physically well founded. This community is indeed becoming like a religious cult, something one of the speakers explicitly denied. Ironically, the very fact that he had to refer to it at all should alarm them.

  12. @DKepler

    Even beginning PhD students will no longer believe that ST is the only framework that reproduces BH entropy.

    That’s interesting, can you give pointers?

  13. DKepler says:

    @David Roberts: There are many references that discuss this from different perspectives. The following articles by Carlip provide a nice overall perspective, drawing attention to the “universality” of BH entropy derived in different approaches to QG – what he refers to as an “embarrassment of riches”.

    1. Symmetries, Horizons, and Black Hole Entropy []
    2. Black Hole Entropy and the Problem of Universality []

    Hope you will find them as a useful starting point.

  14. Cobi says:

    The derivations listed in 0705.3024 seem to fall into three classes:

    – String Theory (D-brane/String state counting, AdS/CFT, Fuzzballs)
    – The LQG “derivation”, involving an ad-hoc choice for the Immirzi parameter
    – Non-QG QFT derivations that are not attempting to identify the microstates

    In particular, i can not find a derivation of any black hole entropy from first principles in quantum gravity outside of string theory.
    The LQG example hardly qualifies if you do not have any way to fix the Immirzi parameter and just tune it to get the desired result.
    Of course there are many ways to obtain the BH formula that do not try to identify the microscopic degrees of freedom in a theory of QG, the original calculation from the 70s being an obvious example.

  15. Tom says:

    Off topic, but the European congress of Mathematics has finished up. The EMS prizes are always a good indication of future Field’s medalists

  16. Log says:


    Ashoke Sen has computed the logarithmic corrections to the Bekenstein-Hawking entropy of various standard black hole solutions in this paper

    It seems that non-stringy approaches to quantum gravity are not very successful at reproducing these universal results.

  17. Peter Woit says:

    Please, no more about the “only string theory can compute the Black Hole entropy” arguments. These have been going on for over 25 years, with nothing new in a very long time. To summarize the situation, now as it has ever been:

    1. Hawking tells you what the semiclassical limit is.
    2. Any consistent theory of quantum gravity should reproduce this semiclassical limit.
    3. String theory calculations that can be done in unphysical situations (wrong dimension, lots of SUSY, extremal black holes) claim to reproduce the semiclassical limit.
    4. LQG calculations claim to reproduce the semi-classical limit.
    5. People interested in string theory/LQG religious warfare argue about the calculations in 3. and 4.

    I’ve personally never understood why showing that (unphysical limits of) your quantum theory gives the expected semi-classical result is anything other than a rather weak consistency check on your theory. Others clearly feel differently and think this is of huge significance.

    If people think carrying on this argument is a good use of their time, they are encouraged to do so elsewhere.

  18. adrianmigueldiego says:

    Dear Peter, I believe you are ‘cherry-picking’ parts of comments. An exchange that was otherwise quite lively and interesting, you paint in the worst possible way. I am not a fan of string theory. I just work on Theoretical Physics topics. I saw the discussions you refer to. You are either blinded or so convinced by the idea that string theory has nothing to say about Physics, that you pick any phrase that adds to your view of things. Please, watch the discussions (there were many) and appreciate that these were honest discussion. People are attempting to understand things, with the tools at hand.

    Is String Theory a Theory of Nature? Probably not if you ask me. But it is clear that these colleagues are attempting to understand topics in QFT and Gravity, with the tools they have at hand.

    Please, do appreciate that negative comments like yours distort the ideas the general public (or even physicists in other areas) have about String Theory, or Theoretical Physics in general.
    Of course, I do not justify equally wrong comments from certain string theorists
    with the exact opposite agenda.

    But this procedure you adopted in this post, is not the best way of understanding science, or communicating it.


  19. Peter Woit says:

    I did watch fully the discussions I wrote about here, also took a look at many of the others. The quotes I gave are accurate, and links are provided so that anyone who wants to can see the full context.

    You’re right that I was cherry-picking, but in the sense of ignoring the huge amount of hype about what great progress was being made, and sticking to writing about certain places where people like Arkani-Hamed or Witten chose to address the real problems (which most of the time are studiously ignored).

    If you’re so concerned about the public or physics community getting misled, surely you’ve contacted the organizers of and participants in the conference’s “Outreach activities”.

  20. adrianmigueldiego says:

    Dear Peter,
    You wrote
    ”If you’re so concerned about the public or physics community getting misled, surely you’ve contacted the organizers of and participants in the conference’s “Outreach activities” ”

    To this I was referring with my phrase
    ” Of course, I do not justify equally wrong comments from certain string theorists
    with the exact opposite agenda.”

    I do not know if outreach sessions like the ones we witnessed in this Strings-conference are the best idea. I think a better service to the public would be made by discussing things that are well established, rather that the permanent focus on ”what is coming/the future” etc. In my view, this anxiety to communicate to the public the latest developments is a sign of certain immaturity. But this is just an opinion. There are other views, it is a matter of taste.

    What I wanted with my post was to point out that (in my view, again), you were selecting particular phrases that sounded negative, in what other wise were quite interesting exchanges among physicists.

    The way I see the string people (not necessarily those who appear in the media, but those who publish in the arXiv) is as a small set of honest and hard working physicists, technically sophisticated, researching mostly on QFT and Gravity, with various approaches.

    I believe I am not misguided. Can they make mistakes? Over-claim? Of course, these are people. But the time, the community (and the arXiv) takes care of these mistakes.

    I believe that your permanent opposing the ‘string hypes’ is good and welcomed. It is healthy. But I believe that in today’s update of this post you were not intellectually honest. This is damaging.
    Once again, this is just an opinion.

    Thanks for reading

  21. martibal says:

    @adrianmigueldiego: as a mathematical-physics, I have been earring for 20 years that string theory is the only game in town, I have seen the “raise” of LQG as an opponent to string, and I witnessed the tentative of string theory to kidnap noncommutative geometry claiming that this was “nothing but the geometry of string theory”.

    So please stop it with this idea that criticising string theory, as Peter does, is not good nor an honest way to communicating about science.
    The string community has been dishonest for at least 20 years, and has absolutely no right to give any lessons about ethics in science. As Peter keeps on repeating, the denial of the string community to acknowledge the failure of string theory as a unification theory (which is nothing to be ashamed off: physics is full of beautiful theory that are not correct) is a shame for the physics community.

  22. Peter Woit says:

    I strongly disagree with you about the issue of the intellectual honesty of my account of what Witten and Arkani-Hamed had to say. Yes, I was ignoring some other things they had to say, for example, Arkani-Hamed’s
    “String Theory remains the most magical structure we have encountered in theoretical physics – and the magic seems directly connected to the particle physics of the real world”
    I think I was doing him a favor by not quoting that kind of thing, which he really should be embarrassed about.

    Note also that I was making no comment about the work people were presenting at the conference, was sticking to discussing particular important points about the current state of the subject being made by the two most talented and hard-w0rking leaders of the field. I don’t think I was misrepresenting those at all.

    The problem with the string theory hype is not that it’s new, exciting, cutting-edge speculation, rather than well-established. String theory is now older than not just my students, but also many of their parents and unfortunately it’s very well-established, sometimes even taught to high school students. The problem is not that it’s too new to be evaluated properly, but that it’s been a failure.

  23. Essenza says:

    In the ending remark, Ooguri makes the observation that the percentage of speakers older than him is roughly a linear function (64-2x)% where x is the number of years after his PhD. While Ooguri has his own interpretation about this observation (incoming flux of young people in the field is as healthy as ever), I wonder what others think about it.

  24. Alessandro Strumia says:

    Following Nima’s final recommendation, an experimentalist and a string theorist hang out together.
    Experimentalist: “I have to compute QCD, Wikipedia says I need to add unphysical ghosts on a local section and their fiberwise duals and project out the quotient Fock space isomorphic to the supergraded BRST cohomology pullback of the field configuration along a vertical diffeomorphism of the principal Batalin–Vilkovisky anafestic G-bundle, as if it were antani. This jargon sounds a bit confusing”.
    String theorist: “indeed, what’s QCD?”

  25. M is for Missed says:

    I’m posting this since I thought it could be useful for those very busy hep-ph physicists who may wish to go directly to (what I & surely many others) considers as the key moment from “perspective” lectures.

    If one knows how to read between the lines, then at this lecture you finally can hear what, in different words, could well be re-phrased as: the search for M-theory using dualities failed.

    Of course even his brightest former student couldn’t accept that:

  26. Ellis_AND_Wooster says:

    The experimental community pretty much politely ignores the string theory community, and has, well, since 1974 or so when the analytic S-matrix failed to predict the charmed quark.

    Blasting is well underway in South Dakota for the $2 billion + LBNF/DUNE facility.

    The real stuff happens in physics without a press release, book, or blog.

  27. André says:


    it is true that experiments are real physics. But what DUNE will find has been predicted years ago: normal neutrino mass order, no additional neutrinos, no proton decay, and no physics beyond the standard model whatsoever.

    These boring predictions, if confirmed by DUNE, will show that also in experimental physics no “real stuff” is happening…

  28. Witten says something about a (possible) theory that is “intrinsically” quantum mechanical, which seems to mean a theory that does not start with a continuous field and quantise it, and which (therefore) presumably cannot be described in that way. Are there any actual attempts in the literature to build such an “intrinsically” quantum theory, or is this just pie in the sky?

  29. Tony says:

    Penrose’s talk is interesting (, as is the discussion between him and string folks afterwards.

  30. Peter Woit says:

    Sorry, but deleted various attempts to carry on the experimentalist vs. theorist mudfight of Ellis_AND_Wooster/André. I suppose I shouldn’t have even let that get started…

    Robert A. Wilson,
    As I’ve pointed out here before (can’t find a link), there’s nothing unusual about a quantum system that isn’t just a conventional quantisation of a classical system. The simplest example is the qubit.

  31. Yes, but I can’t imagine that Witten was thinking of anything as well known and well understood as a qubit.

  32. Peter Woit says:

    Robert A. Wilson,
    Likely Witten was thinking of things like the 6d (2,0) superconformal theory which has not classical limit and its construction is somewhat mysterious. The point though is that there are plenty of much simpler examples, which often can be understood purely in terms of representation theory.

  33. Jackiw–Teitelboim says:

    Hello, Robert A. Wilson
    If you are interested in what field theories Witten’s probably interested in, then I certainly recommend you to follow the on going ICTS’s Quantum Field Theory, Geometry and Representation Theory, with Witten’s mini-course starting tomorrow:

    In order to get a feel what is at stake here, start with:

    Already at p.1 one reads, “The Problem: According to textbooks, the passage from classical mechanics to quantum mechanics is made by replacing Poisson brackets with commutators. However, this is an unrealistically simple description of the situation, even for a basic example such as the classical phase space.”

    Go on and you certainly will be assured that’s not going to be about qubits, Dear Robert.

  34. Peter Woit says:


    That’s about the general problem of quantizing a symplectic manifold, but my impression was that Witten was thinking of the even more general problem of a quantum theory with no known symplectic manifold of which it is the quantization.

    The qubit example is a bit unfair since in some sense it’s the quantization of the sphere, although this sphere is not a classical limit of the quantum theory (and sphere isn’t a cotangent bundle of some configuration space).

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