What the Hell is Going On?

I’ve looked at the talks from a few of the HEP experiment and phenomenology summer conferences. If anyone can point me to anything interesting that I’ve missed, please do so. The lack of new physics beyond the Higgs at the LHC has left the field in a difficult state.

One conference going on this past week and next is the IAS PiTP summer program aimed at advanced grad students and postdocs. This year the topic is HEP phenomenology, and talks are available here. If you want to understand the conventional wisdom on the state of the subject, you can watch Nima Arkani-Hamed’s three and a half hour lecture (here and here) which he starts off by describing as on the topic “What the Hell is Going On?”.

A lot of the first part is historical, starting off with the Georgi-Glashow GUT and the arguments for SU(5) or SO(10) GUT unification first put forward 43 years ago. He then walks the audience through the sequence of steps theorists have taken to solve the problems of such models, after an hour ending up at the landscape, spending a half an hour promoting the anthropic solution to the CC and other problems. The second part of the talk is largely devoted to making the case for his favored split SUSY models, with anthropics and the landscape taking care of their naturalness problems. By the end of the three and a half hours, Arkani-Hamed admits that this scenario is not that convincing, while arguing that it’s the only thing he can see left that is consistent with the idea that theorists have been following a correct path since 1974:

It’s the only picture of the world that I know where everything that we learned experimentally and theoretically for the last 30 years has some role to play in it. But my confidence in it is not so super high, and I definitely think its worth thinking about completely radically different things.

The disadvantage to the trajectory of going with what works and then changing a little and changing a little is that you might just be in the basin of attraction of the wrong idea from the start and then you’ll just stay there for ever.

To me by now the evidence is overwhelming that HEP theory has been in the wrong basin of attraction for quite a while, and the overriding question is what can be done to get out of it. If you’re in the wrong basin of attraction, you need to get out of it by going back to the point where you entered it and looking for another direction. I think Arkani-Hamed is right to identify the 1974 GUT hypothesis as the starting point that led the field into this wrong basin. HEP theory has progressed historically by identifying new more powerful symmetry principles. The move in 1974 was to go beyond the SM symmetries by picking a larger gauge group, then breaking it at a very high energy scale with new scalar fields. The history of the last 43 years is that this idea isn’t a successful one: as this talk shows, it leads to an empty theory that explains nothing. Can one find different new ideas about symmetry that are more promising?

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34 Responses to What the Hell is Going On?

  1. Jon Awbrey says:

    What tack would you tackle?

  2. Peter Woit says:

    Jon Awbrey,
    To me an obvious thing to do is to examine carefully the role of group representation theory (the area of mathematics that studies the implications of symmetries) in the Standard Model, and see if thinking about the Standard Model in that language can inspire new ideas. This is one of the motivations for the book that I just finished writing. I see several different directions of this sort to pursue, that’s what I’m working on these days. I don’t have anything definite enough to write about here. And, I don’t want to turn this blog entry into a discussion forum for speculative ideas, mine or any one else’s.

  3. Alex says:

    Even if you go back to pre-1974 assumptions and reevaluate everything, any new frameworks that emerge will still be of limited utility to physics until the experimentalists get something new. If the collider experiments continue to match up with the Standard Model, the searches for dipole moments and dark matter continue to find nothing, and the interesting anomalies in cosmic ray detectors and gamma ray telescopes get more mundane astrophysical explanations, then there’s little or nothing to test new frameworks against.

    It could be quite intellectually fruitful for mathematics, but physics has little hope without experiments.

  4. Peter Woit says:

    There has been a lot of progress since 1974 in understanding the mathematical structures involved in QFT, so one is not going back to pre-1974. Yes, it would be extremely helpful if experiments break the Standard Model and provide indications of the path forward from the physics side. Bu this is not happening, and as long as this continues, coming at the problem from the more difficult path of studying the mathematics may be the only way forward. Or, one could just give up completely on HEP theory until a new experimental result comes along, and have everybody move into condensed matter theory. Actually, that seems to be where things are headed…

  5. Nakanishi says:

    I remember, though not precisely, the motto of Heisenberg: “The most conservative people can be most revolutional.” QFT is quite successful; therefore, one should adhere to its proper framework as long as possible unless there appears an experimental evidence which undoubtedly contradicts with QFT. The natural representation space of QFT is the indefinite-metric Hilbert space, together with subsidiary conditions. The quantum gauge theories are successful because there exists the BRS invariance condition (the Kugo-Ojima condition). One should investigate whether or not there is any other possible subsidiary condition which guarantees the unitarity of the physical S-matrix.
    SUSY was destined to be ruined because all superpartners must be physical particles. If a supersymmetry in a wide sense could be formulated in the indefinite-metric Hilbert space with a subsidiary condition, all superpartners could become unphysical.

  6. Shantanu says:

    Peter, why isn’t anyone (including string theorists) trying to link theories with non-0 neutrino mass with TeV scales physics and models of EW symmetry breaking. As me and others have mentioned many times, models with see-saw mechanism etc are completely decoupled from rest of BSM physics and I don’t see that situation changing.

  7. A says:

    In the past 10-20 years fundamental physics gave: cosmological constant, neutrino masses, higgs, inflation, cosmology, gravity waves, the fact that past ideas about naturalness seem wrong (which might indicate a cut-off less trivial than an atomic lattice). Despite the difficulties, this is much more interesting than condensed matter, a mature field that can only study more complex materials. I really hope that this is not the future of physics:

  8. Thomas Larsson says:

    It is well known to experts (e.g. Pressley-Segal section 4.10) that the group of gauge transformations in Yang-Mills theory has two different types of extensions: the Mickelsson-Faddeev extension and the central extension. The MF extension is the mathematical formulation of gauge anomalies in QFT, and there is a strong mathematical reason why this extension has to vanish: it admits no non-trivial unitary representations.

    In contrast, the central extension cannot arise as a gauge anomaly within the framework of QFT, because its representations depend on additional data: a privileged 1D curve in spacetime. This leaves two possibilities: either the central extension is unphysical because it is not in QFT, or one needs to modify QFT to allow for all kinds of extensions that are mathematically possible. It is no secret that I believe in the second option, and even if I didn’t succeed to turn this into physics, the math is solid.

  9. David Garfinkle says:

    I agree that going back to the place where a wrong turn was taken and trying to take a different turn is a good idea. However, I’m not convinced that grand unification was the wrong turn. It seems to me more likely that supersymmetry and string theory were the wrong turns. One way to look at grand unification is that (1) in some sense non-abelian gauge theories have better properties than abelian ones, so one might look for a fundamental theory which does not U(1) as part of its gauge group, and (2) in quantum field theory scalar fields are not so tightly constrained as vector fields or fermion fields, so that it’s easier to devise theories with lots of scalar fields and so that e.g. it might make more sense to think of theories where dark matter is a scalar field then where it is a supersymmetric partner.
    I’m also not convinced that more research in representation theory is the best application of mathematics to quantum field theory. What about a more mathematically sensible way to treat the infinities that arise in quantum field theory? One way to look at this issue is the following: partial differential equations (PDE) are much more .complicated than ordinary differential equations (ODE). PDE have more degrees of freedom, and require much more mathematical machinery (Banach spaces, Sobolev norms, energy estimates, etc.) to show existence of solutions, and indeed it is only for particular PDE (hyperbolic, elliptic, parabolic) that solutions exist. Similarly, quantum field theories have more degrees of freedom than either classical field theories or quantum mechanics. It is therefore not that surprising that infinities arise, and that only some theories (the renormalizable theories) are sensible. Perhaps we need more mathematical machinery to make sense of quantum field theory so that one does not get the impression that infinities are being swept under the rug rather than dealt with.

  10. Peter Woit says:

    David Garfinkle,
    Arkani-Hamed explains clearly the logic that non-SUSY GUTs don’t work, leading to SUSY.

    Please, if it’s not about the Arkani-Hamed talk, don’t submit comments, I can’t and don’t want to moderate a general speculative discussion section.

  11. vmarko says:

    Hi Peter,

    “If you’re in the wrong basin of attraction, you need to get out of it by going back to the point where you entered it and looking for another direction.”

    This is all true, but I don’t think that there is a shortage of ideas which other direction to take. The problem is a social one — convincing everybody else (or at least some part of the community) to actually start doing research in some other direction. And I don’t see that happening soon, for two reasons. First, most theorists are egocentric regarding promising ideas (myself included 🙂 ), so they will rather have you and me follow their idea (even if they don’t really have one) than they would agree to follow your or my idea themselves. Second, as usual, physics advances one funeral at a time, and there need to be quite a few of those in order to make room for a sizable number of younger people to actually be able to try out other directions.

    My feeling is that lack of new BSM physics at LHC etc. is simply going to reinforce this tension between old and new ideas, as many big-name lead scientists are becoming more and more of an embarrassment in the eyes of younger generations of scientists. I believe in the end this will turn out to be a good thing, but the hep-th community will feel some pain in the meantime.

    Best, 🙂

  12. Armin N says:

    “I don’t want to turn this blog entry into a discussion forum for speculative ideas, mine or any one else’s.”

    I understand your motivation for not wanting to do this. Would even one post, say, roughly a month, devoted to a speculative idea you find deserves more attention or investigation be too much, even if only as a riposte to those who sometimes criticize you for just pointing out the flaws in the status quo and not offering promising alternatives ideas?

  13. David Garfinkle says:

    In looking over that part of the Arkani-Hamed lecture it seems to me that “logic of why non-SUSY GUTs don’t work” is an overstatement. He looks at things that he considers the simplest and most natural mechanisms for breaking GUTs to the standard model without using supersymmetry and concludes that those mechanisms don’t work. This seems to leave open the possibility that some more complicated mechanism that doesn’t look natural to Arkani-Hamed might work. In cosmology we have abundant evidence that our prejudices as to what is “natural” are a very unreliable guide. Perhaps it is time that we apply that insight to particle physics.

  14. Markus Pfleier says:

    Nima is probably right. Some years ago, I wrote to a Nobel Prize winner for electroweak unification (which I won’t name 🙂 and asked him whether it is more correct to call it “unification” or “mixing”. He answered that “unification” is not the appropriate wording; after all, there are still 2 coupling constants.

    But it was electroweak “unification” that led to grand “unification”. Experiments show us that there are still three coupling constants, not just one. There just is no G”U”T. There is symmetry left to be discovered. We know all elements, we know all particles, we know all symmetries.

    And there is no single symmetry. The answer to the question at the end of the post is what experiments suggest: Renewed progress will be achieved as soon as the gauge interactions are kept and left separate. Renewed progress will be achieved as soon as the search for unified symmetries is abandoned.

  15. Markus Pfleier says:

    The sentence should have been: “There is no symmetry left to be discovered.”

  16. John Baez says:

    Peter wrote:

    Arkani-Hamed explains clearly the logic that non-SUSY GUTs don’t work, leading to SUSY.

    I haven’t looked at his talk, but my impression is that the non-supersymmetric SO(10) GUT is still viable if you don’t mind fine-tuning: there’s a paper by Altarelli and Meloni about this.

    They write:

    We present a renormalizable non-supersymmetric grand unified SO(10) model which, at the price of a large fine tuning, is compatible with all compelling phenomenological requirements below the unification scale and thus realizes a minimal extension of the SM, unified in SO(10) and describing all known physics below the GUT mass. These requirements include coupling unification at a large enough scale to be compatible with the bounds on proton decay; a Yukawa sector in agreement with all the data on quark and lepton masses and mixings and with leptogenesis as the origin of the baryon asymmetry of the Universe; an axion arising from the Higgs sector of the model, suitable to solve the strong CP problem and to account for the observed amount of dark matter. The above constraints imposed by the data are very stringent and single out a particular breaking chain with the Pati-Salam group at an intermediate scale of about 10^11 GeV.

    Is this wrong? I really like how SO(10) unifies all the fermions in each generation and neatly explains many of their properties. So, I’m hoping it’s not quite dead yet.

  17. John Baez says:

    What’s the latest word on the possible failure of lepton universality? This would certainly give particle physics a kick in the pants. Mark Wise says it doesn’t fit into “the story line that the theorists tell”, but it seems that’s exactly what we need: some data that don’t fit into the usual story line.

    The evidence is even harder to swallow given how far lepton universality is from theorists’ expectations of where cracks in the Standard Model might show up. “There’s sort of a story line that the theorists tell,” Wise says, and “this isn’t in the story line.” What’s worse, the proposed explanations for the leptons’ behavior seem ad hoc and unsatisfying. “The kind of models that can fit the…anomalies don’t really do anything else at first sight,” Ligeti says. “For example, they don’t get you any closer to understanding what dark matter might be.”

    Still, he adds, “nature tells us how nature is.” Physicists are increasingly taking note of the violations’ continued persistence, and proposing new theoretical explanations. Experimentalists and theorists alike are also looking to reduce existing measurements’ uncertainties. Ultimately, the biggest revelations will come when LHCb and the next version of Belle produce more data. Physicists are optimistic that within about five years not only will we know whether the effect is real, we will have an explanation for it.”

  18. Peter Woit says:

    If you look at that SO(10) paper, you’ll see that what they’re doing is moving up the unification scale by introducing a truly baroque set of Higgs fields and an intermediate scale. The problem here isn’t really fine-tuning (which all non-SUSY GUTs have) but that to get coupling constant unification you’re introducing lots of new parameters and new choices (in the Higgs sector).

    This shows the generic problem with GUTs: they claim to answer the problem “why SU(3)xSU(2)xU(1)” (or at least change it to a simpler problem, “why SO(10)”), but all they really do is change the problem to “why this Higgs sector (which breaks SO(10) to SU(3)xSU(2)xU(1))?”

    I agree that the fact that a generation fits precisely into the spin rep of SO(10) is a hint about something. But, Spin(10) doesn’t really act on this, or, equivalently the action is”badly broken” in a way we don’t understand at all. Recall that this spin rep can be constructed as the exterior algebra on 5 variables, or, if you like, 5 fermionic oscillators or 5 fermionic qubits. The fundamental problems then become: why 5? why do 3 of them behave one way, 2 another?

    Yes, if the LHCb lepton universality anomaly holds up, that would be a big deal. The history so far is that all similar things have gone away, and there’s no reason to think this one will be different. Unfortunately, it also doesn’t seem that postulating that muons and electrons behave differently in a way that would explain the LHCb results leads to a model that explains anything else (as opposed to just adding complexity to fit this result).

  19. Peter Woit says:

    Armin N,

    I’ll keep this in mind and try and post more often about intriguing new ideas that I see. The depressing thing about the current state of HEP theory is that interesting new ideas are very few and far between.

    I could also just keep repeating my own favorite intriguing ideas, which don’t change much and haven’t seen any progress. I happened to run across an old posting about this from seven years ago, see
    I don’t think there’s been significant developments on those problems since then, the first and third are motivations for things I’m working on now that the book is finished.

  20. Chris Kennedy says:

    Regarding the LHC, I haven’t seen much discussion on the Doubly Charmed Baryon discovered and written about earlier this month? 3520 MeV with a lifetime stated to be shorter than 33 fs. Any new thoughts on that?

  21. Mark M says:

    Hi Peter,

    In response to your “could also just keep repeating my own favorite intriguing ideas”, there have been some progress in trying to formulate a non-perturbative chiral gauge theory, cf:
    Nonperturbative Regulator for Chiral Gauge Theories? Grabowska, Kaplan.
    Phys.Rev.Lett. 116 (2016) no.21, 211602, arXiv:1511.03649 [hep-lat]
    Chiral solution to the Ginsparg-Wilson equation. Grabowska, Kaplan. Phys.Rev. D94 (2016) no.11, 114504 arXiv:1610.02151 [hep-lat]

  22. Peter Woit says:

    Mark M.,

    Thanks! Taking a quick look at those papers, it’s remarkable what a tricky problem this is and I think it’s fair to say that it’s still not clear if it is solved.

  23. srp says:

    In honor of the title of this post, here is the great Vince Lombardi:

  24. George Rush says:

    Good talk from Nima Arkani-Hamed. IMHO it’s basically a reaction to LHC results. They validated electroweak but cast doubt on his favorite version of SUSY. As Nima sees it the key scale is around 10^-17 cm, so he expected / hoped to see new physics here. SUSY can cancel quantum fluctuations, solving the huge CC mis-prediction and other things. But it has to do it around current LHC scale. A natural reaction to failure is to explain why anyone would, and should, have made the same mistake. (Cf. Hold ‘Em players dissecting hands lost on the river to a 1 out of 46 chance.) He goes through the steps since 1974 emphasizing how each was almost inevitable. (Like the poker player proving the only sensible action on the flop was a pot-size bet.) Ties it all together by the idea of finding yourself, by small steps each very sensible, in the wrong “basin of attraction” in solution space. It makes sense and I don’t mean to belittle his story. But it can be viewed as a very common reaction to the loss of what looked like a high-positive-expectation bet: justifying it.

  25. William Nelson says:

    Not sure how you can claim there’s “overwhelming evidence” that HEP people have been on the wrong track, while also noting that there is no evidence one way or the other from experiment. Many of the same people who got it right with the SM don’t think the efforts were misguided at all. Unfortunately, it comes down to opinion at this point. It’s interesting to me that you emphasize mathematical studies of QFT and representation theory, both of which have been going like gangbusters in recent times. Perhaps they are not the exact type of studies that you would like to see, but you can hardly deny that there has been great progress in understand the space of QFTs and many new twists on the appearance of various kinds of representations. Many, many people other than yourself are interested in these things, so it’s not clear to me exactly what you want people to do that they are not doing. It just comes out sounding like sour grapes of some sort.

  26. Yair says:

    Condensing the Arkani Hamed tour the force, to something like “1974 GUT was were we took the wrong turn” does injustice to the talk.

    The talk was a breathtaking and brilliant attempt to cover the current situation – and to theextent any claims were made – it did provide some plausibility arguments in support of an anthropic approach.

    Once a new approach (based on the Pre-1974 “wrong turn” framework) accompanied by calculations that are both consistent with the known experimental data, as well as (hopefully) some new predictions – will be officially published and peer reviewed – will it be possible to cast judgement about the heuristic claims hinted at in the comments section.

  27. Peter Woit says:

    William Nelson,
    I’ve devoted far more time than is sensible to making the scientific case here and elsewhere that the string theory landscape/SUSY GUT paradigm has failed, didn’t see the need to repeat it here. Characterizing things like the LHC negative SUSY results and the forty year history of countless experimental results in disagreement with SUSY/GUT/string theory expectations as “no evidence one way or the other” makes no sense unless you believe that the paradigm doesn’t predict anything, which is exactly why it is a failure.

    Yes, in recent years attention has returned to QFT from string theory, which is a good thing, and there’s a lot of interesting mathematics going into this. My remarks about QFT and representation theory were not a complaint or sour grapes about anything, but intended as a positive response to a commenter asking what I found more promising.

    My comments about the talk obviously weren’t intended to completely cover a nearly four-hour long lecture. The bulk of the talk was devoted to laying out the argument that “if someone put a gun to his head”, Arkani-Hamed’s preferred scenario is the anthropic landscape, with split-SUSY, for no good reason at an energy scale just too high for the LHC to have seen it. As mentioned above, the reasons I think this is a failure are ones I go on about endlessly.

    I wrote about this talk here because I think it’s an unusually clear laying out of the current situation. I’d reserve the terms “breathtaking and brilliant” for a lecture with new ideas, not just a rehash of failed ones. The most interesting thing about the lecture I think was the end that I quoted, where Arkani-Hamed does bring up (without making the case for it) the obvious answer implied by his talk to the “What the Hell is Going On?” question.

  28. X says:

    “I’ve looked at the talks from a few of the HEP experiment and phenomenology summer conferences. If anyone can point me to anything interesting that I’ve missed, please do so. The lack of new physics beyond the Higgs at the LHC has left the field in a difficult state.”

    Well there’s this from the recently concluded Lattice 2017 conference.
    I will discuss the recent B-physics results which indicate intriguing deviations from the Standards Model expectations. I will focus on several New Physics scenario which are currently being explored. I will then go through several flavor physics observables (not only those involving b-quark!) and argue that they too could provide us with access to New Physics provided the hadronic uncertainties are tamed by means of Lattice QCD.)

    This kind of work is an example of what Adam Falkowski (Jester) calls the “2nd approach” to HEP – the `high precision’ approach – as opposed to the `high energy’ approach:
    “By measuring the properties of known particles with great precision, and comparing the results to theoretical predictions, insights can be derived into physical laws at energies inaccessible to collider experiments.”

    The main challenge on the theoretical side is to compute the theoretical predictions with sufficiently high precision to be able to show discrepancy with the experimental results. Taking account of QCD processes is often the ‘hard part’ in getting the high precision theoretical predictions, so Lattice QCD is important here.

    This approach is looking increasingly promising, not only IMHO but also in Falkowski’s view:
    “Shifting the focus away from high-energy colliders toward precision experiments may be the most efficient way to continue exploration of fundamental interactions in the decades ahead. It may even allow particle physics to emerge stronger from its current crisis.”

    (In the past I occasionally berated Falkowski on his Jester blog for wallowing in the lack of new developments from the ‘high energy’ approach to HEP while ignoring the interesting things going on in the high precision approach, so it is amusing to see that he is starting to appreciate it now…)

    However, this kind of work is being done by relatively low-profile people at non-illustrious institutions, so it gets very little attention compared to the latest musings of Nima et al…

    BTW, regarding the chiral gauge theory papers of Grabowska & Kaplan mentioned in a comment above, the approach seems to have been debunked (or at least shown to be very problematic) recently by Suzuki and collaborators here.

  29. Barry Awn says:

    Peter, I would be interested to hear your response to Nima’s comments around 1:37:14 about “… the most boring possible discussion …” in his HEP talk at the IAS PiTP summer program (“Where in the World are SUSY & WIMPS?). I found the comment at: 1:37:14 on https://www.youtube.com/watch?v=dKVXxcbJ4YY

  30. Peter Woit says:

    Barry Awn,
    I thought that whole section of his talk was just outrageous. Given an hour to present to young researchers his take on the state of the field, he spends about 40 minutes going over laboriously exactly the same story about the CC/landscape/anthropics that those in the audience have surely heard a hundred times already (talk about boring!!), then dismisses objections with a wave of the hand as “the most boring possible..”

    If, as seems increasingly all too possible, we’re now at an endpoint of fundamental physics, with the field killed off by a pseudo-scientific argument (“no point in continuing, the multiverse did it”), Arkani-Hamed is one of those who will be most responsible for the situation.

  31. Jim Akerlund says:

    The August issue of “Notices” with the articles about gravitational waves, lists Arkani-Hamed as getting inducted into the “National Academy of Sciences”. I know you get elected by current members, but does the election also include the reason the new member was elected? If so, does Arkani-Hamed reason include non-mathematical contributions, like the subject of the talks above?

  32. Peter Woit says:

    Jim Akerlund,
    Arkani-Hamed is a physicist, not a mathematician, well-known for his work on various topics in HEP phenomenology (as well as some more recent more mathematical work on scattering amplitudes). He has an enthusiastic lecture style, likes to make dramatic, forceful claims, this is one reason he is quite influential in the field at the moment, for better and worse…

  33. Phil says:

    As far as new physics are concerned, strong hints actually appeared this Summer, not from the LHC, but from T2K, which excludes CP conservation in neutrino oscillations at the 95% confidence level using the latest data:
    Perhaps could be worth a new blog post?

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