Space-time, Quantum Mechanics and the Large Hardon Collider

The title of the posting is that of Nima Arkani-Hamed’s public lecture last week at the IAS, with the spelling that of the title at the beginning of the video (available here, lower resolution version here).

The bulk of the talk is devoted to expounding the idea that the central problems of fundamental physics are two hierarchy problems, that of the CC (why isn’t it at the Planck scale?) and that of the Higgs mass (why isn’t it also at the Planck scale?). Given that we don’t understand quantum gravity, and don’t know that the Higgs phenomenon is due to an elementary scalar, it’s not clear to me that these are yet real problems. In any case, Arkani-Hamed gives the anthropic multiverse argument for the CC problem, and claims that if the LHC doesn’t see supersymmetry or large extra dimensions, then we’re stuck with the anthropic multiverse argument also for the electroweak scale.

The LHC only puts in an appearance in the last fifteen minutes of an hour and a half talk. Back in 2005 (see his talk at Strings 2005) Arkani-Hamed claimed that we would know whether supersymmetry solves the hierarchy problem within a year or so of first collisions at the LHC (then scheduled for summer 2007). Now that initial results from the LHC are in, showing no evidence of supersymmetry, his estimate is:

We’re going to have answers one way or another to this question on the time scale of 2020.

One of his slides estimates production of 1 squark/minute given 1 billion collisions/sec, which would mean about 50 squarks already produced in each detector. While it’s true that the LHC won’t be running at full energy until 2014, no explanation is given for why we need to wait until 2020 to find out about supersymmetry. Back in 2005, before the machine was turned on, enthusiastic predictions of quick results were being made. Now that the data is coming in, the story seems to have changed.

Update: Nature News has a new article up by Geoff Brumfiel: Beautiful theory collides with smashing particle data (also available here). While Arkani-Hamed is arguing that one will have to wait until 2020 (the sLHC perhaps?) before knowing whether supersymmetry is at LHC energies, John Ellis appears willing to give up much earlier, maybe the end of next year:

“I’m wouldn’t say I’m concerned,” says John Ellis, a theorist at CERN, Europe’s particle-physics lab near Geneva, who has worked on supersymmetry for decades. He says that he will wait until the end of 2012–once more runs at high energy have been completed–before abandoning SUSY. Falkowski, a long-time critic of the theory, thinks that the lack of detections already suggest that SUSY is dead.

“Privately, a lot of people think that the situation is not good for SUSY,” says Alessandro Strumia, a theorist at the University of Pisa in Italy, who recently produced a paper about the impact of the LHC’s latest results on the fine-tuning problem. “This is a big political issue in our field,” he adds. “For some great physicists, it is the difference between getting a Nobel prize and admitting they spent their lives on the wrong track.” Ellis agrees: “I’ve been working on it for almost 30 years now, and I can imagine that some people might get a little bit nervous.”

The article ends with a very sensible quote from experimentalist Chris Lester, who evidently doesn’t share Arkani-Hamed’s view that it’s SUSY or the Multiverse:

“Plenty of things will change if we fail to discover SUSY,” says Lester. Theoretical physicists will have to go back to the drawing board and find an alternative way to solve the problems with the standard model. That’s not necessarily a bad thing, he adds: “For particle physics as a whole it will be really exciting.”

Update: It seems that the video files have been temporarily removed, presumably for editing. I fear that some poor tech person is having a bad morning…

Update: New video files with typo fixed are now available.

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69 Responses to Space-time, Quantum Mechanics and the Large Hardon Collider

  1. Kea says:

    AAAARRRGH! Nima of all people should be taking the twistor geometry message seriously: ie. the right spaces for calculating twistor amplitudes depend on particle number and holographic principles, NOT on an a priori local spacetime with fairy fields attached.

  2. Chris Oakley says:

    Right – at first I thought that there was a Large Hardon Collider that fulfils all Nima’s dreams/fantasies. Now I realise that it is probably a mis-spelling.

  3. anon says:

    Much in the same way that God implanted fake dinosaur bones in the ground as a test of faith, so too has SUSY arranged to trap the unfaithful. The likes of Ellis will abandon hope over the next few years, and will be left behind when the super-Rapture arrives in 2020.

  4. Thomas says:

    So SUSY or nothing for Arkani-Hamed?
    How many more physicists will give up on their field if there is no SUSY?
    What a waste of talent and experience it is, to just throw up your hands like that.
    What a perverse way of saying that you are wrong; to say that there is no answer.
    And what an arrogant way to do physics; to claim the unknown and call it yours.

  5. realistic says:

    Let’s not get carried away with `Hardon’. Tim Bishop, a Congressman for Long Island, NY, USA, is fighting against budget cuts which might cost 930 jobs at BNL. He speaks of the negative impact this would have on the “Realistic” Heavy Ion Collider.

    “It could also result in BNL shutting the doors on the National Synchrotron Light Source (NSLS) and Realistic Heavy Ion Collider (RHIC) machines, world class research facilities,” according to a Bishop press release.

    One can laugh, or scoff, but that will be small comfort to the 930 jobless at BNL. Even smaller comfort of you’re one of them. And it *will* be a real loss for science, realistic or not.

  6. Peter Woit says:


    I’ll try and write something here about the current situation of the US federal budget when it becomes a bit clearer what the implications for science are, and suggest comments on this topic should wait until then. At the moment, as far as I can tell, no one knows what will happen, with the extreme House budget presumably DOA, and legislators fighting over government funding not for next year or next month, but for this weekend. Strange way to run a society…

  7. Shantanu says:

    Peter or others, what about other implications for models such as technicolor, Little higgs models etc based on current LHC data?

  8. Geoff says:

    Shantanu, many little Higgs models predict W’ and Z’ bosons, and they have been ruled out up to fairly large mass values.

    The experimental situation is simply that there is no deviation from the standard model at all, whereas all theoretical models predict such deviations.

    Both the Higgs and supersymmetry were predicted because the standard model cannot be all there is. Now experiments are showing the opposite. The only thing one can do, if really nothing is found, is to go back to these predictions and check whether they are water-tight.

    In a sense, this is what Peter is doing, especially when he criticizes the supersymmetry fans.

  9. Bernhard says:


    Thanks for the news and the video. One thing I find worth discussing is Alessandro’s comment “This is a big political issue in our field,”

    It’s a interesting point because many people in power right now are SUSY or string proponents and I just hope more people will be willing to take Ellis’s example and throw the towel if SUSY is not found at the LHC.

    At the same time what gives me really high hopes is that eventually these people will go into pension and the new generation will not swallow theories so experimentally discredited (if that really happens, of course). This is the same problem of the multiverse “revolution”, a movement of old theorists giving up altogether to impose that if they couldn’t make it than the answer is likely that there is no answer.

  10. Bernhard says:


    I think nothing really conclusive (at least not new) can yet be said about these models. Differently from SUSY, 35 pb-1 are not enough to improve limits in a significant way:

    Have a look:

  11. says:

    Very smart people whose main concern is winning a Nobel Prize usually finish their unimaginative careers as deans.

  12. SUSY says:

    The news of my death have been greatly exaggerated.

  13. Giotis says:

    It’s not a coincidence that both explanations (SUSY, anthropic) are provided by String theory. This is because String theory is currently the only consistent theoretical framework with enough explanatory power to provide potential answers to such deep questions.

  14. Jens says:


    Or perhaps String Theory has so little explanatory power that it’s consistent with anything.

  15. Tom says:

    I amnot a SUSY advocate & have worked on many BSM ideas over the years so will claim to be an agnostic as to what new physics exists beyond the SM. However it is fair to point out that SUSY is NOT mSUGRA (or even the MSSM). If you give up on the very simple mSUGRA idea there is plenty of parameter space still available where sparticles can be light & missed (so far) by the LHC. The problem is that (the 4-parameter) mSUGRA has been pushed so hard for so long by advocates that people forget about this.

  16. Peter Woit says:


    The initial data has ruled out only a part of even the CMSSM region in parameter space promoted as most likely, so it’s certainly too early to announce the death of SUSY. But, presumably over the next couple years we’ll see much of the rest ruled out, as well as other parts of parameter space.

    Since the MSSM parameter space is so large (and extensions of it even bigger), SUSY diehards will be free in coming years to keep pointing out that possible places for it to hide remain. While this is going on, it seems to me a good idea to look at what the same people were saying before LHC turn-on. I found the difference between Arkani-Hamed 2005 (answer 1 year after first collisions) and Arkani-Hamed 2011 (answer 10 years after first collisions) quite striking, and not fully explicable by the fact that the initial run is at half energy and luminosity is ramping up more slowly than some might have expected.

  17. chris says:

    the tension is mounting not because susy is under scrutiny – our understanding of physics is. regardless of what it might be, we did not see any sign of new physics at LHC yet – safe for some high multiplicity and heavy ion stuff.

    the news is not so much that susy is still not found but that it takes a pretty darn large stick to beat old standard model dead at the energy frontier.

  18. SUSY says:

    People get a bit impacient, don’t they? SUSY is dead with this little integrated luminosity? Gimme a break!

  19. Bernhard says:

    I didn´t save Nima´s talk. Can someone share?

  20. Somdatta says:

    Susy will most definitely be found.

  21. Peter Woit says:


    Sorry, but I don’t think I should use this blog to provide or exchange info about bootleg copies of something belonging to the IAS. I’m assuming that they’ll soon provide the full video again themselves, after a very small amount of editing…

  22. Bernhard says:

    I understand, Peter.

  23. Giotis says:

    If somebody is interested in the anthropic alternative (a la Bousso-Polchinski) to low SUSY for solving the hierarchy problem, Higgs mass etc, he could check this original paper by Silverstein It is well known that low SUSY was never a prediction of String theory. It is just a reasonable hypothesis; a bonus if you like. SUSY could break at String scale for example as the paper explains.

  24. Bernhard says:


    If low SUSY was never a prediction from String it´s even worse. SUSY is a testable theory by the prediction of TeV scale particles. If you are not a fanatic neither pro or against it, one can even say that it can be ruled out. Part of the problem Peter is trying to draw attention to, I believe, is regarding the sudden efforts by some of its leaders to change their minds about this. But that´s point particle theoryl, where one can discuss, agree, disagree and make bets but in the and it will be up to the LHC to define, if SUSY, technicolor or something else.

    With Strings there are no tests to be made, since the theory cannot be falsified. You can get any answer you want. If SUSY is there, good for Strings, if SUSY is not there good for Strings too.

  25. Chris Austin says:


    “safe for some high multiplicity … stuff”

    Are you thinking of the right-hand graphs on pages 8 and 9 of ATLAS-CONF-2010-088, or is there something else at high multiplicity? And if so, where?

    Thanks in advance

  26. Peter Woit says:


    For a while 6 or more years ago, Susskind, Douglas, Dine and others were quite excited about the idea that string theory could predict the SUSY breaking scale statistically: you count “string vacua” and take into account anthropics. It quickly became clear this doesn’t work, see for example

    Like everything else, the SUSY breaking scale can be absolutely anything at all in string theory, with no way to decide among the possibilities. There is one thing predictable here though: if susy does show up at LHC energies string theorists will claim this was a prediction of string theory, but if it doesn’t they’ll claim that string theory made no such prediction.

  27. chris says:

    Chris Austin,

    i was thinking about the CMS observation – it is e.g. the first reference in a paper by Shuryak arXiv:1009.4635.

    most likely it is the first observation of a QGP in pp colissions, so nothing too exciting, but kind of new physics in some sense.

  28. Chris Austin says:


  29. Roger says:

    Geoff says: Both the Higgs and supersymmetry were predicted because the standard model cannot be all there is.

    That is true about Higgs, but not supersymmetry. Supersymmetry was predicted for entirely aesthetic reasons, and not because of any standard model shortcoming.

  30. Eric says:


    Supersymmetry is predicted because it provides a solution to a big problem in the Higgs sector of the Standard Model. A Higgs which is a fundamental scalar receives large loop corrections which should push its mass to the Planck scale. These corrections may be cancelled by introducing supersymmetry partners. This is called the hierarchy problem and is one of the main topics mentioned in this post.

  31. Roger says:

    Yes, the post talks about the hierarchy problem, and beliefs that people have about it. But it is not a problem with the SM or with any experimental outcomes. It is only a problem with the beliefs that some people have. If I am wrong, please point me to something that shows that I am wrong.

  32. Eric says:


    No, the hierarchy problem is not about belief. It is quantifiable, mathematical problem with the Standard Model + Higgs. SUSY provides one way of solving this problem. It is not the only solution, just the one that has been considered most likely. I think what you may be trying to say is that it is a theoretical problem rather than a problem with experiment.

  33. Roger says:

    I am not sure why you would rather call it a theoretical problem than a problem about beliefs. It is not even a theoretical problem unless you have certain unverified beliefs about how things ought to be. At any rate, I guess that you are agreeing that it is not a problem with any observed physics.

  34. Bernhard says:


    As you already know some people disagree that SUSY offers a solution to the hierarchy problem, and as you also know I include myself among them. But in any case let’s not go there, as we know each others arguments (but I believe you agree with me this is a clear object of dispute).

    In any case: “not the only solution, just the one that has been considered most likely.”

    Likely by whom? Are you talking about Ellis, Arkani-Hamed et al? Sure. Or are you talking about particle physics community? Then, not so sure.

    By the way, what is the likelihood of this “solution” being right given the already amount of fine tuning needed? 10%? 50%? 90%?

    Let me know, since you see this as a clear “quantifiable, mathematical problem”…

  35. Eric says:


    Yes, I would assert that supersymmetry has long been considered by the particle physics community as a whole as the most likely solution to the hierarchy problem since at least the 1980’s. The only other serious contender is technicolor, but it has had problems satisfying precision electroweak measurements, as well as other issues. More recently, there have been solutions involving warped extra dimensions, and there is always the possibility of (drastic) fine-tuning with just the SM.

    The degree of fine-tuning in minimal supersymmetric models required at present is rather mild, of the order of 10%. It is still very easy for SUSY models to satisfy all experimental constraints and produce a Higgs mass ~115 GeV. If there happens to be additional couplings which are ignored in the minimal models, even the 10% degree of fine-tuning can go away.

  36. Peter Woit says:

    What I’ve found surprising, for about 30 years now, is that the “hierarchy problem” has been considered something especially significant. It’s only a problem if you have a unified theory you believe in, with quantum gravity at the Planck scale, and electroweak symmetry breaking due to the dynamics of an elementary scalar at the 100 GeV scale. If you don’t think you understand quantum gravity and how it unifies with the SM, or if you think electroweak symmetry breaking may be due to something more interesting than an elementary scalar, there’s no problem. Put differently, if you don’t think you understand what is responsible for the electroweak scale or the Planck scale, that they’re very different is not a “problem”, it’s just part of what you don’t understand.

  37. Eric says:


    The hierarchy problem is a problem in QFT with the Higgs being an elementary scalar. If this is the case, then the Higgs mass will receive large quantum corrections, regardless of whether or not we understand anything about quantum gravity. Unless there is some low-energy scale at which these corrections are cut-off, then there is no reason why the electroweak scale should be ~100 GeV, rather than some arbitrarily high energy.

  38. Peter Woit says:


    Sure, the whole set-up of electroweak symmetry breaking via an elementary Higgs field, with weak-couplings that make perturbation theory valid, has a fine-tuning problem. This just seems to me an argument that probably something more interesting is causing electroweak symmetry breaking. Hopefully the LHC will soon start giving us hints about this.

  39. Seesaw says:

    A simple seesaw already shows clearly the hierarchy problem. No need of quantum gravity or GUTs.

  40. Peter Woit says:


    Again, that’s a hierarchy problem you’ve introduced by introducing some speculative new physics at a speculative new energy scale. Until we have some better evidence for that new physics, it seems to me the “hierarchy problem” is with your speculation, not with the physics we know about.

  41. Bernhard says:

    The most popular model beyond the Standard Model
    is unquestionably supersymmetry. Its motivation is to
    ameliorate, not solve, the gauge hierarchy problem.”

    This is from Frampton in 1997:

    I think that opinions changed drastically after LEP, so I strongly disagree with your vision about what people in general think about SUSY solving or not the hierarchy problem. But OK, this is not a very solid thing to discuss.

    But Peter has as point. This is a problem only if you are sure that electroweak symmetry breaking is caused by some scalar field, that by the way, unless you are talking about bound states, was never seen in nature at all.

    You create a solution to a problem (hierarchy), that solutions has another problem (large quantum corrections to the particle that its essential for your solution (Higgs) and to solve this you create a trick that should solve everything (supersymmetry). If the trick is true you should see evidence of it, but you don´t. So you accept the fact that the trick is not perfect (little hierarchy problem) and go on.

    One begins to wonder if your original solution to the electroweak breaking was correct in the first place. Perhaps there is something else that you missed and makes the need for constantly repairing the theory unnecessary. I don´t have this solution (unfortunately) but it´s conceivable that the LHC data will gives us a good hint.

  42. Roger says:

    I looked at the above Frampton paper. He explicitly says that the motivation for susy is “not from the physics end. It does seem, on aesthetic grounds, that supersymmetry is likely to be used by Nature in Her fundamental theory.” From this I conclude that susy is only likely if you have certain mystical beliefs. If I am wrong, I would like to see the paper showing that I am wrong.

  43. Bernhard says:


    I believe what Eric was trying to say is that if you assume that the cause of electroweak symmetry breaking is a scalar field, then you have a mathematical problem, for you expect a number and you get another one that is way larger than this expectation. This is not a mystical belief. It´s to solve this discrepancy that people invented supersymmetry and if it´s elegant or not, well depends who is looking at. Now, if you should assume that the origin of the electroweak symmetry breaking is indeed this scalar field in the first place is a whole different story, as this might well be the wrong assumption. But this assumption is not mystical in nature.

    If you want to see mystical assumptions in physics talk with String theorists since their best argument seems to be a mystical feeling that the theory is right and that non believers in Witten are infidels 😀 .

  44. Eric says:


    Yes, I agree with you that the argument that SUSY solves the hierarchy problem is on less solid ground since LEP and also with the latest results from the LHC. This undoubtedly causes some people to worry, probably more than they should at this point. My point is that SUSY still can solve the hierarchy problem, though not as naturally as before. However, it is still the best and most likely solution to the hierarchy problem.

    You and Peter are absolutely correct that the hierarchy problem arises only if the Higgs is a fundamental scalar. Technicolor models attempt to avoid this by having the Higgs be a composite state of some strongly coupled sector. Other variants of this idea are top condensates.

    I should also say that there is no requirement that SUSY must solve the hierarchy problem in a completely natural way. The important thing is that it can get the job done.

  45. Seesaw says:

    Eric says “From this I conclude that susy is only likely if you have certain mystical beliefs.” What you call mystical beliefs other people call beauty and theoretical ingenuity. It can fail but it can also bring glory. Examples abound and they should be familiar to all the readers of this blog.

  46. Seesaw says:

    Sorry Eric, I meant Roger!

  47. Seesaw says:

    By the way, it would be good that those impatient people who want to prematurely exclude SUSY remember the long story of failed expectations regarding the top mass value. Of course the theoretical need for the existence of the top affects the picture. Take that need out and many people would have considered the top as a purely mythical belief and bla bla bla. History repeats itself.

  48. Emile says:

    Hi Peter,

    There is something I don’t get about what you wrote: “Again, that’s a hierarchy problem you’ve introduced by introducing some speculative new physics at a speculative new energy scale. Until we have some better evidence for that new physics, it seems to me the “hierarchy problem” is with your speculation, not with the physics we know about.”

    Maybe it is just some distinction between the hierarchy problem and the naturalness problem.

    As you know, the SM, something we’ve tested quite well, includes a scalar field. If we don’t find that scalar field, the SM is wrong. You also know that the corrections to its self-energy increase in proportion to the energy cutoff you use. I don’t need to introduce speculative new physics or hierarchies for this statement to be true. Nothing in the SM prevents me from doing calculations at high energies. Nothing in the SM says that I can only use it below 1 TeV or 100 TeV or whatever. So the SM has a theoretical problem in that the Higgs’ mass is “unnaturally” small. It could be that this is just the way the Univserse rolls… Of course, anthropic arguments will immediately explain the fine tuning 🙂

  49. Peter Woit says:


    I’m not sure what your question is…

    One point is that the “naturalness” problem is just one of several reasons to be unhappy with an elementary scalar Higgs (others are the renormalization group behavior of its couplings, and the way Yukawa couplings are arbitrary). It might be a good idea to wait and see if the LHC shows the Higgs to be an elementary scalar at TeV energies, or gives us a hint about something more interesting. If it still looks like an elementary scalar, I’ll still be unhappy about the state of the theory, with “naturalness” not the most serious problem to my mind.

    If you instead want to look at the problem as a “hierarchy problem”, i.e. why are two hugely different energy scales kept separate, you first need to be sure there’s a second energy scale (i.e. we have no actual evidence for GUT or Planck scale physics).

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