This Week’s Hype

Last month’s Quark Matter 2011 conference was a venue for discussion of new results from the first heavy-ion run at LHC energies last fall. I’ve looked a bit at the slides of the talks, but this is an area far from my expertise. One thing I’ve been wondering about is whether the heavily-promoted application of AdS/CFT to studying heavy-ion physics could possibly be tested at the LHC. Does AdS/CFT make any distinctive predictions about how things will change as one goes from RHIC energies to LHC energies, and have these been checked? Looking at the slides, there seem to be all sorts of interesting things being learned about heavy-ion physics, but little mention of AdS/CFT modeling of such phenomena. Perhaps an expert can help by pointing to pre-LHC predictions, and explaining whether they’ve been tested already, or may be in the future.

Symmetry Breaking magazine today does cover Quark Matter 11, with String theory may hold answers about quark gluon plasma, which appears to mostly contain the same hype about string theory and heavy ion physics that has been current for the last half-dozen years now:

Now, scientists have begun to see striking similarities between the properties of the early universe and a theory that aims to unite gravity with quantum mechanics, a long-standing goal for physicists.

Unfortunately there’s nothing in the article about any LHC test of these ideas. The closest we get to that is this from Krishna Rajogopal (his talk is here):

“String theory is like a gift to us,” Rajagopal said. “We’re challenged with understanding the quark-gluon plasma as a liquid, and while string theory doesn’t give us precision, it can help us get a feel for the shape of the subject.”

So, I gather that AdS/CFT makes no precise, testable predictions, with the best case to be made for it that “it can help us get a feel for the shape of the subject”, whatever that means. A question for experts: if “String theory may hold answers about quark-gluon plasma”, what are the questions for which string theory is giving answers, and what does the LHC data have to say about these questions?

Update: David Mateos has posted a write-up of his Quark Matter 2011 talk here. In it, he explains what the problems are with using AdS/CFT to say anything about QCD. In terms of the question of LHC predictions, he gives an example: the dispersion relation of heavy quarkonium mesons moving through the quark-gluon plasma. Unfortunately, this doesn’t look like much of a prediction:

I emphasize that whether one obtains a visible peak, simply a statistical enhancement or an unobservable effect depends sensitively on many parameters related to the in-medium J/Psi physics. The latter is not sufficiently well understood to make a precise prediction, so all one should take away from figure 3(right) is that there could be an observable effect for some values of the parameters within the acceptable range.

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41 Responses to This Week’s Hype

  1. Derek Teaney says:

    Peter,

    Testable predictions seems to be asking for too much — as the correspondence
    (at its best) is for maximally supersymmetric theories.

    But, it would be certainly unfair to claim that the AdS/CFT has not to heavy ion physics (and vice versa) . For instance, the most striking observations at the Quark Matter conference were the remarkable wealth of new data on hydrodynamic flow (higher harmonics) . In simulating RHIC and LHC events we use a second order hydro formalism which was fully worked out (through work in AdS) by Baier, Romatschke, Son, Stephanov, Starinets. Steve Gubser, recently found an analytic solution to the hydro equations using methods inspired by AdS which will be useful to even the most phenomenological heavy ion smashers, especially those concerned with higher harmonics (myself included). It is also interesting that traditional heavy ion people spoke about work in AdS/CFT. For example, in my case, instead of speaking about flow, I spoke about a new approach to hawking radiation in non-equlibrium geometries which drew heavily upon experience with non-equilbrium field theory (based upon the 2PI formalism).
    I view the AdS/CFT as an extreme limit which can be useful when confused by
    the new data. For example, while it the shear viscosity to entropy ratio may not
    be 1/(4*pi), the new data are placing new constraints which suggest a range (1-3)/4\pi . Berndt Mueller (who could have spoken on any number of current heavy
    ion topics) also gave an interesting talk on thermalization in AdS/CFT
    here.

    In summary, while the correspondence may not provide precise quantitative predictions, it can hopefully provide guidance and insight to complex physics.

  2. Conformal says:

    Whatever the status of AdS/CFT as physics, it has inspired some deep mathematics, e.g. in differential geometry. That means that it has not been a waste intellectually.

  3. Yatima says:

    Could anyone point me to a succint explanation of where the “liquid” or “hydro-” methapor comes from in this case and what idea it is supposed to encapsulate?

  4. Bernhard says:

    Derek Teaney,

    What is very clear from what you are saying is that traditional criteria to handle a certain theory and test it, somehow does not apply to AdS/CFT since it does not provide precise quantitative predictions. “Provide guidance and insight to complex physics” is not enough since one cannot be sure if the theory is correct in the first place, so this guidance could be wrong. Asking for testable predictions is NOT asking too much, is asking for what makes science to actually work. What I would like to see if whether anything being done in heavy ion physics can say if AdS/CFT or ST is correct. Clearly this is not the case, if and when results don’t match with AdS/CFT it does not mean its wrong and even when it does “give guidance” , one is at best confirming a test of equivalence between two theories so no real experimental test was done.

  5. Derek Teaney says:

    Dear Bernhard,

    Regarding “What is very clear from what you are saying is that traditional criteria to handle a certain theory and test it, somehow does not apply to AdS/CFT since it does not provide precise quantitative predictions. ”

    This statement more less discounts models for providing qualitative insight to complex physics. Think back to QCD for how wrong this is — for confinement and asymptotic freedom the Gross-Neveu model; for chiral symmetry breaking we have the linear sigma model and the nambu jona lasinio model; for small x physics (important to heavy ions) the McLerran Venugopalan Model; in energy loss in QCD (important to heavy ions) the Gyulassy-Wang model… None of these models provide “precise quantitative predictions”, but all have been extremely useful at capturing one or more aspects of complicated physics

    Of course AdS/CFT does make precise quantitative predictions for N=4 SYM theory with a large number of colors and strong coupling. So if “only” the N=4 theory could be simulated on the lattice then the predictions of the correspondence could be tested.

  6. A. says:

    At LC2011 last month there were several talks about AdS/CFT and AdS/QCD. The bulk of questions from the audience were from disgruntled older physicists asking, slightly more aggresively than I would have guessed, “But what does this have to do with real physics?” to which there were some refreshingly honest answers, such as “AdS/CFT has probably taught us nothing about real QCD.”

    I hadn’t seen this kind of attack/surrender pattern before.

  7. Bernhard says:

    Dear Derek Teaney,

    Thanks for the answer, that was very interesting. You have a good point that qualitative insight is indeed useful and the examples you gave are very good. But for asymptotic freedom even if the prediction is not all mathematically precise it is certainly not ambiguous and highly testable.

    What I find most interesting is your statement:

    “So if “only” the N=4 theory could be simulated on the lattice then the predictions of the correspondence could be tested.”

    If you care to enlighten my ignorance a bit more, are you saying that if this simulation were possible one could test AdS/CFT in the sense that it could also proved to be wrong? I know this would not test ST in any sense, but it would indeed be an exceptional achievement.

  8. Peter Woit says:

    Derek,

    Thanks for the detailed and informative content. This still leaves me with the same general impression I got from the slides I was looking at: AdS/CFT provides an interesting toy model to play with, but it’s far from being the sort of thing that is in any sense experimentally testable. The descriptions of the situation provided by some physicists to the press are pretty outrageously over-hyped.

    It would be useful if people kept separate the different sorts of issues involved here about confronting QCD with experiment which your last comment to some extent mixes up. By my count, there’s

    1. purely toy models, like Gross-Neveu (which is a 1+1d model), which can’t in any sense be confronted with experiment.

    2. sigma models, which provide a good approximation to the low energy behavior of the theory in certain sectors. These can be confronted with experiment in some regimes.

    3. the McLerran Venugopalan and Gyulassy-Wang models you mention, which I know nothing about.

    4. the comparison of AdS/CFT-based calculational methods with other non-perturbative calculational methods in N=4 QCD (e.g. the lattice), which provide checks on the validity of the methods, but have nothing to do with confrontation with experiment.

  9. J says:

    Derek: I am very happy to see you here making nice explanations about quark-gluon plasma physics.

    And guys, let’s be fair and honest.

    Does string theory or more narrowly the AdS/CFT provide precise, quantitative, and testable predictions for QGP physics? I think the answer is NO. Further I think most people in the field of QGP physics hold the same awareness, despite whether they would like to speak this atitude out or not.

    Does this tool AdS/CFT useful for the QGP physics? YES, actually very much. Again I believe a lot of people (maybe not the majority, but certainly a fair fraction) of QGP field share this opinion. Look, physics is different from mathematics in many aspects: one distinction of course is the experimental test; yet another distinction is that physics often gets boost from conceptually useful ideas and models even some of those are not the complete story or even wrong. I think playing wild cards a bit is healthy and good for the development of physics, and it is fair to say the application of AdS/CFT for QGP physics has produced many intellectually useful developments.

    Of course, over-selling (on purpose or not) is not good. But over-killing is probably another extreme. The only thing that matters, I guess, is that the people who are doing QGP physics know very well the boundary of the usefullness and bullshit-ness for applying AdS/CFT to QGP.

  10. Peter Woit says:

    J,

    Thanks for the comment. I assume most experts in the field do know what’s useful and what’s bullshit in this story. Unfortunately, the press coverage has been uniformly misleading about this.

  11. srp says:

    As a complete outsider, it’s kind of interesting how “wild” the theory of the strong force still is, even after it’s been incorporated into standard physics. It sounds like it’s still very hard to figure out exactly what the theory predicts in any specific case, but I’m a bit confused about just where we are. For example, would it be correct to assume that the proton is the model system where everything can be calculated pretty well from first principles? How about something like a helium nucleus? Is there a quark-gluon derivation of hydrogen to helium fusion?

    Naively, if people are trying to study heavy-ion collisions with zillions of pieces flying around you’d think that the simpler cases had long been solved, but I’m not sure because I’m aware that sometimes messy things have to go first experimentally. In any case, it sounds like there’s lots of important work to do.

  12. Derek Teaney says:

    This is my last (long) post on this

    Dear Peter,

    There are models which are somewhere in between toy models such as Gross
    Neveu and more rigorous approaches such as Chiral Perturabtion theory. For
    instance, the Nambu-Jona Lasino model does provide a picture of chiral symmetry
    breaking and does have a phase transition at finite temperature to a chirally
    symmetric phase. This qualitative feature of NJL models was of course confirmed
    by lattice (i.e. real QCD) only (much) later.

    Lets talk about the Gyulassy Wang model: in this case the medium is
    replaced by random static scattering centers — this is certainly not the real
    quark gluon plasma. Nevertheless, the Gyulassy Wang model can be used to study
    the radiative energy loss of high energy partons propagating through random
    static scattering centers (as opposed to QCD). (Much) later when people worked
    out real QCD (at weak coupling) it turned out that the Gyulassy Wang model
    captured almost all the essential physics, e.g. the probe energy and path
    length dependence.

    One of the qualitative lessons from the strongly coupled N=4 theory is
    the absence of quasi particles. This leads to qualitative predictions for
    spectral densities (i.e. photon production rates) which are markedly different
    from weakly coupled expectations. I (and others) wrote about this
    here . (See figure 5.)
    While no one would reasonably compare the N=4 theory to real QCD
    spectral densities it is interesting wether this qualitative result survives.
    At the quark matter conference, there was, what I would describe as the first
    serious effort to extract the current-curent spectral density here. The lattice spectral functions (which are not without systematic
    effects) are disasterously inbetween the quasi-particle theory and the strong
    coupling theory

    Another lesson from the N=4 theory is the appearance at strong coupling
    distinct inversion of scale, Temperature=T << \sqrt{\lambda} T. lambda =
    g^2Nc is the 't Hootft coupling. In weak coupling non-abelian plasmas the
    scales are exactly reversed \sqrt{\lambda} T << T. The not particularly
    smalln Debye mass (which is proportional to \sqrt(\lambda)) for real
    numbers forces perturbation theory to go to very high orders and to treat m_D
    as variational parameter. This was discussed at quark matter
    here , by Nan Su.
    In fact naive first order application of perturbation theory in
    QCD the Debye mass is several times the temperature. All of this makes the N=4
    theory and interesting theoretical foil to the weak coupling calculations.
    In particular, the scale inversion implies that the dynamics of high energy
    probes are distinctly different depending on how the energy compares to these
    scales, ie. T << E << \sqrt{lambda} T. Or T<< \sqrt{\lambda} T << E . It is
    interesting to look for such inversion of scales in the heavy ion data.

    Peter and Bernhard,

    Regarding hypothetical simulations of N=4 theory at finite temperature (which are basically impossible, but who knows…). It would provide numerical evidence for an amazing web of conjectures starting with the Black Hole entropy and arriving that the pressure of N=4 SYM at Large N is 3/4 of the steffan Boltzman pressure. It also would strongly suggest (What do you think Peter) that a non-perturbative formulation of the string theory exsists.

  13. Peter Woit says:

    srp,

    Yes, calculations in QCD are amazingly difficult, and for many aspects of it we still have no good calculational methods. The proton itself is still imperfectly understood, and calculating nuclear physics quantities (or heavy-ion physics ones) from first principles is currently pretty hopeless. The amazing thing though is that when something can be reliably calculated (perturbatively, lattice calculations, symmetry arguments) there is agreement with experiment.

  14. Peter Woit says:

    Derek,

    Thanks for putting the effort into explaining more about some of these issues.

    In principle, calculations at arbitrary couplings and number of colors of N=4 SYM could show that AdS/CFT works, and that you can approximate the theory in some regimes with strings or with supergravity. Even if you did this though, the only well-defined theory you have is a quantized Yang-Mills theory, a QFT. You still can’t start from the other end of the duality and write down a well-defined theory whose fundamental variables are strings. What one would really like is a version of the duality that allowed you to start from either end, writing down a complete theory in terms of either strings or gauge fields. Right now you’ve only got the QFT half of this.

    One aspect of this is that you can (in principle!) define precisely any QFT, in any background. AdS/CFT is only going to give you a precise theory you may want to call a “string theory” in one specific background. You can generalize this with more general gauge/gravity dualities, but I still don’t see anyone making progress on the question of what string theory in general backgrounds really is non-perturbatively, i.e., what is M-theory?

  15. Peter Orland says:

    In the last few years, I made a few comments on this blog about AdS/QCD. I said something along the lines of:

    The real strong-coupling theory is found by a renormalization group. Take QCD with a cut-off up at the Planck mass. Integrate out all the degrees of freedom with wavelength bigger than 0.5 Fermi. There is no universality (the strong-coupling action depends on the cut-off method) and tons of irrelevant operators in the action. THAT’S the right strong-coupling theory. I was metaphorically excoriated as ignorant or worse, as a result.

    Anyway, strong coupling is an old story. In the 1970’s, Hamiltonian lattice theorists (some of whom later turned to string theory) got a rough caricature of the hadron spectrum, by working at very strong coupling (total confinement without any real gluon dynamics). Their predictions were not that different from the quark model (and they did not get good results for mesons, like the quark model w/o current algebra). Something better came along, namely Monte-Carlo simulations (though it took a long while for them to be much better).

    This just means AdS/QCD is a model and not the actual strong-coupling limit. I gather most comments above are saying precisely this.

  16. chris says:

    As a relative outsider to QGP physics i hope not to offend people in the field, but my general impression is that heavy ion physics and quark matter is basically still a mess. and it’s no wonder why this is so: strongly coupled multi-particle systems are notoriously difficult from the theoretical side. and (non-equilibrium) thermodynamics of a short-lived collision product of hundreds of strongly coupled particles is a really tough experimental job. experimentalists frankly admit that they can’t determine the (pseudo-)critical temperature even. and theorists are fighting about the correct value for years now (on top of which the hydro is really difficult).

    in this general setting, things like AdS/QCD are extremely welcome. i have repeatedly witnessed senior heavy ion people trying to understand the lifetime or equilibration time of the QGP in terms of dual black holes. these people know that the model has its limitations but it seems to me at least that the competition is not doing so much better in terms of rigor. the advantage of string duality methods is of course the relative ease. the competition is lattice gauge theory and hydrodynamic simulations – both really tough and computationally intensive. once the field matures – and that may take a very long time – models will hopefully be obsoleted and replaced by ab-initio understanding. but as it stands, AdS/QCD seems to be one model among many and probably not the worst at the moment.

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  18. Wolfgang says:

    “… the only well-defined theory you have is a quantized Yang-Mills theory, a QFT. You still can’t start from the other end of the duality and write down a well-defined theory whose fundamental variables are strings.”

    I don’t think that’s the right way to think about this.

    N=4 super Yang-Mills should be seen as the nonperturbative definition of Type IIB string theory on AdS5xS5. That definition implies a prediction: that the large-N, large ‘t Hooft coupling limit of N=4 SYM is described by classical supergravity in AdS5xS5.

    That is a surprising and unexpected prediction. (Not totally-unexpected: it was long-conjectured that large-N Yang-Mills is a classical theory; it’s just that no one suspected that the “master field” was a higher-dimensional supergravity.) But it really is a prediction, and it has lots of consequences that have been checked by explicit calculations.

    Furthermore, if you supplement the above with an additional hypothesis: “Strongly-coupled gauge theory plasmas have certain universal features of their behavior.” then this prediction about N=4 SYM has experimental consequences for the QGP.

    Of course, “certain universal features” is a bit vague. That’s why AdS/CFT is, at best, a model for the QGP. Still, it seems to capture a lot of the important features of the QGP, which is why the nuclear theorists are excited.

  19. Peter Orland says:

    “Furthermore, if you supplement the above with an additional hypothesis: “Strongly-coupled gauge theory plasmas have certain universal features of their behavior.” then this prediction about N=4 SYM has experimental consequences for the QGP.”

    Right, this is what people say, and I am certain it is plain wrong. There is nothing universal about BARE strong-coupling theories, where the cut-off is large. So, knowing someone will call me an ignoramus or just plain evil, I will elaborate a bit on my comment above.

    In field theory and critical phenomena there is nothing universal in a random point of the phase diagram (including the coupling as well as the other parameters) of a cut-off theory. Thus, the strongly-coupled theory with a cut-off has NO universal properties. Universality is something that applies to theories near a critical point (where the cut-off is gone!), which in the case of gauge theories means small bare coupling. The renormalized coupling can be large.

    I am convinced this bit of conventional wisdom some AdS people say about the QGP is false. These AdS/QCD theory are models. You can do phenomenology, perhaps even good phenomenology with them. That doesn’t make them universal.

    It is a technical point, but it is important.

  20. Wolfgang says:

    “There is nothing universal about BARE strong-coupling theories, where the cut-off is large.”

    You are right that there is nothing “universal” about the UV behavior of QFTs. QCD is weakly-coupled in the UV.

    But QCD is strongly-coupled in the IR, and it is that IR behavior (at finite temperature) that we are discussing.

  21. Peter Orland says:

    No, I did not mean UV behavior. I meant IR behavior.

    In asymptotically-free theories, the real bare coupling (not the renormalized coupling) goes to zero as the cut-off is removed. It is the properties of the theory with no cut-off that are universal. If you have a large gauge coupling in a cut-off (but otherwise local) field theory, nothing is universal. Not viscosity divided by entropy, not parameters of elliptic flow, not anything!

    If the bare coupling is small, so that the theory is near the continuum limit, you have universality in a quantum field theory. That is not the case here.

  22. Peter Orland says:

    I should add that often it seems there is much confusion concerning the phrase “strong coupling”. There is a difference between a cut-off theory with a large bare coupling and the strong-coupling (IR) limit of a gauge theory. The strong-bare-coupling theory is just a model. It it the strong-renormalized-coupling theory which is unique, i.e. universal. In this theory, the bare coupling is infinitesimal, not big.

    The bare coupling is not physical. It must be taken to zero with physical quantities fixed (this defines the r.g. equation). This is true for any observable, large Wilson loops or other probes of IR included.

  23. Wolfgang says:

    “If the bare coupling is small, so that the theory is near the continuum limit, you have universality in a quantum field theory. That is not the case here.”

    I assume by “here,” you mean N=4 SYM at large ‘t Hooft coupling (since, for QCD, the bare coupling obviously is small). N=4 SYM is conformally-invariant. So you are always near the continuum, at any value of the bare coupling.

    “The bare coupling is not physical.”

    True in QCD, but not true in N=4 SYM.

  24. Peter Orland says:

    But heavy ion physics is not N=4 YM!

  25. Wolfgang says:

    In QCD, the bare coupling is unphysical, and is tuned to zero to obtain the continuum limit. In N=4 SYM, the theory is conformal, the bare coupling is physical, and you are near the continuum, for any value of the bare coupling.

    In neither case do your comments, above, seem to be relevant. When you said

    “If the bare coupling is small, so that the theory is near the continuum limit, you have universality in a quantum field theory. That is not the case here.”

    what did you mean by “here”?

  26. Peter Orland says:

    “Here” is heavy-ion physics, described by (renormalized) QCD. Again, I am not criticizing using AdS methods as models. I just think this talk about universality has no basis.

    If you claim universality, you need a reason. I don’t see why AdS models should give universal answers. There is nothing universal about a strong-bare coupling AdS/QCD model. Certainly nothing universal about an entirely different theory like N=4 Yang-Mills. People say it’s so, but they don’t provide a reason.

  27. K. Kajantie says:

    I think one could draw some parallels to QCD around 1970. Then there was an extremely successful constituent quark model, counting rules, parton model, etc, but really no idea why they worked so well. Now we have some very suggestive observational facts (p=3/4 p_ideal, shear viscosity, pattern of quark energy loss) but again no quantitative theory, only rough idea. Then around 1972-3 came QCD Lagrangian, asymptotic freedom and quantitative predictions for logarithmic scale violations in deep inelastic scattering. It took a few years to verify these and convince everybody of the fact that QCD is the correct theory. This is the stage we are missing today and until a QCD-string theory dual permitting quantitative predictions is there, there is no way to convince everybody. This is, of course, a completely trivial statement. Unfortunately this dual seems extremely remote and also there is much less help from experiment to guide us there.

  28. Wolfgang says:

    ““Here” is heavy-ion physics, described by (renormalized) QCD.”

    Then the bare coupling has been taken to zero, and I don’t see what you are going on about.

    “There is nothing universal about a strong-bare coupling AdS/QCD model.”

    Perhaps I don’t know what you mean by “AdS/QCD”, but in N=4 SYM, the theory is conformal, the bare coupling is physical, and the strong bare coupling behavior IS universal (in the commonly-used sense of the term “universal”).

    You seem to want to apply either the intuition from QCD (where the bare coupling is unphysical) to N=4 SYM (where the bare coupling is physical, and your intuition is incorrect), or you seem to want to work in QCD and take the bare coupling to be strong (which is NOT a sensible thing to do).

    I can’t figure out which of them you have in mind, but neither of them seems correct.

  29. Peter Orland says:

    Wolfgang,

    I made no comments on the validity of calculations for N=4 theories, just for QCD.

    I said only one thing: I see no justification in claims of universal predictions for heavy-ion physics. It is a pretty clear statement, even if you don’t agree.

  30. Wolfgang says:

    “I said only one thing: I see no justification in claims of universal predictions”

    Your statement was about the strongly-coupled lattice gauge theory of QCD. But there is no justification for using the strongly-coupled lattice QCD. To the contrary, one wants to tune the bare lattice gauge coupling to zero.

    Whether or not there is some sort of “universality” in the behavior of strongly-coupled gauge theory plasmas, your statement seems to have no bearing on the question.

    But perhaps I have misunderstood your argument. Why is the strongly-coupled lattice QCD relevant to heavy ion physics?

  31. Peter Woit says:

    Wolfgang,

    Your discussion with Peter Orland I fear is going nowhere. My understanding is that he’s simply objecting to your claim about “universal behavior” in N=4 SYM and asking for justification. What exactly does this mean and what is the argument? Presumably it can’t be the same kind of universality argument one can make for asymptotically free theories by putting them on a lattice, and defining the continuum limit by going to the critical point at zero coupling.

    His reference to strong-coupling lattice theory I take as an analogy: there’s a strong-coupling limit in lattice gauge theory, but there’s nothing universal about it.

    As for your response to my posting: of course AdS/CFT makes a prediction, a very interesting one, but it’s a prediction about a specific QFT (N=4 SYM in a certain limit). Again, what’s well-defined here at all couplings is a QFT and statements can be made about that. You don’t have a method for defining string theory at any coupling. All you can do is, in a very specific background, define away the problem of “what is non-perturbative string theory” by saying that whatever it is, it’s the same thing as a well-defined QFT.

  32. Peter Orland says:

    Wolfgang,

    OK, I’ll try one more time. Here is what I said:

    No arbitrary theory with a cut-off and large bare coupling (which includes both lattice and modified AdS approaches. The latter go by the name of AdS/QCD – not standard N=4 susy YM) can give universal results. These are strongly-coupled theories, but they are the wrong strongly-coupled theories.

    I wasn’t commenting on theories with vanishing beta functions, though it is hard to see why these should produce results in the same universality class either. They have no QCD-type scale, and completely different matter content. There is no reason to think their infrared behavior resembles QCD.

    Summary – I don’t believe AdS methods give solid predictions for heavy-ion physics, though I don’t doubt some good phenomenology can be done.

    I tried to give some justification for this statement in terms of the renormalization group.

    I don’t know what I can add to make the above points clearer.

  33. Peter Orland says:

    Wolfgang: When I posted the last comment, I did not see Peter W.’s clarification. He summarized it very well.

  34. Wolfgang says:

    “… he’s simply objecting to your claim about “universal behavior” in N=4 SYM and asking for justification”

    The only justification is a semi-empirical one: there are a number of 4d theories (not just N=4 SYM), for which one can form an AdS/CFT correspondence. They have different amounts of supersymmetry, different matter content, etc. Those properties (and only those properties) of the corresponding gauge theory plasmas, which agree between these different theories, have some claim to being “universal.”

    “They have no QCD-type scale, and completely different matter content. There is no reason to think their infrared behavior resembles QCD.”

    Their zero-temperature behavior is completely different from QCD. The claim is about (certain features of) their finite-temperature behavior.

    “All you can do is, in a very specific background, define away the problem of “what is non-perturbative string theory” by saying that whatever it is, it’s the same thing as a well-defined QFT.”

    To the extent that you can do computations in that QFT (by whatever techniques you, as a field theorist have available), you are computing the behavior of the corresponding string theory in an AdS background.

    You surely didn’t expect that determining the behavior of the string theory required no computation whatsoever! So the fact that the computation has been reduced to a field theory computation (in lower dimension) is about as simple a result as one could possibly hope for.

    And that computation carries with it lots of predictions about the field theory, too: in certain limits, certain “known” features of the string theory ought to emerge from the field theory calculation (the supergravity limit was one such limit, the BMN limit is another). So one learns not just about string theory in AdS, but about (suprising!) features of field theory.

  35. Peter Woit says:

    Wolfgang,

    My objection here is one of language, not about the science. When you write “quantum field theory” this is a well-defined term: at least conjecturally, for the relevant class of theories, we know what this means (i.e. how to non-perturbatively define the theory). When you write “string theory”, you don’t: what is really meant is “something (M-theory?) we don’t even have a good conjectural definition of that has a specific limiting behavior that can be written down as a quantization of strings”. I don’t believe that the fact that certain qft’s appear to have strong-coupling limits that can be identified with weakly coupled strings changes this and makes “string theory” a well-defined term.

  36. Peter Orland says:

    Universality is a very strong statement. It means that there are universal numbers.

  37. Wolfgang says:

    “My objection here is one of language, not about the science.”

    You do have a point.

    People speak of AdS/CFT as a duality between the string theory in the bulk and a field theory on the boundary. Usually, when we speak about a “duality”, we have two independently-defined theories, which we claim are equivalent. Here, as you say, the field theory, on the boundary, has an independent definition, but the bulk theory does not.

    At best, it has a set of properties it should satisfy. In the IIB case:

    * It should reduce, at low energies, to type IIB supergravity.
    * It should have an S-duality symmetry, which acts on the axio-dilaton by fractional linear transformations
    * It should have a certain set of BPS extended objects (branes), permuted by the S-duality symmetry.
    * etc.

    The “right” way to think about AdS/CFT is to take it to be the DEFINITION of the bulk theory. Then these properties become predictions about the field theory on the boundary.

    It’s certainly true that it would be nicer if there were independent definitions for both sides of the duality. Then one would get testable predictions in both directions (not just in one direction).

    But, in the absence of such an independent definition, it is still true that AdS/CFT provides a nonperturbative definition of string theory in AdS. And it is nontrivial that this definition is consistent with all of the properties that we expect string theory to possess

  38. sanjay sood says:

    Dear Yatima,
    The “hydro” refers to the hydrodynamics of strongly coupled quark gluon plasma that one is interested in studying. AdS/CFT allows one to calculate certain properties of this plasma such as viscosity to entropy density ratio of QGP in the IR region of QCD, where it is now known to be strongly coupled. The asymptotic freedom in QCD means that gauge coupling would decrease exponentially at lambdaQCD limit. This is what makes LHC such an interesting machine. In a few years, when gets up to full power, it would be able to push beyond the lambdaQCD limit and (hopefully) one would then observe the hydrodynamics of weakly coupled plasma! Since Ads/CFT only applies for the strong coupling case, it would not be suitable (in its current form) to study this new plasma. There is some conjecture though, that AdS/CFT applies for weak coupling as well. This is what’s known as the ‘weak’ statement for AdS/CFT.

  39. Charles says:

    Rumor has it that Atlas has a “Higgs”-like excess at slightly more than 160 GeV. It has not hit the rumormill, but will do so soon. Also an excess of leptonic events at 950 GeV…..something like a Z’.

  40. Peter Woit says:

    Thanks Charles!

    It would be pretty funny if ATLAS finds the Higgs right where the Tevatron experiments claim they have excluded it…

    Anyone from ATLAS who feels like posting the abstract of any internal documents about this is encouraged to go right ahead.

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