Hype or Not Hype?

The high point of my expertise in condensed matter physics was about thirty years ago, when I studied the subject in order to pass one of the general exams at Princeton. At the party after the test was graded, Phil Anderson came up to (after a fashion…) compliment me, noting that he was glad to see that even though I hadn’t been able to solve one of the condensed matter problems, I had known enough to realize that the calculation I was trying to do was giving a result that couldn’t be right and had written that on the test.

Since then, my little understanding of the subject has slowly decayed over the years, so I’m in no position at all to evaluate claims made about new advances. Recently there has been a lot of interest in applications of gauge/gravity duality to certain condensed matter systems, and this week there’s a new article out in Science (not available on the arXiv itself, but based on this arxiv preprint), together with a press release from MIT. This has led to news stories headlined String Theory Explains Superconductors, and String theory and black holes show a possible path to practical superconductors. This latest story starts off:

A leading candidate for room temperature superconductors is the copper compound cuprate, but no one knew how cuprates facilitated superconductivity…until some brave souls looked inside a black hole and broke out the string theory to explain how they work.

So, hoping that there might be someone expert on this out there and willing to comment, what’s the verdict: hype or not hype?

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65 Responses to Hype or Not Hype?

  1. Professor Elephant, Ph.D. says:

    Hype. Without much doubt.

  2. Dr Ron says:

    Well, it’s 30yrs since I worked in condensed state physics for my PhD, and it had pretty well lost its way back then. To my casual glance, this is the application to the condensed state of some peculiar substances of some of the well-established mathematics that has been developed in the course of studying string theory, just like we happily fiddled with Green functions way back when. Perfectly plausible. Damn all to do with String theory per se, but it makes a better headline.

  3. Peter Orland says:

    It sort of makes sense to use a new tool (i.e. AdS-string methods) to try to study as many problems as possible, and that’s my impression of what is going on.

    Like you, Peter, I not a condensed-matter insider, but I try to keep up with cond-mat. In the two or three years many people have tried to use string theory in condensed matter physics, but I don’t think there is a definite outcome. The big problem in high-T_c (from my ignorant standpoint) is non-Fermi-liquid behavior (in many contexts). I am not aware of any definite progress on this front using any method. I don’t have any specific comment on the paper you mention, though.

  4. Peter Orland says:

    OK, I just glanced at the paper. It seems to be an attempt to describe a non-Fermi-state using anti-deSitter methods. It is not a microscopic approach, though. With no microscopic theory (e.g. a Hamiltonian for the electrons/holes) it isn’t clear to me what it all means.

  5. Peter Orland says:

    I suppose one can have an effective theory (like the Ginzburg-Landau approach). I can’t see whether this string-AdS model is even an effective theory in that sense, since it seems to just start with some high dimensional system. Maybe a real non-Fermi-liquid should behave the way the authors claim, but it is not clear to me.

  6. neo says:

    I don’t know whether it is hype, but it seems simple to decide. Theorists need simply to show how string theory provides a single prediction about superconductivity that is not predictable without string theory but which is or eventually can be confirmed. Then everyone will pay attention.

  7. Peter, you are going soft on us. The p.r. even has Polchinski saying physicists can throw in some gauge/gravity duality whenever they cannot understand a system. Do you really think that there is some chance that black holes will explain the conductivity of rare metal compounds?

  8. Anonymous says:

    It would be interesting to find out Xiao-Gang Wen’s opinion about AdS/Condensed matter. Originally a student of Ed Witten, now a professor in the condensed matter theory group at MIT, his research has spanned string theory, quantum gravity, and (NON-string related) high Tc superconductors. (Though I’m not sure whether he still follows string theory.)

  9. spin fluctuations says:

    I have recently been involved in the new iron superconductors. Judging by who gets invited to give lectures at conferences, it seems theorists who do first principles calculations have made important contribution in this field than people who study various models. There are some papers written by Sachdev, Viswanath, etc. about superconductors, but most people don’t seem to pay attention to them.

    In superconductors, the most important questions are what are the pairing interaction and symmetry. I don’t think string theory has anything interesting to say about that and people in condensed matter theory and experiment don’t seem to care about AdS/CMT correspondence.

  10. nbutsomebody says:

    Hype 75%
    Not hype 10%
    Who knows ? 15%

  11. nbutsomebody says:

    My previous rating was for the usefulness of AdS/CMT activity. If the question is about pairings in cuprate etc. (what peter asked) then

    Hype 99.5%
    Who knows ? 0.5%

  12. anonymous says:

    The claims are overstated in the press release, but AdS/CFT does provide an interesting handle on strongly coupled field theory at finite density, a subject that goes back several years. The MIT group’s paper as well as one or two other papers in this area have found some of the hallmarks of strange metal behavior, but in models that are known not to be the correct theory for the cuprates or any other real material. The hope is that some aspects of the AdS/CFT theories can survive an extrapolation to real materials, but there has not been a concrete proposal for a systematic way to improve the AdS/CFT models to make them realistic.

  13. ab says:

    Not hype.

  14. Avidan says:

    You gotta be kidding, of course its hype. I hear numerous AdS talks on this superconductor crap and unfortunately read more on high-Tc than most of the speakers (and their advisors).

    The definition of high-Tc superconductors according to string- theorists is: any gravity background, which if applied on with the correspondence, that is unlikely to work except for N=4, will produce something with funny dispersion relation.

    Oh God, what has science has come to. God bless wall-street for giving these people something to do once they figure out that there are no jobs in this swindle.

  15. nbutsomebody says:

    ab,

    Most of it is hype. Are you kidding ?

  16. Smurf says:

    The authors of the paper managed to construct a gravity background which seems to be dual to a system with non-Fermi-liquid-like behavior. (This is not the first paper on the subject though.) Everything else is hype, but what is not hype in today’s media?

  17. nbutsomebody says:

    Although unrealistic the non-fermi liquid part is reasonably interesting. However any connection with cuprate superconductors and claim that gravity duals may be used to understand the high-Tc pairing mechanisim is pure bullshit !

  18. somebody says:

    “However any connection with cuprate superconductors and claim that gravity duals may be used to understand the high-Tc pairing mechanisim is pure bullshit !”

    Its not clear to me that in high-Tc superconductors (which are strongly coupled) “pairing” is a good description because particles are an inherently weakly coupled notion. At best “pairing” can count the charge of the condensate in terms of the fundamental charge, but beyond that it is not really useful.

    Anyway, the message that I draw from the holographic superconductor business is that many of the features of high-Tc superconductors are universal, and captured by gravitational systems via AdS/CFT. This is scientifically interesting. Nobody in the field is under the illusion that they have a theory for any *specific* cuprate. The problem is morally as hard as finding the standard model among the vacua of string theory.

    The hype in the public media (if it exists in this context) is such an omni-present problem that I will leave that fight to braver souls.

  19. Anonymous says:

    AdS/QCD is remarkable because it is the *only* theoretical tool (not counting lattice QCD) which calculates quantities such as entropy / viscosity ratio. According to the MIT press release, AdS/CMT is also the *only* model that offers an explanation of the behaviour of “strange metals” such as linear dependence of resistivity on temperature. (In the article, Liu says “There’s really no theory of how to explain that”) If this is indeed the case, I would say they are probably on to something!

  20. Peter Orland says:

    Anonymous,

    A strong-coupling calculation of the ratio of entropy to viscosity is found analytically (not numerically), without using the AdS correspondence in

    http://arXiv.org/abs/0810.4181

  21. Peter Orland says:

    My point is not that one can’t learn from the AdS correspondence in some contexts (I believe one certainly can). It is that it is just one tool among many (such as that mentioned above for the entropy/viscosity ratio), and is heavily oversold as solving heretofore unsolved problems. It’s high time to solve some of these strongly-coupled problems, not to sell the proposed solution.

  22. anonymous says:

    There are already multiple ways to explain linear resistivity in the holographic setting (the one in the MIT press release, and Lifshitz theories and generalizations to name two). This may be theoretical progress, but the effective theory of real high Tc materials is an open problem, and still of great interest.

  23. somebody says:

    Sorry but I might be missing something. Seems to me that strong coupling expansions of the kind in that paper (0810.4181) are believable only a-posteriori. It is essentially a purely classical computation (even though naively it uses the path integral in terms of Wilson loops). It is not clear what renormlaization means here (in a theory like QCD this is not forgivable), it is not clear where the confinement-deconfinement transition is etc. The latter is essential because it is only in the deconfined phase that one can talk about viscosity to entropy ratio.

    I agree it is not a worthless calculation, but to call it “another technique” seems a bit too much. Why did nobody do it until AdS/CFT? The authors quote the AdS/CFT result explicitly and one of the authors has written papers on string theory …

  24. nbutsomebody says:

    “Anyway, the message that I draw from the holographic superconductor business is that many of the features of high-Tc superconductors are universal, and captured by gravitational systems via AdS/CFT. This is scientifically interesting. Nobody in the field is under the illusion that they have a theory for any *specific* cuprate. The problem is morally as hard as finding the standard model among the vacua of string theory.”

    really ?, many features of high-Tc superconductors are universal, and captured by gravitational systems via AdS/CFT ? This is the most standard hocus-focus most AdS/CMT people will write in the introduction of their papers (well, I have to do it too :)). The whole point is that there is no reason to believe such claims.

  25. nbutsomebody says:

    It is undeniable that viscosity and entropy ratio calculation is a big triumph for string theory. Other applications of AdS/QCD, while not exact, are also sufficiently interesting. AdS/QCD is a standard part of various QCD and lattice literature. Most reviews contain some holographic results etc. atleast for a comparison purpose.

    However the question here not so much with AdS/QCD but with AdS/CMT. Is it possible to learn something novel about condensed matter system by applying ideas of AdS/CFT ? I would like to stick with a resounding nooo…

  26. chris says:

    somebody,

    your argument is really priceless. yes, 0810.4181 does make uncontrolled approximations – but at least it starts out with the right theory.

    but well, if you think that this is far worse than some estimate in an N=4 maximally supersymmetric extension of QCD…

  27. somebody says:

    ” many features of high-Tc superconductors are universal, and captured by gravitational systems via AdS/CFT ? This is the most standard hocus-focus most AdS/CMT people will write in the introduction of their papers … The whole point is that there is no reason to believe such claims.”

    Really? All you have is that ideological pronouncement?

    Before AdS/CFT, how often has anyone been able to theoretically produce even the condensate curve of a strongly coupled superconductor? Getting generic (universal, if you prefer) condensates, conductivity, gap, etc. are simple via AdS/CFT. I am not repeating a talking point I read somewhere: it is simple and I know how to do it. The point is that this is still not enough in understanding any *specific* high-Tc superconductor.

    To check what you call a “claim” for its veracity, you just need to open one of the papers and read beyond the “hocus pocus” in the introduction.

  28. somebody says:

    ” … but well, if you think that this is far worse than some estimate in an N=4 maximally supersymmetric extension of QCD…”

    All this is at finite temperature, so SUSY is broken. The bigger problem potentially is that N=4 SYM is conformal, whereas QCD is confining. But at finite temperature conformal invariance is broken and the deconfined phase is what we are after.

    But yes, I agree that it is a matter of judgment.

  29. Peter Orland says:

    somebody,

    We’ve been over this before. Yes strong-coupling approximations are not controlled approximations. But AdS/QCD is also a strong-coupling approximation.

    This is a renormalization-group issue. The only correct way to make a strong-coupling approximation is to integrate out all the short-wavelength degrees of freedom. Unlike at weak coupling (near the fixed point, where the theory becomes simple), this means the effective theory is filled with irrelevant operators. Until that is done, I can’t trust the claim that a lattice or stringy strong-coupling method comes from QCD. I don’t know if you work on AdS/ACD, but many who do work on it, to their credit, understand this point.

  30. Peter Orland says:

    Anyway, the only reason I brought up the lattice paper on entropy/viscosity is to point out that AdS is not the *only* (claimed by one of the many Anonomouses) method for doing analytic calculations. The real problem is that there is *no* analytic method which is clearly correct.

    I think the situation is similar in condensed-matter physics. Ultimately the goal is to calculate using a microscopic Hamiltonian, not a model. Does this mean string methods are not useful? Of course not. It just means they haven’t solved these problems yet.

  31. somebody says:

    Peter O., I am not sure I understand what you have in mind. For instance, it is not clear at all to me what you mean by microscopic description. From one of your previous messages it looks like what you want is to understand high-Tc superconductors in terms of a Hamiltonian for electrons and holes. Electrons and holes are useful only when the theory is close to a free limit. It is meaningless to talk about them away from it where the degrees of freedom can be completely different. If it was a UV free theory or something, there would be some meaning to saying that they *are* the true degrees of freedom (like quarks in QCD), but here we only care about the IR of the theory.

    In fact, this is the whole reason why it is not a Fermi liquid: high-Tc superconductors have charge carriers that don’t behave like free particles on a fermi surface. It is a mistake to think that the microscopic Hamiltonian for superconductivity is comprised of electrons and holes: the microscopic Hamiltonian is that of the standard model of particle physics. It is indeed an unsolved challenge to see how the (dressed) electrons and phonons of BCS theory arise from the standard model. But we don’t care, because that is not what we mean by solving superconductivity. What we mean when we try to “solve” superconductivity, is precisely to identify the right degrees of feedom where the problem can be understood quantitatively: we are trying to GUESS the degrees of freedom, because deriving it is impossible. This is what BCS did, for usual (weakly coupled) superconductors: they guessed that they were phonons etc. In a strongly coupled high-Tc system, if a gravitational/stringy system can be the right degrees of freedom, then that answers the problem as completely as BCS theory. Yes, a dual black hole is more dramatic than a phonon, but that is hardly an argument against it.

    Wanting to have a theory in terms of electrons and holes, as you say in a previous message is simply not the right approach, IMHO. If there is a string vacuum which can reproduce a /specific/ superconductor, then that /is/ the theory we want, and the problem is solved. I personally think it is a looooooong shot, but it does seem useful for understanding some generic features.

    PS: As a final comment, I use the word “hole” loosely, because you used it. Doping and superconductivity seem somewhat distinct to me: but I hardly know any condensed matter, so what do I know?

  32. nbutsomebody says:

    “To check what you call a “claim” for its veracity, you just need to open one of the papers and read beyond the “hocus pocus” in the introduction.”

    Actually, I wrote few of them 🙂

    AdS/CMT does not produce a condensation curve which may not be reproduced by Landau-Ginzberg theory. However you are quite correct that calculation of conductivity and especially the mass gap etc. is indeed a novel result. I personally find it very interesting. However the results one gets is rather unphysical and mass gap is actually zero. It changes to a non-zero value as we shift to a non-abelian p-wave holographic superconductor. I do not want to go into more technicality as Peter does not like a lot of technical discussion in this blog. However the bottom line is quite clear that it is unclear (sorry for the pun) whether there is something generic about strongly coupled superconductors. Hence we may not say much about high-Tc etc from the study of holographic superconductors. The goal in high-Tc business is to learn about some specific superconductors. Generic results possibly do not have much role to play.

    Also look at Peter Orland’s post about uncontrolled nature of whole AdS/CFT business. Another point is that here the system we are looking at is rather different than conventional superconductors. It is a lorentz invariant system, there is no lattice, it is a large-N gauge theory and the list goes on and on. The whole holographic superconductor business is actually just the study of bose condensation in a holographic context. That is why it is sometimes called holographic superfluid and has more closer relation to flavor superfluid phase in QCD phase diagram.

    To set the context correct there was no debate about the relevance of studying holographic superconductors. It is possibly interesting and relevant. Although it is unlikely that it would provide a deep insight in cuprates.

  33. Peter Orland says:

    “Wanting to have a theory in terms of electrons and holes, as you say in a previous message is simply not the right approach…”

    It’s not? I thought it was the goal. Isn’t that what we’re supposed to do is describe phenomena in terms of the most basic theory we can?

  34. nbutsomebody says:

    Peter Orland,

    “It’s not? I thought it was the goal. Isn’t that what we’re supposed to do is describe phenomena in terms of the most basic theory we can?”

    I think so. That is what I have been informed by my colleagues in condense matter.

  35. somebody says:

    Peter O., contrary to the comments of the other somebody, there is strong experimental evidence that the system cannot be captured by a weakly coupled Fermi liquid. That is, free electrons etc. cannot be a good description in a high-Tc superconductor. (for eg. linear resistivity above Tc cannot arise from a weakly coupled description.)

    I emphasize that even in the BCS theory, the degrees of freedom (not sure why you call them microscopic, they are strictly effective) were *guessed*. (Dressed) Electrons and phonons there, are effective, in no sense fundamental, and also we do not know how to derive them by integrating out some fundamental theory.

    The thing about BCS that made things useful, was not that it was fundamental, but that these effective field theory d.o.f were weakly coupled.

  36. somebody says:

    “Actually, I wrote few of them ”

    So you were being disengenuous, and not ignorant. You have got me between the devil and the deep sea …

    “AdS/CMT does not produce a condensation curve which may not be reproduced by Landau-Ginzberg theory.”

    No condensate curve can ever be /not/ produced by some Ginzburg-Landau theory. So this point is moot.

    “However you are quite correct that calculation of conductivity and especially the mass gap etc. is indeed a novel result. I personally find it very interesting. ….”

    You seem to be going back and forth between this and the other extreme. About your other comments like p-wave etc.: the real high-Tc superconductors are d-wave, so I would say it is encouraging that a hard gap can be found by going from s to p.

    But anyway, there are other problems when you get to d-wave because the spin is too high etc. My point: that some general characteristics of superfluid/superconductors can be captured by holographic methods is not bad. This already warrants further inevstigation. I am not going to do it myself pbbly, because I prefer more fundamental questions, but I am glad someone is doing it.

  37. Peter Orland says:

    Yes, somebody (capitalized Somebody?), we know it isn’t a Fermi liquid. The disagreement is not a technical one, so there is no need for a technical justification.

    Physics is supposed to explain things, as well as describing them. It’s fine to derive Green’s functions from a scheme, but you need to derive them from a theory. Schemes and theories are not (always) the same.

    AdS methods are another tool and more power to the people who use them. The point people are making on this thread is that AdS/CMT is not enough. You have to justify it on the basis of something you know about matter. Similarly AdS/QCD needs to be justified from QCD. That, in a nutshell, is the problem.

  38. somebody says:

    I do not agree that the Fermi liquid issue is merely a technical point. The fact that high-Tc is a non-Fermi liquid is of fundamental significance in this discussion. You seem to want to apply the particle/hole/phonon idea in a situation where I see no reason why it should be applicable.

    Also, why are you not unhappy that nobody knows how to derive BCS theory from standard model or QED? Why is BCS any more explanatory, than descriptive in that sense? Is it because you have a mental picture of a metal with free electrons etc. in it?

    “The point people are making on this thread is that AdS/CMT is not enough. You have to justify it on the basis of something you know about matter.”

    From my side, it looks like you are just uncomfortable with the idea of a dual description, where the original degrees of freedom have re-organized (or at least re-expressed) into something completely different. You rationalize that discomfort by looking suspiciously at AdS/CFT.

    Wanting a full proof of AdS/CFT is a noble thought and certainly interesting. But note that in the history of QFT it would’ve been a bad idea to first make the path integral rigorous. Working with it instead, was what lead to progress especially in the second half of the last century. My strong prejudice is that trying to prove AdS/CFT in a full way is a bad idea at this stage. I am convinced that AdS/CFT is correct, not just because of the specific and detailed tests, but also because the beautiful way in which too many different things (like details of gauge theories, gravity and black hole thermodynamics) tie together.

    Also, this thread is really about the specific issue of condensed matter applications, not the general viability of holographic methods. I am not interested in the latter discussion because having worked with various aspects of gauge-gravity duality, I think a blog discussion is unlikely to change my opinions on it.

  39. Peter Orland says:

    Somebody, my discomfort isn’t with dual descriptions (which I have worked on, by the way). I am not trying to cast suspicion on this idea or that idea. I am trying to cast suspicion on declarations of victory.

    I and nbutsomebody (whoever he or she may be) are just trying to make a non-technical, unbelievably simple, point.

    In physics we try to understand how behavior emerges from theories. No one is saying we should derive BCS directly from the standard model, which would be a very foolish thing to try to do. But B, C and S derived superconductivity from a theory of Nature (electrons and phonons). We can understand how this theory arises from QED, another theory of Nature. This in turn arises from the standard model, yet another theory of Nature. There are some mathematical gaps (the second law of thermodynamics, crystallization), but we have a pretty good picture of how it all this fits together.

    Ultimately, we would like to understand non-Fermi-liquid behavior from a Theory of Matter. Phenomenological models are stepping-stones, not the answer. This is very basic to science.

    Here is a better analogy than BCS. Kepler’s laws are great, but they aren’t enough to explain how the solar system works. A theory of mechanics and gravity is more fundamental. Maybe in the 17th Century, someone thought science should settle for Kepler’s laws. Wasn’t it a good thing some scientists didn’t believe that Kepler’s laws “explained” astronomy? I think so.

    Having taught courses for non-physicists, I know that many non-technically-trained people understand this. Don’t you?

  40. somebody says:

    “But B, C and S derived superconductivity from a theory of Nature (electrons and phonons). We can understand how this theory arises from QED, another theory of Nature.”

    No, we don’t know how to get BCS from any more fundamental theory. This is the point. You are just comfortable with the idea that BCS *might* be derivable from QED, because the ingredients are not too exotic. The degrees of freedom are not the same as in QED, the vacuum we are working with is horrendously complicated (a metal), we really have NO idea. BCS were clever to ask the correct (effective) questions to get the correct (effective) answer.

    If (a big if) holography delivers, this will precisely be the situation with high-Tc. The only difference is that the description of the system is in dual variables, in terms of black holes in AdS. It will be phenomenological, exactly to the same extent that BCS theory is. The comparison is not at all with Kepler’s laws. It would indeed be great if we could derive effective theories from fundamental theories, but that is generically not possible with most mesoscopic phenomena.

    In particular, it is not clear to me that the phonons etc. are somehow useful for any purported “microscopic” description.

    “Having taught courses for non-physicists, I know that many non-technically-trained people understand this. Don’t you?”

    🙂

  41. Peter Orland says:

    What a lot of baloney!

    Somebody, if you don’t think that science uncovers deeper descriptions, then why do it?

  42. Peter Orland says:

    … and since you don’t want to argue the real issue and prefer to hide in a smokescreen of jargon…

    BCS applied mean field theory and a Bogoliubov transformation to a model of electrons and phonons. They didn’t start with the answer, they derived it! BCS is not a phenomenological theory. I don’t know where you came up with the nonsense above.

  43. somebody says:

    “Somebody, if you don’t think that science uncovers deeper descriptions, then why do it?”

    Its money for nothin, and chicks for free in physics. Thats why. 🙂

    You will have better luck with me, if you phrased your commentaries as commentaries and not rhetorical questions. Lets not re-interpret my criticisms about specific misunderstandings you had, into some deep and profound differences in our “views” on science. Science is of course about deeper truths, what I am saying is that these deeper truths do not fit with your prejudices.

    I imagine that there will be hell to pay for my glib comment above: Woit gets all politically correct when string theorists are being tongue-in-cheek. So I will be bowing out.

  44. Peter Orland says:

    My questions were not rhetorical. I was really curious.

    You misinterpret my disagreement with you as a technical issue. It’s really not technical. That’s why I tried to turn the conversation to plain words, not jargon.

  45. somebody says:

    “BCS applied mean field theory and a Bogoliubov transformation to a model of electrons and phonons. They didn’t start with the answer, they derived it! BCS is not a phenomenological theory. I don’t know where you came up with the nonsense above.”

    Aaaaaargh, you drag me in again! 🙂

    Where do you have phonons and dressed electrons in any fundamental theory, sir? QED doesn’t have them! Is this so difficult to understand? What you call a derivation is what I call BCS theory already. That derivation is trivial, and one can essentially do away with it by just writing down the relevant operators once one identifies the degrees of freedom. The point is that it is forbiddingly difficult to derive it from a (microscopic) theory like QED where the degrees of freedom look different: photons, fermions, etc, are different from the phonons etc. of BCS. This is why BCS is an *effective* theory.

  46. Peter Orland says:

    Yes, the degrees of freedom look different. No, we can’t rigorously derive electron-phonon systems from QED. But that doesn’t mean we have no understanding of how they arise from QED.

    I apologize if I have made you so frustrated. My friends (it’s hard to believe I have any, I admit) and relatives all tell me how frustrating I can be.

    There are outstanding problems in deriving certain results from deeper models. Some of us fall back on approximations, and sometimes hand-waving arguments. A small number of people try to prove propositions and theorems to make some of the connections more carefully (like how states of matter appear in more fundamental theories). I am not saying it is easy.

    Your viewpoint seems to be that we can’t really explain anything. Well, it’s very tough, but occasionally we can.

  47. somebody says:

    Phew, at least we seem to be beginning to understand each other…. lol! Thanks for playing and thanks for staying honest.

    Just one comment:

    “Your viewpoint seems to be that we can’t really explain anything.”

    Not all phenomena, just mesoscopic ones. In general I think trying to truly derive mesoscopic phenomena can be quite hard, but I am not sure thats the equivalent of surrendering. It is usually a detail thing, rather than a concept thing – or so we hope. Because usually, we in particle physics regard the low energy physics as somehow understood, even though it is not really understood. From what you say, it seems like this reductionist hope is ultimately what you vote for too.

    But the interesting thing about AdS/CFT is that it is the great equalizer in energy scales: quantum gravity can be dual to things at lower energy scales. Its a bit like concept and detail are dual to each other. This reverse direction of the arrow is what is interesting from a fundamental physics point of view. A detailed explanation of high Tc superconductivity is secondary (at least to me) – no matter how interesting it might be, from a practical point of view.

  48. onymous says:

    Maybe an actual condensed matter expert can correct me, but my understanding is that the CM community doesn’t have even a toy calculable model that exhibits all the basic features of the cuprate phase diagram — antiferromagnetism, the pseudogap, superconductivity, a Fermi liquid phase at higher doping — or a good guess at the universality class of the suspected underlying quantum critical point. So if AdS/CMT could give a toy model that encompasses all of these phenomena, it might be of interest. My (not thoroughly up-to-date) understanding is that AdS/CMT can give toy models of critical non-Fermi-liquid-like phenomena, tunable to achieve any desired scaling behavior, and hence isn’t so close to doing this. But one could imagine it might be possible, and would count as a success.

    As far as matching to the correct theory, though, AdS/CMT is dual to something with large N and large ‘t Hooft coupling, and thus is in the same situation as AdS/QCD; it’s a toy model that exhibits a lot of the right phenomena, but it’s very far from being the system one is interested in, and there’s little concrete hope for bridging the gap in the foreseeable future. This doesn’t make it useless, though; toy models are a worthwhile part of physics.

  49. nbutsomebody says:

    Somebody,

    “No condensate curve can ever be /not/ produced by some Ginzburg-Landau theory. So this point is moot.”

    It can be found. I am surprised that you do not know it.

  50. nbutsomebody says:

    “So you were being disengenuous, and not ignorant. You have got me between the devil and the deep sea …”

    No I was not. It is natural in a scientific literature to speculate when it is explicitly mentioned as so. It becomes disingenuous when those speculations are advertised as a important scientific finding in the popular media.

    “My point: that some general characteristics of superfluid/superconductors can be captured by holographic methods is not bad. This already warrants further inevstigation.”

    Not bad and not too great either. There are various investigations going on even on less important matter. Surely it deserves some investigation and no hype.

    “You seem to be going back and forth between this and the other extreme.”

    I am not the center of the universe, nor are you. Just the fact I find something interesting does not mean it is extremely important.

Comments are closed.