String Theory Finds a Bench Mate

There’s a nice article this week in Nature about AdS/CMT, entitled String Theory Finds a Bench Mate. According to the article, the whole thing is (partly) my fault:

But in 2006, string theory took a public battering in two popular books: Not Even Wrong by Peter Woit, a mathematician at Columbia, and The Trouble With Physics by Lee Smolin, a physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. Both books excoriated the theory’s isolation from experiment.

“It’s hard to say whether the interest in condensed-matter applications is a direct response to those books because that’s really a psychological question,” says Joseph Polchinski, a string theorist at the Kavli Institute for Theoretical Physics in Santa Barbara. “But certainly string theorists started to long for some connection to reality.”

The main point of the story is to tell about what is probably the hottest topic in hep-th these days, attempts to use AdS/CFT to say something about some models in condensed matter physics. For some idea of what this is all about, see the review article What can gauge-gravity duality teach us about condensed matter physics? by Subir Sachdev, and take a look at the online talks from the KITP workshop Holographic Duality and Condensed Matter Physics.

The article does go into the history of this in some detail, including its roots in efforts to use AdS/CFT to say something about heavy-ion physics phenomena observed at RHIC (for the string theory promotional campaign surrounding this, see e.g. here). I had expected to see a lot about this topic when higher energy results from heavy-ion collisions at the LHC were released earlier this year, but it seems to have gone quiet, perhaps because of the kind of comparison of data with AdS/CFT predictions that Sabine Hossenfelder points out here:

As the saying goes, a picture speaks a thousand words, but since links and image sources have a tendency to deteriorate over time, let me spell it out for you: The AdS/CFT scaling does not agree with the data at all.

My knowledge of condensed matter theory is minimal, and the hype level surrounding string theory makes it hard to know whether to take at face value many of the claims being made. On general principles, this looks a bit more promising than the heavy-ion case, since there are many different kinds of systems one might look at, and the connections are more to QFT than to string theory. Experts quoted in the Nature article give opinions ranging from:

Polchinski admits that the condensed-matter sceptics have a point. “I don’t think that string theorists have yet come up with anything that condensed-matter theorists don’t already know,” he says. The quantitative results tend to be re-derivations of answers that condensed-matter theorists had already calculated using more mundane methods.

to condensed matter theorist Andrew Green’s:

“Maybe string theory is not a unique theory of reality, but something deeper — a set of mathematical principles that can be used to relate all physical theories,” says Green. “Maybe string theory is the new calculus.”

Time will tell whether this suffers the same fate as in the case of heavy ions.

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9 Responses to String Theory Finds a Bench Mate

  1. Nige Cook says:

    Here in England, tonight BBC2 TV just screened a “Faster than Light” program with Michael Duff giving some string theory hype to explain the alleged 60 ns “faster than light” neutrinos. Duff stated that the results could be explained by neutrinos leaving our 4-d brane, taking a super-fast short-cut through the 11-d bulk, and then appearing again on the 4-d brane nearer the detector. Eventually, after claims about the experiment being “proof” of string, they showed Duff making a more sensible comment that he wasn’t actually hyping the experiment as proof of superstring or even confirmation of a falsifiable prediction from string theory. But it left the viewer in no doubt that superstring offers the only “real” good explanation. Duff illustrated the 4-d brane with a slice of bread, using a loaf for the 11-d superstring bulk.

  2. SteveB says:

    Gerard ‘t Hooft has updated his web site. Similar to your quote from Andrew Green, in the quantum gravity section ‘t Hooft now says:

    …many of my colleagues are convinced that “string theory solves the problem”. But why does this happen? How does string theory resolve the paradox? Curiously, string theorists themselves do not quite understand this. But I think I might understand this now. String theory is just an instrument to do calculations in regions of a theory that are otherwise inaccessible.

  3. Peter Woit says:

    Duff can really be relied upon when string theory hype is needed. There’s a page about this program here:

    It seems to have been produced in record time.


    ‘t Hooft is referring to AdS/CFT’s supposed role in resolving the information paradox about black holes, which is a somewhat different topic. Whether it does this is independent of whether it provides a useful approximation method for certain condensed matter systems.

  4. Spinons says:

    As a condensed-matter theorist, I don’t really appreciate the AdS/CFT papers. Although they might be useful in the vague sense of being a phenomenological model, they don’t really solve anything fundamental. For example, this Gauge/Gravity paper on Science: Faulkner et al., 329 (5995): 1043-1047 talks about their findings of a class of non-Fermi liquids through the AdS/CFT correspondence, with a tantalizing application for the normal state of high-Tc superconductors. However, such approach couldn’t give any clue at all about the mechanism of the high-Tc superconductor, or even what makes the electrons non-Fermi liquid. It is beyond their capability.

    With the arXiv flooded with this kind of take-the-easy-route papers, I was wondering whether this correspondence between AdS and CFT is really well established as rigorous mathematical theorems ? Probably not…

  5. lun says:

    Spinons, while I am skeptical of AdS/phenomenology in general, I do not think your argument works: With the same reasoning, you would have also had to ditch the Ginzburg-Landau theory of superconductivity, since it also offered no clue as to the origin of the scalar field responsible for spontaneusly breaking U(1).
    Your namesake origin, spin-charge separation became famous well-before any microscopic explanation was even proposed, if I remember correctly.

    The 10^n dollar question is what are the necessary requirements for a physical system to be described by a holographic theory. How do we know if a generic field theory is dual to SOME string theory?
    For QCD, the “obvious” answer is that classical gravity is only good for SU(N), and hence one either needs to quantize gravity or concentrate on things that dont change between N=3 and N=infinity. I would be interested to know if something similar exists in condensed matter.

  6. Spinons says:

    lun, but Ginzburg-Landau theory is indeed a phenomenological theory, and is indeed very useful. So as I said, the AdS/CFT might be useful in this sense if this gravity/gauge equivalence is really well established.

    The GL theory finds even more applications than superconductivity. But as far as the (old) superconductivity problem is concerned, only BCS theory provides the true explanation and advances our understanding of superconductivity (electron-phonon coupling, cooper instability, etc).

    As you must have known, the GL theory can be derived from BCS theory. Without this connection, I believe the GL approach is not much different from what engineers are doing.

    I guess what GL approach also symbolizes is the “universality” aspect of physical systems. Different microscopic models share the same field theory at the critical point. The AdS/CFT is also similar in this spirit. But again, it makes more sense to talk about universality when you have a bunch of explicit model systems which show similar behavior.

    I’m not so sure about what you said “spin-charge separation became famous well-before …” But in 1D this is well known theoretically with explicit lattice models long ago. Although there are a lot of talks about fractionalized excitations in high dimensions, but without at least some lattice models or (better) experiments, it is indeed not much different from stringy speculations in my opinion (fortunately, we have several such models already)

  7. lun says:

    Agree with spinon. Having said all that, for Peter, see my comment
    at your link

    While I am convinced QCD is fundamentally different from any theory with a classical gravity dual, I think ruling out AdS/CFT as phenomenology for heavy ions is premature,

  8. Derek Teaney says:

    Dear Peter,

    I posted this remark at Bee’s web site.

    I think this post of Bee is off the mark, when it comes to
    heavy ion collisions.
    In particular the remarkable aspect of the
    new data is the success of hydrodynamics
    in describing the new data on the higher
    harmonics flow when the shear
    viscosity is of order 1-3/4\pi . This was the truly
    new (and remarkable) aspect of the new data from
    the LHC. It is certainly very difficult
    to reconcile this with weak coupling . I spoke
    about the higher harmonic flow at the
    AGS Users meeting:

    (see Current Theory Perspective, D. Teaney)

    Certainly when the momentum becomes
    large p_T >> Lambda QCD \sim T it
    becomes increasingly dubious to use strong
    coupling methods. So, although there
    are many reasons to be skeptical of the
    AdS/CFT this example (which Bee used) does not change
    my opinion one way or the other.

  9. David Brown says:

    My translation of “Confutatis maledictis, flammis acribus, voca me cum benedictus” is “When the cursed sinners are condemned to the hot flames, may the Lord call me among the blessed.”