Strongly Correlated Physics in a Superposition State

It's been a while since I posted anything science-y, and I've got some time between flipping pancakes, so here's an odd thing from the last few weeks of science news. Last week, there was an article in Nature about the wonders of string theory applied to condensed matter physics. This uses the "AdS/CFT" relationship, by which theorists can take a theory describing a bunch of strongly interacting particles in three dimensions (such as the electrons inside a solid), and describe it mathematically as a theory involving a black hole in four dimensions. This might seem like a strange thing to do, but sometimes the black-hole problem turns out to be more tractable, allowing the calculation of useful things that would be difficult in the original model.

By itself that's not odd-- AdS/CFT has been a big deal in string theory circles for years, and "string theory a success!" has been a reliable way to sell magazines for longer than that. What's odd is that nearly simultaneously (according to some observer moving at a speed close to that of light, anyway), Bee posted an article at Backreaction showing AdS/CFT failing miserably in its original domain. The original problem it was supposed to solve involved properties of the quark-gluon plasma produced in heavy ion colliders like the Relativistic Heavy Ion Collider on Long Island, or some of the experiments at the LHC. In the plot Bee shows, though, the theoretical prediction from AdS/CFT doesn't come anywhere near the experimental points.

So, it's sort of odd to see these two things crossing in my newsfeed. AdS/CFT is a miracle for a non-particle-physics branch of science! But it's failing badly in the particle context! Umm, OK? I guess it's quantum...

I've seen a lot more of the Success! side of this, because it was in Nature, and has been re-shared about a thousand times on Google+, Twitter, and Facebook. (As an aside, I would really like a way to tell my social networking services "I've seen this, please don't show me the next five hundred reshares of the same article/ video/ cat picture." This would cut my Google+ time by an order of magnitude or more.) I think the contrast is interesting, though, so here's my tiny bit of signal boost for that side of the story.

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glad then I didn't share the story on G+...

Also, as I understand it, it should also be noted that this approach hasn't yet led to major new breakthroughs in superconductivity. (and this article doesn't mention any either). I know also some people working on superconductivity who are a bit critical of the use string theory. But it's too early for judgements, and in any case intriguing to see where this approach might go!


I stumbled over this statement in the Nature article

"...some of the testable predictions from string theory look a tad bizarre from the condensed-matter viewpoint. For example, the calculations suggest that when some crystalline materials are cooled towards absolute zero, they will end up in one of many lowest-energy ground states. But that violates the third law of thermodynamics, which insists that these materials should have just one ground state..."

so it seems that some basic properties come out wrong.

I looked briefly at the post at backreaction, expecting to find a reference to a paper which I can then read. I did not. Instead there is a quick claim (based on one transparency from a talk) that a specific model of AdS/CFT, with a whole host of other assumptions, as applied to one specific observable, give poor results. This is such a faint signal that it leaves everyone the freedom to interpret it according to their a priori inclinations, and indeed they do.

For what it is worth, I think it is clear to anybody in the string community that holographic methods would not apply to any observable whatsoever, and making it "failing miserably" by misapplying it is not all that hard. Much more interesting and productive is to see where such methods can be useful, instead of being faux-Popperian and try to "falsify" calculational methods which show a clear potential but whose range of applicability is not yet clear.

So in the end, it is not all that different story from your…. Like that story, this one has an autonomous existence in the blogosphere and pop science circles. I doubt either one of this stories will have too much impact on the actual scientific discourse on either one of these subjects.

Hi Moshe,

Maybe look again, there's a reference at the end of the post, proceedings from the same guy who gave the talk, but in the paper the figure doesn't contain the AdS/CFT line. I also wrote that yes, there are additional assumptions that go into the fit, heavy ion physics is a messy business, but it clearly doesn't look good. I think we'll probably hear more about that at some point. Best,