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A prediction from string theory?

Image from Barbara Jacak’s talk. Credit:

By Margaret Harris

Skeptics find much to complain about in string theory, but perhaps their most stinging criticism has been its inability to be falsified by experiment. A few years ago, one string theorist even told me that a particle accelerator big enough to “see” a string would be so large that its opposite ends would be causally disconnected. So this is not a problem we’ll be solving any time soon.

Yet even if we’ll never see a string in the lab, it turns out that string theory does make a few predictions about how matter should behave at the quantum level — and now physicists from the apparently unrelated disciplines of heavy-ion collisions and cold fermions are coming tantalizingly close to testing them.

According to Clifford Johnson of the University of Southern California in Los Angeles, the theory of black holes in 4+1 dimensions (four spatial dimensions plus time) makes a prediction about the ratio of a “perfect” fluid’s viscosity to its entropy. For reasons he didn’t have time to explain in a 15-minute talk, this ratio must be greater than 1/(4 pi).

Now, scientists at Brookhaven National Lab have long been interested in studying the viscosity of the quark-gluon plasmas they create at the Relativisitic Heavy Ion Collider (RHIC). Barbara Jacak of Stony Brook University noted that RHIC data have contained a number of surprises in recent years, including the fact that debris clouds from relativistic gold ion collisions behave more like a liquid than a plasma, with a near-zero viscosity.

Unfortunately, this “quark soup” only exists for tiny fractions of a second after each collision, which makes accurate viscosity measurements extremely difficult. This is where the cold fermions come in. John Thomas of Duke University works on cooling and trapping samples of ^6^Li atoms down to less than one microkelvin above absolute zero. At such temperatures, the Li atoms behave like a near-perfect liquid, too — and they stick around long enough to let Thomas and his group measure their viscosity and entropy.

Thomas told me that they’re still working on refining their measurements, but preliminary data in his talk showed minimum viscosity-to-entropy ratios tailing off at right around the critical value. It’s not quite a confirmation yet, but as Jacak noted, these results are exciting enough to get researchers who “don’t even drink at the same bars, let alone attend the same conferences” talking seriously.

Maybe the orthographically-challenged cat in the photo (which Jacak showed in her talk) is onto something.

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  1. Hi Margaret,
    Thanks for the post on this. A few key remarks are in order:
    (1) I did not focus on the “minimum” issue largely because that is not really the issue at all (and further, I prefer to use the word “natural” value, not “minimum” value, since it is only a conjecture that it is a minimum).
    (2) The reason lots of us are excited is because the experimental results are creating a phase of matter in a regime where the traditional techniques for those disciplines do not do so well at describing their unexpected properties (it is a strongly coupled system), or at least use extremely complicated methods to make sense of small portions of it. On the other hand, rather simple computations in these models constructed using string theory with quantum black holes in higher dimensions seem to naturally describe just the kind of fluid phases with the unusual properties experimentally observed. This is exciting. Good open-minded physics uses good robust tools wherever they can be found, and there is an emerging picture (that still needs more research to see how far it goes) that says the the right organizational tool for getting at the physics in question is string theory, which is rather nice for a lot of reasons. How far this can be pushed is a matter of further research, as should always be the case with any methodology.
    (2) You, perhaps unintentionally, made it seem like there was is just one little number that is being discussed. Actually, it is a _whole_ _class_ of behaviour that is being captured experimentally, and described theoretically by the string theory methods, with several measurable quantities associated with them. The precise behaviour of the energy density of the fluid with the temperature is one of the first signs that it is not behaving as an ideal gas. This is one of the simplest analytical computations you can do with the string models, and it comes out quite readily to match what was eventually inferred from the experiments. The behaviour of probe quarks as they shoot through the fluid displays a striking shock wave – again something that you see beautifully in the string computations…. and so on and so forth. For further reading on the RHIC side of things, I recommend looking at a review by, for example, Ed Shuryak.
    (3) An important point to emphasize overall is that there’s something quite marvellous emerging. The string theory work shows that there is a reasonably wide class of fluids with very specific (and from some perspectives) unexpectedly peculiar properties. That’s nice in itself, but the great thing is that examples of this class of fluid seem to be showing up in two (so far) experimental contexts. That’s, I think, the best way of stating the content of the prediction, if the term provocative term “prediction” need be used at all. From a physics perspective, I find this rather exciting. New behaviour of matter is found in the lab and there is a theoretical framework in which this behaviour readily emerges and can be described.
    I talk a lot more about this on my blog, by the way (including further reading, etc.) For example, see a report I did on a 2007 workshop on some of this:

  2. A few minor clarifications on your very nice post:
    (1) It’s the ratio of viscosity to entropy density that is predicted to be 1/(4 pi). This is the value when the coupling in the fluid is as strong as possible, so the conjecture is that all real fluids must have a ratio of viscosity to entropy density exceeding 1/(4 pi). There have been some string-theoretic calculations of counter-examples to this bound, so it’s a very interesting topic for further research.
    (2) Entropy density is entropy per unit volume. For those trying to work this out at home, please note that the value of 1/(4 pi) is in natural units with hbar and Boltzmann’s constant k set equal to 1. In standard units, the bound is hbar/(4 pi k). This might suggest a relation to the uncertainty principle, which is correct.
    (3) The value of the viscosity in the plasma is very large, rather close to that of glass at the annealing point. But the entropy density of the plasma is also huge, so the ratio turns out to be very small and quite close to the conjectured bound. This might seem like a bait-and-switch, but it’s actually the ratio of viscosity to entropy density that determines the response of the fluid, much like the engineer’s “kinematic viscosity”. For more details, see

  3. Zou Ling-Nan

    Interestingly, a jet of fine sand impacting a target also seems to behave as a near-perfect fluid. The origins appears to be purely kinematic and independent of the sorts of interactions involved. See Cheng et al., PRL, 99, 188001 (2007); (also available at Disclosure: I’m a colleague of the authors.


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