A session on results from the LHC at last week’s AAAS meeting has generated some news reports about results from the heavy ion run, see here and here. Under the heading “String theory supported”, MSNBC reports:

Previous experiments conducted at another particle accelerator, the Relativistic Heavy-Ion Collider in New York, showed that quark-gluon plasma took on the form of a liquid. Some scientists expected the plasma to go to a gaseous state at the higher temperatures achieved by ALICE, but it didn’t. Instead, it was a “perfect liquid, which flows without resistance and is completely opaque,” Schutz said.

That in itself was a big surprise. But Schutz told me that the results were consistent with what had been predicted by a particular variant of string theory known as AdS/CFT correspondence, which also addresses such mysteries as quantum gravity and extra dimensions. “I’m surprised that they can make a prediction and that it matches what we measured,” Schutz said.

String theory is a long-debated conception of the subatomic world that envisions matter as being composed of incredibly tiny strings or membranes that vibrate in an 11-dimensional universe. Skeptics have criticized the concept as being untestable and unfalsifiable, but if findings from the LHC can confirm some hypotheses and falsify others, that could increase string theory’s acceptance.

The campaign to deal with the failure of string theory unification by confusing it with AdS/CFT as an approximate calculational method continues. No matter how successful or unsuccessful AdS/CFT is at describing heavy-ion collisions, this has nothing to do with string theory as a unified theory of gravity and the Standard Model. I am curious though about the question of how well AdS/CFT does work as an approximation for describing heavy-ion physics. Can anyone point me to distinctive AdS/CFT predictions about what the LHC should see that are now being tested? The news reports just seem to refer to evidence that at LHC energies the quark-gluon plasma seems to continue to exhibit the perfect liquid behavior seen at RHIC.

**Update**: See the comment section for an extensive discussion by someone expert in the field (Hans Juergen Pirner) relevant to the question I was raising.

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Peter,

String Theory is the best paradigm to build a theory of quantum gravity. I don’t know why, like you, but what I’ve read about ST, I can say, she has the potential to solve the unification problem in physics. ST models can unify the gravity force with the other three fundamental forces in an only consistent framework, namely, ST is the way to build the Theory of Everything. ST is the natural heir of quantum field theory. So far, ST is just a paradigm. A paradigm with the adequate features to describe all known fundamental forces. A paradigm that is compatible with special relativity and quantum mechanics. A paradigm where you can quantize the Einstein field and to obtain consistent results with the calculations. Theoretical Physicists who are working in string theory are also discovering new math tools useful in other branches of physics. How time do you think is enough to complete the TOE? I think a hundred of years would be optimistic ;-). By now I just can say, ST is spectacular.

El Cid,

Even if one were inclined to swallow all of your hype, that wouldn’t change the fact that the data about heavy ion collisions collected by ALICE has nothing to do with it.

Peter,

I think these results about the quark-gluon plasma rather behaving like a fluid than a gas are not new but discovered and reported about last year already…

So, it seems that Your google alert works not perfectly well sometimes :-P, ha ha …

Amused,

The relevant LHC ALICE results may not be new (I first mentioned them here), but the MSNBC hype is fresh this week.

(Note: I’m a big fan of the blog and appreciate the often lonely battle you’ve waged against string theory hype.)

Peter,

I went to a popular talk about string theory today by Brian Greene, expecting that all I would hear was unabashed hype. To my surprise and delight, at several points in the talk he specifically said that string theory was speculative in the sense that it didn’t yet have experimental support. I find it encouraging that, due to just criticism from people like you, some of the greatest popularizers of string theory (e.g., Brian Greene) have started conveying some of the theory’s difficulties. For instance, he mentioned the 10^500 possible geometries, and said that some people had taken the “justifiable step” of quitting work on string theory, that some were still trying to figure out a way to make it unique, and that others had resorted to the multiverse interpretation, which he openly acknowledged was very speculative and controversial. On the subject of AdS/CFT, he mentioned the recent Relativistic Heavy-Ion Collider result as being an “encouraging” result for string theory, but he certainly didn’t tout it as being proof that the theory is correct.

I don’t mean to make this about any one popularizer of string theory, but I think it shows that your critiques are having an impact, and that the public might be beginning to get a more balanced view of the theory from some of its popularizers. Good work!

(Side note – given the lack of progress in string theory, I for one would love to hear any news you might have about progress of other approaches…non-commutative geometry, loop quantum gravity, etc. Thanks!)

Reg. heavy ion physics at LHC & AdS/CFT, here’s one paper I know of

Di-Jet Conical Correlations Associated with Heavy Quark Jets in anti–de Sitter Space/Conformal Field Theory Correspondence

Jorge Noronha, Miklos Gyulassy, Giorgio Torrieri

http://arxiv.org/abs/0807.1038

Abstract: “We show that far zone Mach and diffusion wake “holograms” produced by supersonic strings in anti–de Sitter space/conformal field theory (AdS/CFT) correspondence do not lead to observable conical angular correlations in the strict $N_c\to\infty$ supergravity limit if Cooper-Frye hadronization is assumed. However, a special {\em nonequilibrium} “neck” zone near the jet is shown to produce an apparent sonic boom azimuthal angle distribution that is roughly independent of the heavy quark’s velocity. Our results indicate that a measurement of the dependence of the away-side correlations on the velocity of associated identified heavy quark jets at the BNL Relativistic Heavy Ion Collider and CERN LHC will provide a direct test of the nonperturbative dynamics involved in the coupling between jets and the strongly-coupled Quark-Gluon Plasma (sQGP) implied by AdS/CFT correspondence.”

There was also something about jet quenching and elliptic flow, but can’t recall the details. Maybe this paper: http://arxiv.org/abs/1009.2286

Brathmore,

It is of course a good thing that (e.g.) Brian Greene is prepared to be honest about the failure of String Theory; what is retarded is that the conclusion of all of this seems not to be that one should give up on it and try something else.

I heard an AdS/CFT talk recently which claimed that the thermalisation time of the QGP should depend on the boost factor of the heavy ion (I forget the precise dependence, 1/gamma?), so there was an effective prediction for the functional dependence of the thermalisation time on the centre of mass energy of the initial state.

Not an expert so don’t know to what extent this is correct, a particular feature of AdS/CFT, or a particular feature of the model used. It remains the case that N=4 SYM in the N_c -> infinity limit is not a systematic approximation to QCD.

Some proposed predictions by Casalderrey-Solana et al

http://arxiv.org/abs/0912.3717

http://arxiv.org/abs/0907.4012

Iancu, Mueller and Collaborators have come with a picture for energy loss and momentum broadening at strong coupling.

All this builds strongly on the work of Gubser and collaborators, to mention just another prominent name.

I am currently reading Brian Greene’s new book. In it, as in the talk Brathmore attended, he candidly admits there is no evidence for string theory, as yet. However, he still asserts that the evidence is around the corner at LHC. His view remains that string theory is a huge success on all fronts, except for that little matter of experimental evidence…

The fact that ADS/CFT has some fairly strong qualitative resembelance to nuclear physics seems remarkable to me. I don’t understand why you insist that S.T. must provide a cannonical explanation of such a compicated interaction before it can at least be admitted that the correspondence provides some level of evidence that the theory is on the right track.

Brathmore,

Thanks for the encouragement. I should point out that Brian has always taken pains to make sure to acknowledge the lack of experimental support for string theory. Unfortunately his caveats often get dropped in stories that make it into the press. My main disagreement with him has always been on the issue of whether optimism about this situation changing is justified.

Unfortunately I don’t know of any news about anything promising coming out of non-commutative geometry or loop quantum gravity. Personally I’m excited by some ideas about representation theory coming out of the Langlands program that I think might go somewhere, but I’m still a ways away from understanding this well enough to start promoting these ideas here. Maybe soon though…

Sam,

I don’t insist string theory provide a canonical explanation of nuclear physics. I’m quite willing to believe that it provides a useful approximation scheme in this field. However, the ferocious level of hype in this subject is pretty discouraging and makes it very hard for a non-expert to figure out exactly what works and what doesn’t work. Again, it would be very interesting to see exactly what AdS/CFT based approximations predicted about what the LHC would see and compare to what the experiments are finding. Maybe such a comparison is out there, but I haven’t seen it.

All of this though has nothing at all to do with string theory’s failure as an idea about unification. Arguments otherwise seem to me to be nothing more than attempts to mislead people and avoid owning up to this failure.

PW, you keep bashing string theory for its failure to provide a unified model for gravity plus the gauge forces. Why don’t you post an explanation of how, in your opinion, the correct model to describe such an unified reality is *excluded* from the scope of string theory?

hv,

Because no one knows precisely what “string theory” is, you can’t prove that there is no way it can lead to a successful unified theory. However, you can look carefully at what is understood about it, what the problems are keeping it from success so far, and what the history of attempts to overcome those problems is, and make an educated judgment based on those facts. That’s what I’ve tried to do in the book I wrote and in many of the blog entries here.

If they want to, people are always going to be able to come up with some reason to hope that string theory’s problems will be overcome, no matter how bad things look.

These claims about ADS/CFT predicting anything in these experiments sound very suspicious. Where are the monte carlo/detector simulations showing that? Does one really want to use vague or qualitative arguments to support this? Or did I miss the point? I doubt very much any studies on this direction exists at all.

I’m sorry, but when you say that ‘no one knows precisely what “string theory” is’, you’re wanting an exact and rigorous nonperturbative definition of string theory. However, you know exactly how that unknown theory looks like in several perturbative limits. In addition, duality relations between those limits confirm that the they are different points of view of the same physical formalism. But even if that wasn’t the case, each perturbative limit of string theory is known and well-defined.

So, reformulating my question, how would you argue that a realistic ‘gravity + standard model’ model is *excluded* as a vacuum of any one of these perturbative string theories?

While you say that ‘you can’t prove that there is no way it can lead to a successful unified theory’, I say that it is strong enough to show that no perturbative vacuum of the theory contains ‘gravity + standard model’ as a low energy limit. If you fail to prove this, it means that there is a possibility that the opposite statement is true. There’s also the possibility that it is false. Our inability for technically proving an assertion or its negation is not synonymous of unprovability.

hv,

You can’t show that a realistic unified theory is excluded as a solution of string theory for two reasons:

1. Even if you assume that couplings are small and you’re in a region where a perturbative expansion about some hopefully consistent background works, you can’t actually compute what you want, since the number of such things is exponentially large, and each individual computation is too difficult (you need to be able to do things like have explicit Calabi-Yau metrics).

2. Assuming you can solve 1, you still would not have shown that a realistic solution to string theory is excluded, because you don’t know what the theory is outside of limiting cases. If the realistic solution is not in a region close to one of these limits, you don’t even know what it is you are supposed to compute.

If perturbative string theories are quantum and consistently contain both gravity and gauge forces (as they do), and if, as you argued, one cannot exclude that they may contain a realistic unified model, that means that they *potentially* harbor one valid complete description of reality. And that’s enough motivation to continue doing research in string theory. You only listed the technical difficulties in dealing with the theory. Difficulties that warrant a continued research program, not its abandonment.

IMO, the necessary condition for a valid realistic unified theory is to contain gravitational and gauge forces in one consistent framework. String theory does that. The sufficient condition is for it to contain at least one specific model that is consistent with the low-energy reality and is consistent with new experimental high-energy results.

Other proposals, like loop quantum gravity, fail already at the level of the necessary condition, let alone the sufficient condition. Which is why string theory is the only framework on the table to a complete description of reality.

Theorists are yet to find a realistic unified model within string theory, yes, but that only means that it is premature to ask for experimental predictions. But the fact that no one has ever found one realistic model doesn’t mean that none is there. It might not be there, true. But potentially it is. And that’s what matters.

For example, Yang-Mills theories are a good framework to describe non-gravitational forces. But it is only a framework, just like string theory is. The match with reality in Yang-Mills theory resides in choosing the correct gauge group that describes reality. Luckily, there aren’t as many Lie groups as there are Calabi-Yaus, so it was relatively easy to figure out that SU(3) describes the strong interactions, for example.

hv,

You’re just repeating standard hype. The facts of the matter are that thousands of people have worked for more than a quarter century trying to do what you suggest, and what they have learned is that:

1. They can’t extract any specific predictions out of this framework. Not one. Nada. Zip.

2. There are clear reasons why this is true (see my earlier comment), and no one has any reason other than wishful thinking to believe that these reasons will disappear and the theory will become predictive.

Comparing this situation to that of the Standard Model is just silly.

PW, if I’m repeating anything, it is standard LORE, not HYPE. With respect to the predictability of string theory, I think you are mistaken:

1. A theory is *wrong* if it is predictive but at least one of its predictions does not fit experimental data.

2. A theory is *incomplete* if the prediction for some physical phenomenon that could/should be in its scope is empty.

3. A theory is *non-predictive* if the prediction for some physical phenomenon is multivalued.

Unless you can demonstrate any these points, you can’t dismiss any theory! And since no one, including you, has ever shown the multivaluedness or emptyness of the generic predictions of string theory (on any vacuum of the theory, toy or realistic), you can’t dismiss it as wrong. Nor as non-predictive. Nor as incomplete.

One thing is certain: the theory may be technically hard, but none of the points above is undecidable. And that’s why research continues.

hv,

Your definition of “non-predictive”, predicts multiple things, is what I would call “inconsistent”. My definition of “non-predictive” is that it’s a vacuous idea that can’t now be used to predict anything, and such that there are good arguments that it never will. String theory fits this definition well. There are lots and lots of vacuous speculative ideas out there that don’t and can’t lead to a predictive theory. String theory is just one more.

Dear Dr. Woit

You have asking for information on Ads/CFT and QCD. I have been following with our research the Ads/QCD and AdS/CFT debate. My preliminary conclusions are as follows:(Hopefully this is not too long)

It is very difficult to model AdS/QCD breaking conformal symmetry in such a way that it reproduces the equation of state of Lattice QCD and the Debye Mass as a function of temperature. We have tried with a dynamical dlilaton in a 5-d two-derivative gravity action and with a Nambu-Goto action for the string to reproduce QCD as accurately as possible. See our reference “Trouble finding the optimal Ads/QCD (Veschgini, Megias,Pirner) Phys Lett B 696: 495-498,2011, see also very extensive work by Kiritsis et al. before us.

In this paper we also describe further work on the optimization program. (See the papersQCD-Thermodynamics using 5-d gravity and Thermodynamics of Ads/QCD within the 5-d dilaton-gravity model)

Concerning the heavy ion physics there are two aspects to be investigated further:

1)low energy flow (parameter v2) which documents the strongly interacting Quark gluon plasma according to most theoreticians

2)Jet quenching which should also indicate the strong interaction of the parton with the plasma

We only worked on the second topic there is a paper in review

“Jet Evolution in the Quark Gluon Plasma from RHIC to LHC” by Domdey, Kopeliovich and Pirner where we calculate two scenarios:

1)A conventional QCD parton cross section with the plasma particles which adds to the DGLAP evolution and then at the critical cross over temperature an absorption mechanism for the freshly formed prehadrons in the resonance gas.

2)An enhanced (K-factor =8, purely hypothetical) cross section ( a la strongly interacting plasma) and no final state prehadronic cross section

Note, however, perturbative QCD has a problem to choose the coupling scale, since the shower virtuality is high , but the temperature in the plasma is low. We can fit with both scenarios RHIC data. But at LHC scenario 1 is favoured. In our prediction we cannot reproduce the strong transverse momentum dependence seen in the ALICE data between 5 GeV and 20 GeV. Our prediction for pion suppression is above the data for charged particles between 5-10 GeV. In our opinion this necessitates a more careful absorption calculation. Our conclusion would be: from jet quenching there is no strong indication for an ADS/CFT like strongly interacting plasma with the partons. Anyhow AdS/QCD is so bad an approximation to QCD concerning the running coupling and the string tension and the equation of state and the Debye mass that in our opinion it is not worth to be discussed seriously.

One definitely needs to break conformal invariance. Further progrees depends on coupling the world sheet to the dilaton and perhaps increase the number of derivatives in the gravity dilaton action. In total I think low energy QCD up to 10 GeV is a really good candidate for string theory and 5 dimensions may help to do a good job as Polyakov already pointed out very early. The arguments for this theory have to bottom up and at the moment not so much top down, but more work on the relation between a nonconformal string theory and the corresponding gravity is urgently needed.

Best regards

Hans Juergen Pirner