Wednesday, October 11, 2006

Does String Theory explain Heavy Ion Physics?

Earlier this week, we could follow a quite heated debate about the applications of the AdS/CFT (Anti-deSitter/Conformal Field Theory) correspondence to strongly coupled QCD (Quantum Chromo-Dynamics, the theory of quarks and gluons) as observed in relativistic heavy ion collisions.

Or, in a more catchy phrase, whether or not "string theory explains RHIC physics". Or -- even more provoking -- as it was formulated in the recent Nature issue:

"When the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York, first produced a hot quark gas, it was string theory that correctly predicted, retrospectively, some of the gas's properties. "

Nature 443, 491(5 October 2006), Theorists snap over string pieces, by Geoff Brumfiel, see also here.

(Okay, I take the word 'explain' in the title instead of 'predict', but I want to bring that quotation with the alleged prediction somewhere. - If it's unavoidable. - It is.)

This is quite an experimental post written by both of us, trying to understand what is there about these claims. If you ever want to test how much your marriage can take, try to write a blog post together. When you see remarks in brackets, these were the issues we couldn't settle.

In an earlier post, Bee reported on a talk about the applications of AdS/CFT to heavy ion physics that she heard at the KITP. She was thrilled to see string theorists trying to get in touch with experiments! And isn't it ironic that after several decades string theory has come back to where it started from: explaining features of the strong interaction? That was, before it was promoted to be a promising and promising and promising approach to the theory of everything (TOE), which would become important at unobservable energies. (Isn't that too sloppy? - It is called cynicism.)

The use of the AdS/CFT correspondence for strongly coupled QCD is an extremely interesting and exciting project, and probably one of the hottest and densest topics that is currently out there. (haha - sorry, could not resist the temptation) It can provide us with a lot of important insights into QFT. But one should be realistic here:

From the side of the string theorist, realistic about what it can possibly tell us about string theory as a TOE, and what it can't. From the side of the nuclear physicist, what it can possibly tell us about heavy ion collisions. And what it can't.

So, this is an attempt to explain some of the physics involved, from the point of view of relativistic heavy ion physics - and since since Stefan has some background there, we figured we would make a good team, but he's definitely the one to ask what a horizontal flow is. (I should know that by now, but I keep forgetting it. - Does that mean I have to answer all the comments?!)

Here's the outline:

1. What is this all about?
2. Is it a Hype?
3. What does it mean?

1. What is this all about?

Heavy Ion Collisions have one big goal: To map out the phase diagram of nuclear matter. The question one would like to answer is: Under which conditions of temperature and density is nuclear matter made up of hadrons (of nucleons like neutrons and protons, of hyperons like Sigmas and Lambdas, and so on), and when and how will one find the constituents of hadrons, the quarks and gluons, as the relevant degrees of freedom? Where in this diagram is the phase boundary between the hadron gas and the quark-gluon plasma, as the state where confinement is lifted and quarks and gluons can move freely is called? And moreover, what are the properties (the equation of state, or transport properties such as viscosity) of the quark-gluon plasma?

(Phase diagram here? - Good idea. - Where's the figure from your talk? - Where's yours?)

On the experimental side, there is only one tool available: Accelerate nuclei of heavy atoms such as gold or lead, and let them collide. At the collision, the kinetic energy of the nuclei is dissipated, and goes into the compression and heating of the nucleons in the nuclei. If heating and compression are high enough, a quark gluon plasma can be formed.

On the side of theory, there is QCD which describes the interaction of quarks and gluons. There is only one big problem: QCD is a complicated theory, and its low energy limit, which contains the hadronic ground states, the protons and neutrons and so on, can not be handled analytically. The same is true for the deconfinement transition from the hadronic world to the quark-gluon plasma: There is no analytical method to describe deconfinement and hadronization in QCD. What one can do instead is to use lattice QCD, or apply approximation schemes that approach hadronization from high densities or high temperatures, where the theory is asymptotically free, and perturbative methods can be used. There are different techniques available to describe QCD at temperatures above deconfinement, with hard thermal loop re-summation as one example. This is a very active area of research in current nuclear theory. For the regime of heavy ion collisions, there still remains on problem: At temperatures above the deconfinement temperature Tc, say for T = 1 - 3 Tc, QCD is not yet completely free. Lattice calculations of energy density and pressure show a clear difference to the Stefan-Boltzmann limit, which corresponds to an ideal gas of quarks and gluons. So, this temperature regime is difficult to study with standard QCD techniques. Unfortunately, it is just this temperature regime that is reached in heavy ion collisions at RHIC, the relativistic heavy ion collider at Brookhaven.

(Do you have some fundamental problem with entering paragraphs? - But the context belongs together! - It looks completely unreadable. - Who reads that anyway? - I think I don't like your attitude.)

There was one big surprise in the experimental data from RHIC: it seems that the quark-gluon plasma created in the collisions has a very low viscosity, or is a most ideal liquid. At least, that is what can be concluded from the success of hydrodynamical simulations of RHIC collision simulating the quark-gluon plasma as an ideal liquid.

Here, one point is important to note: There is no way to measure the viscosity of the quark-gluon plasma directly. You have to infer it from the momentum distribution of final state hadrons, in this case, of the anisotropy of the momentum distribution of hadrons in the transverse plane for non-central collisions, which is called the elliptic flow.

(Insert explanation, link, graphics. - Good! Where is it? - I'm at home, can't access the journal. - Okay, lets do that tomorrow.)

Large values of elliptic flow are observed at RHIC, larger than what was expected from an extrapolation of the results from the CERN-SPS, where the collision energy is lower. As mentioned before, this RHIC elliptic flow can be reproduced using a hydrodynamical simulation of an ideal (zero viscosity) fluid for the deconfined phase. So, the conclusion is, the viscosity of the QGP is very low.

Here, there is one point to keep in mind: the actual viscosity is not known for sure, and model assumptions about the QGP go into it: Assumptions about the initial state used for the hydrodynamics simulation, for the equation of state and the properties of the hot and dense system, for hadronization, and for hadronic rescattering, i.e. the interactions of the hadrons in the still dense, but late phase of the collision. Moreover, the hydrodynamics code in use only now start to systematically investigate the effects of actual viscosity on the expansion dynamics.

The simulations using ideal hydrodynamics that are so successful in the reproduction of the elliptic flow use a so-called Glauber-dynamics initial state for the codes to run. But this initial condition is not the only game in town. For example, the so-called colour glass condensate (one model assumption for the high density, high-temperature initial state of the nuclear matter, where gluons are the main players) produces very high initial transverse momenta, which produce an elliptic flow consistent with data only if a viscosity is taken into account which is markedly higher than in the ideal fluid models used so far. So, a definite, uncontroversial answer about the the actual viscosity is still out. Obviously, lots of issues are not yet completely settled here.

When the first data on elliptic flow larger than expected before become known, Edward Shuryak pointed out that the very low viscosity which data seem to imply (but keep in mind that this fact as such is not completely waterproof yet) would be consistent with predictions of a very low viscosity of a supersymmetric Yang-Mills theory, and that this low viscosity corresponds to an absolute minimum of viscosity derived from the AdS/CFT duality and superstring theory. Hence, the term "most ideal liquid" was coined for the QGP created at RHIC, and it was argued that the strongly coupled QGP can be described using the analogy to the supersymmetric Yang-Mills theory.

Shuryak is a brilliant physicist, but it is also fair to say, we would say, that he is known in the community as someone who strongly promotes his ideas. And his ideas are often contested - as in this case, the idea of the "most ideal liquid" has been contested a lot, especially from the side of the promotors of the colour glass condensate. So, there is an ongoing debate in the community about these questions, the press releases about the ideal liquid notwithstanding. Anyway, this is our impression of how AdS/CFT entered the heavy ion community.

(It this the one who...? - Yes. - Do you really want to write that? I mean, I don't usally comment on people. - That is fair to say, believe me. And for the heavy ion people it's a compliment.)

Now, what does the AdS/CFT say, and where can it be applied? In brief - and corrections of experts on this are welcome - it helps to write down correlation functions in strongly coupled gauge theories from a duality to the dynamics of strings in an 10-dimensional AdS background with a boundary. Strings end on the boundary, which is Minkowski space, and end points of strings correspond to particles in the gauge theory. Problems of the mathematical exactness left beside, this is a unique and ingenious way to get information about correlation functions, which are very hard to obtain (or are not obtained yet) by lattice gauge theories or thermal field theory techniques.

Where has this duality been applied? The first case has been mentioned before: To calculate the viscosity of hot gauge theories, with the famous universal lower value of 1/4 pi. There are two more situations where it has been applied: To determine the screening of the interaction potential of a heavy quark-antiquark pair in a system moving in a background of hot gauge theory (An AdS/CFT Calculation of Screening in a Hot Wind by Hong Liu, Krishna Rajagopal, Urs Achim Wiedemann, hep-ph/0607062), and for jet quenching calculations, that is, to determine the energy loss of fast particles travelling through a hot medium. (Calculating the Jet Quenching Parameter from AdS/CFT, by the same authors: Hong Liu, Krishna Rajagopal, Urs Achim Wiedemann, hep-ph/0605178, now accepted as a PRL). As a sidenote, Wiedemann and Rajagopal are not string theorist, but have worked in heavy ion theory, QCD and nuclear theory. Hong Liu and Dam T. Son, one of the authors of the main viscosity reference, and also not a string theories by formation, will have plenary talks at Quark Matter, the main conference of RHIC physics.

Can these things be observed in heavy ion collisions? For the case of viscosity, we have discussed it before: there are some caveats, since viscosity can not be measured directly - you have to reproduce elliptic flow, and the inverse problem is not unique. The hot quark-gluon system may be a most ideal liquid, it may be something else, we do not know yet for sure. Screening of the potential is relevant for the so-called J/Psi suppression, but this is also something that has to be inferred backwards from the measured J/Psi yield, which is influenced by many other factors (the original idea iabout this is twenty years old now - however, there are still many open questions left).

At RHIC, there are chances from photons that may make these signals more waterproof than at CERN-SPS, but currently,. ambiguities remain. Jet quenching and energy loss is also a point where many calculations and models exist, but the inverse problem is very hard. So, we would say in all these three cases, you may have a very beautiful application of AdS/CFT to QCD at strong coupling, but the connection to experimental data is difficult and ambiguous.

You should not be disappointed: that is, unfortunately, very common in heavy ion physics. Take the original idea about J/Psi, or disordered chiral condensates, and many other examples: Signatures to check beautiful ideas are often washed out by lots of dirty QGP soup and hadron gas wind effects.

2.Is it a hype?

Does the Global Positioning System (GPS) work because of General Relativity? One often hears this statement in discussions of General Relativity, and it is meant, we guess, to create an awareness that this arcane theory is true, and moreover has applications to down-to-earth technologies which are in every-day use. And as a matter of fact, it is true: The systematic effects on atomic clocks in orbit when observed from points on the surface of the earth as predicted by GR are incorporated into the system, and it all fits perfectly well!

On the other hand, to say that the GPS work because of General Relativity is an oversimplification which neglects the actual intricate details of the system, and which are, from a practical point of view, equally important for the workings of the GPS. A look in a technical description of the GPS will discuss lots off effects of the ionosphere and the atmosphere on the propagation of the satellite signals that have to be taken into account and corrected for - relativity often is not even mentioned! So, clearly, to say that the GPS work because of General Relativity is not wrong, but it is not the whole story: It is a catch phrase to show that GR is not some abstract mathematics, but plays indeed a role in the real world.

There is also a kind of inverse problem in the GPS example: Could one reconstruct GR from the GPS system alone? Could clever physicists derive GR from the systematic deviations in GPS, if they would not have been taken into account from the beginning? Well, they would only be partially successful, since, in fact, only some form of the equivalence principle is tested with the GPS, and not the full Einstein equations. To establish GR, more observations, such as the perihelion precession, are necessary.

To us, this seems to be very analogous to the situation of AdS/CFT in RHIC physics (To us? It was your comparison. I really like it but I like to point out it was your creativity at work here!) : There are applications of this fundamental duality to the physics of hot and strongly coupled QCD, and they probably contribute to the outcome of experiments. But there are many more, mundane effects coming into play, which influence final state hadronic data, and which make it very difficult to solve the inverse problem - to unambigously conclude an initial state.

For sure, AdS/CFT does not explain all of RHIC physics, so far, it seems, in our understanding, applicable to the regime of strongly coupled QGP above the deconfinement transition. What does it say about hadronisation, for example? Can it say something about this? That would be extremely cool, but it seems that there is no solution yet. Moreover, there seem to be open quastions in how far results derived for the supersymmetric Yang-Mills theory can, indeed, be carried over to QCD, see for example hep-ph/0608062.

These limitations of the AdS/CFT approach should also be mentioned, in our opinion, if only to avoid the misleading impression of string theorists showing up on the scene like the FBI agents with suits and sunglasses, take over the case from the dumb local police, and solve immediately what the locals have been unsuccessfully investigating for years.

(Couldn't find a nice pic of FBI agents, but I wasted some time on that. -- This is great!)

Besides this one should keep in mind that the AdS/CFT correspondence is an outcome of years of research of string theory. But it is not equal to string theory. Even if the current results show the usefulness of this correspondence, and make use of many developed techniques, what could this possibly tell us about string theory as a TOE? And then, the spacetime used there isn't really one that we would be interested in as a description of the world we live in - we come back to this in the last point.

These are some words of caution, but as Clifford pointed out:

"[...] applying string theory ideas - the whole shebang of strings, branes, black holes, gravity, etc - to understanding the new forms of matter being discovered at Brookhaven. This may welll be a great way of testing the remarkably intricate structures that string theory puts together and give us lots of clues about how to develop the theory better."

(You sure that's a good idea? - I don't want that to come out wrong either, I really like his point of view, esp. regarding the teapots and so on. And the fig jam. But I think it's okay, I mean we've made quite clear we aren't anti-string in any regard. - You really sure? - I have the comments forwarded on my BB, you think I want the beeps to keep me up all night?)

And this is understandibly something to get really excited about! Nevertheless, we can't avoid having the impression that string theorists must be pretty desperate if they try to justify their work with the calculation of a viscosity using a conjectured (unproven) side effect of the theory they have been working on. I am not aware of any work on how it would be possible to learn something about string theory as the TOE from observables in heavy ion collisions.

(Isn't that a bit hard, desperate? - Yes. I am not a nice girl in case you haven't mentioned. I want them to get the message. There's no need to be desperate. Nobody wants to kill string theory. But they should stay realistic.)

3. What does it mean?

So, to us, it seems that AdS/CFT is a cool application and we would be happy to understand more about it. (? - !) On the other hand, there are caveats, and experimental verification of great ideas is difficult, as always, in heavy ion physics. But then, there is, we think, a more fundamental question about the ontological status of this duality: It is merely a computational tool, or should we really belief that the quarks and gluons we know and love are just the endpoints of strings in a 10-dimensional AdS×S5 space? (I am not really very much concerned with the ontology, honey. If it's the same, then it's the same, what's the point?)

This is probably a very general question, that can, and should, already be asked for the Ising model and the mother of all dualities, the Kramers-Wannier duality. In an experimental realisation of a two-dimensional Ising system, the elements of reality which are described by the Ising Hamiltonian are the magnetic moments of atoms. Or aren't they? Taking the duality serious, we could as well argue that no, not the magnetic moments are the real thing, but the dual plaquette variables. But does this make sense? Apparently not, especially since duality works for the Ising variables, the magnetic moments, but not for all other real things in the system, the atoms with all their electrons, and their nuclei.

Coming back to AdS/CFT, if it works for strongly coupled QCD, should we believe that the dual side, the strings, are real? Maybe, but then, the duality should work for all kinds of particles, not just strongly coupled quarks and gluons. Then, one could not discern any more between both sides of the duality mirror, and both sides could claim the same right to be the real thing. Or are we fundamentally wrong here? (Are we? What is reality anyhow? - I don't want to get into this right now. Can we just finish this &$%@ post?)

To summarize: Heavy Ion Physics does not equal strongly coupled QCD, and String Theory does not equal AdS/CFT. The calculations done using the AdS/CFT correspondence are wayleading and exciting. But the connections to string theory as a theory of everything, explaining quantum gravity, the parameters of the standard model, and more, are so far very weak and require more investigation.

Update: See also More on Ads/CFT and RHIC

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  1. Wow, you too. :)You sound like a nice couple.

    I felt like there was a struggle going on within myself:)The anima and animus, depending on whose gender would speak and the other who wouldn't listen.

    IN Viscosity State Production is ?

    Far be it from the layman's view here that one should extend the topic a little bit further?

    It was a intuitive leap for me. Properties of the supeconductors?

    The Blackhole as a SuperFluid?

    Thank you to for your explanantion as this is "where my head was at", but there is more as I relay. Help me:)

  2. But the connections to string theory as a theory of everything, explaining quantum gravity, the parameters of the standard model, and more, are so far very weak and require more investigation.

    you've missed the point. that's what we don't need: more investigation of that type.

  3. This link provided by Plato:

    will be important to compare the "Photon" transposition, via viscocity.

    I'm going to take a backseat on this one and see how this thread progress's ! ;)

    P.S. just let me quote Anglenene from a Futurama episode:Flexo..Flexo..Flexo its the Episode where Bender and Flexo battle in the factory that produces Non_compressible/Unbendable girders, somewhat like the RHIC and CERN factories !

    Great thread again, and best wishes to you both,paul.

  4. Paul, Plato,

    thanks for the links - interesting as they may be, they have no direct connection to the topic of this thread. Please, stay more focussed. Thank you,


  5. thanks for the very interesting post. I would like to ask some clarifications.

    If I understood, true QCD (3 colors, no SUSY) is approximated with a gauge theory with many colors and 4 supersymmetries, that can be exactly computed with AdS/CFT.

    Why this approximation should be good, at high density?

    How accurate should it be? (100%? 30%? 1%?).

    How accurately has it been tested so far? (I think you indicate that uncertainties are still at the 100% level)

  6. Hi Anonymous,

    as a fact, it is a good approximations at high temperatures, most easily to see in the reference above to Kovtun's work

    Banana, see first plot or slides mentioned there.

  7. Okay Stefan,

    While I recognize the badge of the Peace Officer(section ... of the Criminal code) of the RHIC, I wanted to show that the extension of the knowledge you have been showing, is part of the picture being shown by RHIC and string theory.

    And your saying "no connection?"

    May I use your badge picture?

    Thank you for your patience.

  8. Again, about QCD versus N=4 SYM: I am not at all an expert in this, but the limit "Number of Colours to infinity" is a very common tool for calulations in QCD, so this probably poses no problem.

    Moreover, energy density and pressure as a function of temperature for N=4 SYM are very close (on the some percent level) to their QCD counterparts - that's what shown in the plots mentioned in the comments of Bee and "another Anonymous".

    However, the quantitative agreement for viscosity over entropy density between QCD and N=4 SYM may not be so close. This is discussed in hep-ph/0608062, where they conclude that for QCD near the transition point, this ratio "is several times larger than the viscosity bound".

    (Thanks to Jörg for pointing out this paper - the introduction is very readable, indeed!)

    But there is also a lot of activity to formulate AdS/CFT dualities for more QCD-like theories. I am aware of some papers that make connections to QCD (e.g. hep-ph/0501128, hep-ph/0602229), but can not comment anything about them.

    Best, stefan

  9. Hi Plato,

    sorry, I was a bit harsh, maybe - but I just thought discussions of black holes, even the dual ones connected to RHIC physics, would fit better to other posts...

    Best, stefan

    PS: I also like the badge very much - again and again I'm amazed at the creativity of my wive :-)

  10. > string theorists must be pretty desperate

    I don't know, I think it is pretty exciting that there is a way (indirect , via duality and based on a conjecture, but still) for the first time to test quantum gravity and black hole physics.

    As I understand it, AdS/CFT allows us to understand the information loss problem (i.e. that there is no such loss) and in this sense is very important for string theory as TOE.

  11. Hi Wolfgang,

    I think it is pretty exciting that there is a way (indirect , via duality and based on a conjecture, but still) for the first time to test quantum gravity and black hole physics.

    If you know more than I do, then please explain: what do we learn about quantum gravity from the AdS/CFT result for the viscosity?

    As explained above: besides the experimental problems, there is the question what the data even in the best case could tell us about the underlying model - in this case AdS/CFT. Again, take the best case, and consider AdS/CFT proves to be a model able to predict to good accuracy all observed properties of the strongly coupled phase. Then what does this tell us about quantum gravity? Does this even tell us that there IS something like quantum gravity?

    No. Why not? Because the correspondence applied there has nothing to do with the 'real' quantum gravity. Not even indirect, via duality. The QGP is not a dual description of our spacetime and it's properties.

    But yes, if string theory is correct, then these observations allow to test whether developed tools have been applied appropriately. That IS exciting! And it is great that years and years of research suddenly become applicable to explain experimental facts :-)

    But nevertheless, this can't settle the question whether or not string theory is the right approach to quantum gravity.

    Reg. the information loss problem, I share the opinion that there is no information loss. But besides the statement that the information in AdS/CFT isn't lost, it doesn't help me to understand where it goes, and how it gets there. Please tell me: how do you describe a black hole collapse, the following evaporation, it's endstate, and how does an observer after complete evaporation recover the information that was in the initial state?




  13. Wolfgang, I know the paper, and also the previous ones by Nastase. What are you trying to say?

  14. I really wanted to post this one 8-)

  15. > What are you trying to say?

    Your distinction between the "real" string theory (as TOE) and the "not so real" AdS variant is in my opinion not valid.
    AdS is the full string theory compactified as 5d + 5d instead of 4d + 6d.

  16. > I share the opinion that there is no information loss.

    The good news is that this is not necessarily an issue of "opinion".

    IF the AdS/CFT conjecture holds (and the RHIC experiments could be a valuable test) you *know* at least one example where a black hole does not indicate 'information loss'.

  17. Wolfgang,

    a) Where does the information go that falls into a black hole in our spacetime, say, in the center of our galaxy. I don't see how Nastase's work is helpful there. I have very much sympathy with Hawking's recent argument (inspired by AdS/CFT), but I also have my problems with it. In short, I think he's omitting to actually answer the most important point, that is: even if he shows inf. can't get lost, how does it work?

    b) AdS is the full string theory compactified as 5d + 5d instead of 4d + 6d. If you have a problem with my formulation, then please explain what in your terminology is 'the full string theory'. Does it, in a low energy limit, reproduce the SM?



  18. Hi Wolfgang,

    IF the AdS/CFT conjecture holds (and the RHIC experiments could be a valuable test) you *know* at least one example where a black hole does not indicate 'information loss'.

    Yes, I agree with you on that. If the conjecture holds, and RHICs data support it, then we have an example of a dual black hole that doesn't loose information. But that doesn't help me to understand what happens to the information that vanishes in the black hole in the center of our galaxy. Best,


  19. > Does it, in a low energy limit, reproduce the SM?

    If you are lucky and find 'the one' compactification in the haystack.
    I hear that Volker Brown is working on it 8-)

    But it seems one does not need a theory which reproduces the SM in order to learn a bit about black holes (even if it is not enough for you 8-).

  20. I am sorry.
    Volker Braun of course.

  21. By the way, I should add that I agree with you that it is way too early to state that AdS/CFT describes the QGP accurately.
    I was about to write something about this on my blog (now I dont have to), because it is IMHO not so clear yet what really happens in the RHIC 'fireball'.

  22. Hi Bee,

    "... to say that the GPS work because of General Relativity is not wrong, but it is not the whole story: It is a catch phrase to show that GR is not some abstract mathematics, but plays indeed a role in the real world."

    On the topic of mathematical models in general, see Feynman:

    ‘It always bothers me that, according to the laws as we understand them today, it takes a computing machine an infinite number of logical operations to figure out what goes on in no matter how tiny a region of space, and no matter how tiny a region of time. How can all that be going on in that tiny space? Why should it take an infinite amount of logic to figure out what one tiny piece of space/time is going to do? So I have often made the hypothesis that ultimately physics will not require a mathematical statement, that in the end the machinery will be revealed, and the laws will turn out to be simple, like the chequer board with all its apparent complexities.’

    - R. P. Feynman, Character of Physical Law, November 1964 Cornell Lectures, broadcast and published in 1965 by BBC, pp. 57-8.

    Now you want me to explain a candidate mechanism I suppose...

    Nothing works because of a mathematical model, and until quantum gravity is included general relativity won't even be a complete mathematical model ;-)

    You might as well claim that that people meet and marry because of the equation 1 + 1 = 2.

    Underlying general relativity, there are real dynamics. If it is analogous to a Yang-Mills quantum field theory, exchange radiation will behave differently in the universe than in an atom or nucleus, due to redshift ;-)

    Smolin et al. show in LQG that a path integral is a summing over the full set of interaction graphs in a Penrose spin network. The result gives general relativity without a metric (ie, background independent). Next, you simply have to make gravity consistent completely with standard model-type Yang-Mills QFT dynamics to get predictions:

    Over short distances, any Yang-Mills quantum gravity will be unaffected because the masses aren’t receding, so exchange radiation won’t be upset. But over great distances, recession of galaxies will cause problems in QFT gravity that aren’t physically included in general relativity.

    I don’t know if gauge boson’s are redshifted as or slowed down (background independence upsets SR, and Maxwell's model is hogwash since his displacement current equation which depends on vacuum polarization can't occur in a QFT unless the electric field strength exceeds the IR cutoff, which corresponds to about 10^18 v/m, FAR higher than the field strengths of Hertz' radio waves which he lying claimed to prove Maxwell's equations correct), but that simply doesn't matter: either way, it’s clear that between two masses receding from one another at a speed near c, the force will be weakened. That’s enough to get gravity to fade out over cosmic distances.

    This means G goes to zero for cosmology sized distances, so general relativity fails and there is no need for any cosmological constant at all, CC = 0.

    Lambda (the CC) -> 0, when G -> 0. Gravity dynamics which predict gravitational strength and various other observable and further checkable phenomena, are consistent with the gravitational-electromagnetic unification in which there are 3 dimensions describing contractable matter (matter contracts due to its properties of gravitation and motion), and 3 expanding time dimensions (the spacetime between matter expands due to the big bang according to Hubble’s law). Lunsford has investigated this over SO(3,3):

    ‘... I worked out and published an idea that reproduces GR as low-order limit, but, since it is crazy enough to regard the long range forces as somehow deriving from the same source, it was blacklisted from arxiv (CERN however put it up right away without complaint). ... my work has three time dimensions, and just as you say, mixes up matter and space and motion. This is not incompatible with GR, and in fact seems to give it an even firmer basis. On the level of GR, matter and physical space are decoupled the way source and radiation are in elementary EM. ...’ - drl

    Nobel Laureate Phil Anderson:

    “... the flat universe is just not decelerating, it isn’t really accelerating ...”


    Hence Lunsford's model is right. Note that this PRECEDES experiment. I got a publication in Electronics World Oct 96, which is for a dynamical model.

    When you think about it, it’s obviously correct: GR deals with contractable dimensions describing matter, and one time dimension. Lunsford simply expands the time to three dimensions hence symmetry orthagonal group (3,3). The three expanding time dimensions give the cosmological recession! The Hubble expansion then becomes a velocity variation with time, not distance, so it becomes an acceleration. Newton’s laws then tell us the outward force of the big bang and the inward reaction, which have some consequences for gravity prediction, predicting G to within experimental error!
    We already talk of cosmological distances in terms of time (light years). The contractable dimensions always describe matter (rulers, measuring rods, instruments, planet earth). Empty space doesn’t contract in the expanding universe, no matter what the relative motion or gravity field strength is. Only matter’s dimensions are contractable. Empty spacetime volume expands. Hence 3 expanding dimensions, and 3 contractable dimensions replace SO(3,1).

    The question is, how long will stringers with only hype be defended by non-falsifiable predictions about soft scatter of heavy ions? Similar to predictions of large extra dimensions?

    BTW, if you want to contribute a cent to determining experimentally whether redshifted light suffers a velocity change, go over to LM's blog. ;-)


  23. Just one nitpick, and it is highly off-topic (apologies in advance).

    "Yes. I am not a nice girl in case you haven't mentioned." - Bee, this thread.

    "Hey, I am a nice girl!"
    Bee - here

  24. Experiment: I strongly suggest that everybody interested reads RHIC papers, or at least abstracts, instead of over-hyped press releases: you will find that there are many model-dependent ways of interpreting data. There is no experimental evidence for QGP.

    Theory: AdS/CFT relies on supersymmetry. QCD is not supersymmetric. There is no reason to believe that N=4 SYM is in any way related to non-perturbative QCD.

    Sociology: QCD people are delighted because of influx of new personnel while string theorists can claim that AdS/CFT is useful. This explains the source of hype....

  25. Hi Arun,

    *lol* very observant. The sentence from PW's blog actually rang in my ears when I wrote the above post. Let me put it this way: a) I am contradictory b) I find being nice very complicated. And it's unfortunately very often in conflict with being honest. I wasn't feeling particularly nice yesterday. Ever tried to tell a friend nicely his new hair cut is *humm* maybe a bit too short?

    Best, B.

  26. Hi anonymous,

    I mostly agree with you. But regarding your remark: There is no reason to believe that N=4 SYM is in any way related to non-perturbative QCD. I have to say I am somewhat more pragmatic here. If it proves to predict observables to good accuracy, then it's a useful model to describe nature, and worth the investigation.

    Dear Wolfgang,

    I would really look forward to read a post of yours on the subject!

    If you are lucky and find 'the one' compactification in the haystack.

    That was what I meant when I said, it's not the string theory as a TOE that's under discussion for the application of AdS/CFT at RHIC. How could it, since we don't have it. And all the RHIC data isn't going to help with the search in the haystack. It could help us to confirm and extend our present knowledge though.



  27. To echo Burton Richter, some of our distinguished theorists not only fail to understand the difference between an observation and an explanation, but between the application of a theory and the application of (a part of) its mathematical formalism to a problem in a different domain.

    No wonder string theory has become such a hopeless boondoogle.

  28. "Ever tried to tell a friend nicely his new hair cut is *humm* maybe a bit too short?"

    LOL!, you say "it will look even better in a couple of weeks!"

  29. Anonymous,

    I strongly suggest that everybody interested reads RHIC papers, or at least abstracts

    good idea :-), and maybe also the introductions, which often are quite helpful to get the bigger picture.

    you will find that there are many model-dependent ways of interpreting data.

    Exactly! This is extremly important to keep in mind when discussing heavy ion data, since most information you can get stems from hadrons, which are produced late in the collision events. So, what one does is to constuct models, such as hydrodynamical models, or parton transport models, or whatever, and then one tries to figure out whether the outcome correspomds to the data. Interpretations are difficult. For this reason, your statement

    There is no experimental evidence for QGP.

    is true in the sense that there is not one single, completely unambigous signature which can tell you whether there was a QGP or not - at least, such a signal, if it would work out, would be "the holy grail" of heavy-ion physics, I think.

    However, I would say that the statement "no experimental evidence" is very misleading: There are a lot of signals which are very hard or impossible to explain in purely hadronic terms: Strangeness production, heavy quark signals, jet quenching, flow signals, and so on.

    Especially, one finds systematic devations of signals from nucleus-nucleus collisions from the extrapolation of proton-nucleus collisions (or deuteron-nucleus collisions, at RHIC). This means that something new is happening in nucleus-nucleus collisions.

    Whether you call that evidence or not is, for sure, a question of judgement, or the rigour you are demanding. There are many very good and strong reasons to believe that a QGP has been formed at RHIC, and also at the CERN-SPS at full energy.

    QCD people are delighted because of influx of new personnel

    ... you know, at least in the heavy-ion community, we are always open to new ideas ;-). My impression is that not so many string theorist start doing heavy-ion physics (at least, I would call it like that ;-)), but that some nuclear theorist start looking how the AdS/CFT stuff and all that may be useful to gain new insights into QGP physics.

    So, I would say, there is more an exchange of ideas than of people.

    Best, stefan

  30. To summarize: Heavy Ion Physics does not equal strongly coupled QCD, and String Theory does not equal AdS/CFT.

    Dear Stefan and Bee,

    thank you very much for this clarifying and fair status report! I have really been puzzled how much there is about it and many of the refererences string theorists like to give do only increase my confusion. Now I do understand better why the exitement! Yours,


  31. Stefan:However, I would say that the statement "no experimental evidence" is very misleading: There are a lot of signals which are very hard or impossible to explain in purely hadronic terms: Strangeness production, heavy quark signals, jet quenching, flow signals, and so on.

    Yes I cannot benefit in the ways that you all do( I try) but as far as being from the big picture this does not escape me:)

    Have we seen strange Quark matter?

    The collisions are strange: PHENIX can identify particles that contain strange quarks, which are interesting since strange quarks are not present in the original nuclei so they all must be produced. It is expected that a Quark-Gluon Plasma will produce a large amount of strange quarks. In particular, PHENIX has measured lambda particles. There are more lambda particles seen than expected.

  32. "They claim that there is no experimental support (true) or hope for experimental support (how can they know that?) for string theory… but they ignore the fact -they intentionally don’t tell you, dear reader- about the interesting work going on by a huge percentage of the field to use string theory to study the structure of nuclear matter." by

    "My impression is that not so many string theorist start doing heavy-ion physics (at least, I would call it like that ;-)), but that some nuclear theorist start looking how the AdS/CFT stuff and all that may be useful to gain new insights into QGP physics." by Stefan here are somewhat at odds.

  33. Hi Plato,

    strange matter and strangelets are a very interesting topic, but, unfortunately, there has been no experimental evidence for them so far. They are not really connected to string theory either, besides the fact that it was an early paper of Witten that resuscitated interest in them with nuclear physicists, I think.

    Strangelets have been thought of as possible culprits for RHIC disaster scenarios (besides the ubiquitous black holes ;-), and as responsible for potential cosmic ray particles beyond the GZK cutoff.

    But as far as I know, there has been no experimental verification of any of these ideas (and the world still exists: RHIC has produced no greedy strangelets which would have eaten up the Earth).

    In the case of the potential quark star you cite, RX J185635-375, again, and unfortunately, as far as I remember, it came out that the radius determination was not completely safe. Bottomline was that this star could be well understood as a common neutron star. I am not completely sure, though, about the current status of this object, whether it is thought to be a quark star or not.

    Anyway, it is a good example for an exciting observation which is reported in the press, but which has to be partially revisd later - only that these revisions don't make in the press releases. I guess it would often be quite interesting to have a kind of follow-up reporting, where one could read what is, eventually, the fate of some discovery that has been announced in the press.

    The strange particles I was talking about are not strangelets, but the common hadrons with strangeness, especially the Ξs and the Ωs, with two and three strange quarks, respectively. These are the particles that I had mentioned in my earlier post, and whereof I should finish the second part, finally ;-). You typically find much more of these particles in nucleus-nucleus collisions than in (properly scaled) nucleon-nucleus collisions, which is a strong indication for an intermediate QGP state, where stange-antistrang quark pairs can easily be produced.

    Best, stefan

    PS: Keep on trying :-)

    Hi Rob,

    thanks for the nice words :-)

  34. "but the limit "Number of Colours to infinity" is a very common tool for calulations in QCD, so this probably poses no problem."

    I'd say that that limit is a qualitative caricature of QCD. Semiquantitative at best, when it does work it usually works at the 200% error level.

  35. Hi Arun,

    you shouldn't take that apparent discrepency too serious - there is probably just some gap of perception as to what you would call heavy-ion physics ;-)

    I found it very revealing when I scanned the literature for dibaryons (I had used my simulation code to study the production of dibaryons at hadronization and was looking for experimentally known facts) and found a paper with the very interesting title dibaryon spectroscopy. It turned out to be, well, not exactly what I had expected.

    My guess is that such differences of perception between hard-core theorist and more phenomenologically oriented people are quite common, and not restricted to string theory...

  36. Hi anonyomus,

    I'd say that that limit is a qualitative caricature of QCD. Semiquantitative at best, when it does work it usually works at the 200% error level.

    I don't know actual numbers there. Can you give me a reference?

  37. Hello Stefan,

    These are only my generalizations that were "lead by the physics."

    I just wanted to share some of my own research in the direction you had been talking.

    Thanks for the encouragement to keep trying.:)

  38. Hi,

    I would like to point out two recent papers related to the "strongly coupled" QGP discussion and the viscosity bound. I have found them very interesting and readable:

    The Letter "s" (and the sQGP) by Jamie Nagle (nucl-th/0608070) gives a nice discussion of all the different notions of strongly interacting matter and strong coupling involved in the debate about the hot and dense nuclear matter created at RHIC.

    On the Strongly-Interacting Low-Viscosity Matter Created in Relativistic Nuclear Collisions by Laszlo P. Csernai, Joe Kapusta, and Larry McLerran (nucl-th/0604032) presents a new and fancy way to plot the phase diagram of fluids in the vicinity of the critical point: These heavy ion/nuclear theory veterans plot the ratio of shear viscosity to entropy density as a function of temperature for different pressures.

    These curves seem to look quite similar for all kinds of fluids, with a kink when hitting the critical point. Moreoever, this kink corresponds to the absolute minimum of eta/s for the fluid under discussion, and it is allways higher than the AdS/CFT lower bound, which seems to be quite a universal feature indeed!

    This paper was published as a PRL last week, and it seems to have been directly motivated by the Kovtun-Son-Starinets PRL.

    However, it does not (if I did not miss that point somehow) discuss whether AdS/CFT can say anything more specific about RHIC matter or not. But it seems that hot and dense QCD matter does not go down in eta/s until the AdS/CDT bound.

  39. "And as a matter of fact, it is true: The systematic effects on atomic clocks in orbit when observed from points on the surface of the earth as predicted by GR are incorporated into the system, and it all fits perfectly well!"

    I believe this to be a myth. I once tried to track the origin of this myth years ago and all I found was some paper where the author made dubious claims about General Relativity. But I could not go further. The reason I am writing is to verify, not to blame you, but just to ask, if you checked the actual workings of GPS, the code etc, to verify to your satisfaction that the statement is true? Again, this is nothing against your claim. I have no proof against this. I am just guessing that you are repeating someone's authority here. As usual great post. Thanks.

  40. Hi Pioneer,

    stop being ridiculous. Redshift of electromagnetic radiation in the gravitational field is an established fact that has been repeatedly experimentally tested to great accuracy. If you open a textbook on General Relativity you will find the references - to start with try googling for Pound-Rebka.

    The GPS is not the device to measure the effect, the point is you need to take these corrections into account to analyze the signals. You don't need to know the hardware code to figure out whether its functionality is sensitive to GR effects, you just need to know the basics of the transmitted signal sequence, satellite's orbits, and position reconstruction. And yes, I once checked an estimate on the required precision that is needed for the GPS and whether it is sensitive to GR. I couldn't repeat it now (was some years ago), but I found no mistake in the argument.



  41. Yes. I know this is ridiculous. I have no faith in physics experiments. But thanks, I remembered that was the Pound-Rebka experiment I was reading about and then it led to a more recent paper that now I have to find and read. At the time I was naive enough to believe that I could actually refute a physics experiment by duplicating it. I did not know that physics can never be wrong, it is the law.

    My point is that, in GPS, or in any other astronomical computation of position, you subtract computed from observed. My impression was that physicists called some unidentified residual a general relativistic effect. Do I have a proof of this? No.

    Thanks for your reply. I didn't get the chance to read your blog for a while, you added a lot of interesting posts. Keep up the good work!

  42. Hi Pioneer,

    I did not know that physics can never be wrong, it is the law.

    The Pound-Rebka experiment has been repeated several times with increasing precision. If the original finds had turned out to be faulty, you'd have found an erratum on it.

    My point is that, in GPS, or in any other astronomical computation of position, you subtract computed from observed. My impression was that physicists called some unidentified residual a general relativistic effect. Do I have a proof of this? No.

    My impression is that you didn't even understand the point. The GPS is sensitive to GR effects. That means, you need the GR prediction to use it, otherwise you can't analyse the data. It is not that the device receives some 'unidentified residua' that naive physicists think are caused by GR, but these 'residua' are exactly the ones predicted by General Relativity.



  43. Thanks for the clarification. Since this was long time ago I cannot claim to know the experiment as well as I should.

    So I need to locate the paper and read it before I comment again.

    To me most physics experiments are suspect unless they are duplicated by independent parties. Nowadays this does not happen in physics. So I have to check if those more precise experiments that you mention had been confirmed or they entered the canon by the authority of the physicists.

    Thanks again for stimulating comment.

  44. Hi Pioneer,

    You are welcome. I looked into the details of the experimental situation of GR for my thesis, but unfortunately, it is in German. You might find this living review helpful

    which is also on the arxiv

    see p. 12. Best,


  45. Thanks for the link. Yes, it was another Living Review article that I was studying, I remembered:

    The subject is interesting but I will have to study General Relativity to understand these papers. And studying GR to me is like studying COBOL. I don't see why. So I thought about this clever scheme: I established the First Global Pioneering Physics Awards! I already have a corporation called Global Pioneering which does nothing useful. This award will be a good excuse for it to be appearing to be doing some business. This may even be considered educational.

    The award (USD$500) will be given to the best proof proving that without GR GPS would utterly fail. I think some physics students may be interested in this challenge.

    I wanted to ask you if you would be kind enough to help drafting the rules for the award? What would physicists consider a proper proof for this question?

    I created a wiki here: and wrote down some rules. It was great fun. I spent a lot of time with this over the weekend. Please take a look.


  46. Dear Pioneer,

    the link you provided above requires a log-in.

    Regarding your question to advertise you 'doing some business', you can't possibly be serious with that?

    Just to make sure, aren't you the guy writing this blog

    In case you are: from what you write it is obvious that you don't know very much about natural science today. Knowledge is not identical to intelligence, and you are always welcome with your questions over here, I do my best trying to answer them. But I can't approve of you ranting about science and scientists without having any idea what you are talking about. You don't even bother with describing personal experiences, probably because you don't have any. You don't provide any evidence for your negative view.

    Everything I have occasionally read on your blog is insulting for every scientist. It is a completely nonsensical, paranoid, diatribe that I attibute to you being frustrated about not understanding science. You frequently state completely stupid things like 'Physics = scholasticism' or 'Doctors of Philosophy did corrupt physics.' or 'The academic system is based on crushing human intelligence.' Most of what you write is so absurd and confuse I don't even know what to say about it.

    Among other very nice things you write 'That’s why Lisa Randall and her ilk are professional shamans in the payroll of the Big Media.', and YOU ASK ME if I 'would be kind enough to help drafting the rules for the award'. Have you ever meet Lisa Randall? Have you ever talked to her? Or at least heard her giving a talk? Did you read ONE SINGLE PAPER?

    And you are seriously ASKING if I would even consider advertising any of your crap? What planet are you from?


  47. Thanks for your reply. I made the wiki public, you should be able to look at it now.

    Of course, business part was not serious. It was an attempt to make a joke. But the Award is serious and I believe it may interest and help some students and even help change my bad attitude against physicists.

    Yes, I write Freedom of Science blog.

    I agree, I know almost nothing about "natural science" today or past inclusive. That's why I read Backreaction and write my blog.

    I cannot agree more about "knowledge is not identical to intelligence." But today most of our knowledge comes from the media and for media the unit of knowledge is intelligence (genius). Thanks for making this distinction. I added it to my slogans page.

    About not bothering with describing personal experiences: in your blog you tie very nicely personal experiences with physics and you make them interesting. I don't think I have that talent. But I have been working on the history of my research which is meshed with personal experience:

    About having negative views about physics: I agree with that as well. I thought about it a lot and I don't like it myself. Why be negative instead of moving forward with original research? The reson apparently is that physics is not independent of physicists. Physics is full of culture. And the content of physics is defined by cultural elements: media, institutions which award the grants, and politics. I believe that the process of elimination of cultural elements from physics is science. This is what Galileo did. This is what Fermat did. And both made new discoveries after eliminating cultural elements from their field of study.

    About Lisa Randall. No, I have not read any of her papers. I watched several videos of her lectures and public appearances. I just wrote a reply to a comment in my blog on this topic: I believe that her technical papers and popular books are isomorphic and there is no meaning lost in translation. So it is enough to read the book. This is my opinion.

    I also agree with your criticism of my blog because it treats Doctors of Philosophy as a homogeneous class with members having identical characteristics. I believe this is wrong. Others also commented on it. I will address this issue soon. For instance, I believe that the academic physics education is a common characteristic of this professional class but shamanism is not. Thanks for mentioning this.

    Yes. I am seriously asking if you would help draft as a respected physicist the rules for an educational project which will do good for physics. If you take a look at the wiki and you want to contribute I would send you a password for editing.

    And many thanks once again for replying to my questions here. I added your links about GR/GPS on the wiki.

  48. Hi Pioneer,

    the only thing that I can convince myself of doing for you is not to delete your comment. Good luck,



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