Lookin’ For Some Hot Stuff

Hot, hot, hot, hot stuff
hot, hot, hot
hot, hot, hot, hot stuff
hot, hot, hot

– from “Hot Stuff”, by Donna Summer (1979). I refer to not only the physics but the c. 100 oF temperatures we’ve been having here every day recently.

On my way back from the conference, I spotted this book (below left) last Saturday in Foyles (the booksellers) in London1. Quark Gluon Plasma book It is a collection of reprints of a lot of the papers forming the foundations of the physics of the quark-gluon plasma (QGP) idea, going back the early to mid 1970s with such papers as Collins and Perry (Gosh, I had no idea Malcolm was one of the early workers on this idea. He’s much more thought of as associated with black holes, gravity, strings and so forth, ideas which – ironically – have recently turned out to be relevant to the discussions of the physics too. See my recent post, and there are also various popular articles to be found2).

Putting aside the usual ridiculous price that Springer Elsevier charges for books, I found myself in two minds about this book, in view of the surprises being uncovered about the properties of this remarkable state of matter at the RHIC experiment. Is this collection of early papers a useful working tool, or is it now just of historical interest, since many of the basic expectations about the properties of the plasma seem now to be incorrect?

rhic collision of gold ionsWell, after a bit of thought, I decided that the latter view would be way too hasty. First and foremost, on a general level, even if some of the computations in some papers were done in the “wrong” light (it’s a strongly coupled liquid that flows, not a weakly coupled gas of quarks and gluons), much of their content will still be useful in many ways – good and correct calculations last for all time, it is the sense of the words decorating them that may crumble over time. More specifically, one can worry about whether there were assumptions (and approximations based on those) that went into the computations that will render entire works invalid. I’m sure that will be true in some cases, but not all, and even in those cases where this is true, there is still (I find) value in reflecting upon such results too. (Image above right: Snapshot of the particles ejected after one of the RHIC heavy ion collisions.)

[Update: In the comments, Stefan also points out that there are lots of later works in the collection that are of considerable value and remain (to date) the state of the art: For example, papers about the phenomenolgy and analysis of heavy ion collision experiments that are used to deduce the properties of the QGP, whether it be liquid or gas.]

Finally, more reasons to not be hasty center around the fact that the explorations (and surprises) really have only just begun. The strongly coupled liquid phase of the quark-gluon plasma might not persist forever – at much higher temperatures the originally anticipated gas phase (with all the expected properties) could well appear, and then there’d be direct relevance for all those earlier papers. Also, there’s the experimental window afforded by the CERN’s Large Hadron Collider (LHC) yet to come (see the ALICE experiment) – it is to be hoped that we’ll get confirmation of what has been learned at RHIC, and then more information about what lies beyond.

I love the way science works. You never know for sure what ideas will be useful, and in quite what way, until you’ve consulted with Nature.


(And now I’ve got that song stuck in my head, I’m off to buy the album.)


  1. …After having a nice lunch there with regular commenter Candace and her partner Paul. Good to meet face to face with some of the readers of the blog from time to time. [return]
  2. There’s also an article by Tim Folger in February’s Discover magazine (2007) on the issue. (I borrowed the above RHIC collision image from it.) Overall it is a nice layperson’s guide, with interviews and so forth, but beware (for example) of the statements about how close to QCD (the standard theory of nuclear) the string theory computations currently can get. There’s still a lot to learn and a lot to do, and we’re still surprised about how well some of the things are working, and it is not yet understood why. This has to change before we can be quite so bold about what we’re doing. See my earlier blog post for more discussion and thoughts. [return]


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8 Responses to Lookin’ For Some Hot Stuff

  1. ” love the way science works. You never know for sure what ideas will be useful, and in quite what way, until you’ve consulted with Nature.”

    This one reason why I am always rather hesitant to pay €100 for a textbook on a subject matter that is in a constant rate of flux 😉

    This is perhaps one of the reasons why electronic text have been so quickly adapted by the scientific community.

  2. stefan says:

    Dear Clifford,

    thank your for that interesting post! When I heard about this book, I was very tempted to get it, but indeed, the prize is by a factor 3 or so higher than what I am ready to spend for it… by the way, it is published by Elsevier – Springer still has a somewhat more reasonable policy concerning prices.

    As for the content, the collection features many “classical” papers which explain basic facts and background knowledge one should know if one works in heavy ion physics, and which one might have a hard time to find somewhere else.

    And then, there is much more to heavy-ion physics than the current “strongly coupled fluid” versus “ideal gas” issue – these findings may concern mainly chapter 2 on the Perturbative QCD Plasma. The chapters on Fluid Dynamics and Flow, Strangeness, Charm, Electromagnetic Probes, Parton Energy Loss, and Density Interferometry deal with topics which remain extremely important for the phenomenology and analysis of heavy ion collisions – we have to keep in mind, for example, that statements about the viscosity of the QGP can be made only indirectly by, say, an detailed analysis of flow patterns.

    As far as I know, only the chapters about Disoriented Chiral Condensates (never observed, to my knowledge) and Phase Transition Dynamics and Cosmology may be a bit outdated – the latter especially since most of this work was based on the assumption that the hadronisation transition in the Early Universe (at zero chemical potential) is first order, thus opening the exciting possibility of bubble formation which might have been seeds for cosmic structure. Alas, current wisdom is that this transition is a cross-over, yielding no dramatic signals.

    As for the exploration of the QCD phase diagram, let me mention that besides the heavy ion programs at RHIC and the LHC, there will be the CBM (Compressed Baryonic Matter) experiment at FAIR, the new Facility for Antiproton and Ion Research at the GSI in Darmstadt, Germany. The purpose of this experiment is to study nuclear matter at nonzero chemical potential, and hopefully probe the region of the phase diagram in the vicinity of the critical point. Anyway, that experiment may yield valuable new insight in the transition from hadronic matter to the QGP, and to hadronisation.

    Best regards,


    BTW, I didn’d know either that the Perry in Collins-Perry and Myers-Perry are one and the same…

  3. Clifford says:

    Well, if it is any good, new advances in a subject don’t make a textbook (or collections of papers) wrong all of a sudden. Just less complete. They can still be useful. I like printed books and collections. I like to hold them in my hands and see them collected together. Electronic access (and texts in various forms) are useful and convenient, but I still like to take something off the shelf and get that tactile experience… and the ability to rifle through and see what is next to what in a way that is not really available electronically….


  4. Clifford says:

    Hi Stefan,

    Thanks…. indeed there is a lot in there that is useful, as I said in the post – and there are several chapters that I’d love to study to learn more about the background of the central ideas. Thanks also for the addition to the survey of more of the experiments (I was planning a blog post about that for later). Non-zero chemical potential is of considerable interest in a lot of the (stringy and lattice) computations now. [update: … and hence in addition to upcoming earth-bound experiments, I will try at some point to mention more about neutron star observations. Marvellous the way astrophysics feeds into this too, and of course ironic that we may learn a bit about the physics at the cores of neutron stars by using (dual) black holes.]



  5. Frank says:

    Dear Clifford,

    Are the basic properties of qgp known (even theoretically), like its density for instance.
    I understand the creation of qgp is a phase transition so what about the latent heat?

    All the best,
    Good ol’ Frankie boy

  6. Clifford says:

    Yes, lots of properties are known, by indirect deductions. Maybe one of the most famous of these is the extraordinarily low viscosity I discussed in the pprevious “Exploring…” post.. Have a look in the review paper of Shuryak that I referenced in the “Exploring…” post and references therein, for a start.

    The transition you might be thinking of is the deconfinement phase transition. This is a transition which rather dramatically changes the number of available degrees of freedom of the system available to carry energy. This is where the plasma allows the quark and gluon degrees of freedom to participate in the dynamics as independent (more or less) degrees of freedom. The classic way of presenting this these days is a plot of the energy density divided by temperature (raised to the power four) vs temperature- the equation of state. The plot comes from lattice simulations. A flat line on this curve represents the stephan-boltzman law (from which you can deduce the specific heat in the usual way), and the notable thing is that there is a jump in the curve at the deconfinement temperature
    (about 175 MeV), representing the increase of energy density due to increased degrees of freedom (the deconfinement), and then the line is flat for a good while. The interesting thing here is that the flat line is about 0.8 the height it should be if it were a weakly interacting gas. 11 year old stringy computations show that the plasmas we study using the methods described in the earlier post are just of this sort (the key number is 0.75, see Klebanov, Gubser and Peet 1996)… anyway, it is believed that the sQGP phase has set in just above this temperature and this is what is being seen. It is hard to study the transition directly in the experiments, as I understand things, but the results extracted so far as believe to be not in contradiction with the features of the lattice data for the equation of state that I mentioned.

    I’d consider reading some of the talks at last year’s Quark Matter 2006 meeting (http://www.sinap.ac.cn/qm2006/) for actual numbers, and much more about the status of the measurements of various properties, as well as results from lattice and other approaches. See also the proceedings of of last week’s Cambridge conference I mentioned.. Shuryak’s summary talk there is nice, with audio and slides and so forth…. The Quark matter meeting has other good summaries too, I think.



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