Saturday, July 29, 2006

Surprise, and Pride

Earlier this week, when I had a cursory look at my mailbox at the institute, I was a bit surprised: Usually, it contains only boring information leaflets or updates of phone lists, but this time, there was a big white envelope waiting to be picked up. I was even more surprised when I recognized the sender: Peter Hoyng, a researcher at SRON, the Netherlands Institute for Space Research. I had completely forgotten about him, and for sure, I had not expected that he would, indeed, send me what was in the envelope: a copy of his book, Relativistic Astrophysics and Cosmology - A Primer.

The story had begun in November 2004, when I got an email form someone I had never heard before working at a Dutch research institute I had never heard before. Peter Hoyng told me that he was preparing a textbook on astrophysics and cosmology, condensing into a book the course he has been teaching at the University of Utrecht since several years. He was looking for an illustration of a heavy ion collision that he would like to use in the part on the early universe where the transition from the primordial quark gluon plasma to a hadron gas is discussed. By chance, he had found a snapshot from a simulation of a lead-lead collision at the CERN-SPS in an online talk I had prepared for my PhD advisor a year before. Now, he was interested in a more detailed explanation of the figure, and asked for the permission to use it in his book.

Of course, I was extremely pleased by this request. I provided him with the explanation and a colour version of the figure file, and asked him to tell me when the book will be in print. The next time I heard from him was half a year later, last July, when he contacted me again. He told me about delays in the publishing procedure because of a change of the publisher, and asked me for a black-and-white version of the figure, following the request of the new publisher. I was happy to help him, and completely forgot the whole story - until this week, when I found his book, together with a short note, in my mailbox.

Obviously, there was one more change of the publishing house, since now, colour is used again for the illustrations. The book is very neatly produced, as a part of the Astronomy and Astrophysics Library at Springer. It starts with a motivation for the need of general relativity in astrophysics, introduces the geometry of Riemann spaces and general relativity, and goes on with the Schwarzschild metric, compact stars, and black holes. Two chapters discuss gravitational waves and Fermi-Walker transport (including a discussion of Gravity Probe B). The remaining chapters are devoted to cosmology: the Robertson-Walker metric, the evolution of the universe, observations, the early universe, and inflation. You can download the detailed table of contents on the publishers website of the title.

So far I have only had a cursory look a the book, so this is not a review. But from what I have seen, it looks very interesting and worth reading. I especially appreciate the discussion of interferometric gravitational wave detectors, and Gravity Probe B. Moreover, I am happy to see the onion-like diagrams of light paths in the expanding universe, which you may know from Ned Wrights web site. I have seen these types of diagrams for the first time in a paper by Ellis and Rothman in the American Journal of Physics. I found them extremely useful to develop some kind of visual understanding of the expanding universe, and I wonder how any textbook about cosmology can be without them.

All this, of course, was not the first thing I looked up in the book. I searched the index for quark-gluon plasma, and on page 241, I found my illustration:

It shows a snapshot from a simulation of a collision of two lead nuclei immediately after an off-center impact with an energy of 17.4 GeV per nucleon pair (corresponding to the CERN-SPS), as calculated with the code I have used in my thesis. Unaffected so-called spectator nucleons are white, while deconfined quarks and antiquarks are represented as coloured spheres. There are no gluons in this model - the effect of glue is all subsumed in a linear, confining potential, which is used to describe the interaction between quarks. It comes out of this simple model is that quarks quickly team up in colour-neutral quark-antiquark or three-quark configurations, which are mapped to mesons and baryons, respectively. For better visibility, the figure is stretched in the beam direction by the gamma factor to undo the Lorentz contraction of the colliding nuclei. The gamma factor at this collision energy is of order 10, and the spatial configuration of the collding system in the centre-of-momentum frame is already quite pancake-like...

I guess I will have to write in much more detail about my simulations of the quark-gluon plasma, and this exciting topic in general. I will do so some time... But for now, I am just proud to see this figure of mine reproduced in a textbook.

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  1. Hi Stephan, I've added a link to this post from my blog.
    Hope that is ok with you
    Laters ... Q

  2. You three have a wonderful and educative site here.

    As a layman I have been studing on my own and have learnt a lot. I first became aware of you when I was looking for information on "backreaction" in the laval nozzle.

    Do you know of it?

    I work by "analogy in relation to experimental science" reductionism to help push my perspective and the conditions of blackholes are very interesting to me in this regard.


  3. Stefan,

    The snapshot is very nice and the simulations you mention look quite interesting indeed. My thesis was on N-body gravitational simulations, but I am interested in simulations on other physical phenomena as well. Is your thesis available in electronic format? If so, I would appreciate to receive a copy by email or download it.

    The textbook looks interesting as well.

    Best wishes
    christinedantas [at]

  4. Hi quasar9,

    thank you for adding the link - I am also proud of links to my posts ;-)

    Hi plato,

    thank you very much for your nice words - that is very encouraging!

    This laval nozzle... I now remember I've read about it in last December's Scientific American, in this article about acoustic black holes by Jacobson and Parentani... This was about the analogy between the transition from subsonic to supersonic flow in this laval nozzle and the horizon of a black hole. If you are interested in more technical details, maybe you know gr-qc/0601079?

    But I guess Bee is more qualified to comment on this topic ;-)

    Hi Christine,

    indeed, when struggling with the code I have used, which was not really fast, I have often thought, these astronomers, they run similar codes for much much larger systems, probably I could learn some clever tricks from them...

    You can download my thesis from my homepage, follow the link More than you ever wanted to know about it: Modelling ultra-relativistic Heavy Ion Collisions with the quark Molecular Dynamics qMD. There are also some more illustrations there...

    Best regards, Stefan

  5. ... OK, one should check the links before submission if on cannot edit the comments... my homepage is here.

  6. How splendid! It's really gratifying when someone else publishes your work (with appropriate acknowledgement, of course).

  7. Stephan - A slightly OT question for you. In a recent interview Leonard Susskind claimed that string theory was the "best explanation" of heavy ion collision physics. Any comment?

  8. Hi Bee, thanks for that link to the higher dimensional artwork, it certainly brings a further perspective on xtra dimensions on different scales. love it.
    Here's hoping you are having a good week. Cosmic Variance has been producing a flow of sequential logic posts the last few days, more tightly knit in its theme than it sometimes is. The advantage of several contributors is one can have more material to post, but it is good to resist the temptation to post on a whim, and draft posts which can then be used reused and rehashed with updates chronologically.
    Compliments on your research and work, and compliments on Backreaction the blog.

  9. Hi CIP,

    where was this link supposed to go? I am sorry, I can't take Susskind's remark serious. It goes glub, glub, glub to the bottom of the sea.

    I will think about it when string theorists are able to explain something. And, btw, AdS/CFT is not equal string theory and QCD does not have N = infinity, and heavy ion collisons don't take place in thermal equilibrium.



  10. Hi quasar :-)

    Thanks for the nice words, glad you like the pictures. Unfortunately, my week is pretty shitty so far. I could need a vacation, say, for the next five years or so. Yep, I noticed the incredible activity over at CV. It's just too many letters, I don't even have time to read it all.

    All the best,


  11. Hi CIP,

    Bee has sent me the Susskind interview yesterday night, but I have not listened to it completely yet. Does he really say that

    string theory is the "best explanation" of heavy ion collision physics?

    Aeh - quite a strong statement, and I doubt that many heavy ion physicists would subscribe to it...

    The problem with heavy ion physics is that it involves so many tricky things, and that there is not such a thing as the one explanation for everything:

    What is the initial state, and how do you describe it? You can use phenomenological PDGs and pQCD, yielding parton models, or the colour class condensate, or classical gluon fields because of the to high gluon occupation number and particle production from them... What is the dynamics, and how is thermal equilibrium reached in a collision, if at all? Do you really create a QGP in euqilibrium? And then, how does hadronization happen? How and where do you cross the phase boundary from the QGP to hadrons, and what about finite size and finite time effects?

    At nearly all the stages of a collision, and for searching for answers for all these issues, some modelling will be involved, and some analogies are usually helpful.

    There is the dual black hole and AdS/CFT, there is the analogy to the Unruh temperature for the initial state and the issue of thermalization. And, of course, there is the model of the hadronic string and its descendants, which preceded QCD and can be seen as the origin of string theory, and which was pioneered by people like Susskind, and only later interpreted as something like colour flux tubes...

    So, some aspects of heavy ion collisions are definitely well described by stringy things, or models involving strings. But that does not necessarily mean, it seems to me, that string theory provides the best explanation, nor yields a complete picture.

    Unfortunately, there is not one explanation of heavy ion collisions, but a whole patchwork of explanations, each tailored for different stages or different aspects of a collision.

    Lots of stuff to think and write about...

    Best, Stefan.

  12. Hi Stefan,

    Many thanks for the link to your thesis, I am downloading it right now.

    N-body gravitational simulations of galaxies and larger structures (the dark matter component) have some peculiarities of implementation since they are collisionless systems. I don´t know how far these (or other) techniques could be useful in other physical contexts, and it would be interesting to learn if that is so.

    Best wishes,

  13. Stefan and Bee,

    Thank you for the kind explanation.


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