The write-up of Larry McLerran’s summary talk at Quark Matter 2006 has now appeared. This talk created a bit of a stir since McLerran was rather critical of the way string theorists have been overhyping the application of string theory to heavy-ion collisions.
McLerran explains in the last section of his paper the main problem, that N=4 supersymmetric Yang-Mills is a quite different theory than QCD, listing the ways in which they differ, then going on to write:
Even in lowest order strong coupling computations it is very speculative to make relationships between this theory and QCD, because of the above. It is much more difficult to relate non-leading computations to QCD… The AdS/CFT correspondence is probably best thought of as a discovery tool with limited resolving power. An example is the eta/s computation. The discovery of the bound on eta/s could be argued to be verified by an independent argument, as a consequence of the deBroglie wavelength of particles becoming of the order of mean free paths. It is a theoretical discovery but its direct applicability to heavy ion collisions remains to be shown.
McLerran goes on to make a more general and positive point about this situation:
The advocates of the AdS/CFT correspondence are shameless enthusiasts, and this is not a bad thing. Any theoretical physicist who is not, is surely in the wrong field. Such enthusiasm will hopefully be balanced by commensurate skepticism.
I think he’s got it about right: shameless enthusiasm has a legitimate place in science (as long as it’s not too shameless), but it needs to be counterbalanced by an equal degree of skeptical thinking. If shameless enthusiasts are going to hawk their wares in public, the public needs to hear an equal amount of informed skepticism.
Another shamelessly enthusiastic string theorist, Barton Zwiebach, has been giving a series of promotional lectures at CERN entitled String Theory For Pedestrians, which have been covered over at the Resonaances blog.
Zwiebach’s lectures are on-line (both transparencies and video), and included much shameless enthusiasm for the claims about AdS/CFT and heavy-ion physics that McLerran discusses. His last talk includes similar shameless enthusiasm for studying the Landscape and trying to get particle physics out of it. He describes intersecting D-brane models, making much of the fact that, after many years of effort, people finally managed to construct contrived (his language, not mine, see page 346 of his undergraduate textbook) models that reproduce the Standard Model gauge groups and choices of particle representations. Besides the highly contrived nature of these models, one problem with this is that it’s not even clear one wants to reproduce the SM particle structure. Ideally one would like to get a slightly different structure, predicting new particles that would be visible at higher energies such as will become available at the LHC. Zwiebach does admit that these contrived constructions don’t even begin to deal with supersymmetry-breaking and particle masses, leaving all particles massless.
He describes himself as not at all pessimistic about the problems created by the Landscape, with the possibility that there are vast numbers of models that agree to within experimental accuracy with everything we can measure, thus making it unclear how to predict anything, as only “somewhat disappointing”. He expects that, with input from the LHC and Cosmology, within 10 years we’ll have “fully realistic” unified string theory models of particle physics.
The video of his last talk ran out in the middle, just as he was starting to denounce my book and Lee Smolin’s, saying that he had to discuss LQG for “sociological” reasons, making clear that he thought there wasn’t a scientific reason to talk about it. I can’t tell how the talk ended; the blogger at Resonaances makes a mysterious comment about honey…
Finally, it seems that tomorrow across town at Rockefeller University, Dorian Devins will be moderating a discussion of Beyond the Facts in Sciences: Theory, Speculation, Hyperbole, Distortion. It looks like the main topic is shameless enthusiasm amongst life sciences researchers, with one of the panelists the philosopher Harry Frankfurt, author of the recent best-selling book with a title that many newspapers refused to print.
Update: Lubos brings us the news that he’s sure the video of the Zwiebach lectures was “cut off by whackos” who wanted to suppress Zwiebach’s explanation of what is wrong with LQG.
Update: CERN has put up the remaining few minutes of the Zwiebach video.