Wednesday 31 January 2007

Barton Zwiebach for Pedestrians

Barton Zwiebach recently wrote the highly acclaimed string theory textbook. This week at CERN he gave a series lectures entitled String Theory for Pedestrians. A pedestrian is only slightly better than a dummy.

Barton is a very good speaker. His priority is to discuss difficult problems using a simple language and intuitive pictures. The downside is that technical difficulties are swept under the carpet.

His choice of topics reflects the change of season that seems to be happening in the string theory world. The emphasis was on applications of string theory to real physical systems (hadronic spectra, quark-gluon plasma) or to hypothetical ones but potentially observable in the near future (cosmic strings). Strings as a candidate for a fundamental theory was only briefly discussed towards the end of the last lecture. That's good.

The beginning was quite original. The entire first lecture was devoted to the properties of the classical relativistic string (usually, one jumps straight into the quantum theory). It turned out to be a funny physical system, very interesting in its own. The classical theory can be applied to the dynamics of fundamental strings stretched to cosmic sizes.

Most of the second and third lecture was devoted to string theory as an effective description of strongly interacting systems. Barton started with a bosonic string and its potential to explain the hadronic spectra and the quark-quark potential. He stressed that it is hard to imagine a physical system other than the quantized relativistic string that could reproduce Regge trajectories (the relation M^2 ~ J between the excited hadron mass and spin). After a brief introduction to AdS/CFT, he moved to the application of string theory to quark-gluon plasma. He spent quite some time discussing the RHIC experiment and the meaning of elliptic flow and viscosity. Then he talked about predictions for the viscosity and jet quenching parameters derived from a string theory dual to supersymmetric QCD. This is presently the hot topic and a front of intensive research. The reason is that high temperatures break supersymmetry, so that there is a chance that supersymmetric versions of QCD (which string theorists know how to handle) yield relevant predictions for the real world experiments. The fact that the quark-gluon plasma has small viscosity, if confirmed, would indicate that QCD at strong coupling has some sort of a gravity dual. That would be something. What is often missing in this discussion is the clear statement: even if some version of string theory will prove to be a good effective description of strongly interacting systems, it does not mean that string theory is the fundamental theory of gravity. The QCD string and the fundamental string are two different things, albeit described by the same framework.

He closed with a few bitter remarks about loop quantum gravity. He also expressed his feelings towards the famous string theory critic and fellow blogger. Those string guys seem to hold some kind of grudge ;-) The last question was about honey, seriously.

See for yourself. The transperencies and the video recording are available here.

Update: For (shamelessly) less enthusiastic remarks about the same event, see this post at Not Even Wrong.
Update2: Yet another view on the story is available on the Reference Frame, who apparently does not like changing seasons. Come on, spring is coming soon ;-)
The Supreme Court for Canton Geneva ordered that the full video recordings of the lectures should be made available on the web. You can now enjoy Barton's opinions about LQG, but the honey part is still censored. The legal battle will continue...


Anonymous said...

Harry Lipkin had a soft spot for pedestrians, his 'Lie groups for' did us all a service

Blake Stacey said...

Have you noticed that the video recording cuts off just when Zwiebach starts talking about LQG? Clearly, he said something we aren't supposed to know.

But I've already said too much, so I better be quiet now.

Anonymous said...

"he said something we aren't supposed to know."

Secrets in a public lecture.... I was there, and he said LQG doesn't work as of yet as it hasn't been shown that it leads to gravity. It is also clearly written on his slides.