Well, my work at the Quantum Gravity school over (see previous posts here, here and here), I hopped on a plane yesterday, in order to return to Los Angeles. It was an excellent time. I enjoyed being in Mexico again, and seeing a new part of it (see posts to come), and the school was well organized and very enjoyable. The students were very receptive overall, and I spent a huge amount of time chatting with them about physics between sessions, over lunch, and on the bus back and forth between the campus and the city (and on the excursion on Sunday for a while). A number of students seemed very interested in some of the quantum gravity phenomena that are possible to describe using string theory – surprised in most cases (since the background independence issue that keeps being waved about by some as a naive (in my view) and bluntly used principle seems to have served as a block to many) – and it was a pleasure to be able to open them up to new physical ideas. It worked pretty well because I gave them three lectures on the basics of string theory, and so could build on that material in the one to one discussions to explain more detail.
Dynamical changes of spacetime dimension is one thing that captured a lot of interest, as well as string theory’s various ways of showing in model examples how spacetime is an emergent, classical approximation to an underlying quantum description that does not have manifest spacetime geometry. The latter is something that all approaches to quantum gravity hope to realize in one way or another (since those sorts of ideas and fantasies about what quantum gravity should be appeared in the mid 20th Century), and exactly how it emerges in string theory is a beautiful story. Who knows, perhaps they will take these hints from string theory about how quantum gravity can work and develop them further and/or find their own realizations of them in whatever approach to quantum gravity that they ultimately pursue.
Here’s a technical note on approaches that was interesting. I tried to emphasize that much of the way of thinking about how all these things work at a technical level is to “follow the light states” – essentially using intuition of effective field theory to organize one’s thoughts. It is one of the first things many people working actively in the string approach to many of these issues (and related) reaches for, helping figure out what theory, spacetime, dimension, etc, that you’ve been driven to by the dynamics. One thing I’ve noticed is that this is not as widely used a way of approaching the physics as one might think, but then I recalled that it was not so long ago that it was less common in the string world too. I think a lot happened in our field in the 90s as a result of many young people in the field learning to think that way, following the influence of people like Polchinski, Witten, and Seiberg, among others. Prior to that it was also not the way the average young person in the field organized their thoughts, and then I recall watching how it all rapidly changed. You could see it spread geographically quite clearly too, catching on fast in the larger centers in the USA (where some of the aforementioned giants of the field worked), and eventually spreading over to the rest of the USA, and Europe and beyond. But it seems (I could be wrong; this is from my conversations with the students and also from the lectures and papers I’ve looked at – the latter at a relatively basic level) that the language of effective field theory which we use so much, coming more from condensed matter and particle physics (fields which with string theory has long shared a lot of language, ideas, and techniques) is used as much in, for example, the loop quantum gravity approach, which has somewhat different traditional roots (i.e. in relativity and geometry). Perhaps that will change, and it will find more use (especially with things like spin foam approaches and so forth, which sound to my naive ears more more in the spirit of modern approaches to field theory from the ground up). I’m sure others with more knowledge about the other approaches can comment further.
To the far above right is an overhead shot on the first day after the first session, with some of the students and faculty waiting for the lunch to begin in the UNAM (Morelia) Radioastronomy and Astrophysics Centre building (which is stunning in form and colour, as you can see from two other photos). The lectures are in an auditorium nearby. You can see at left a shot from later in the week showing Per Kraus (UCLA) lecturing about aspects of the fundamental ideas behind AdS/CFT. I’m sitting in one of the wings on the side right at the front part that is elevated over the stage. I sort of thought of them as like the royal boxes at an old opera house.
Some Related Asymptotia Posts (not exhaustive):