I am on my way back from the Emergent Gravity conference at MIT, contemplating what I've heard and learned. The aim of this meeting was to bring together condensed matter physicists with those tireless seekers looking for a fundamental theory unifying classical gravity with quantum field theory. Should such a theory exist then the features we observe might only be collective variables, emerging from a more basic underlying structure. Much like the properties of liquids are eventually a consequence of the dynamics of its molecules, the spacetime we live in might only arise in a macroscopic limit from a more fundamental theory. One would expect then both areas, condensed matter and quantum gravity, to share common approaches when going from a microscopic to a macroscopic description, and there to possibly be similar features like phase transitions or modifications of symmetries in the small distance limit.
This past week we have heard about emergence of gravitons on a quantum bosonic model (Zheng-Cheng Gu, arXiv:gr-qc/0606100v1), the emergence of diffeomorphism (Jorge Pullin, arXiv:gr-qc/0606121v1) and the emergence of spacetime and matter in Group Field Theory (Daniele Oriti, arXiv:0710.3276v1), to only mention a few. My head is still spinning, and I don't really know where to place all this information, so please don't ask for details. Fotini gave an update on Quantum Graphity - A condensed matter model of emergent geometry (arXiv:0801.0861v2). I wasn't very convinced when I first heard of that model, but they've made some refinements to the approach and I see a real chance that maybe some day I indeed manage to make sense of it. Lee talked about his paper with Joao about the possible observational consequences of a phase transition in the early universe from a non-geometrical to a geometrical phase (arXiv:astro-ph/0611695v3), and my talk this morning was just a summary of my Minimal Length model that you of course know all about so I won't bother you with the details.
A highlight was certainly Stephen Wolfram's talk on Tuesday evening in which he introduced his idea that the universe and everything we know and like fundamentally arise from a cellular automaton. He advertised his book "A New Kind of Science", in which, allegedly, the emergence of the universe and all theories we usually deal with is explained. Stephen's claims are certainly bold. By choosing the right updating rules for the automaton, so he says, he gets not only a manifold with Lorentzian symmetry, but also Einstein's Field equations. Unfortunately, I couldn't quite follow his argumentation - the talk was a bit confused (to put it mildly) and about 1 hour over time, but it was certainly interesting. I think I will need to have a look at this book before I come to any conclusions.
Thursday afternoon, we had a panel discussion with Jorge Pullin, Max Tegmark, Xiao-Gang Wen, and Bei-Lok Hu, moderated by Olaf Dreyer. The topic circled around the question what are aspects of emergence, why it is so hard to quantize gravity, and whether this is the right path to follow at all. The issue of background indepence was briefly touched, so was the question of observable predictions and whether or not time is fundamental. It was an interesting exchange, though with a certain lack of disagreement.
I had prepared a 5 min bonus to my talk because I thought I might finish earlier, but then I finished remarkably in time and didn't need it. It was to mention some of my thoughts on the merits of emergence and our quest for a fundamental theory on a very general level, probably quite bloggable. So I thought, you'd get the bonus instead - if I come around to writing it up that is.
To brag a bit: we were staying a the Kendall Hotel in Cambridge, right next to MIT. A very nice place that I can warmly recommend.