I just returned from an afternoon, evening, and late evening at Quantum Theories and Symmetries 7, a massive theoretical physics conference in Prague.
Martin Schnabl et al. have invited about 400 participants and 350 of them or so are giving a talk in this week. I attended about five of them - and then a dinner (which grew from a planned session of 2-3 people to a collective physicists' event) in a needlessly expensive classical Czech-style restaurant in a basement. The prices of food went up to $400 (even though this one was some animal for 10 people) and they were so expensive exactly because the place attempts to mimic the cheapest Czech pubs where the prices are more than an order of magnitude lower, in some cases. ;-) I prefer the original ones.
The participants understood that Andy Strominger's talk on the map between Einstein and Navier-Stokes equations was a luxurious event at the conference so the large hall (at the Nuclear and Physical Engineering Faculty of the Czech Technical University) was overcrowded when he spoke.
It was pleasant fun to meet Martin, Andy, but also others - like Amihay Hanany and Shahin Sheikh-Jabbari of Tehran who is also (believe me or not) my friendly ex-co-author etc. ;-)
(Sorry: if you don't see your name here, it doesn't mean that I didn't enjoy meeting you!)
We chatted about all kinds of things. Now I am somewhat tired, at 2:20 am, but even tomorrow, I will probably sketch just a selection of the impressions etc. because the reproduction of life into the online diary form is not a genre I enjoy too much, especially when it comes to private and semiprivate issues. ;-)
I needed a clear break that separated the post-midnight and morning text. ;-) Uncle Fido tells them that of course they will have to go to the bed. Good night. "Good night, Uncle Fido. Good night, children." ;-) One of many TV cartoon fairy-tales of my childhood.
Andy's talk made lots of sense and it could clearly articulate many things that may become hidden if you just read papers. So everyone had the opportunity to get the idea that the ultimate link between the Einstein and Navier-Stokes theorem is kind of inevitable because there are not too many equations that could match so with a proper choice of the details, they have to match exactly.
The near-horizon limit of the black hole becomes mapped to the hydrodynamic limit of the Navier-Stokes equations - one may extract a non-relativistic theory out of it. This whole duality is a long-distance, classical statement that you should view independently on AdS or any UV completion because it is independent of them. If you try to embed it to AdS/CFT, you must realize that you first take the hydrodynamic limit of the CFT which erases the whole UV/microscopic physics such as quarks and gluons: it's really about the IR limit only.
Many political things were discussed as well. Because the people around were from countries such as Ukraine/Spain, Italy/Brazil, Iran, Israel, etc., I asked lots of things about what's happening with Tymoshenko, Spanish budget, and so on, and so on, you get the point.
Andy thinks that the field is in a healthy shape and only those who irrationally expected a theory of everything to be completed within a year may be disappointed. There were many things in which the people at the conference (including your humble correspondent) agreed about and others where they didn't.
For example, I am very puzzled by Andy's comments about the entropic gravity. While he agrees that there's no theory under this title that actually works at this moment, he seems to be persuaded about some broader point that I view as having been ruled out as well.
He thinks that because there are some hints in the empty AdS that motion in curved spacetime (=generalized gravity) is dual to some process with an increasing entropy, it should be true more generally and even Newton's force should have an interpretation in terms of some increasing entropy (which may however be entanglement entropy, so that Verlinde's picture would be equivalent to Van Raamsdonk - I am convinced that the authors would agree with me that they're papers are not to be meant equivalent at all - and he adds many other possible future mutations).
Andy's picture in which the entropy is supposed to grow seems self-evidently wrong to me. He just taken an object in AdS so that on the three-sphere, its wave function looks like a packet that spreads. Because it spreads, the entropy (as seen in the CFT) increases, Andy says, while the dual AdS description sees an object that moves away from the center of the AdS space - which is because of the effect of gravity. So (some generalized) gravity is dual some (some) increase of entropy.
Except that if you define the entropy - and ensembles of macroscopically indistinguishable microstates - in a conformally symmetric way (and you may and you should), it's totally obvious that the entropy is not increasing in this process at all. The fact that something visually spreads doesn't mean that the entropy goes up. In the bulk perspective, it's totally obvious that the time evolution is just moving an object around, and an object at a different place obviously has the same entropy as it had before.
There's no increase in entropy and there can't be any increase of entropy in reversible phenomena such as orbiting of gravitationally bound objects because the entropy growth is irreversible. Andy apparently doesn't want to hear such things and says - very generally - that one can't possibly rule out any hypotheses in physics. That's interesting because I think that physics and science is doing it all the time and falsification of a hypothesis is indeed the main step upon which the scientific method is found.
Strange but of course, be aware that this exchange was cherry-picked because the readers TRF could be interested in some disagreements.
The question whether the many-worlds interpretation is physically valid split the physicists, too. Martin Schnabl offers a story about alternative worlds that makes him believe that MWI is right and in my eyes, it resembles the stories by which Christians spread the gospel. ;-) A Korean physicist agreed with me about MWI.
A few words were said about some new people at Harvard etc. A.S. thinks that Jacob B. - whom I never met but he has already significantly impacted TRF - is a "character" over the "amplitudes". Even though A.S. surely had to help him to get him into the Society, I think that he severely underestimates Jacob B. Jacob B. also seems to be an amazing string theorist - and string model builder in particular. Surely one of my top 10 physics people below age [insert his page plus one] in this world who makes me say Wow because I can't get possibly rid of the clear impression that he or she is smarter than I (now). ;-)