Monday, June 25, 2018

Nima on the end of spacetime

...and arrow of time, quantum foundations, and other things...

David Barrera sent me a link to a five-day-old lecture by Nima Arkani-Hamed at SLAC, the Stanford's linear accelerator center:



If you had doubts, the host is Michael Peskin, a co-author of a very widespread quantum field theory textbook. Nima talks about the demise of the spacetime, simplification in QFT, amplituhedrons which turn scattering amplitudes into high school geometry volumes, and other things.

He formulates it in such a way that I would agree with almost everything but I still feel that there's a potential tension or contradiction with what e.g. Juan Maldacena says on the other side. Nima thinks that the disappearance of spacetime notions as precise ones is "mandatory" – while Maldacena (and perhaps Witten, Seiberg, and others – and maybe me) – thinks that it's just "possible" to suppress the spacetime language to construct a different, equivalent description. But with some good enough stuff – degrees of freedom – that are placed in the spacetime (perhaps including extended objects like strings and branes, bilocal objects such as wormholes, and other things), it may very well survive as an exact concept, too.




You should remind yourself of the wonderful progress in the scattering amplitudes and the exotic mathematical structures underlying them, special functions, shapes in auxiliary spaces, permutations etc.




There are some questions at the end. Somewhere between 1:24:00 and 1:27:00, Nima answers a question about the quantum foundations – and TRF readers must think that they must have heard about such a thing somewhere. ;-) Nima says that 30 years ago, 99.9% of active physicists – and it's still true for reasonable physicists today – would agree that there's absolutely nothing physically interesting going on in the research of the "foundations of quantum mechanics". For some reason, he adds, this statement has become "somewhat politically incorrect" in recent years.

Exactly!

Incidentally, Edwin Steiner – who also shares Nima's and my views on the foundations of quantum mechanics – listened to me and joined an LHC tracking Kaggle contest a day ago. After 1 submission, he is 2nd among 453 contestants. Wow! The gold-silver and silver-bronze gaps look robust enough so that I would guess that if nothing changed, the top 2 folks would remain the same in the final leaderboard (based on the remaining 71% of the data). But we all root for Edwin, good luck!

Also, another question deals with the speed of the progress. Nima says that it may be fast or slow but the progress (mainly in the amplitudes, he thinks, it seems to me) in the last 10 years has vastly surpassed his expectations.

He's also asked whether the end of the spacetime violates the second law of thermodynamics. Well, the end of the spacetime has various meanings. He has discussed one of them – the need to replace the "spacetime" by another concept (and the author of the question apparently misunderstood this very point of the whole lecture). If you just use a different language, it cannot violate a law. At most, it can make the law ill-defined if the law has depended on the deprecated concept. But in this case, even this is untrue. The second law doesn't really depend on the spatial structure of the degrees of freedom – the existence of some "time" is enough.

The second interpretation of the "end of the spacetime" is some actual geometric end somewhere – a boundary at some spatial point or the big crunch as the end at some moment of time. The discussion would be different. But the second law follows from the special initial conditions, and those unavoidably evolve to less special ones by the evolution. This is always true (the H-theorem may be proven in general), regardless of the spacetime interpretations, so just like your humble correspondent in many blog posts, Nima assured the author of the question that the arrow of time has nothing to do with cosmology (with special assumptions about cosmology). Nima also agrees with the corollary that e.g. Sean Carroll is full of šit on these matters but he didn't want to be too comprehensible. ;-)

For some reasons, Nima never gets discouraged by getting questions that had nothing to do with his fascinating things of replacing the spacetime and quantum mechanics by not-quite-quantum and not-quite-local objects that may be combined to the local quantum amplitudes. Also, at some moment, he said that he can't be bulšiting for 24 hours a day because some of his colleagues are "intolerant". I guess that he talks about some formal theorists at the IAS Princeton. So Eduans or whatever are your names :-), you're insane. At most, you could push him to be bulšiting really 24 hours a day, he could get living with it. Why don't you prefer to discourage the people who are really bulšiting?

You may also watch another 90-minute video, one from the World Science Festival in February 2018:



This video has over half a million views. Brian Greene is the host. Between 1:15:00 and 1:24:00, Mark van Raamsdonk in particular does a very good job of explaining the ER-EPR correspondence, the ideas he helped to pioneer that the entanglement is the glue of the spacetime. Various intuitive interpretations and perhaps generalizations of that statement were given.

Gerard 't Hooft was there as a co-father of the holographic principle but it seemed clear that those ideas from the early 1990s were much more heuristic than the present ones and he isn't quite following what's going on.

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