Sunday, January 28, 2018 ... /////

Nima on the doom of spacetime

Let me embed an exciting, two-hour-long, December 1st, 2017 talk by Nima Arkani-Hamed at PSW, the Philosophical [by purpose scientific] Society of Washington D.C.

Don't forget that buttons are available to play the video at a double speed (or other speeds) and many of you are fast enough listeners to be OK with that.

After PSW announcements, rituals, and various off-topic comments by the PSW officials, Nima is introduced between 11:00 and 13:00 and starts to talk at 13:15. He was introduced as a part-time string theorist associated with IAS Princeton and with Cornell but he chose IAS and Beijing on his first slide. ;-)

He introduced his field as the oldest, 2000-years-old, deepest science that was modernized by Galileo and Newton and that recently undergoes a true revolution once in a century. Time may be ripe for another revolution and that revolution could depend on the doom of spacetime. A nice and concise summary of the whole history of human thought! ;-)

Arkani-Hamed sketched some of the important lore showing that the spacetime must become approximate. When you try to probe ever shorter distances, you increase the energies (like at the colliders – the inverse relationship holds because the de Broglie wavelength gets shorter when the momentum is higher due to the uncertainty principle) but that strategy fails once the energy is too high and you produce black holes. By adding energy, instead of probing ever shorter distances, you produce a larger black hole. The resolution just doesn't get better than a Planck length.

Also, you can't measure things too precisely in quantum gravity, even when things are big, so the limitations of the spacetime affect long-distance physics, too. When you want an accurate measurement, you need large gadgets and very fat experimenters to reduce the risk of fluctuations and random evaporation of finite objects, but such gadgets and fat men may collapse into a black hole as well when they're too fat, so the experiment breaks down.

So the spacetime must be approximate, it must emerge. He showed the AdS holography as an example of physics in a tin can. After healthy doses of a useful review of these gems of contemporary research, he asked a question: How could an old physicist usefully exploit the information from a traveler in the future that determinism is dead? Well, such a general surprising thesis doesn't immediately lead to a new theory, quantum mechanics.

A good strategy would be to reformulate things he knew in a way that doesn't depend on the old dogma that is known to be wrong – determinism in this case – too much. He could figure out that the principle of least action is a good starting point and rediscover Feynman's path integrals in this way. That's the strategy that Nima wants to use with the information he got from a time traveler from the future – that the spacetime is doomed (I tried to watch the Hitchhiker's Guide to the Galaxy last night but I failed to complete it again – it just seems like too many seemingly unrelated silly would-be jokes).

The application of this strategy means to find a way to calculate the known amplitudes in a way in which the spacetime and perhaps quantum mechanics don't look central. So he outlined the amplituhedron program, the picture that the amplitudes have so many complicated terms because the amplitudes are really volumes of a polytope in an auxiliary space and the polytope is cut to many complicated pieces in some ad hoc ways. This program interprets a scattering process as a generalization of the process where just numerical labels scatter – and their scattering means a permutation.

At the end, there were questions. Most of the questions ended with the answer "no, I don't think that there's a known direct relationship between the amplituhedron and a god ABC in the religious sect XYZ", if I exaggerate just a little bit. Over the years, I've been getting so many questions of this kind – questions that are insanely far from the topic I talked about and that indicated that I was mostly wasting my time – that I grew disillusioned about the usefulness of popular talks in general.

One question led Nima to an important and cool answer saying that from the old-fashioned perspective, the scalars' $j=0$ scattering is the simplest one, gluons with $j=1$ are harder but doable, but $j=2$ graviton scattering is ludicrously hard even at the basic level. But from the novel emerging viewpoint, it's the other way around. Gravity sees the new simplifying structure most clearly and things get messier as you lower the spin.