## Tuesday, January 28, 2020

### A remarkable bullish, correct PBS video on Heisenberg, Bohr, bootstrap, and amplituhedrons

Peter F. sent me a link to a new PBS SpaceTime video
Hacking the Nature of Reality (17-minute-long YouTube video)
and told me that it would make me smile. I was very skeptical about that claim. Everything that other people systematically create to bombard me/us with seems frustrating these days. But holy cow, Peter was right. I was smiling soon and despite additional skepticism about my continued smile, I did continue to smile. ;-)

The video is amazing. This part of PBS was clearly conquered by someone who knows the stuff, who is on the right side of everything when it comes to discussions about modern fundamental physics.

I have previously discussed numerous PBS Space Time videos on this website and they got a better than average ratings. While they were still a mixed baggage, I have upvoted numerous videos. However, this was the first time when the probability was over 98% that I would upvote the video, assuming that the probability is computed by weighting some random mood swings of mine.

Remarkably enough, the video was also upvoted by more than 98% of the viewers who expressed their opinions and within less than a day, the video got over 160,000 views, showing that even the truth may be embraced by the public and it really matters what the public is served.

OK, the video starts with a rare defense of Heisenberg and Bohr. People were trying to do their old-fashioned classical "reductionist" things, asking what are the exact components of the atom etc. and what they're doing. Radically and ingeniously, we're being told, Werner Heisenberg (escaped from hay fever to Heligoland and) realized that those things were unobservable, perhaps even in principle, and we need to describe the atom in terms of things that are actually observable.

So he was led to invent matrix mechanics, the oldest realization of quantum mechanics. That's exactly how it was but you won't hear this truth too much around 2020, i.e. 95 years after Heisenberg's pioneering discoveries. You know, Heisenberg realized that the probabilities of transitions seem to be measurable, there's some interference so it might be sensible to replace the probabilities by their square roots with a phase, the amplitudes (they were first "intensity amplitudes" but Born made it clear that "probability amplitudes" are more fundamental), and Heisenberg has been looking for the transition (unitary) matrices with elements $$U_{jk}$$ ever since (Heisenberg first used the indices $$j,k$$ for the energy eigenstate basis because it was already experimentally clear that the energy levels were quantized in the proper sense). The role of Bohr is mentioned.

Needless to say, I would subscribe every sentence of that monologue including some subtle kicks and omissions that could really look controversial within a broader contemporary community of raters. Aside from matrix mechanics, we hear, some other versions of quantum mechanics also emerged soon, like wave mechanics. That one was suppressed enough so that even the name of Erwin Schrödinger was omitted, as far as I could hear, and rather deservedly so. Wave mechanics appeared both later and it was created without the full understanding of the new philosophy that physics had to adopt since Heisenberg's discoveries.

This is so fair and appropriate yet so unusual these days. Heisenberg's dictum "describe the relationship between things that can actually be observed and allow the perhaps-unobservable things to be classified as unphysical" was ingenious, radical, new, yet very crisp, logical, internally consistent, and easy to precisely formulate and it's one of the most important ideas underlying modern physics. Needless to say, almost every presentation of these topics rewrites the history, places Erwin Schrödinger and wave mechanics into the cradle of quantum mechanics, and Heisenberg and Bohr are often either omitted or downright spat upon. Wave mechanics is then usually spun in a way to persuade the listeners that the wave function is just another classical field (a misconception that was started by Schrödinger himself, with some early help from de Broglie and Einstein). It's completely and fundamentally wrong but it has become the default claim of almost all pop science presentations of quantum mechanics in the postmodern epoch.

Not here, however. The video was completely right. Then it continued by saying that most scientists ignored the teachings by Bohr and Heisenberg – their insight that one should focus on talking about the observable things and the other events in between could be meaningless physically even in principle. Well, I had some 0.1% probability to change the upvote to a downvote at that moment because Bohr and Heisenberg really did define the mainstream for decades and there were lots of brilliant, achieved men who mainly differed by not being quite the true fathers of this idea but who agreed (Jordan, Born, Pauli, Dirac, Wigner, von Neumann, Pauling, Kramers, Weizsacker, and then the next generation with Feynman and many others). But the video made a good point, I think, when it reframed some early constructions of quantum field theory around 1930 as efforts to return to the reductionist, and morally classical, world view.

But I quickly returned to the positive judgement because the comment seemed sort of right, and nontrivially for me: maybe I learned something, it clicked. After all, I did want to indicate that the focus on "QFT of 1930" is a sign of a misunderstanding of the key lessons of the modern physics (of Einstein and Heisenberg) – focus on the principles and the principles may include the unobservability of many things (objective simultaneity of events, events inside atoms). It's how the "QFT of 1930" people think today and maybe many of them did even in 1930. So the video was really plausibly right and continued with history that makes this point meaningful.

Heisenberg wasn't a hasbeen yet and did pioneer the S-matrix or bootstrap approach to physics: S-matrix isn't an emergent quantity calculated from fundamental things. Instead, the S-matrix is the object or the law itself, something that we directly search. That approach – which naturally followed from the same "intermediate is unphysical" philosophy that Heisenberg promoted at lest since 1925 – temporarily failed and string theory was temporarily anti-bootstrap and then also pro-bootstrap but the final words hadn't been said yet and contemporary string theory and Arkani-Hamed's group's amplituhedron things were the most modern justifications for the bootstrap view. Lots of QCD Feynman diagrams and similar things appeared during the monologue.

The ratio of correct-vs-wrong-or-misleading claims, from a good expert's viewpoint, is so much higher in this video than in almost all previous videos that it makes it rather clear that some really competent guy discontinuously took over the PBS Space Time program, at least temporarily. I wonder who he or she was. I am not sure whether I want to know. If more smile could be likely assuming that I won't know, I would prefer not to know and treat the author of the uprising at PBS Space Time as an intermediate process in quantum mechanics – whose details you shouldn't ask about! ;-)

Not surprisingly, I think that this intervention could have come directly from Nima Arkani-Hamed. If that's true, then it also means that he has promoted himself. Arkani-Hamed's name has appeared among 6 or so important physicists – he was quoted as a guy from the group Bohr, Heisenberg, Wheeler, Chew, Veneziano – but of course it is right for Nima to promote himself because it's right to promote good stuff, even for himself. ;-) I am just not sure whether he would do so. If it wasn't directly Nima, it was someone who was intellectually close to him. However, that someone had to have a sufficient clout to persuade the PBS show to do everything right which is unheard of these days.

Is there someone else who would have the energy to push them and who would be listened to at the same moment? Well, for this reason, I think that there had to be a real Nima behind this video. At any rate, it was a refreshing change. Matt O'Dowd, the talking head, has hinted new episodes on the newer topics like the amplituhedron stuff and rearrangement of QCD diagrams etc. which could be fun.