## Saturday, February 29, 2020 ... //

### Freeman Dyson, 1923-2020

Freeman Dyson died at age of 96+ in a hospital near Princeton, New Jersey, due to complications after he fell, yesterday. He was a legendary English-born theoretical physicist (see Google Scholar: S-matrix, the first reviews of QED, spin waves, statistics of eigenvalues, and models for phase transitions, among other topics), futurist, astrobiologist, and many other things. In recent years, he was a well-known climate realist (liking to say that things are complex etc.).

The cropped photograph is still near the top of Google Images for "Freeman Dyson" and I took it in an important Harvard physics classroom, J253, I think. On that day, I was his host and we had interesting chats about the extraterrestrials' strategies and other things.

He was old enough to remember Albert Einstein from the Institute for Advanced Studies at Princeton.

Dyson has been a real top physicist and reformulated Feynman's derivation of the Feynman diagrams into the operator language. Switching to the interaction picture, the S-matrix (or evolution operator) is something like$S = T\exp\zzav{\int dt\,\zav{-iH_{\rm int}}}$ and some expansion involving the fields appearing in the interaction picture Hamiltonian produce the propagators (from pairing; the time-ordered character of the propagators emerges because of the time ordering in the formula above which is needed because the infinitesimal evolution operators that combine into the exponential are ordered) and vertices (from their being inside the same $$H_{\rm int}$$).

When I was getting started with quantum field theory, I found this derivation of Feynman diagrams cleaner than the original derivation using the path integrals and for some time, I didn't really actively know that Dyson's derivation was newer than the path integral one. Today, I am completely democratic in my sympathies for both approaches. They're equivalent in simple enough cases although some complications may be easier in one of the pictures, usually in the path integral picture.

He's been doing lots of things, writing popular and technical books, fighting against wars, proposing various original ideas and speculations – and, perhaps ironically, fighting against things that he saw as speculations himself. Once he and Feynman drove from the East Coast to New Mexico. During those cool four days, they also encountered a flood in Oklahoma which was a wonderful excuse to spend some quality time in a brothel. Would such a visit in a brothel be sufficiently politically correct for Princeton physics professors today? Are there any brothels in Oklahoma at all today? And if universities discourage professors from going to brothels, why are they surprised that they have so few people of Feynman's and Dyson's caliber?

He also invented the catchy concept of the Dyson Sphere: an extraterrestrial civilization may surround its star by a big sphere of solar panels or something of the sort to catch all the star's energy. It's simple, beautiful, and I would bet that there's not a single one in the visible Universe as of now. At some moment, it's extremely impractical and civilizations that aren't obsessed with being visible just ultimately don't need so much energy (look at the crippling environmentalism today, it's all about saving). Do we really "want" to rebuild huge planets and galaxies? I think that most people surely don't (and probably most extraterrestrial Americans don't want it, either) and sometimes I am not sure whether I would find it a priority. A Dyson sphere could be a good source of energy for a huge, superplanetary collider.

Also, I think that the construction of such a thing contradicts the capitalist economy. Just look at the size: It would probably cost a majority of their GDP. If it were a public project, it would mean that they redistribute most of their wealth and such a socialist economy could never get close to something like building Dyson Spheres (on Earth, even when we are planning to build a modest 100-km tunnel with magnets, a new collider, we face a bunch of obnoxious anti-science hecklers!). In this sense, the Dyson Sphere could even be considered a communist propaganda, a flawed promise what a forcedly united mankind could be able to build. Also, there is really no good reason why the whole sphere should be filled with planets – a small part of this structure seems enough (there is no qualitative advantage or irreducibility of the completion). For those and related reasons, I think that it is really just a simple meme with a "beautifully" simple geometry that is unrealistic for all the "dirty" reasons.

Of course, Dyson's voice will be missing on the sane side of the climate debate although I believe that his main weapon was his authority that he had earned elsewhere, not some very deep new ideas or careful analyses of difficult questions. His full name appears in 47 currently uncensored TRF blog posts.

RIP, Prof Dyson.