I’m back from Amplitudes 2019, and since I have more time I figured I’d write down a few more impressions.

Amplitudes runs all the way from practical LHC calculations to almost pure mathematics, and this conference had plenty of both as well as everything in between. On the more practical side a standard “pipeline” has developed: get a large number of integrals from generalized unitarity, reduce them to a more manageable number with integration-by-parts, and then compute them with differential equations. Vladimir Smirnov and Johannes Henn presented the state of the art in this pipeline, challenging QCD calculations that required powerful methods. Others aimed to replace various parts of the pipeline. Integration-by-parts could be avoided in the numerical unitarity approach discussed by Ben Page, or alternatively with the intersection theory techniques showcased by Pierpaolo Mastrolia. More radical departures included Stefan Weinzierl’s refinement of loop-tree duality, and Jacob Bourjaily’s advocacy of prescriptive unitarity. Robert Schabinger even brought up direct integration, though I mostly viewed his talk as an independent confirmation of the usefulness of Erik Panzer’s thesis. It also showcased an interesting integral that had previously been represented by Lorenzo Tancredi and collaborators as elliptic, but turned out to be writable in terms of more familiar functions. It’s going to be interesting to see whether other such integrals arise, and whether they can be spotted in advance.

On the other end of the scale, Francis Brown was the only speaker deep enough in the culture of mathematics to insist on doing a blackboard talk. Since the conference hall didn’t actually have a blackboard, this was accomplished by projecting video of a piece of paper that he wrote on as the talk progressed. Despite the awkward setup, the talk was impressively clear, though there were enough questions that he ran out of time at the end and had to “cheat” by just projecting his notes instead. He presented a few theorems about the sort of integrals that show up in string theory. Federico Zerbini and Eduardo Casali’s talks covered similar topics, with the latter also involving intersection theory. Intersection theory also appeared in a poster from grad student Andrzej Pokraka, which overall is a pretty impressively broad showing for a part of mathematics that Sebastian Mizera first introduced to the amplitudes community less than two years ago.

Nima Arkani-Hamed’s talk on Wednesday fell somewhere in between. A series of airline mishaps brought him there only a few hours before his talk, and his own busy schedule sent him back to the airport right after the last question. The talk itself covered several topics, tied together a bit better than usual by a nice account in the beginning of what might motivate a “polytope picture” of quantum field theory. One particularly interesting aspect was a suggestion of a space, smaller than the amplituhedron, that might more accuractly the describe the “alphabet” that appears in N=4 super Yang-Mills amplitudes. If his proposal works, it may be that the infinite alphabet we were worried about for eight-particle amplitudes is actually finite. Ömer Gürdoğan’s talk mentioned this, and drew out some implications. Overall, I’m still unclear as to what this story says about whether the alphabet contains square roots, but that’s a topic for another day. My talk was right after Nima’s, and while he went over-time as always I compensated by accidentally going under-time. Overall, I think folks had fun regardless.

Wouter Meeussenit’s kind of cute that your name never appears in your posts. I take that as a hallmark of problem-centered versus self-centered sience. Applause. Keep it up, you’ll get ‘there’ in the end. Where? I dunno. What’s your take on Sabine Hossendelder’s standpoints (http://backreaction.blogspot.com/)?

Wouter.

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4gravitonsPost authorThanks! I do promote my own stuff on this blog sometimes, but yeah overall the blog isn’t supposed to be about me.

Regarding Hossenfelder, I think she’s right that social dynamics and cognitive biases play a big role in science, and wrong that pointing this out works either as an argument against particular scientific ideas or as a way to get people to change. I’m more pessimistic than her in that even with perfect objectivity I don’t think we’d be much further ahead, and more optimistic in that I think a good enough new idea can still shine through. I think she has some silly ideas and some good ones (I particularly like her idea of dedicated grants to help scientists switch fields).

Regarding the specific topic of whether to build the next big collider, I’m unsure. I have the impression there are Standard Model predictions that only such a collider can test, parts of the Standard Model that we can’t be confident in from just our current knowledge. But the argument I have in mind is a bit odd and I’d want to discuss it with an expert before stating it publicly. And regardless I have no way to assess the tradeoffs of the collider proposals, since I don’t know which funding agencies would be supporting them and what if anything they would cut to do so.

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