It’s that time of year again, and I’m at Amplitudes, my field’s big yearly conference. This year we’re in Dublin, hosted by Trinity.
Increasingly, the organizers of Amplitudes have been setting aside a few slots for talks from people in other fields. This year the “closest” such speaker was Kirill Melnikov, who pointed out some of the hurdles that make it difficult to have useful calculations to compare to the LHC. Many of these hurdles aren’t things that amplitudes-people have traditionally worked on, but are still things that might benefit from our particular expertise. Another such speaker, Maxwell Hansen, is from a field called Lattice QCD. While amplitudeologists typically compute with approximations, order by order in more and more complicated diagrams, Lattice QCD instead simulates particle physics on supercomputers, chopping up their calculations on a grid. This allows them to study much stronger forces, including the messy interactions of quarks inside protons, but they have a harder time with the situations we’re best at, where two particles collide from far away. Apparently, though, they are making progress on that kind of calculation, with some clever tricks to connect it to calculations they know how to do. While I was a bit worried that this would let them fire all the amplitudeologists and replace us with supercomputers, they’re not quite there yet, nonetheless they are doing better than I would have expected. Other speakers from other fields included Leron Borsten, who has been applying the amplitudes concept of the “double copy” to M theory and Andrew Tolley, who uses the kind of “positivity” properties that amplitudeologists find interesting to restrict the kinds of theories used in cosmology.
The biggest set of “non-traditional-amplitudes” talks focused on using amplitudes techniques to calculate the behavior not of particles but of black holes, to predict the gravitational wave patterns detected by LIGO. This year featured a record six talks on the topic, a sixth of the conference. Last year I commented that the research ideas from amplitudeologists on gravitational waves had gotten more robust, with clearer proposals for how to move forward. This year things have developed even further, with several initial results. Even more encouragingly, while there are several groups doing different things they appear to be genuinely listening to each other: there were plenty of references in the talks both to other amplitudes groups and to work by more traditional gravitational physicists. There’s definitely still plenty of lingering confusion that needs to be cleared up, but it looks like the community is robust enough to work through it.
I’m still busy with the conference, but I’ll say more when I’m back next week. Stay tuned for square roots, clusters, and Nima’s travel schedule. And if you’re a regular reader, please fill out last week’s poll if you haven’t already!
Why are you folks better at calculations where two particles start from far away?
Good question! There are two answers I could give: why Lattice QCD is bad at that kind of calculation, and why Amplitudes is good at it.
Because Lattice QCD does computer simulations, their calculations essentially have pixels: they’re looking at a finite lattice of points, limited in size by how powerful their computer is. If they want to look at particles traveling over a long distance they have to make their lattice points further apart, so their “resolution” decreases, and they can’t see what’s happening at small scales when the particles actually collide. That tradeoff means they have to use smarter tricks (like those Maxwell Hansen talked about) to reinterpret a calculation in a small box in terms of particles coming in from far away.
Amplitudes, in contrast, specifically focuses on calculations where particles start from far away. As long as the particles have enough energy we can think about them using order-by-order approximations, for example with Feynman diagrams. Basically, while we both use approximations, our approximation is to assume we’re dealing with forces that aren’t too strong, while Lattice QCD instead breaks space into pixels, and the former works much better when you have a wide range of distances to think about.
There was a good-looking introduction to lattice QFT today!
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