The day started with the Event Horizon Telescope press release conference, which I watched at Flatiron (but could have watched at NYU or Columbia; a huge fraction of the community was watching!). It really is a beautiful result, and the data analysis looks (on cursory inspection of the papers) to be excellent and conservative. It is just incredible that we can observe a photon sphere, if that really is what it is! It seemed like such a thing of legend and story.
Interesting to think about language: Is this the first observation of a black hole? Or image of one? I'd say not, because any image of a quasar is just as much an image of the radiation around a black hole as this is. I think maybe it is the first image of the parts where strong gravity is acting (photons are orbiting!). But these are not objections in any way to the importance of the result! Just musing on the language. In what sense is this the first time we have taken an image of a black hole? And is it that? And etc.
In the afternoon, Kate Storey-Fisher and I went to the board and got confused about 6-dimensional integrals. We need them to understand correlation-function estimators. The “RR” term in the correlation function estimators is a 6-d integral over an outer product of space with space!
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