Only research today was a talk by Gwenael Giacinti (APC, Paris) about anisotropies in the local distribution of cosmic rays. It ended with a short argument (in the audience) about lensing of the intensity field, with various people confused and not confused about the meaning of the point that lensing conserves phase-space density of photons. The point is trivial in one sense, but in an equally important sense, it is not at all trivial, as evidenced by confusion among astrophysicists about what it can and can't mean. The argument inspired me to write something about it.
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