Final lecture before Easter

I just put out the final lecture before Easter, to be discussed on Monday. It's slightly longer than usual -- but that allows us to finish the discussion of black holes before the break (also, the last part is really more just a technical comment; you can certainly skip over the proof unless you are really interested in the details).

_{[NB: If anyone has experience with handling mp4 files, and manages to merge GR_lecture_20_2a.mp4 and GR_lecture_20_2b.mp4 into a single mp4 file, that would be greatly appreciated. I managed before, for similar setups (using ffmpeg), but for some unknown reason this doesn't seem to work in this case...]}