A Farewell to Black Holes

Yesterday here at USC was my last lecture in the class about black holes (see also here). We’ve got to move on to other topics (Cosmology, Einstein’s equations, etc) and so cannot do any more. It was a fun last lecture though. I pulled together a few scraps of things I did not finish in the previous lecture (such as the extraordinarily high percentage of binding energy per unit rest mass you can extract with rotating black hole orbits – just what you need to power things like quasars) and then finished with:

  1. A taste of Hawking radiation, the Bekenstein-Hawking entropy and the wonderful and beautiful subject of black hole thermodynamics that opens up when you combine gravity with quantum mechanics*, followed by…
  2. A quick discussion of the Penrose process for extracting energy from rotating black holes. (I’m sure that all (past, present or future) super-advanced civilizations must be using them as the ultimate emissions-free means of generating energy for heating their homes and so forth. No, really.)

*Of course, all undergraduates commonly hate it when you dare bring in stuff from other classes, so to combine a class on relativity and gravity with quantum mechanics was probably perturbing enough. To add insult to injury by opening up the whole subject of statistical mechanics and thermodynamics was no doubt a bit of a shock! Nevertheless, I never shy away from presenting things like this since this is where the actually interesting cutting edge ideas often lie – at the intersections between subjects. Despite such traditional resistance tendencies, my group seemed to enjoy themselves quite a bit yesterday, I was happy to see.

Tonight I’m preparing the midterm that they will take tomorrow. It seems like it was only just the other day that I was writing the first midterm, which actually went rather well. I hope this one does too. Slightly worryingly, the signs that I started preparing it too early have begun to show: The tasks I first thought of for them to do seem all too boring and mechanical, and so I am toying with devising a much more “interesting” and “instructive” midterm, where they learn some new physics as they go along. This can lead to trouble sometimes (er… see the words in quotes), but when it works it is always a happy event. (At least for me, and I hope for them too.)

So here I go, head down, brain open, cup of tea on the side, pencil and paper at the ready. Let’s see what I come up with!


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4 Responses to A Farewell to Black Holes

  1. adam says:

    I rather liked it Tuesday when “other” physics started popping up. it sort of brings back the bigger picture and helps us remember that black holes indeed do not exist in some obscure physics corner all by themselves.

  2. Clifford says:


    Glad to hear that… as I said in the post, it did seem that the class was engaged and quite interested in the connections, which was good to see (and does not always happen, in my experience, so I was pleased!)


  3. David says:

    Just finnished my gr course this week. Exam already written, can’t wait to see how they will do. High hopes..
    My indulence was to do Kaluza-Klein at the end of the course and why not, I think they got it.

  4. Clifford says:

    Good luck! Yes, that is a tempting indulgence indeed.