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I First post plus a question re rotating black holes

  1. Dec 1, 2016 #1
    Hi there!

    This has been on my mind for a while now, maybe someone here can help me understand.

    First of all, non-rotating black hole is an idealized concept, right? Why the term then? Other objects are not normally referred to as rotating stars, rotating planets, etc...

    And the main question: in the case of a black hole, what is rotating? As far as I understand it's all energy in the form of curved space-time in there. Does the curved space-time rotate in that region? Rotating space - ok, can live with that. But time? This hurts brain, please explain or the hamster dies...

    Thanks :)
  2. jcsd
  3. Dec 1, 2016 #2
    Also, how accepted is the black hole firewall hypothesis? I recently read about it in 'The Black Hole War' by Leonard Susskind.

    If I understand correctly: Stephen falls into a black hole and, according to the rest of the universe outside the black hole, gets fried at the firewall. But according to Stephen, he passes the horizon and lives to see his 100th birthday while traveling towards the singularity. Both fates of Stephen are real, but can never be observed simultaneously. Is this right?

    Sounds like some kind of relativistic version of Schrödinger's cat, doesn't it?
  4. Dec 1, 2016 #3


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    Staff: Mentor

    The black hole firewall hypothesis doesn't work the way you're thinking. These's only one fate, and infalling Stephen and we on the outside agree about what it is. If the hypothesis is correct, he burns up and if the hypothesis is incorrect he falls through the horizon and dies at the central singularity. We won't see either outcome because light from whatever happens won't reach our eyes, but that doesn't mean it didn't happen - lots of things happen that we don't see. (Schrodinger's cat also doesn't work quite the way that you're thinking, but that's a topic for the quantum physics forum - we have many threads there).

    Stephen doesn't get to celebrate his 100th birthday on the way towards the singularity, unless he's already 99 and happens to pass through the horizon just before the stroke of midnight the night before his birthday. And even then it will be a short celebration - he only has about 100 microseconds between passing the horizon and reaching the central singularity.

    For your main question, exactly what is rotating, the answer is that we don't know. We start with a star. It's rotating, so it has angular momentum. When the star collapses to form a black hole, the rotating matter ends up at the central singularity. We don't know exactly what happens to the rotating matter there, but angular momentum is conserved so we know the angular momentum doesn't go away and black hole still has it.

    If the angular momentum is small enough that we can ignore its effects or irrelevant for the problem at hand, we say we have a "non-rotating black hole" and we use the Schwarzschild solution as a good approximation. If the angular momentum is large enough to matter, we say we have a "rotating black hole", and we use the more complicated Kerr-Newman solution (Google for "Kerr solution") to the Einstein Field Equations to analyze it.
    Last edited: Dec 1, 2016
  5. Dec 1, 2016 #4
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