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All rotating black holes will eventually stop?

  1. Nov 23, 2009 #1
    According to Wikipedia:
    Objects and radiation can also escape from the ergosphere. In fact the Penrose process predicts that objects will sometimes fly out of the ergosphere, obtaining the energy for this by "stealing" some of the black hole's rotational energy. If a large total mass of objects escapes in this way, the black hole will spin more slowly and may even stop spinning eventually.

    By this logic, won't all rotating black holes eventually stop spinning? Or are there processes by which a black hole can gain angular momentum? If so, what are these?
  2. jcsd
  3. Nov 23, 2009 #2


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    In the example of a stellar-mass black hole that has an accretion disk of material that it's eating from its companion star, conservation of angular momentum says that any material that vanishes out of the companion must give its angular momentum to the black hole. I would expect the typical black hole to either have constant angular momentum (if it has no companion and no accretion disk) or increasing angular momentum (if it has an accretion disk).

    I could be wrong, but I believe the Penrose mechanism is somewhat more speculative. I think it's one of the mechanisms that's been proposed to explain relativistic jets. Relativistic jets come out in a narrow beam along the rotation axis, so I wouldn't expect them to carry away much angular momentum...?
  4. Nov 23, 2009 #3


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    A black hole is a bit complicated. What is it that you think is rotating? The accretion disk is defintely rotating, but, not the singularity. What do you think would have a braking effect on the singularity? The singularity sheds mass via Hawking radiation, but, is otherwise oblivious to the universe.
  5. Nov 24, 2009 #4


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    This is incorrect.

    The black hole itself can have angular momentum, not just the accretion disk: http://en.wikipedia.org/wiki/Kerr_black_hole

    The singularity can exchange angular momentum with the rest of the universe. If it couldn't, then its angular momentum would be undetectable.
  6. Nov 24, 2009 #5
    So a black hole eating a companion star will have a continuously increasing angular mometum?
  7. Nov 24, 2009 #6
    As bcrowell said, yes it DOES have angular momentum down to the singularity. It's that very problem that lead to Einstein and Rosen postulating the (mathematical) existence of a ring singularity and their "bridges".

    Secondly, the black hole is not "oblivious to the universe". One way or the other it interacts with the universe (maybe behind an event horizon, maybe not). It emits thermal radiation and can lose, and by "feeding" gain mass. Infalling matter carries its own angular momentum, but in the absolute absene of infalling matter and continuing Hawking Radiation... a black hole that is rotating will not stop doing so as a result of evaporative thermal process. What happenes to black holes when they fully radiate... IF they do... is a subject of conjeture. Probably they go "boom".

    Imagine the future thusly... the universe continues to expand (which it may), and all matter is eventually consumed by black holes, stars, pulverized in impacts, ablated by radiatin and decay... etc. You would expect black holes to have a similar endpoint if they obey thermodynamic laws; they should eventually release all of their mass as thermal energy, leading to a net increase of the entropy of the closed system that is our universe. Neat, eh?
  8. Nov 24, 2009 #7
    Pretty much every answer here is encapsulated in the following post from the archives of this site.

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