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Do blackholes always rotate?

  1. Nov 14, 2011 #1
    There are two representations of blackholes that I see in the popular media. Could you help me find the answer to a (or two) question about this please?

    1. Feeding blackhole with an accretion disk - I assume that the accretion disk is a product / consequence of rotation (in addition to the feeding) ... but we all know what they say about assumptions ... ?

    2. A blackhole passing in front of a star field, causing gravitational lensing. I don't know if these are rotating, or where to start looking for discussions on the topic. Any ideas?

    Basically, do blackholes always rotate? Any pointers / suggestions would be greatly appreciated.


  2. jcsd
  3. Nov 14, 2011 #2


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    Realistic black holes will always rotate. This is because the stars which give birth to them always rotate, and that angular momentum becomes the rotation of the black hole during the collapse.

    Note: Lensing doesn't really have anything to do with rotation. It's an effect observed even in a nonspinning black hole.
  4. Nov 14, 2011 #3


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    The accretion disk is not a product of the balck hole rotating, but of the angular momentum of the matter being attracted to the black hole.

    Suppose you a non-rotating black hole with a star close enough to the black hole so that some of the gases from the star's outer layers is being pulled towards the black hole. The star would have to be revolving around the black hole so that itself is not pulled in, all at once. Therefor the star has an angular velocity, or a forward speed around the black hole. As the gases get closer and closer to the black hole, they still keep this forward motion, and thus spiral around the black hole forming the accretion disk.
  5. Nov 15, 2011 #4
    Thanks Nabeshin. Can I just ask, is "realistic" a classification - I can't find any references to it when I search for it.
  6. Nov 15, 2011 #5

    D H

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    "Realistic" as in what one would expect to see. Think of it probabilistically: What is the probability that a black hole has a rotation rate of that is exactly zero? I would venture that it is exactly zero.
  7. Nov 15, 2011 #6
    Thanks 256bits.

    Of course! Sorry, I should have seen that.

    Am I right in thinking that this infalling material would cause the (non-spinning) black-hole to start to rotate? Also, on a different note, would the infalling material start accelerating to the speed of light once it crosses the event horizion?

    Thanks again 256bits.


  8. Nov 15, 2011 #7
    OK, I think I understand. Thanks D H.
  9. Nov 19, 2011 #8

    They must. As results of stellar system they were once the sun of..They have A/momentum.
    The speed of rotation is interesting.. the event horizon bulges as in an oblate sphere at very high rotation.

    DH the probability is .. zero. Though that statistics and my field is stochastics related to humanity
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