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B What does mean that a black hole is spinning?

  1. Jun 3, 2016 #1
    The angular momentum is related to the rotation. And when a black hole has angular momentum, it is said that it is a "rotating black hole". But what does it mean? A black hole does not have a conventional surface, like a basketball. How should we interpret that angular momentum?

    I asked this question previously in a Spanish forum, but nobody know :( And I think it is a very simple question.

    Thank you and sorry for my basic English.
     
  2. jcsd
  3. Jun 3, 2016 #2
    The angular momentum of the black hole is simply the angular momentum of the body that it formed from. It is detectable via the phenomenon of Frame Dragging.

    Point of order: If I can answer it, it isn't advanced ;)
     
  4. Jun 3, 2016 #3
    Of course, it can change as it engulfs infalling material.
     
  5. Jun 3, 2016 #4
    Thanks. It seems to be advanced for Spanish forums, since nobody know :D
     
  6. Jun 3, 2016 #5
    That's a good question, and personally I don't know if we can relate the angular momentum or "spinning" of a black hole with the casual meaning of rotating or spinning.

    From a quantitative perspective one applies the conservation laws to determine that the angular momentum of a black hole plus that carried by any material blasted away at the formation is equal to the angular momentum of the star that collapsed. Then later any type of matter or energy with angular momentum that is absorbed or emitted by the black hole will add or remove angular momentum from it. But is it (the singularity?) rotating in the usual everyday meaning of the word? No idea, the question itself might be meaningless.
     
  7. Jun 3, 2016 #6
    Maybe, the angular momentum can be understood as a distortion in the adjacent space-time around the black hole, not the black hole itself. That is: the angular momentum is a property of the adjacent space-time around a black hole. But I have no idea, actually :D
     
  8. Jun 3, 2016 #7
    From the perspective of an outer observer all the material that builds the black hole is stuck on the horizon because of the infinite time dilation at its surface. This material is, from that perspectice, frozen right above the horizon, but angular momentum has to be conserved so it becomes space itself that rotates (with time standing still on the surface). Because there is, still speaking from the outer perspective, nothing inside black holes (not even spacetime, see the Schwarzschild paraboloid, which has a hole in the middle where the Schwarzschildradius begins), what you get is a rotating shell. For the infalling particle the story is a bit different, but as long as we are talking about the black hole from the outside.
     
  9. Jun 4, 2016 #8
    There is an interpretation of spin in Einstein-Cartan theory. In this theory, the spacetime has an additional property of torsion. Not only curvature, but also torsion. The torsion is coupled to the curvature and this is known as frame-dragging.

    Rotating black hole (or in some interpretations even elementary particles) has nonzero torsion.
     
  10. Jun 5, 2016 #9

    PeterDonis

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    I don't think this is correct. Frame dragging is an effect that appears in standard GR, where the connection is torsion-free.

    This doesn't look correct either. A rotating black hole is described by the Kerr solution in standard GR, again with a torsion-free connection.
     
  11. Jun 5, 2016 #10

    PeterDonis

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    This is not correct. A black hole is a vacuum solution; there is no "material" anywhere.

    The rest of your post does not look correct either. It is very important in this subject to look at the actual math, not at heuristic pop science descriptions.
     
  12. Jun 5, 2016 #11

    PeterDonis

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    This is actually a good heuristic way of understanding, not just the angular momentum, but the mass of the hole (or indeed of any isolated object). If you go far away from the object and measure the geometry of spacetime, you will find certain aspects of that geometry that correspond to "mass" and "angular momentum". For example, you can put test objects in orbit about the isolated object and measure their orbital parameters, and from those measurements you can compute the mass and angular momentum of the isolated object.
     
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