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Black hole spin

  1. Apr 19, 2010 #1
    Can someone satisfy my curiosity...

    Black holes are considered to be things that sit and gobble up anything that comes close, just getting bigger and bigger.
    What if this view is the wrong way round? In a different scenario, the big bang could have produced nothing but black hole matter. As this black matter moved away from the start point it could have started spinning, with the centrifugal force detaching pieces of black matter from the parent. Some of these pieces that were detached from the spinning object would have been non viable (ie not massive enough to remain as black matter). They would have turned into stars. If the black matter was non-viable it might be expected to expand rapidly. This rapid expansion could have "lit" the stars. Others pieces would have remained as black matter, remaining as black holes or until they were caused to spin. This explains the catherine wheel look to a number of galaxies. It is not hard to imagine a black hole spinning with a slew of stars being thrown off. As the critical spin would be achieved at different times this provides an explanation as to why galaxies are not all the same age.

    Is this sense or nonsense, orthodox view or what?
     
  2. jcsd
  3. Apr 19, 2010 #2

    mathman

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    Spinning black holes do not throw off anything by centrifugal force. The escape velocity is greater than the speed of light.
     
  4. Apr 19, 2010 #3

    Nabeshin

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    So, there is nothing called "black hole matter".

    Like mathman says, black holes do not "throw things off". You may have heard of the concept of critically spin for black holes, above which the singularity would purportedly poke out above the event horizon. However, one cannot spin a black hole up to this requisite angular velocity no matter how hard one tries (like trying to accelerate a particle to the speed of light).

    Also, welcome to PF! As a new member, you should read the forum rules, particularly those pertaining to overly speculative posts:
    https://www.physicsforums.com/showthread.php?t=5374

    Cheers!
     
  5. Jul 3, 2010 #4
    Black holes can have spin. Yet they are posited to be singularities. Isn't that a contradiction? How can a singularity have spin?
     
  6. Jul 3, 2010 #5

    Nabeshin

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    How can a point particle, such as an electron, have spin?

    The analog between what we call spin, either for an elementary particle or black hole, and the classical version of angular momentum is shaky at best in my mind.
     
  7. Jul 3, 2010 #6

    mathman

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    The black hole singularity comes from pushing general relativity to the limit without taking into account quantum theory. When physicists try to use them both inside a black hole, there is a breakdown in the math - the two theories don't work together here.
     
  8. Jul 3, 2010 #7

    nicksauce

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    This is a very strange statement. What physical properties is a singularity supposed to have?
     
  9. Jul 3, 2010 #8
    Well, theoretically the only property I can think to assign would be in the case of a ring-singularity, but that's still pure conjecture. I think this is a simple misunderstanding of the notion of a black hole as a WHOLE, and instead thinking only of the singularity.
     
  10. Jul 3, 2010 #9
    Is there actually EVER an escape velocity for a black hole?
    i.e. Curved space, won't matter how fast.
     
  11. Jul 3, 2010 #10
    I have to ask, and probably because I'm truly exhausted... is the title of this thread a play on the song, "Black Hole Sun"? I really need to get to bed. @_0
     
  12. Jul 5, 2010 #11

    nicksauce

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    Kerr black holes do have a ring singularity!!
     
  13. Jul 5, 2010 #12
    What else is there beside the singularity? Yet a point can have no spin because it has no radius.
     
  14. Jul 5, 2010 #13
    There is the entire region within the event horizon (which may contain a ring singularity nicksauce, but that's a theory of a theory where a theory breaks down. I'm not convinced), the event horizon itself, and then one could argue that the ergosphere and accretion disk are also part of the black hole.
     
  15. Jul 6, 2010 #14
    But the event horizon has no mass. It is spacetime. Its shape and spin is dictated by the shape and spin of the singularity. Which has no shape, volume or spin. The EH's size is dictated by the black hole's mass. Which, I think, is the only physical property of a bh.

    The accretion disk has its own angular momentum, which is maintained as it falls towards the event horizon. I don't think that can be used to infer the black hole's spin.

    So how do astronomers know a black hole has(can have) spin?
     
  16. Jul 6, 2010 #15
    The 'quantity' of spin for a rotating black hole can be established by looking at the relationship between the event horizon, ergosphere and the prograde marginally stable orbit (MSO) which normally coincides with the inner edge of the accretion disk. For a stable black hole (where a=0), the MSO is at 6M, for a rotating black hole, this can reduce to M for a maximal black hole (a/M=1) though MSO have been detected at <2M which implies a spin parameter of a/M>0.94. The MSO is reduced for a rotating BH because the frame dragging effect contributes to the tangential velocity of an orbiting object which means it can orbit closer. This also results in a retrograde MSO for objects that orbit against the frame dragging which is much further out than the prograde MSO.
     
  17. Jul 6, 2010 #16
    I was adressing the issue of how you define a black hole: is it just the singularity, is it a thing or a series of events? I would define the ergoregion as being part of the black hole, and the accretion disk as not being part of it, but created by it. Given that as you say, it is mass deforming spacetime, it might be fair to say that a black hole can be defined as the region of influence within which gravity leads to paths that a star of the same mass would not create.

    In practice, you measure the "size" of a BH based on the EH because it does reflect its mass. A BH also can have spin as stevebd1 says, and CHARGE. It is also possible that as a solution to the Information Paradox, the EH fluctuates in such a way that it encodes information of infalling matter that is not included in HR.
     
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