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Can you scoop out part of a black hole?

  1. Nov 28, 2009 #1
    If the moon was formed by some kind of off center impact with earth could something similar occur with the collision of two black holes. My naive knowledge is that, except for Hawking radiation, nothing can escape a black hole. If two black holes have a glancing impact at extremely high velocity what happens?
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  3. Nov 28, 2009 #2
    A lot of enery in form of gravitational waves can escape.
    Last edited by a moderator: Sep 25, 2014
  4. Nov 28, 2009 #3
    Is it really the case that gravitational waves escape from within the black hole? The acceleration of an electron create an electromagnetic wave but is it "escaping from the body" of the electron? I thought the waves formed by the acceration of an object involved the fields surrounding the objects.

    In either case, when I look at the various thing on the web relating to colliding black holes they seem to focus not on the bodies themselves but on the gravitational waves. They simply show a new larger sphere.

    What I want to know is, if two black holes slung towards each other at an angle ,such that they don't hit head on, but their event horizens briefly intersect, what happens? Can little blobs of black holes be flung off from this.
  5. Nov 28, 2009 #4


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    To rephrase as a weaker question: can a black hole, oddly shaped or not, ever split into two black holes?
  6. Nov 28, 2009 #5


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    In Leonard Susskind's recent book for laypeople, he says that around the time leading up to the prediction of Hawking radiation, one of the ideas in the air was that microscopic black holes could be radiated out from the event horizon in a kind of extremely asymmetric fission. I don't think anything like this can happen in classical gravity, though.
  7. Nov 28, 2009 #6
    Lets assume black holes can be thought of as point masses with an accompanying Schwarzschild radius. A Schwarzschild radius is the event horizon, nothing can escape once it passes this radius because it would need an escape velocity greater than the speed of light.

    Therefore if one blackhole entered the radius of another, it could not escape because it cannot exceed the speed of light.
  8. Nov 29, 2009 #7

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    They merge.
  9. Dec 1, 2009 #8
    The is suprising. I presume if they just miss by a planck's length they can continue on their merry way if they have enough momentum/speed. Is it that only light can escape just outside the event horizon and mass has to be farther away to escape or is it that just the slightest touch drags in a black hole as if it were a rigid object?

    So if two black holes pass at a large distance, and at their closest approach, a lot of other tiny black holes just happen to come in from different directions between the two big ones and form a beaded necklace where each just barely touch the next one then somehow this thin chain of blackholes can pull together these other two huge black holes moving in opposite direction. If A merges with B and B merges with C then ...
  10. Dec 1, 2009 #9
    In this article author mentioned that naked singularities can form as a result of collision of two black holes. That means mass can be ejected from a black hole as a result of collision. Because if black holes collide such that there event horizons overlap then mass in the superimposing region gravity would be much weaker two black holes pulling in opposite directions with almost same force, so this mass can escape or it may fall in any one of the black holes.
  11. Dec 1, 2009 #10


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    That's a speculation based on numerical simulations, I don't think there are any known exact solutions which can prove beyond a doubt that this is what GR predicts.
    Why would it necessarily mean that? A naked singularity could have a mass equal to the sum of the two black hole singularities.
    But gravity is not a "force" in GR, so this kind of analysis isn't sound. You'd need a more detailed analysis of the spacetime curvature as two black holes collided to see if anything that was formerly inside an event horizon could find itself outside of it (I imagine the answer is no, since an event horizon always seems to be moving outward at the speed of light as measured in a locally inertial frame of an observer infinitesimally close to it).
  12. Dec 1, 2009 #11
    I agree with your 1st point but numerical simulations are the best possible way to understand such phenomena at the moment.
    It didn't say it necessarily means that, but it is a possibility. Further in the same article author mentioned that even a fast rotating black hole can tear itself apart.
    In GR gravity curves space and matter moves just moves in it. Imagine a prticle on the event horizon. In the presence of one black hole space would be curved to only one side, But in the presence of 2 black holes space would be curved on both sides of the test particle and it would be in a sort of unstable equilibrium and would be able to go anywhere.
  13. Dec 1, 2009 #12


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    In GR gravity actually curves spacetime rather than space, with freefalling objects following paths through curved spacetime that locally maximize their proper time--the bent-trampoline image of matter curving the fabric of space is just meant as a vague analogy. Curved spacetime is not so easy to visualize, and even if there was a region of spacetime near the midpoint of the black hole's centers which was close to flat, it might well still be surrounded by more curved regions that prevented the possibility of anything escaping. Anyway, visual analogies in this context probably aren't very trustworthy, we'd really need to do the math.
  14. Dec 1, 2009 #13
    Although the "force of gravity" in the region between two nearby black holes is weaker, the gravitational potential is actually stronger in that region because potential is additive. Increased potential means increased curvature and time dilation in that region. Rather than the event horizons overlapping, I imagine a single hour glass shaped horizon forms around both black holes as they merge and nothing escapes that combined event horizon. Using the crude rubber sheet analogy, the region between two neighbouring masses is depressed deeper than regions far away from the masses. The math on the other hand would be very complex because this is a very non static situation.
  15. Dec 1, 2009 #14


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    Isn't it lower potential that's associated with being deeper in a gravity well, not increased potential? And being deeper in a potential well doesn't necessarily mean increased curvature, I remember reading that the region of spacetime inside a massive hollow sphere would be flat, but the potential inside the sphere is lower than outside, so clocks inside the sphere would tick slower than clocks outside (see this thread).
  16. Dec 1, 2009 #15
    Thanks Jesse. Correct on all points... I was being too casual. I should have said increased (negative) potential, but I think the point I was trying to make is still basically valid.

    Another observation is that if we had a very large black hole of mass M and a very small black hole passing within 3M of the larger black hole, then it would be within the photon orbit radius of the larger black hole and doomed to fall in and merge. If on the other hand we had two large black holes each of mass M, then their Schwarzschild radii would be overlapping when their centre to centre distance was less than 4M and in this case it is conceivable that they could escape each other, but some very complex calculations would be required to be sure of that.
    Last edited: Dec 1, 2009
  17. Dec 2, 2009 #16


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    A general relativistic law pertinent to this question is Hawking's area theorem [1], stating that the horizon area of black holes cannot decrease as a result of any physical process satisfying the null energy condition (a classical black hole collision being one of them).

    This implies that a black hole cannot split in two smaller black holes, however it allows for some mass exceeding the sum of the initial areas to be radiated away. The creation of new independent black holes as a side effect of the collision is not forbidden either.

    [1] S. W. Hawking, "Gravitational radiation from colliding black holes", Phys. Rev. Lett. 26, 1344-1346 (1971).
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