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How does a black hole know how big it should be?

  1. Dec 11, 2012 #1
    I was reading an article the other day about the second largest black hole found, something around 17 billion solar masses big, with an estimated event horizon 11 times the orbit of Neptune and it got me thinking. If all the matter generating the gravity lies in the singularity, how does that fact communicate itself thru the event horizon, where no communication can occur, to the area outside of the event horizon? Essentially, how does the outside universe know how much mass is contained in the singularity if there is no way for anything inside an event horizon to interact with anything outside of it?
     
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  3. Dec 11, 2012 #2

    phinds

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    Black holes interact with the outside world via gravity and the resulting gravitational lensing.
     
  4. Dec 11, 2012 #3
    As long as the object stays still, the gravitational field around the object doesn't have to propagate, and thus is not affected by the speed of light. It is only changes in the gravitational field that have to propagate at the speed of light, and therefore cannot escape the black hole. So, I suppose the gravitational field is somehow frozen in from when the black hole formed. I'm just guessing.
     
  5. Dec 11, 2012 #4
    Gravity is not "something" such as a photon or a piece o matter. Gravity is bending of space-time. Light cannot escape because light has a maximum speed. The fabric of space has no limiting speed as far as I know. Although that's irrelevant. Suffice it to say that gravity is nothing you could see or touch, it's an indirect result of weight. When you sit down on your bed the mattress bends beneath you, gravity does the same thing in a 3D perspective.
    The black hole essentially punches a hole through the space. A hole to where? Nobody knows.
     
  6. Dec 11, 2012 #5

    Drakkith

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    Changes in gravity do propagate at c. Note that in a static field nothing is moving. Nothing even needs to come out of the black hole at all for its gravity to be felt.

    Gravity waves themselves don't even need to "exit" the black hole itself either depending on how you look at it. A gravity wave that meets the event horizon of a black hole will alter the black hole itself in such a way that the event horizon stretches and bends just slightly, which will alter the metric and the resulting gravitational wave from the event horizon bending back and forth will be indistinguishable from a gravitational wave passing right on through the black hole.

    One way I've heard it described is that infalling material never reaches the event horizon from out point of view, so you don't even need to consider what happens from "beyond" the event horizon because nothing ever passes beyond it anyways. It will be redshifted beyond detection however, so you would still be unable to actually see the material that is very very close to the event horizon.
     
  7. Dec 11, 2012 #6
    Wow you lost me :-). The way people have explained it to me is that space has be altered at any speed.
    Isn't that the idea behind NASA's faster-than-the speed of Light project?
     
  8. Dec 11, 2012 #7

    Drakkith

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    Hmmm. I admit I don't know enough to comment on it. I know GRAVITY propagates at c. If there are other types of metric changes that don't obey that, I am unaware of them.
     
  9. Dec 12, 2012 #8
    Your mistake lies in the assumption that "there is no way for anything inside an event horizon to interact with anything outside of it". Are you saying gravitational forces can't escape the event horizon because.... gravitational forces pulls them back in? Hmm...
     
  10. Dec 12, 2012 #9
  11. Dec 12, 2012 #10

    PeterDonis

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    Because the outside universe knows how much mass originally collapsed to form the black hole. That is where the information about the mass comes from: from the past history of the object that collapsed. It doesn't come from inside the hole.
     
  12. Dec 12, 2012 #11

    PeterDonis

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    This is more or less correct. A better way to say it is that, as the object that originally formed the hole collapses, the field in the vacuum region outside the object becomes more and more "frozen"; and once the collapsing object has formed an event horizon around itself, the field outside the horizon is entirely "frozen".
     
  13. Dec 12, 2012 #12

    PeterDonis

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    But this is all due to the field outside the horizon, which is formed by the original collapsing object. Nothing has to "propagate" from inside the horizon.
     
  14. Dec 12, 2012 #13

    PeterDonis

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    Yes, it does; it's the speed of light. "The fabric of space" is just another word for spacetime (or at least that's the only meaning I can usefully assign to that phrase), and the limiting speed in spacetime is the speed of light. The reason that doesn't stop the black hole's gravity from affecting objects outside it is that the gravity doesn't come from inside the hole; it comes from the object that originally collapsed to form the hole. Information about that collapse, before the horizon forms, can travel to the rest of spacetime outside the hole at the speed of light, although that's not really the best way to phrase it since the information doesn't "travel"; a better way to state it is that the field at a given point outside the hole is entirely determined by what's in the past light cone of that point, which includes the history of the collapsing object outside the horizon.
     
  15. Dec 12, 2012 #14
    From an earlier discussion posted by PeterDonis:

    + my note: or gravitational curvature

    edit: oops..I see Peter posted while I was finding his prior explanation.....good job, Peter! they seem consistent!!
     
    Last edited: Dec 12, 2012
  16. Dec 12, 2012 #15

    PeterDonis

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    Thanks! I've been getting plenty of practice... :wink:
     
  17. Dec 12, 2012 #16

    turbo

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    Nature does not "know" anything. Rules are rules, and the rules do not allow for anthropomorphism WRT to black holes, planets, etc. These bodies do not "know" anything regarding their formation.
     
  18. Dec 12, 2012 #17

    PAllen

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    What about General Gaia Theory? :wink:
     
  19. Dec 14, 2012 #18
    Interesting. But now I don't understand how NASA could build what they were talking about in the article that I posted. Could you explain that? Thanks :).
     
  20. Dec 14, 2012 #19

    PeterDonis

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    The Alcubierre "warp drive" is highly speculative; I would not take the fact that NASA has a project to try to build one as good evidence that one can actually be built. :rolleyes:

    That said, the "warp drive" spacetime does not violate the law I stated; in other words, it does not involve anything moving outside the local light cones. What it claims to be able to do is to warp the light cones themselves. This in itself is not speculative; ordinary gravity does the same thing. But ordinary gravity, meaning gravity caused by ordinary matter or radiation, can't warp the light cones enough to allow anything that looks, globally, like "faster than light" travel; that requires a kind of substance called "exotic matter", which most physicists do not believe is physically possible--at least not in the quantities that would be needed for an FTL ship. When the article talks about a "negative vacuum energy ring", it's talking about trying to generate this kind of substance, which, if it were to exist, could warp spacetime in a way that ordinary gravity can't. Personally, I'll believe it when I see it.
     
  21. Dec 14, 2012 #20
    Ahh... I can't say I fully understand but at least I have a better idea. Thank you yet again good Sir.
     
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