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Can gravity carry information out of a black hole?

  1. Nov 15, 2008 #1
    Here's a question I've had for some time.

    I imagine that you can transfer information via gravity just as you can with light. For example, if I wave my hand back and forth (in any direction), and you had a sufficiently precise instrument, you could record the movement of my hand from a distance. I could transfer information to you by perhaps a binary system (left = 1, right = 0). Of course, you'd have to be sure you aren't measuring gravity of stuff other than my hand, like a bird flying by or something like that.

    For the sake of this experiment, let's use something other than my hand, like a wrecking ball that moves up and down. You are trying to measure where it is from just 10 feet away. (I'm pretty sure there are gravitimeters(?) this precise... links appreciated). Now I can move the wrecking ball up and down and you could, knowing the mass and shape of the ball, know whether its 20ft or 20ft down or whatever.

    Now, I'd like to know why this isn't possible for the ball to be inside a black hole, and you outside the blackhole. If any particle with mass inside a black hole is moving around, then you should be able to detect its movements from outside the blackhole, correct? Black holes do occupy a volume, correct? I at least know they have a radius. If the answer to my question (can you transmit information via gravity out of a black hole) is "no" then I assume that everything inside the event horizon cannot move, or is completely homogenous, otherwise you should be able to measure whatever has mass inside the black hole moving (which would be information escaping).

    My intuition tells me this has been though of before and there's a reasonable explanation for it all.

  2. jcsd
  3. Nov 15, 2008 #2


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    i think that this is an interesting question. it seems to me that at least the information of how much matter got sucked into the BH "escapes" it because, i think we can measure its mass by the "amount of gravity" it emits (or the curvature of space-time in its neighborhood).
  4. Nov 16, 2008 #3
    Interesting question indeed. The immediate problem I see is that gravitational waves propagate the same way light waves do, namely, at the speed of light. Since waves (electromagnetic, gravitational, or otherwise) cannot escape from inside the black hole, no information can come out as the wave its self. In other words, the internal mechanics of the area enclosed by the Event Horizon are hidden. Good thing too, because there's a nasty singularity in there.

    However, just as Photon pairs leek Hawking radiation out of the Black Hole, so too might Graviton pairs leek out. As far as I can tell, all this calls for is Graviton Pair Production, which is of course highly speculative at best. Cool idea, though.

  5. Nov 16, 2008 #4


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    No; you're accelerating away too fast, the information will never reach you. If you weren't accelerating, you'd fall into the black hole -- don't forget that from the perspective of someone hovering just over the black hole, the event horizon is also moving towards you at the speed of light.
  6. Nov 16, 2008 #5
    Gravitational waves do not propagate the same way light waves do. For one, they aren't affected by gravity :). Light is electromagnetic waves, which travel at the speed of light, and are affected by gravity. Gravity is simply a part of space time, whose changes travel (coincidentally, as far as I can understand) at the speed of light.

    See "How can gravity escape a black hole" http://www.faqs.org/faqs/astronomy/faq/part4/section-11.html
  7. Nov 16, 2008 #6
    This applies to electromagnetic waves, but not gravity... otherwise how do black holes have mass?
  8. Nov 16, 2008 #7
    It IS an interesting question, one regarding information. And how information emerges, whether it does, from a black hole is a major unanswered question in physics. I believe Hawking may have rescinded his strong belief that information is forever lost, that is hidden, but I'm unsure just where he stands. Charles Seife. Decoding the Universe,2006 which is largely an information study of physics, says information IS lost...this seems to violate conservation of information...all that is available is mass, angular momentum and charge.... (Temperature can be inferred from these three). Other information appears inaccessible, but has it been destroyed? nobody knows. General relativity and quantum theory break down under the extreme conditions encountered.
  9. Nov 16, 2008 #8

    Jonathan Scott

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    No, that's wrong. Gravitational waves propagate through space-time following the same path as light waves would, and travelling at the same speed.
    Last edited: Nov 16, 2008
  10. Nov 16, 2008 #9


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    The short story from the classical theory is that spatial radius outside the black hole becomes time inside the black hole. From this follows that, while everything inside the event horizon can move normally, no information can exit since "to exit" from inside a black hole means "to go backwards in time". Moreover, no information can stay inside forever since it must fall in the singularity. The quantum story is much more rich and it is believed, though not fully proven, that most of the information comes out with the quantum radiation and that there is no singularity.
  11. Nov 16, 2008 #10


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    I don't see the connection. Why would this topic have anything to do with whether black holes have mass?

    Incidentally, black holes also have charge, and have electromagnetic fields.
  12. Nov 17, 2008 #11
    What I meant was, at the same speed.
    Also, both are waves in Gauge fields.
    It's no coincidence that all waves in Gauge fields propagate at the speed of light.
    Last edited: Nov 17, 2008
  13. Nov 17, 2008 #12
    How is it that black holes cannot emit gravitational waves?
  14. Nov 17, 2008 #13
    Is it just me or was this already addressed?

    Argument #1:
    A black hole can't accelerate, nor can it change shape.
    How can something emit gravitational waves without accelerating or changing shape?

    Argument #2:
    Say there is a gravitational wave, approaching the Event Horizon from the INSIDE (a strange thought).
    It will climb the infinitely steep curvature of space and never get out of the gravity well. Ever.
    Last edited: Nov 17, 2008
  15. Nov 17, 2008 #14
    I guess what I don't understand is given argument 2, how anything inside a block hole can gravitationally influence things outside the black hole. Say there's a black hole of 1,000 solar masses, and a 5 solar mass star falls into it. How does that black hole now have 1,005 solar masses if as soon as the star falls into it its gravity cannot reach outside of it?

    Conversely, if the black hole loses mass due to Hawking Radiation (which I know next to nothing about) how is it that the black hole now exerts less gravitational influence (albeit by a minuscule amount)?

    Also, in terms of black holes not being able to accelerate, what about binary systems of black holes and stars? Are the black holes not accelerating?
  16. Nov 17, 2008 #15
    It gets bigger.

    It gets smaller.

    What I mean is that one black hole doesn't accelerate all by it's self. It can't send a signal to you by jiggling the way I can like this *does a stupid dance*.
  17. Nov 17, 2008 #16


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    The gravity doesn't have to "reach outside of it" -- the gravity was already 'outside of it'. Before the star crosses the event horizon, the system lies in a gravitational well of 1,005 solar masses*. After the star crosses the event horizon, the system still lies in a gravitational well of 1,005 solar masses.

    Changes in the gravitational field are caused by waves -- where are you going to find a gravitational wave that cancels out those extra 5 solar masses?

    *: This is imprecise, but it should cover the idea.

    Hawking radiation is not a purely general relativistic effect -- it arises from (heuristically) combining GR's predictions with QM's predictions. When we leave the confines of GR, it's statements are no longer strictly accurate.

    For the record, it's believed that Hawking radiation is purely thermal -- it should contains absolutely no information about what goes on inside the black hole.
  18. Nov 17, 2008 #17
    From what I've read, many people still contest the thermal nature of Hawking radiation.
    Since Hawking himself conceded his famous bet with Preskill, I'd say the community is at least divided on the subject.
    For some interesting thoughts on alternative ways to solve the information paradox, read "The Black Hole War" by Leonard Susskind. He's a bit of a string nut, but the book is interesting.

  19. Nov 17, 2008 #18

    Jonathan Scott

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    Some people still contest the existence of black holes on the grounds that Schwarzschild's original solution didn't have them and it was only when Hilbert made a different (mathematically plausible but physically questionable) assumption and changed the interpretation of the radial coordinate that the possibility of black holes arose.

    I think the best explanation of this situation (and least over-stated) is an article by Salvatore Antoci and Dierck-Ekkehard Liebscher "Reinstating Schwarzschild's Original Manifold and its Singularity", also available at arXiv:gr-qc/0406090.

    These ideas have been forcefully rebutted on-line in a thread "Flogging the Xprint" on sci.physics.research by "T. Essel", and also in a paper by Malcolm MacCallum at arXiv:gr-qc/0608033. However, although in both cases the "rebuttals" appear very authoritative, it appears that they are based on stating weak arguments very forcefully, which I don't find very convincing, and assuming that any fault with the opposing argument means that their own position is correct.

    I believe that basically the situation is that at present there's no physical theory which allows us to determine the location of the mass point in the Schwarzschild vacuum solution. Schwarzschild assumed it to be at his original r=0, which is now R=2GM in the Schwarzschild radial coordinate defined by the proper areas of spheres. Hilbert assumed it to be at R=0 because it seemed to be mathematically valid to do so.

    It was only a few years ago that Leonard S Abrams called attention to this difference of assumptions, which had apparently been ignored since Hilbert "corrected" Schwarzschild's original assumption. Some people have argued that since there are a lot of weird things in black hole theory, Schwarzschild's original assumption must have been correct. I'm inclined to sympathize with this point of view, but it's not a proof. On the other side, the people such as "T. Essel" who argue against them say that most of their arguments against Hilbert's assumption are invalid (which I'd probably agree with), and hence that Hilbert was right (which doesn't actually follow).

    As far as I can see, Antoci and Liebscher's article referenced above gives a more balanced view, giving the background and explaining how the difference arose, and showing that Schwarzschild's original solution does appear to be better in many ways.

    This situation could perhaps be resolved by experiment. There is some evidence in both directions, in that at least one super-massive black hole candidate appears to have a significant intrinsic magnetic field, which is not consistent with black hole theory, but objects which could according to black hole theory be near the borderline between neutron stars or black holes appear to show differences in X-ray emissions suggesting that there is some threshold being crossed (although Abhas Mitra has published a paper saying that this threshold doesn't necessarily imply a black hole, but could be some other form of phase change or similar).
  20. Nov 17, 2008 #19


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    The original semiclassical Hawking radiation is indeed purely thermal, but there are strong indicators that the semiclassical theory is not applicable to this problem. Most attempts towards a purely quantum treatment (both in string theory and LQG), while still speculative, point generally to corrections to thermality and no information loss.
  21. Nov 17, 2008 #20
    I'm confused by this, if gravity cannot escape an event horizon the same way light does not, how would it exert a gravitational pull on an object outside the horizon? Or in relativity terms, how would gravity curve space outside the event horizon if it cannot escape the event horizon.
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