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A question on Black Holes

  1. Jun 4, 2010 #1
    I read in "Simply Einstein; Relativity Demystified" that if someone were to obeserve an object falling into a black hole, they would never actually see it reach the black hole.

    Is this true?

    If so, then lets say for argument sake, we made this object an Encyclopedia and we "the universe" observed the book falling towards the hole. Then we would never actually see it reach the black hole... so the universe would never actually lose the information.
    But then I geuss the blackhole would never gain any mass, since we obeserve everything that goes into it, so we never see any mass actually go into the blackhole. Which is contradictory or, atleast seems it, as many phenomena "depend upon" black holes "eating" matter for us to explain them..

    But i babbled on a bit there; do we really never see anything going into a blackhole? Seems odd to me.
     
  2. jcsd
  3. Jun 4, 2010 #2
    Time slows down near a black hole, I didn't understand this either until I saw this program on TV..
     
  4. Jun 4, 2010 #3

    DaveC426913

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    The photons emitted from the book as it is falling have to climb up a steeper and steeper gradient, getting redshifted as they go. As it nears the event horizon, the photons will effectively be infinitely red-shifted; the last photons we receive from the book will - to us - make it appeear that the book has slowed to a stop there. At least, it would if the image of it weren't too dim to see - fewer and fewer photons will be escaping to reach us. Eventually, just a photon every eon. If you can still call this one-photon-in-a-blue-moon an "image" of the book, then yeah, it appears to slow to a stop.

    But rest assured, the book continues falling freely to the centre in nearly no time flat.
     
  5. Jun 5, 2010 #4
    This may be a little confusing to newcomers. Whenever relativistic effects are discussed, the frame of reference must be given. In the above explanation when Dave is talking about fewer and fewer photons escaping until just a photon every eon, he is referring to the reference frame of a distant observer. But at the end where he says "But rest assured, the book continues falling freely to the centre in nearly no time flat.", he is talking about the reference frame of the infalling book.

    The irony of his statement is that to the infalling book, time is indeed flat, but to the distant observer time is very curved, curved in the sense that the closer the book gets to the event horizon the more time slows down from the point of view of the distant observer. This is not just an optical illusion due to the red shifting of the photons. Clocks that have been exposed to a change in the gravitational field and returned to their origin, do not agree with clocks that have not been moved.
     
  6. Jun 5, 2010 #5
    So, let me see if i've got this right;

    With respect to our reference frame ("the observing universe") we never see anything travel past the event horizon; i mean we get information from this book forever no matter how little the information we get from it is.. it still does give off photons. Which means it musn't be beyond the event horizon as the photons could still not escape.

    But the book observes nothing different about how its time elapses and falls straight on through the event horizon?
     
  7. Jun 5, 2010 #6

    DaveC426913

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    Correct.
     
  8. Jun 5, 2010 #7
    So we don't see anything fall in, but the mass of the blackhole still increases? :S
     
  9. Jun 6, 2010 #8

    DaveC426913

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    Correct.

    You may be looking at the "increase" wrong. Remember, it's not about mass so much as its about the curving effect on spacetime. The dimple-in-spacetime-that-is-the book was always there in spacetime, heading toward the BH. All that's happened once the book crosses the EH, is that the book-dimple has merged with the BH-dent.

    The moment the book reaches the EH, its dimple indistinguishable from the BH's dent. This is consistent with our observation.

    Think about whirlpools in a stream. When a tiny whirlpool merges with a big whirlpool, it's simply a matter of the two funnels merging so that the tiny one cannot be distinguished from the large one (actually, the tiny one is smeared across the circumference of the large one).
     
    Last edited: Jun 6, 2010
  10. Jun 6, 2010 #9
    Another way to look at it is to remember that just as time is dilated close to the event horizon, space is contracted in the radial direction. Thus as an object approaches the event horizon its thickness in the radial direction approaches infinitesimal. Even if the object never crosses the horizon in the time frame of the distant observer, due to its infinitesimal thickness, the object becomes indistinguishable from the other matter at the horizon.
     
  11. Jun 6, 2010 #10
    Thank you Skeptic and Dave that helped alot :)
    Still find it very werid; but i geuss that is SR and GR.. not intuitive

    Cheers
     
  12. Jun 6, 2010 #11
    I think Leonard Susskind would have liked your use of an encyclopedia as an example for the information paradox.
     
  13. Jun 6, 2010 #12
    Yeah :P I used it because thats what Stephen Hawking and the others who he had a bet with used

    Tehehe
     
  14. Jun 7, 2010 #13
    It appears to me as if the explanation is wrong. If fewer photons are emitted and they are increasingly red shifted to the distant observer it would appear as if the book was accelerating rapidly. So wouldn't the distant observers point of view be that the object accelerated to the speed of light? Then disappeared?
     
  15. Jun 7, 2010 #14

    DaveC426913

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    What clues would the observer have that would suggest the book was accelerating rapidly? It's not like it gets smaller.

    Are you suggesting that the observer would literally interpret the red-shifting as extreme recessional velocity? We only infer that with objects we cannot directly observe, such as distant stars and galaxies, where all we receive is the undifferentiated light from them.
     
  16. Jun 7, 2010 #15
    How would you measure the size as distant observer. Since photons travelling out of the well spread since the lip has a greater circumference than the interior where the book is the book will appear smaller to the distant observer. So not only would its red shift indicate that it was accelerating away from us its size would also indicate that it was accelerating away from us.
     
  17. Jun 7, 2010 #16

    DaveC426913

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    And you're sure about this are you?
     
  18. Jun 8, 2010 #17
    I think this! I think it is a logical conclusion that the book would indeed be further away from us in a curved space time then it's projection into a flat space time would have indicated. I am not sure of my conceptual pictures that I use to make sense of complex mathmatical systems and I depend on appropriate instruction from experts to help me understand where I am mistaken. My mental model tells me that the book to an external observer accelerates away from the observer getting smaller and smaller more and more red shifted until it disappears. I would appreciate help in understanding where I have failed to grasp the concepts correctly.
    Bob
     
  19. Jun 8, 2010 #18
    I think a really interesting question would be if you were entering a space time curvature would the rest of the universe appear to be increasingly accelerating away from you? I am not saying that this is what we are currently measuring beause I would expect some anamoly in the CMB that we could detect in this scenario. However it is an interesting thought experment.
     
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