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Black Holes and Relativity

  1. May 27, 2009 #1
    I enjoy studying QM, but my knowledge of black holes is basically limited to the high school explanations as depicted below.

    3570613811_c32a05e432.jpg

    As a body approaches the event horizon the gravitational forces on that body accelerate it to a velocity approaching c. The math implies that mass crossing the event horizon is actually moving faster than the speed of light. This is expressly forbidden by special relativity, but I had always assumed that general relativity somehow provided the answer via curved space-time that allowed such speeds to be obtained "in appearance only"...or something.

    After reading a couple of (non grad-level) books on GR, the problem has not been resolved for me. Sticking strictly to my SR knowledge, my interpretation of black holes has been that they never actually form; as matter begins its collapse from a neutron star, for example, that matter becomes "frozen in time" as its velocity approaches c. I'm pretty sure the SR time dilation math shows that the "outside world" clocks move to infinity during a black hole's formation, yet we seem to readily postulate that black holes currently exist...

    Can someone that understands black holes please explain?:confused:
     
    Last edited: May 27, 2009
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  3. May 27, 2009 #2

    malawi_glenn

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    They will form, but we will never see it. When the matter has passed the horizon according to its own proper time, no light signal can reach us. You have to distinguish from a what falling observer will experience and what global fix observers will see by register light signals from that falling observer.
     
  4. May 27, 2009 #3
    Malawi: thanks, but I'm not asking what we would "see" as observers. I'm asking "when" black holes would form as calculated from outside the black hole region itself and, unless GR provides an answer beyond my simple SR understanding, I calculate "when" to be never.

    I conclude that I am naive, and I want someone to explain how we can so easily accept that black holes currently exist if their formation takes an infinite amount of time from our perspective.
     
  5. May 27, 2009 #4

    George Jones

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    As malawi_glenn has noted the surface of a sufficiently massive collapsing star quickly passes through an event horizon. Observers who are outside the star (even far outside) can pass through the event horizon by heading towards the star. A hovering observer outside the star doesn't see the star's surface pass through the event horizon, and doesn't see plunging observer's pass through the horizon. So what? :smile:

    Consider the attached spacetime diagram for special relativity. The axes are coordinate axes for a particular inertial reference frame, the black curved is the worldline of observer A who undergoes constant acceleration, the red dashed line is a lightlike asymptote for A's worldline, and the straight worldline is the worldline of observer B who moves with constant velocity with respect to the inertial reference frame.

    Observer A never sees B or any other observer, cross the lightlike asymptote. Do you see why? Does mean that B never crosses the red dashed line, and that events above the asymptote don't exist, never happen?
     

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    Last edited: May 27, 2009
  6. May 27, 2009 #5
    George: this isn't a question about SEEING anything. I'm not claiming that an event does not happen just because it cannot be observed from some perspective. I'm asking WHEN the unobservable event of crossing the event horizon happens from the outsider's perspective (because I think the answer is that it "never" happens, and I want someone to explain why this answer is wrong). I had expected a definitive explanation to what seems to me to be an elementary question of such a foundational cosmological subject. :confused:
     
  7. May 27, 2009 #6

    malawi_glenn

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    "As a body approaches the event horizon the gravitational forces on that body accelerate it to a velocity approaching c. The math implies that mass crossing the event horizon is actually moving faster than the speed of light."

    Can you can you tell where you read this? Because it is not true. It is only being accelerated to local speed of light as measured by a global, distant, observer. The falling observes own speed, will be smaller. You have to differ between the falling observers proper time and proper motion with the global and distant observers proper time.

    Now your answer is correct, and I said so in my first reply. However, you said that you did not wanted to know when we SEE the black hole form, but in the last post here you are telling us "I'm asking WHEN the unobservable event of crossing the event horizon happens from the outsider's perspective", but that is the same thing... are you aware of this?

    So now let's come to your attitude, you want someone to tell you why are wrong? How do you know that you are wrong??

    Secondly, we have given you a definite explanation, it is just that you don't understand that we have given you it...
     
  8. May 28, 2009 #7
    Malawi: I understand that seeing and calculating are essentially the same thing (i.e. Relatively-speaking, observing a slowly moving clock and calculating its time dilation are equivalent), but I was making a distinction between "seeing" and "calculating" only because I anticipated that someone was going to say that the body which attempted to cross the event horizon only APPEARED to be frozen in time from out perspective but that this is not what really happens at all. What you are saying is that this "freezing in time" actually IS what occurs from the outside perspective which is my whole point. (On a side note, I find it unusual that you think I have an attitude when I'm presuming that I am wrong on something and asking others to help me :wink:)

    Here is the same question in a different wording: if, from OUR frame of reference here on Earth, the time that it takes matter to cross an event horizon of any distant black hole is infinite, then how can we realistically postulate that black holes currently exist from our frame? In other words if the "creation time" of a black hole from a given frame is infinite how can they exist at all in that frame?
     
  9. May 28, 2009 #8

    malawi_glenn

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    we will still notice their gravitational effects....
     
  10. May 28, 2009 #9
    Yes, I do not question that. But I question whether or not an event horizon, or even a singularity, is ever created.
     
  11. May 28, 2009 #10

    malawi_glenn

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    yes they do, but not in our reference frame, recall that space and time is relative in Einsteins theories.
     
  12. May 28, 2009 #11
    Yes, I should've been clear on this. I question whether or not an event horizon or a singularity are ever created from our frame of reference as outside of the effective sphere of influence of the black hole. If it is true that black holes cannot be created from our frame, then how can we postulate that they currently exist from our frame?
     
  13. May 28, 2009 #12

    malawi_glenn

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    I guess that this is more of a philosophical question, and who has made that claim anyways?
     
  14. May 28, 2009 #13

    diazona

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    If you fell in, there's every reason to expect that you would find a singularity (apart from the obvious problem of, y'know, dying ;-). But if you stay well outside the black hole... I guess you could say an event horizon would be created after an infinite time. (Which is different from never, believe it or not.) It's kind of meaningless to ask about the existence of a singularity from outside the black hole, though, because we could never observe such a thing. The volume inside a black hole's event horizon is a "black box" from anywhere outside it.

    Note that although it may take an infinite amount of time (from an external frame) for a black hole's mass to be compressed to the Schwarzschild radius (which is the radius an event horizon would have, if it exists), it takes only a short time for the mass to be compressed close to that radius, and that's really all you need. The gravitational effect at some distance from the black hole is the same whether the mass is actually inside the Schwarzschild radius or not. And since any radiation emitted from close above the Schwarzschild radius undergoes an immense gravitational redshift, the black hole becomes invisible long before it fully collapses.
     
  15. May 28, 2009 #14
    diazona: Excellent answer, thank you so much. I understand that, "functionally", the difference between the Schwarzchild radius ever being reached or not is irrelevant to the outside observer, but it seems to me that other apparent problems are resolved if we assume that the singularity is never formed:

    1) Relativity holds without any kind of space-time inversion mumbo jumbo (which is the type of response I was expecting)
    2) Avoiding information loss does not require any special form of Hawking radiation
    3) The world may yet be homeomorphic(?)

    I'm not saying this is the way it is, and I'm not saying that you agree with me fully, I'm just appreciative of you directly answering my question and acknowledging my logic.
     
  16. May 28, 2009 #15

    George Jones

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    Yes, it is.
    It seems to me that this is exactly what you're claiming.
    It's not that it never happens, it's that a certain class of observers never sees it happen. Other observers do see it happen. This is completely analogous to my special relativity example. How are the two situations different?
    What is the difference between "APPEARED to be frozen" and "actually IS what occurs from the outside perspective"?
    What does "frame of reference" mean in general relativity?
    Contrary to a popular myth,there is no "space-time inversion" inside a black hole.
    I think you have something in mind, but I don't that "homeomorphic" is the term for it. Two topoplogical spaces are homeomorphic if there exists a continuous bijection between them whose inverse is also a continuous bijection.

    See also the article "Evidence for the Black Hole Event Horizon,"

    http://arxiv.org/abs/astro-ph/0310692.
     
  17. May 28, 2009 #16
    Maybe not. If the black hole evaporates in finite time but the event horizon forms after an infinite time, I think you could say the event horizon never forms.
     
  18. May 28, 2009 #17
    Who has made the claim that black holes currently exist? I thought I was the bold one asking if it was possible that they do not!

    George: the universe is not homeomorphic if a singularity is allowed to form. The rest of your post has an argumentative whiff to it and I don't feel like playing; besides, others in this thread have understood my question perfectly and responded accordingly.
     
  19. May 28, 2009 #18

    malawi_glenn

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    now you are mixing such general statements with physical statements. Where is the argument that black holes have been formed in our frame of reference? There is a difference in claiming that black holes exists and that they have been formed in our frame of reference. You have to understand this.
     
  20. May 28, 2009 #19
    OK, fair enough...so when Astrophysicists speculate about the giant black hole that exists in the center of the Milky Way they are actually referring to the black hole that does not currently exist but will form after an infinite amount of time has passed (from our perspective)? Something tells me that this is not popular sentiment.
     
  21. May 28, 2009 #20

    malawi_glenn

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    welcome to the world of relativity!
     
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