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Black hole time dilation

  1. Apr 15, 2015 #1
    Would the universe end if you entered a black hole? What I mean by this is that due to time dilation would time elapse so fast for the universe outside the black hole relative to you inside it that all the stars would burn out and all that wouild be left would be other black holes?
     
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  3. Apr 15, 2015 #2

    phinds

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    I do understand you question, and am not sure of the answer, but if you entered a BH, the universe would end for YOU, that's for sure :smile:
     
  4. Apr 15, 2015 #3
    Haha ok so assuming I can survive entering the black hole, is it possible that time would elapse so slowly there relative to the universe outside the black hole (apart from other similarly massive black holes) that the universe would essentially "run its course" during the time you spend in the black hole.

    Edit: Oops misread your post I thought you said you didnt understand the question
     
  5. Apr 15, 2015 #4

    phinds

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    I remember seeing other threads covering this so you might try a forum search.

    The problems is not surviving since for a supermassive BH, there is nothing to survive out near the EH. Spaghetification doesn't occur w/ a SMBH until much closer to the singularity. You would not even notice crossing the EH. The problem is that you can't hover once inside, you HAVE to proceed to the singularity whether you like it or not. This does not, however, really answer your question, so keep looking.
     
  6. Apr 15, 2015 #5

    Chalnoth

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    Unfortunately, once you've entered the black hole, you are drawn towards the center in a short, finite time (from your perspective). Even if you were (somehow) able to survive the trip towards the center, that trip wouldn't take enough time for you to observe much that goes on outside the black hole.
     
  7. Apr 15, 2015 #6

    timmdeeg

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    You would see the things far away extremely blue shifted due to gravitational time dilation (stars burning out) in case you would be hovering outside the black hole close to its event horizon. But it makes a big difference if you enter the black hole in free fall. Then (regardless if you are still outside or already inside the black hole), falling away from the stars outside causes redshift which is more or less canceled by the gravitational blueshift. I'm not sure about the net effect however.
     
  8. Apr 15, 2015 #7

    Chronos

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    The external universe viewed by an infalling observer inside the event horizon is redshifted by a factor approaching 2. It is not blueshifted because you are falling away from the external universe, and you can only view photons approaching you from behind. There is no trajectory that permits photons from other directions to reach your eyes.
     
  9. Apr 15, 2015 #8
    OK, so our indestructable observer has just fallen through what is the horizon as far as external observers are concerned.
    He can observe only red shifted light coming from behind him, and that,s all.
    If he could make sense of what he see behind him what does he see? - everything vastly sped up, though still looking very red.?
    Galaxies and solar systems forming then dissipating in seconds?
    Is there any idea of how much time passes from his own point of view until arrival at the dreaded singularity?
     
    Last edited: Apr 15, 2015
  10. Apr 15, 2015 #9

    Orodruin

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    This would depend on the initial conditions. There is a continuum of different coordinate velocities you can pass the horizon with. And there are of course also different black hole sizes.
     
  11. Apr 15, 2015 #10

    Nugatory

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    The proper time he experiences from horizon to singularity, assuming free fall, is something on the order of 15 microseconds for a sun-sized blackhole.

    It scales linearly with the mass of the black hole.
     
  12. Apr 15, 2015 #11

    PAllen

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    To answer the unanswered question, red shift correlates with observed clock rates. This must always be true, because a light emission process is effectively an oscillator clock. So, for the initial condition Chronos described, you would visually see the external universe going at half speed as you approached the singularity.
     
  13. Apr 16, 2015 #12

    timmdeeg

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    If just falling away was to be considered one should expect that the infalling observer would see the stars outside infinitely red shifted while crossing the horizon at the speed of light. On the other side the light of the stars gains energy entering the gravity well of the black hole. But I'm not sure if this reasoning is correct. Anyhow, how do you calculate the factor 2?
     
  14. Apr 16, 2015 #13

    Chronos

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    You need to chose the proper coordinate system to calculate the shift in frequency between emitted vs received photons. In Schwarzschild coordinates the infalling observer views the external universe at z=2 at the event horizon. Unfortunately, you cannot validly extend Schwarzschild coordinates beyond the event horizon. You need something like Panleve coordinates to do that - re: http://arxiv.org/abs/gr-qc/0001069. I believe this is what Hamilton used in his presentation at http://casa.colorado.edu/~ajsh/schw.shtml
     
  15. Apr 16, 2015 #14

    George Jones

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    An earlier post of mine:
     
  16. Apr 17, 2015 #15

    timmdeeg

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    Great, that's what I have been looking for, thanks George Jones and also Chronos.
    So, the factor 2 is clarified.

    Unfortunately my intuition is still a bit behind.
    A sees B (hovering) extremely redshifted and C (in free fall) even more redshifted, because in the latter case the redshift consists of a gravitational and a kinematic component additional.
    Passing by, C sees B extremely redshifted (purely Doppler) but sees A at z = 2 according to the formula you have shown. Would it be correct to say that also this redshift (z = 2) consists of a kinematic (not purely Doppler, because Doppler depends on a well defined relative velocity which in curved space-time is possible only locally) and a gravitational shift. So, it seems that the kinematic effect prevails over the gravitational effect. But why?
    Kindly correct my reasoning.
     
    Last edited: Apr 17, 2015
  17. Apr 17, 2015 #16

    PAllen

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    Timmdeeg, I'm not sure there is a satisfying 'why', especially for what is observed by a freefaller past the horizon. Note that gravitational time dilation is only well defined for stationary spacetimes, and SR Doppler is well defined only over a spacetime region that may be considered flat (either due to small size or distance from any massive bodies). You are asking to extend these concepts beyond their applicability, especially to the interior case (the spacetime is not static or stationary, and curvature becomes unbounded as the singularity is approached).

    It won't help your intuition necessarily, but there a couple of universal approaches to red shift that don't involve gravitational redshift at all (which is strictly a non-essential, derived concept in GR, that is useful only for the special case of stationary spacetimes).

    1) Establish the 4-momentum of the emitted light pulse (it will be a null vector, whose time component will be the energy if the vector is expressed in a local frame of the emitter). Parallel transport this 4-momentum along the null geodesic path (must specify path, because, of course, parallel transport is path dependent) to the receiver. Compute the dot product with the receiver world line's tangent vector (4-velocity) at the event of reception. The ratio of the energy specified in emitter local frame and this dot product will be the red/blue shift.

    2) Equivalently (not obvious at all), you can parallel transport the emitter 4 velocity along the null geodesic to the reception event, express the transported 4-velocity in the local frame of the receiver world line at that event, and apply the pure SR Doppler formua (as if the emitter were local, with transported 4-velocity).

    Both of these methods are universally applicable in any GR situation, no matter how complex. They explain why there is really only one fundamental phenomenon, not two. So called pure gravitational redshift for static observers in a static spacetime falls right out of either of these computational methods. They also cover any combination of non-static observers, in any spacetime, as noted.
     
  18. Apr 17, 2015 #17

    bcrowell

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    This doesn't sound right to me. There can't be a fixed, finite Doppler shift for a free-falling observer observing an external signal, because two different free-falling observers could have different states of motion.
     
  19. Apr 17, 2015 #18

    bcrowell

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    I think this is one of those questions that is very easy if you understand how to use Penrose diagrams, very hard otherwise. The question here is basically whether or not an event inside the event horizon has timelike infinity i+ in its past light cone. The answer is no if you look at the Penrose diagram.

    By the way, it's a common misconception that the fate of all matter in the universe is that it will ultimately end up in black holes. Not true. In the distant future, due to the acceleration of cosmological expansion, there will be many cases where you have a single subatomic particle that is all alone inside its own cosmological horizon.
     
  20. Apr 17, 2015 #19

    PAllen

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    I assumed what was meant was the classic free faller from infinity. This is unique - the limit of free falling at zero speed from a static observer, as the static observer approaches infinite distance from the BH.
     
  21. Apr 18, 2015 #20

    timmdeeg

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    Thanks, PAllen, for your explanation. Yes, I think it is important to know that there is one phenomenon only, the observed frequency-shift, which is invariant.

    Frankly speaking, I was inspired by authors like Peacok and Chodorowski,

    http://arxiv.org/PS_cache/arxiv/pdf/0809/0809.4573v1.pdf
    http://arxiv.org/PS_cache/arxiv/pdf/0911/0911.3536v3.pdf

    who think of the cosmological redshift as being combined of kinematic and gravitational shifts. In the expanding FRW-model co-moving galaxies fall away from each other. Since the same is true in the Schwarzschild case regarding objects in radial free fall, I started reasoning why not thinking of said decomposition as well. Just to get a better feeling or intuition of the situation that the observer at R = 2M sees the far away exterior redshifted at z = 2 only, though moving locally with c.
    But, of course, it is questionable whether the decomposition of the redshift makes any sense, because there is no invariant definition of a gravitational and a kinematic shift, which in combination yield the measured redshift.
     
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