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Blackhole's Effect on Time

  1. Oct 22, 2005 #1
    Is it plausible that the effect of a blackhole is the removal of time on something?

    Example: A rock is thrown and it passes a blackhole, the blackhole forces the object to stop in time and space, therefore staying suspended at that moment in it's same form forever (well not really forever but it would not move ahead in time ever again). This would be considered time travel but it would not be useful as the object would be frozen in that moment.

    Does that make sense?
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  3. Oct 22, 2005 #2


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    Black holes cause time to pass infinitely slow compared to a reference frame outside of the event horizon. This means the infinitely more time will pass in reference frames outside of the black hole, putting any matter in the black hole in the infinite future of the reference frames outside. (another reason you can't leave a black hole, how can anything exist in the INFINITE future of the universe??) So people from outside, if they could see inside the black hole, would see the object frozen in time. The rock, if it could see out, would see the infinite future of the other reference frames. Someone please correct me if I'm wrong, I'm no expert:)
  4. Oct 22, 2005 #3


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    I would suggest the sci.physics.faq What happens to you if you fall into a black hole for some information on this topic.

    Ted Bunn's Black Hole FAQ is also a useful reference http://cosmology.berkeley.edu/Education/BHfaq.html
    The situation is this. If you free-fall into a non-rotating black hole, you will reach the event horizon in a finite time by your clock, and you will not see the entire history of the universe while doing so.

    So it's not really true that a black hole stops time. It is true, however, that an observer far away from the black hole never sees you cross the event horizion, and that the Schwarzschild coordinate time t at which you cross the event horizon is infinite.

    While no material body can "hover" at the event horizon, in theory a rocket with an unbounded acceleration capacity could hover arbitrarily close to it. Such a rocket could achieve an arbitrarily large time dilation factor by "hovering" arbitrarily near the event horizon of a black hole. (At the event horizon, infinite acceleration would be required, so a rocket could not achieve a time dilation factor of zero).

    So _falling_ into a black hole does not allow you to see very much of the future history of the universe, while _hovering_ near the event horizon does.

    Note, though, that any rocket capable of such a hovering feat is also quite capable of achiving a similar effect simply by flying around in free space and taking advantage of the velocity induced time dilation from special relativity.

    I should probably add that there is no way to orbit a black hole inside the photon sphere which is at 3/2 the Schwarzschild radius - a rocket must be used to "maintain station" inside the photon sphere.
    [end add]

    Note that this is all for a non-rotating black hole. Rotating black holes are more complicated in that they may have an inner horizon as well as an outer event horizon. The existence and stability of the "inner horizon" are still a matter of some debate last I checked.
    Last edited: Oct 23, 2005
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