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Just for Fun: Blackholes Live Forever?

  1. Nov 26, 2013 #1
    Hey all,

    This is a just for fun question, as im sure there is no definitive correct answer. But here it goes

    Blackholes to my knowledge, generate the strongest gravitational fields in the known universe. While thinking about gravitational time dilation, and the sheer magnitude that black holes affect gravity, does this mean that black holes will outlive EVERYTHING in the universe? I know blackholes, more specifically, the singularity within the blackhole, are considered to have an infinite density made possible by a volume = 0 im assuming (any mass/0 volume = infinite density). So all in all, I guess this question is asking, is the gravitational time dilation factor of blackholes large enough to allow them to outlive everything else?

    Blackholes are extremely fun to think about. I truly believe that they hold the answers to everything about the origins of our universe, and possibly other universes. But thats a whole different subject! Let me know what some of you guys think about my proposed question!

  2. jcsd
  3. Nov 26, 2013 #2
    When the cosmic background radiation cools enough (billions of years in the future) black holes will eventually start radiating more energy than they acquire from the background radiation and start evaporating. Some stars are too small to form a black hole when they run out of fuel and end up as cold bodies, and these bodies might possibly outlive the black holes as they are not subject to Hawking radiation. The high gravitational time dilation just outside the event horizon of a black hole means that an observer at this location will generally measure less elapsed time between two co-located events than an observer further out in space. *There is a caveat here, the two observers should be stationary with respect to each other.*

    If you look up Hawking radiation, you can see that once a black hole starts evaporating, the rate that a black hole evaporates increases exponentially as they get smaller and this evaporation rate is calculated at infinity and will appear even faster close to the black hole. The onset of the evaporation process is dependent on the size of the black hole. The bigger the black hole, the colder the background radiation has to get, before the process starts, so the small black holes start evaporating first. Perhaps the radiation from dying small black holes may contribute to the background radiation and be enough to preserve the larger black holes. All this assumes that the universe will continue expanding and that the universe suffers a 'heat death', which is possible debatable. That aspect is probably better discussed in the cosmology forum.
    Last edited: Nov 26, 2013
  4. Nov 26, 2013 #3


    Staff: Mentor

    You're confusing the gravitational time dilation factor with how long an object "lives". The two are not the same. For example, suppose you and I are both in a space station far above a black hole. I climb into my rocket and go down very close to the hole and "hover" there for a while, then I come back up to the station. We will find that the elapsed time on my ship's clock is much less than the elapsed time on the station clock; but does that mean I have "outlived" you? (To make the question even more pointed, assume both you and I are immortal; neither of us will ever die of natural causes. That is a better analogy to the black hole case anyway.)
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