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I Time to form a black hole

  1. Jul 15, 2016 #1
    Is it correct that, from the perspective of an observer, time slows down and ultimately stops at the event horizon of a black hole, implying that no black holes have had time to form in the universe ?
     
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  3. Jul 15, 2016 #2

    PeterDonis

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    No. A massive object only takes a finite time to collapse to a black hole, according to an observer falling in with the collapse; and an observer falling into a black hole that already exists only takes a finite time by his own clock to reach the hole's horizon.
     
  4. Jul 15, 2016 #3
    Thanks - but the observer I was referring to was not falling in with the object, but rather was watching from a distance. So in particular if a black hole is formed by a collapsing star, from the perspective of the rest of the universe does the black hole ever form? (I understand that from the perspective of the collapsing star it only takes a finite time).
     
  5. Jul 15, 2016 #4
    I think the observer would see what had been the star becoming increasingly red shifted until it was no longer visible.
    However if it is gravitationally bound, say in a binary system, it could be detected indirectly that way, and possibly there might be a hot and visible accretion disk of remnant material surrounding the event horizon of what now is a black hole.
     
    Last edited: Jul 15, 2016
  6. Jul 15, 2016 #5

    PeterDonis

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    There is no such thing as "from the perspective of the rest of the universe". The black hole is a feature of the global spacetime geometry; either it is there or it isn't. If the collapsing object forms a horizon in a finite time, or an infalling observer reaches the horizon in a finite time, then the black hole is there. The fact that the distant observer can't see it doesn't change that.
     
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