As an external observer at infinity, I observe a star collapsing. But because the surface of the star will be infinitely redshifted, I will never see the formation of the black hole. For me, there will be always some matter outside, it never crosses the event horizon.

So, can I observe the Hawking radiation if I never observe the formation of the black hole ?

  • 2
    $\begingroup$ If you were infinitely far from the star, wouldn't you have to wait an infinite time before you could see it? $\endgroup$ – RedGrittyBrick Feb 3 '16 at 11:00
  • $\begingroup$ @RedGrittyBrick - the observer at infinity is a synonym for the asymptotically flat observer $\endgroup$ – Yukterez Oct 18 '18 at 17:56

You are quite correct that any external observer will never see a true event horizon form, however they will see an apparent horizon and an apparent horizon generates Hawking radiation.

So yes you will observe Hawking radiation from an object that is on its way to becoming a black hole.

  • $\begingroup$ Thanks. That brings few more questions. 1- From what I understand, the apparent horizon depends on the slicing of spacetime. So for an observer at infinity, are we sure that this horizon necessarily exist. And more generically, if we can vanish this horizon, can we imagine observers which will not see Hawking radiation. 2- Is the apparent horizon "inside" the event horizon, which will imply that the radiation emitted from this horizon will have to cross some matter before reaching us. Because the radius of the star collapsing is larger than the event horizon (for the observer at infinity). $\endgroup$ – anubis Feb 3 '16 at 12:00
  • $\begingroup$ If you're at infinity then the horizon you see is a true one by definition. You only see an apparent horizon if you are a finite distance away. $\endgroup$ – John Rennie Feb 3 '16 at 12:03
  • $\begingroup$ So if I'm an observer at infinity, I don't see the formation of the apparent horizon (because it is local) but the radiation I see comes from it ? $\endgroup$ – anubis Feb 3 '16 at 12:06

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