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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 ?

anubis
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1 Answers1

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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.

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John Rennie
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  • 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). – anubis Feb 03 '16 at 12:00
  • 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. – John Rennie Feb 03 '16 at 12:03
  • 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 ? – anubis Feb 03 '16 at 12:06
  • @JohnRennie So we can receive hawking radiation from a black hole, but correct me if I'm wrong, we would never receive Hawking radiation from an object that is as compact as his gravitational radius + an as small as you want, correct? This means that even if this compact object and a black hole look exactly the same to us (2 compact objects that don't reach their gravitational radius) we could in theory distinguish between them thanks to Hawking radiation? i.e. if we receive it then it's a black hole if we don't then it's just a compact object – AnOrAn Sep 24 '20 at 17:57
  • @AnOrAn good question. I guess you're right though I've never thought about it. – John Rennie Sep 24 '20 at 18:18