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If time stops inside a black hole, due to gravitational time dilation, how can it's life end after a very long time? If time doesn't pass inside a black hole, then an event to occur inside a black hole needs infinite time relative to the outside. Thus, it will never age.

Please keep it simple..

Qmechanic
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Force
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4 Answers4

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I assume you're asking how a black hole can evaporate due to Hawking radiation. The answer is that the Hawking radiation does not come from the event horizon, but instead comes from a region just outside the event horizon so time has not stopped at its position.

If you were to watch a black hole form then evaporate, you would never see an event horizon form. That's because in your co-ordinates the event horizon would take an infinite time to form. You would see the infalling matter slow and red shift, then be re-emitted as Hawking radiation without the event horizon ever having formed.

John Rennie
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  • The question is quite vague. In fact, I thought the same Hawking radiation before looking into DJ Bunk's answer. I think the title is different from the body ;-) – Waffle's Crazy Peanut Apr 12 '13 at 17:04
  • @John why will I never see the event horizon form, isn't it the larger sphere where light never escapes surrounding the zero dimension singularity? :) I have always imagined a black hole to consist of both.. – Force Apr 12 '13 at 17:16
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    @Force: you need to ask that as a new question. The answer is too long for a comment! – John Rennie Apr 12 '13 at 17:20
  • @John Alright anyways, shouldn't there be a part that doesn't get affected by time. At least the singularity? – Force Apr 12 '13 at 17:22
  • The problem is that there is no universal time in GR (or SR for that matter). External observers never see the black hole form, so there can't be any place where timer stops. If you jump into the black hole you pass through other horizon and hit the singularity in finite time, so again there is no place time stops. – John Rennie Apr 12 '13 at 17:43
  • @John I agree with you that according to any observer, his own time would appear the same to him, as we observe ours right now.. But what I meant all along is regarding the outside observer. The original gases from the star trapped in the singularity if they had a clock, would appear to tick normally to them, but would we be seeing that clock from the outside click any soon? I hope I understood your statement that "there is no place time stops" correctly as to one's own time not the outside observer, wouldn't you agree? – Force Apr 13 '13 at 07:35
  • Please check out the final paragraph in section 1.1 "General Relativity" http://en.wikipedia.org/wiki/Black_hole If time inside a singularity is not moving to the outside observer which is what matters. And which is the frame at which we are measuring the 10^100 anticipated years for the BH life to end, but then we have to wait forever for any aging to occur.. – Force Apr 13 '13 at 07:51
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The time dilation you speak of is a description of the apparent time an observer outside witnesses someone falling into a black hole. That is, if you are standing outside the black hole (some distance away) and you watch you buddy jump in, it will look at if he slows down as he reaches the horizon, and never quite gets there. As for your buddy, he sees his watch ticking away normally and he notices nothing special as he crosses the horizon.

This is just another specific case of one observer noticing how the clocks in another observers reference frame appear to tick. The same is the case for 2 people moving in flat space at constant velocity with respect to one another. Each observer sees the others clocks running slower, but sees their own clocks run 'normally'.

David Z
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DJBunk
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  • Yes but 1 clock tick inside a black hole will take forever on the outside, correct? then no possible event will actually occur inside to be measurable in our time. The other unfortunate guy for example will not age (if he survives) one second before the universe ends and even beyond.. – Force Apr 12 '13 at 16:55
  • That assumes you can somehow observe the clock ticking on the inside of the black hole while standing on the outside. So I don't know what sort of meaning you can ascribe to clocks ticking on the inside of a black hole. Perhaps an expert (which I am not) can step in. – DJBunk Apr 12 '13 at 16:58
  • His clock -if we somehow were able to see it- won't tick at all – Force Apr 12 '13 at 16:59
  • You are making answer not accounting for BH evaporation to a question which explicitely asks about evaporation. -1. You can never reach the horizon before the BH evaporates. – Anixx Jun 21 '15 at 18:14
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Please check out the final paragraph in section 1.1 "General Relativity" en.wikipedia.org/wiki/Black_hole If time inside a singularity is not moving to the outside observer which is what matters. And which is the frame at which we are measuring the 10^100 anticipated years for the BH life to end, but then we have to wait forever for any aging to occur.. – Force 9 hours ago

Time does not stop inside a black hole. As others pointed out already, for the observer inside a black hole, time simply goes on.

Time does not stop for observers outside of the black hole "looking in", either. It merely slows down to an extreeeeeemly slow crawl. That's a subtle but important difference. Time keeps ticking away, but it is infinitely (no pun intended) slow: It is so slow that it would take an infinite amount of time before the outside observer can see someone actually fall in. The fact that it takes an infinite time however does not mean that time stops.

user1459524
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  • Yes it would take infinite time which is still larger than 10^100 years, the inside would still not get interrupted by finite time events from the outside, nor lose any thing, i.e radiation, don't you agree? I also don't want to start with the gravity that doesn't allow anything to escape, but that's off topic. – Force Apr 14 '13 at 01:06
  • A number like 10^-99999999999 is -as small as it is- infinitely larger than zero.. – Force Apr 14 '13 at 01:10
  • If time is a dimension shouldn't any thing obey it's flow in a certain reference frame? – Force Apr 14 '13 at 01:16
  • As John pointed out above, Hawking radiation does not come from within, so "the inside" would "not lose anything". That's why time (and gravity) inside the black hole do not influence the radiation. The black hole disappears to the outside observer after getting smaller and smaller and brighter and brighter. I don't think anyone really knows what happens inside the black hole. – user1459524 Apr 14 '13 at 11:18
  • @Force, check out John's answer above, he really already answered it all. You seem to cling to a notion of universal space time that is existing for everyone, but there is no such thing in General Relativity, and therefore there is no central reference frame. "Now" means something totally different to you here on earth vs. for someone in a distant galaxy vs. for someone hovering near a black hole vs. for someone inside it. – user1459524 Apr 14 '13 at 11:24
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Say you're a distant observer having your own clock and you're observing a clock falling into the black hole. As the clock approaches the black hole, the time measured by you is much slower than your own clock. As it nears the event-horizon, it gets red-shifted so much and at its closest distance to the horizon (somehow you managed to see the red-shifted clock), you'll see the clock freeze and stands still there. Because, further photons can't escape from the black hole's strong curvature and you'll see the photons that are trying to escape from the blackhole. This results in a scene that you'll see the clock stay there forever, never reaching the event-horizon. (The same thing happens during the formation of a blackhole)

But if you are an "in-falling observer" along with the clock, you'll see that your clock is normal. Even after you've fallen into the blackhole, you can see your own clock tick. But, you can't quite determine whether you've crossed the horizon or not..! So, this kinda (local) observation does not account for the blackhole's age anyway.

On the other hand - if you're interested in the age of blackhole, Hawking radiation is useful to determine (as mentioned by John). In its principle, the blackhole evaporates by quantum gravitational effects which makes use of virtual particle pairs and the rate of this emission is proportional to the mass. But, the problem here is (due to quantum scale) that the evaporation is very very low (not simply some billion billion years as for other phases of stars)...

Waffle's Crazy Peanut
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