# Black hole question.

1. Jul 8, 2008

I was reading wiki's blckhole page and its diseription of what happens as you approach/enter the event horizone does not seem to make since to me. Spocificaly things like

"An infalling object takes a finite proper time (i.e. measured by its own clock) to fall past the event horizon. This in contrast with the infinite amount of time it takes for a distant observer to see the infalling object cross the horizon."

It seems to me that even though a distant observer would say the falling man's watch is really slow, the falling man would still be moving at a really fast speed across the event horizon.

Even the discription of inside the event horizon seems off to me. Is this what is generaly accepted and if so is there somewhere I can read on why we believe these things to me true?

2. Jul 8, 2008

### DopplerDog

Keep in mind that the distant observer cannot see inside the black hole. The observer sees no light coming out from inside the horizon. As the victim falls closer to the horizon, light coming from him is increasingly red-shifted to the observer, and the victim's clock appears to run slower and slower (relative to the observer), up to the point where he hits the horizon, at which point his clock appears to have stopped (though the observer wouldn't see this as by then the light would have been infinitely red shifted, i.e. invisible).

The observer, of course, feels nothing strange as he crosses the horizon. He does see that he's accelerating towards the centre (and possibly feeling the effects of tidal forces).

3. Jul 9, 2008

### DopplerDog

That should be "the victim... feels nothing strange". I placed the observer far away from the horizon, and the victim travelling towards it.

4. Jul 9, 2008

### MeJennifer

There is no contradiction since the model assumes an asymptotically flat space at infinity.

5. Jul 9, 2008

### JesseM

I don't think this has anything to do with it--they aren't talking about the time needed to fall in from infinity, but just the time needed to fall in from some finite radius outside the event horizon, which would still appear infinite to the outside observer if he could see light of arbitrarily large wavelengths and if light were emitted continuously rather than in a discrete series of photons (see the discussion here). Pretty sure the same would be true for a black hole in a non-asymptotically flat universe, such as a closed universe with positive curvature.

6. Jul 9, 2008

### MeJennifer

A closed universe cannot possibly have black holes.

7. Jul 9, 2008

### yuiop

I have heard this elsewhere. Does that come from the definition of a black hole?

8. Jul 9, 2008

### MeJennifer

There would simply be not enough time for an event horizon to completely form in a closed universe.

9. Jul 9, 2008

### JesseM

I don't think that's correct, do you have a reference? For example, I know that it is possible to model a black hole which forms and then evaporates due to Hawking radiation in a finite time--an outside observer will not see any object reach the horizon until the moment the hole evaporates completely (including the original material that formed the black hole in the first place, presumably), but from the perspective of the infalling observer, they do cross the event horizon in a finite time and there is a singularity inside. See the discussion here. So, it seems plausible that the same sort of thing would be true of a black hole in a closed universe, outside observers couldn't see anything cross the horizon at any moment before the black hole was destroyed in the big crunch, but infalling observers could cross it in a finite time.

10. Jul 9, 2008

### MeJennifer

Last edited by a moderator: May 3, 2017
11. Jul 9, 2008

### nrqed

Why? Why can't a supermassive star have the time to go supernova and produces a black hole in a closed universe??

12. Jul 9, 2008

### JesseM

The full article isn't available unless you're a subscriber, but the abstract doesn't appear to say that event horizons can't form in closed universes, in fact it says that by defining a black hole in terms of trapped surfaces one can talk about black holes in closed universes:
It does say that other results such as the black hole area theorem (that black holes never decrease their cross-sectional area, ignoring Hawking radiation) cannot be extended to closed universes, though.

Also, googling "trapped surface" and "black hole" and "closed universe" I came across this google book search result which says:

Last edited by a moderator: May 3, 2017
13. Jul 9, 2008

### Antenna Guy

I'd have preferred an argument based upon the second law of thermodynamics.

Does a black hole do work while concentrating energy within an arbitrary closed system?

Regards,

Bill

14. Jul 9, 2008

### MeJennifer

I do not think it is worth arguing about it, if you think that closed spacetimes can contain black holes then so be it.

Last edited: Jul 9, 2008
15. Jul 9, 2008

### MeJennifer

Yes that works as well.

16. Jul 9, 2008

### JesseM

I doubt either one of us is qualified to make independent judgments on this issue, so the question here is what professional physicists think about it. The quotes I posted seem to say that black holes can indeed form in closed universes--do you disagree with my understanding of what the quotes are saying, or are you trying to say that you think these physicists are wrong? Presumably their statements are based on study of mathematical models in GR, not qualitative arguments.

17. Jul 9, 2008

### JesseM

The behavior of a black hole is thought to increase entropy in accord with the 2nd law of thermodynamics, if that's what you're talking about--see black hole thermodynamics.

18. Jul 9, 2008

### Antenna Guy

Am I not reading correctly, or does the "second law of black hole mechanics" imply that an arbitrary closed surface (that does not grow with time) cannot be constructed about a black hole that gains energy?

Clearly, "a closed universe" is not equivalent to a black hole's event horizon.

Regards,

Bill

19. Jul 9, 2008

### JesseM

Why do you think it says that? The second law concerns the area of the event horizon, it doesn't say anything about a larger surface around the black hole.
Who said they were equivalent? What would it even mean for them to be equivalent? I don't follow you at all.

20. Jul 9, 2008

### Antenna Guy

What does it say about the area of the event horizon?

No offense, but I didn't claim that the wiki page supported my argument.

Regards,

Bill