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Author Topic: Time slowing and Black holes  (Read 4865 times)

Offline thedetective

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Time slowing and Black holes
« on: 11/05/2005 18:27:33 »

I read somewhere that as you get closer and closer to a Black hole, the intense gravity distorts time so that to the observer (approaching the black hole) time will slow, and there is a point, just before the event horizon where tens of thousands of years could pass relative to a few minutes back on Earth.  I know that moving really fast (close to C) will slow down time, but how exactly does the gravity of the black hole slow down time?  I know it has to do with General Relativity, but I don't exactly get it.  

And how much energy would it require to remain stationary at that point?  I'm not looking for an exact amount of energy needed, just an idea of how powerful the gravitational pull would be that close to the event horizon.



 

Offline gsmollin

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Re: Time slowing and Black holes
« Reply #1 on: 11/05/2005 19:02:07 »
For an observer distant from the black hole, your time would appear to slow down, and then as you reached the event horizon, it would stop. You would appear frozen at the event horizon, because your speed would have assymptotically approached c.  That's if you can be seen at all, of course. The intense gravity would impose a red shift on the reflected radiation at the edge of the event horizon, so that you would not actually be visible in visible light.

At the event horizon, no amount of energy is enough to hold you back. There is a radius just outside the event horizon that allows a stable orbit, with complicated properties that depend upon the black hole's rotation.
 

Offline DoctorBeaver

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Re: Time slowing and Black holes
« Reply #2 on: 17/05/2005 14:04:50 »
gsmolin - if the light is red-shifted, what's all this talk about blue curtains round black holes. Surely they would be red curtains

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Offline gsmollin

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Re: Time slowing and Black holes
« Reply #3 on: 17/05/2005 18:22:06 »
I can't say exactly, you would have to give me your source. I can conjecture that a "blue curtain" would be the accretion disk surrounding the black hole. The accretion disk is not black! It radiates highly energetic particles and photons. This is the power of the quasar and other types of active galactic nuclei.
 

Offline DoctorBeaver

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Re: Time slowing and Black holes
« Reply #4 on: 17/05/2005 20:04:02 »
I read it in a book I borrowed from the library a few years ago so I can't give the source. It was something to do with the build up of photons at the event horizon when observed from a distance: to an outside observer the photons would be stuck at the horizon forever.

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Offline gsmollin

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Re: Time slowing and Black holes
« Reply #5 on: 18/05/2005 18:34:00 »
That might be the "last stable orbit". One can orbit a black hole as easily as any star. As the orbit gets close to the event horizon, the orbital velocity become relativistic. This is the source of the synchrotron radiation from charged particles orbiting the black hole. There is a narrow band around the event horizon where radiation can orbit with velocity c. It cannot escape, and it does not go into the event horizon because it is in a stable orbit. Maybe that's the blue curtain. It would not appear blue from a distance, however, since it would be strongly redshifted by the gravity of the black hole.
 

Offline DoctorBeaver

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Re: Time slowing and Black holes
« Reply #6 on: 18/05/2005 21:38:18 »
No, it wasn't to do with orbits. I'll google it to see if I can find anything else about it

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Offline DoctorBeaver

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Re: Time slowing and Black holes
« Reply #7 on: 18/05/2005 21:45:49 »
Google didn't come up with much. I'll pop down the library in the next few days to see if I can find the book. I'd recognise it if I saw it again.

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Offline chimera

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Re: Time slowing and Black holes
« Reply #8 on: 19/05/2005 21:33:07 »
I think you can save yourself the trouble: lots of artist's impressions use blue for the cool effect, but it has no basis in reality - remember, no pics of black holes exist, so far:

http://hubblesite.org/newscenter/newsdesk/archive/releases/2002/30/image/a

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Offline DoctorBeaver

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Re: Time slowing and Black holes
« Reply #9 on: 20/05/2005 11:58:48 »
Rob - this wasn't anything to do with artists' impressions. It was about particles being accelerated by the gravitational pull of the black hole but seemingly coming to a halt at the event horizon. A build up of particles would occur as, to an outside observer, they would all remain stuck at the horizon. It's just the actual mechanism of the blue shift I can't remember.

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Re: Time slowing and Black holes
« Reply #9 on: 20/05/2005 11:58:48 »

 

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