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Author Topic: Events that Occur on the Event Horizon of a Black Hole  (Read 3335 times)

Offline shadowdrummer46

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Firstly I'm new here so, hello to all of you! ;D

In a Episode of "Through the Wormhole" they were using a thought experiment using "Bob" and "Jane".  As Jane approached the black hole Bob saw her slow until she stopped, while Jane continued past the event horizon.

My question is this:
Why don't we see the frozen image of everything the black hole has "eaten" over its history? Does the relativity of time mean only Bob can see Jane frozen in time forever?


 

Offline neilep

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Events that Occur on the Event Horizon of a Black Hole
« Reply #1 on: 31/07/2010 02:15:43 »
A very warm welcome shadowdrummer46 ! This is a great question. I once asked a similar one asking that if am observer saw someone approach the event horizon and then another object or another person approached the event horizon at the precise same point would the images become mixed ? I do wish you well here and hope the answer comes soon. Best wishes .. Neil
 

Offline LeeE

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Events that Occur on the Event Horizon of a Black Hole
« Reply #2 on: 31/07/2010 12:08:43 »
While Black Holes (BH) are probably the simplest structures in the universe, what happens to objects near their vicinity is very complex, with some of the factors being fairly well understood but with some of the others being much more obscure.  In the end, nothing you're likely to see on TV will be able to explain everything that happens, much of which is still open to debate anyway, and whatever is shown will be incomplete and therefore inaccurate.

As you approach a BH the shape of space-time changes from being nearly flat to being highly curved and what this means is that not only will freely moving objects move in curves instead of straight lines (unless they're moving directly along the gravitational gradient) but that time will appear to pass at different rates when compared with something/someone further away from the BH.

Both of these effects are due to the curvature of space around the Earth, because the Earth, just like a BH or any other mass, come to that, distorts the shape of space-time around it.  This is one of the fundamental aspects of Einstein's theory of General Relativity.

As something, or in this case, someone i.e. Jane, approaches a BH time will slow down for her, although she, of course, will not be aware of it directly; she could only tell that she was 'running slow', as it were, by looking back out from the BH towards a clock further away from the BH, presumably held by Bob, where she would see Bob's clock appear to speed up.  If Jane is holding a similar clock towards Bob as she approaches the BH, then Bob would see Jane's clock appear to slow down.

This slowing of time as Jane approaches the BH doesn't just mean that the only effect is that her clock appears to slow down though.  The only reason that Bob can see Jane at all is because she is reflecting or emitting light back towards him, because you can't see anything if no light reaches you from whatever you're trying to look at.  However, the frequency, or colour of the light, and the energy it carries, is function of time so if Jane is also carrying a big floodlight to illuminate her as she approaches the BH, or is holding a torch pointing back at Bob, then the light from it will gradually become more and more red coloured as her rate of time slows down, and will eventually pass through the infra-red region, where it would become invisible to the naked eye, and end up as ultra low-frequency radio waves.

Bob then, wouldn't see Jane slow down and stop, apparently 'frozen' in time, but instead the light reaching Bob from her would initially start to become more red coloured and, because the energy of light is also dependent upon its frequency, dimmer too, until she was only reflecting/emitting ultra-low frequency radio waves, just before she reached the BH's Event Horizon, at which point her rate of time relative to Bob would have dropped to zero, with the result that the frequency of the light she was reflecting/emitting would have also dropped to zero, corresponding to zero energy, and she would have finally (and terminally) disappeared.

More gruesomely, and if Bob is actually watching Jane from the side instead of looking directly down on her, he may see her become 'spaghettified' before she fades out of sight due to the difference in the gravitational forces acting on the side of her that is nearest the BH and the side of her that is furthest from it.  Even in orbit around the Earth, the difference in gravitational force over relatively small distances i.e. a km or two, is enough to put tension in a long 'cable' that is oriented towards the Earth and keep it taut (a couple of experiments have proved this) but in the gravitational gradients around a BH the difference in the gravitational force over just a few cm could be great enough to tear her apart.
 

Offline shadowdrummer46

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Events that Occur on the Event Horizon of a Black Hole
« Reply #3 on: 06/08/2010 02:56:23 »
Tanks for the warm welcome and replies.

It's funny you used the word "Spaghettified" because I almost used it in OP but I wasnt sure how scientific this forum was. Also, I realise that BH are a hotly debated issue in physics at this point, and television isnt the best source for information.  ;)

"However, the frequency, or colour of the light, and the energy it carries, is function of time so if Jane is also carrying a big floodlight to illuminate her as she approaches the BH, or is holding a torch pointing back at Bob, then the light from it will gradually become more and more red coloured as her rate of time slows down, and will eventually pass through the infra-red region, where it would become invisible to the naked eye, and end up as ultra low-frequency radio waves."

Is this what we see as radio waves being emitted from black holes? I'm not sure if BH do emit radio waves, but I think I remember reading it somewhere, although I may be mistaken.

"Even in orbit around the Earth, the difference in gravitational force over relatively small distances i.e. a km or two, is enough to put tension in a long 'cable' that is oriented towards the Earth and keep it taut..."

Could you further explain this please?
« Last Edit: 06/08/2010 02:58:24 by shadowdrummer46 »
 

Offline LeeE

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Events that Occur on the Event Horizon of a Black Hole
« Reply #4 on: 06/08/2010 22:06:05 »
Is this what we see as radio waves being emitted from black holes? I'm not sure if BH do emit radio waves, but I think I remember reading it somewhere, although I may be mistaken.

You don't get anything emitted from a BH, at least not directly - have a read of: http://en.wikipedia.org/wiki/Hawking_radiation

Quote
"Even in orbit around the Earth, the difference in gravitational force over relatively small distances i.e. a km or two, is enough to put tension in a long 'cable' that is oriented towards the Earth and keep it taut..."

Could you further explain this please?

Have a read of these articles: http://en.wikipedia.org/wiki/Tidal_stabilisation, http://en.wikipedia.org/wiki/Space_tether#Tidal_stabilization
 

Offline shadowdrummer46

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Events that Occur on the Event Horizon of a Black Hole
« Reply #5 on: 08/08/2010 00:05:51 »
Thanks for the refrances ill be sure to take a look. Also, thanks for the correction.
« Last Edit: 08/08/2010 00:07:42 by shadowdrummer46 »
 

Offline Farsight

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Events that Occur on the Event Horizon of a Black Hole
« Reply #6 on: 08/08/2010 14:44:43 »
In a Episode of "Through the Wormhole" they were using a thought experiment using "Bob" and "Jane".  As Jane approached the black hole Bob saw her slow until she stopped, while Jane continued past the event horizon.
I'm afraid wormholes are speculative, shadowdrummer. And Jane doesn't really continue past the event horizon because the gravitational time dilation is infinite at that point. It takes forever to get past it. What Bob sees is what actually happens. Jane's clocks and observations and thought processes go slower and slower until they stop. But she doesn't know that she's stopped... because she's stopped. Unfortunately what tends to be taught is the Misner/Thorne/Wheeler "Geometrical Interpretation" of general relativity. This matches what you described and is so prevalent that most people don't appreciate that there's an alternative called the Weinberg "Field Interpretation". You can see mention of it in this Formation and Growth of Black Holes article. The article sides with the Misner/Thorne/Wheeler version, but not convincingly:

"Incidentally, I should probably qualify my dismissal of the "frozen star" interpretation, because there's a sense in which it's valid, or at least defensible. Remember that historically the two most common conceptual models for general relativity have been the "geometric interpretation" (as exemplified by Misner/Thorne/Wheeler's "Gravitation") and the "field interpretation" (as in Weinberg's "Gravitation and Cosmology"). These two views are operationally equivalent outside event horizons, but they tend to lead to different conceptions of the limit of gravitational collapse. According to the field interpretation, a clock runs increasingly slowly as it approaches the event horizon (due to the strength of the field), and the natural "limit" of this process is that the clock just asymptotically approaches "full stop" (i.e., running at a rate of zero) as it approaches the horizon. It continues to exist for the rest of time, but it's "frozen" due to the strength of the gravitational field. Within this conceptual framework there's nothing more to be said about the clock's existence. This leads to the "frozen star" conception of gravitational collapse. In contrast, according to the geometric interpretation, all clocks run at the same rate, measuring out real distances along worldlines in spacetime. This leads us to think that, rather than slowing down as it approaches the event horizon, the clock is following a shorter and shorter path to the future. In fact, the path gets shorter at such a rate that it actually reaches (our) future infinity in finite proper time. Now what? If we believe the clock is still running just like every other clock (and there's no local pathology of the spacetime) then it seems natural to extrapolate the clock's existence right past our future infinity and into another region of spacetime. Obviously this implies that the universe has a "transfinite topology", which some people find troubling, but there's nothing logically contradictory about it (assuming the notion of an infinite continuous universe is not itself logically contradictory)".

Why don't we see the frozen image of everything the black hole has "eaten" over its history? Does the relativity of time mean only Bob can see Jane frozen in time forever?
Because the infalling object doesn't keep on emitting light, and after it got spaghettified by the tidal gradient it gets totally flattened by the radial length contraction. And not much light comes out of a black hole anyway.

 

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Events that Occur on the Event Horizon of a Black Hole
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