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2019-12-26, 04:06 AM (ISO 8601)
- Join Date
- Feb 2016
Re: If a black hole evaporates, where or how does the rotational energy go?
Assuming that additional matter falls in from the outside faster than the black hole loses mass to hawking radiation, shouldn't we, in practice, observe infalling matter corssing the event horizon in a finite period of time, due to the fact that even though its position becomes arrested in our reference frame, the infalling matter causes the event horizon to expand (and, crucially, will eventually to expand past the position of the object in question provided that it is given enough infallig mass)
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2019-12-27, 12:34 PM (ISO 8601)
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- Sep 2019
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- Florida
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Re: If a black hole evaporates, where or how does the rotational energy go?
Yora's point was about not seeing anything past the event horizon. In the case you describe one would see things falling into the event horizon, but they wouldn't see anything of the distribution of matter within the event horizon.
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2019-12-27, 01:28 PM (ISO 8601)
- Join Date
- Jun 2013
- Location
- Bristol, UK
Re: If a black hole evaporates, where or how does the rotational energy go?
I have not been talking about doughnuts. When it comes to eating doughnuts, I prefer by a long way jam doughnuts (though the jam is more like fruit sauce than normal jam), with no hole, and they don't have a cavity because they have jam in them.
The singularity of a Kerr (spinning) black hole has a hole. The event horizon has a cavity, but no hole (also, unlike the jelly doughnut, the cavity doesn't connect to the outside at all). There is absolutely no way to see the singularity or the cavity.
Adding angular momentum will shrink the event horizon and make the cavity bigger. However the outer and inner event horizon approach being the exact same shape at the exact same place as you add angular momentum. Once they're at the same place, they cancel and there is no event horizon anywhere. THe mass in the singularity would have nothing forcing to be a singularity and would just be mass at that point.
What's interesting me about evaporating black holes is that in the last few seconds a huge amount of mass is being lost very quickly, and if there is any angular momentum there seems to be a strong risk of the angular momentum relative to the mass rising very fast, which by your explanation would give rise to a naked singularity, and by mine would lead to a toroidal event horizon.The end of what Son? The story? There is no end. There's just the point where the storytellers stop talking.
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2019-12-30, 03:19 PM (ISO 8601)
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- Sep 2019
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- Florida
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Re: If a black hole evaporates, where or how does the rotational energy go?
Good question; I was unclear. A spinning black hole has two event horizons and a region between them. Within that region everything must be moving downward, time doesn't work the same way, and kinetic and potential energies have an imaginary component.
I would imagine the event horizon being a given distance (that distance mostly depending on the mass of the singularity) from the singularity, which would mean that the event horizon moved outward at the equator, and inward at the poles (equator and poles being relative to the rotation of the singularity).
As the black hole changes from no spin to maximum spin the event horizon shrinks to half it's radius. The ergosphere keeps the same equator as the original event horizon, but it's poles pinch in to meet the poles f the new event horizon.
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2019-12-30, 05:10 PM (ISO 8601)
- Join Date
- Jun 2013
- Location
- Bristol, UK
Re: If a black hole evaporates, where or how does the rotational energy go?
Wikipedia disagrees:
https://en.wikipedia.org/wiki/Black_hole#Event_horizon
The shape of the event horizon of a black hole is always approximately spherical.[Note 4][86] For non-rotating (static) black holes the geometry of the event horizon is precisely spherical, while for rotating black holes the event horizon is oblate.[The end of what Son? The story? There is no end. There's just the point where the storytellers stop talking.
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2020-01-09, 03:57 PM (ISO 8601)
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- Sep 2019
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- Florida
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Re: If a black hole evaporates, where or how does the rotational energy go?
This is a case of Wikipedia contracting itself since it also says the event horizon of a spinning black hole is a sphere. I'm personally more inclined to go with the page that has equations, but I'm not a proper authority so feel free to disagree with me.
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2020-01-09, 08:13 PM (ISO 8601)
- Join Date
- Jun 2013
- Location
- Bristol, UK
Re: If a black hole evaporates, where or how does the rotational energy go?
The end of what Son? The story? There is no end. There's just the point where the storytellers stop talking.