Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Renee Roberts on 27/09/2008 10:53:11
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Renee Roberts asked the Naked Scientists:
Hello, Naked Scientists!
Could you tell me how non-rotating black holes form? What happens to the spin impulse of the material falling into them?
Thanks for a great podcast! (http://www.thenakedscientists.com/HTML/podcasts/) Naked Science rocks! ;-)
What do you think?
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Renee Roberts asked the Naked Scientists:
Hello, Naked Scientists!
Could you tell me how non-rotating black holes form? What happens to the spin impulse of the material falling into them?
Thanks for a great podcast! (http://www.thenakedscientists.com/HTML/podcasts/) Naked Science rocks! ;-)
What do you think?
What does exactly mean this question? I mean, how much non-rotating? Exactly non-rotating is impossible, so you should establish an angular speed's upper limit.
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lets split the question up.
How do black holes form?
Are there spinning, and nonspinning black holes?
If there are, do they form differently?
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As lightarrow said, non-rotating black holes can't exist.
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Isn't a Schwarzschild black hole non-rotating (i.e. has 0 angular momentum)?
http://en.wikipedia.org/wiki/Schwarzschild_metric#Singularities_and_black_holes
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As far as I understand it, the situation is this:-
The Schwarzschild solution to Einstein's equations is the simplest possible form and simply proves that black holes can exist. It neglects angular momentum & charge for the sake of simplicity.
The Kerr solution incorporates angular momentum, and is a more comprehensive solution for this. Going beyond the Kerr solution is the Kerr-Newman solution which incorporates angular momentum & charge.
Rotation is believed to be the norm for any compact object, and that includes black holes.
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The Schwarzschild Blackhole is an exact solution of Einstein's equations, so it is not neglecting anything. Same thing canbe said about the Kerr and other Kerr-Newman solutions.
One way in which a non-rotating blackhole could form is by slowing down the rotation of an already rotating blackhole. The so-called "penrose process" is capable of extracting rotational energy from a rotating blackhole. But in general I guess we would expect physically realistic blackholes to be rotating.
One should never mistake the solution of an equation for a real physical object.
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The Schwarzschild Blackhole is an exact solution of Einstein's equations, so it is not neglecting anything. Same thing canbe said about the Kerr and other Kerr-Newman solutions.
My misunderstanding. Thank you for correcting me.
One way in which a non-rotating blackhole could form is by slowing down the rotation of an already rotating blackhole. The so-called "penrose process" is capable of extracting rotational energy from a rotating blackhole. But in general I guess we would expect physically realistic blackholes to be rotating.
Would the Penrose Process be capable of stopping the rotation of a BH in less time than the age of the universe? I have always understood that it couldn't.
Could a close encounter with another BH be capable of effecting it?
One should never mistake the solution of an equation for a real physical object.
That's really the point I was trying to make; I just got my facts a bit wrong. Sorry [:(]
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The Schwarzschild Blackhole is an exact solution of Einstein's equations, so it is not neglecting anything.
Yes, it's an exact solution, but it doesn't contemplate the case of angular momentum or charge; I don't understand why Doctor Beaver's statement was wrong.Same thing canbe said about the Kerr and other Kerr-Newman solutions.
One way in which a non-rotating blackhole could form is by slowing down the rotation of an already rotating blackhole. The so-called "penrose process" is capable of extracting rotational energy from a rotating blackhole. But in general I guess we would expect physically realistic blackholes to be rotating.
One should never mistake the solution of an equation for a real physical object.
Infact; it's impossible for a real BH to be a Schwarzschild one, and even if you extract energy from its rotation (always possible, of course) you will never be able to do it completely. For this reason I asked: "how much non-rotating?"
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I did some searching to figure out why non-rotating black holes can't exist in nature, and I couldn't find anything. Wikipedia suggests that you can extract all angular momentum via the Penrose process:
The process obeys the laws of black hole mechanics. A consequence of these laws is that if the process is performed repeatedly, the black hole can eventually lose all of its angular momentum, becoming rotationally stationary.
although the one link they have doesn't address that claim, and I don't have the text on gravitation that they reference.
So why can't a real black hole be non-rotating? Or do you know of sources where I can read up on this?
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The Lense-Thirring effect is very small. I doubt we could stop a BH to a perfect rest.