- #1
MattRob
- 211
- 29
I've been reading Kip Thorne's "Black Holes and Time Warps," and it mentioned something rather counter-intuitive; apparently, when material forms an accretion disk and falls into a spinning black hole, it increases the angular momentum of it.
Now, let's take a gas cloud, and put a spinning black hole in it.
The gas cloud has zero angular momentum as a whole ([itex]L_{cloud} = 0 [/itex]), and the black hole itself has [itex]L_{hole}[/itex] angular momentum.
From what I know of classical mechanics, the momentum of the system shouldn't change; once the black hole accretes all the gas of the cloud, it will, according to classical mechanics, spin slower, and the total momentum of the system will stay the same.
[itex]∑L_{final} = L_{cloud} + L_{hole} = L_{hole} = ∑L_{initial} [/itex]
by classical mechanics.
So, am I understanding this correctly, that that classical mechanics' description of the system doesn't agree with General Relativity, and that a full GR description of the same spinning black hole and gas cloud will end with the entire system having more angular momentum?
[itex]∑L_{final} > L_{hole initial} + L_{cloud initial} = L_{hole initial} = ∑L_{initial}[/itex]
by General Relativity.
If that's so, then how? The book didn't go into detail much on how that's the case, but I find it really odd/interesting and would like some insight on how it happens.
Now, let's take a gas cloud, and put a spinning black hole in it.
The gas cloud has zero angular momentum as a whole ([itex]L_{cloud} = 0 [/itex]), and the black hole itself has [itex]L_{hole}[/itex] angular momentum.
From what I know of classical mechanics, the momentum of the system shouldn't change; once the black hole accretes all the gas of the cloud, it will, according to classical mechanics, spin slower, and the total momentum of the system will stay the same.
[itex]∑L_{final} = L_{cloud} + L_{hole} = L_{hole} = ∑L_{initial} [/itex]
by classical mechanics.
So, am I understanding this correctly, that that classical mechanics' description of the system doesn't agree with General Relativity, and that a full GR description of the same spinning black hole and gas cloud will end with the entire system having more angular momentum?
[itex]∑L_{final} > L_{hole initial} + L_{cloud initial} = L_{hole initial} = ∑L_{initial}[/itex]
by General Relativity.
If that's so, then how? The book didn't go into detail much on how that's the case, but I find it really odd/interesting and would like some insight on how it happens.