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this post was submitted on 28 Sep 2023
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The black hole and the stuff outside it constitute a single system, and within that system, angular momentum is conserved. So as objects cross the event horizon, their angular momentum is transferred to the black hole.
Well.... does it? If all the stuff falls in and only the volume remains, who could say that it's spinning? How could you detect it?
For one thing, the size and shape of the event horizon change depending on the black hole’s spin.
What would happen if one were to stop spinning? Could one even stop spinning?
Yes—it’s called the Penrose process.
I see. Thank you. So, you can use light to infer the mass, and then the volume information to infer the spin? Easy enough.
That would be a general method, if we were close enough to observe the shape of the event horizon (which we aren’t).
The article is describing another way, which only works in this case because the black hole is precessing so extremely that the changing axis of rotation is frame-dragging the polar jets along with it.
I thought nothing actually crossed the event horizon and was essentially frozen approaching a complete stop in time in a kind of 2f representation of 3d reality until it slowly leaked out trillions of years later as hawking radiation?
From an outsider's perspective, you would see an object approach and then freeze. It would red-shift dimmer until it disappeared. From an in-falling perspective, I don't think you'd notice anything at all.