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Obviously I am not a physicist.

I have seen What is black hole spin? but this is not what puzzles me.

If I understand correctly, black hole has only three features (please correct me):

  • angular momentum, i.e. spin.
  • charge
  • mass

My problem is to understand what spins. The matter that might spin is disconnected from Schwarzschild radius due to limit of the light speed.

Schwarzschild radius in another hand, is the mathematical concept, it doesn't spin, does it?

So please could you explain WHAT spins exactly?

EDIT: I see that my question was corrected so "angular momentum, i.e" is added but angular momentum of what?

dllhell
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    Does this answer your question? https://physics.stackexchange.com/q/503799/ – g s Apr 10 '22 at 19:25
  • @gs possibly but "The curvature singularity in the center of the space-time is a ring" the ring of what? I am more stupid than you expect. – dllhell Apr 10 '22 at 19:35
  • A ring of curvature singularity. By "is a ring", Void means "is ring-shaped". – g s Apr 10 '22 at 22:37
  • sigh You can see influence of whatever is inside of black hole via its angular momentum just like with gravity or charge. Spacetime itself doesn't care about event horizon - in fact black hole even drags surrounding spacetime with itself while rotating. – Mithoron Apr 10 '22 at 23:28
  • " in fact black hole even drags surrounding spacetime with itself while rotating." i think this is a key point. the spacetime around the event horizon is what is spinning. – shai horowitz Apr 11 '22 at 02:36

1 Answers1

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My two cents of the euro.

Lets take a specific observation of black holes the LIGO merger,seen here .

The video is a complicated solution from the data gathered with the experiment, but rotation is detected. It is an animation using the data.

What is rotating? The background stars to the event are distorted through the distorted space-time and thus the motion of the two black holes can be detected/calculated.

A spinning black hole will be changing the spacetime about it and it is the distortions in the background that show it off. If it is at rest with the background stars there would be no motion of the images. From conservation of angular momentum, it must be the rotating black hole that is creating the rotary motion of the images, so the black hole is assigned a spin.

but angular momentum of what?

At the level of general relativity, where the singularity is modeled as a point it is useless to model a spinning superheavy point. If/when gravity is quantized the singularity will disappear and become a fuzzy region as the one imagined for the beginning of the universe, where all the mass will be in a volume defined by a probability space-time region, where a spin with respect with the background starts can be assigned/measured.

anna v
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  • calling that SXS simulation, a simulation “based on LIGO data” is pretty disingenuous (LIGO PR’s fault). It is a simulation with the same orbital configuration as inferred from a LIGO observation.
    • – TimRias Apr 12 '22 at 09:16
  • A rotating black hole does not cause a rotating distortion of the background ( as can be easily inferred from the fact that the Kerr metric is stationary).
    • – TimRias Apr 12 '22 at 09:20
  • The singularity in the Kerr metric is not modeled as a point in GR.
    • – TimRias Apr 12 '22 at 09:21