<p>If the mass and orbital period for two neutron stars is known before collapse to a black hole and the rate of spin is known after collapse, 100 revolutions per second for instance, that will correlate mathmatically to a specific radius.</p>

<p>Does this radius correlate to a physical feature of the black hole such as a surface? Does this radius fall between the radius of the physical substance and the Schwarzschild radius? Is it the largest, intermediate, or the smallest of these?</p>

CC BY-SA 4.0
2019-09-18T21:46:27.257Z
Black holes are characterised by an angular momentum, not an angular velocity (which is unknown/undefined).
ProfRob
CC BY-SA 4.0
2019-09-19T10:38:34.727Z
A black hole doesn't *have* a physical surface, but it has one or more horizons. See https://en.wikipedia.org/wiki/Rotating_black_hole
PM 2Ring
CC BY-SA 4.0
2019-09-20T07:51:45.903Z
@RobJeffries There *is* an angular velocity associated to a black hole $\omega_{H} =\frac{a}{2M r_{+}}$, which is often interpreted as the angular velocity of the horizon. Specifying $M$ and $\omega_H$ is of course full equivalent to specifying $M$ and $J$ or $M$ and $a$
TimRias
<black-hole>
Does the spin rate of a black hole correspond to a physical feature?
DMac
# Does the spin rate of a black hole correspond to a physical feature?

(Asked by: username_0 on 2019-09-18)

If the mass and orbital period for two neutron stars is known before collapse to a black hole and the rate of spin is known after collapse, 100 revolutions per second for instance, that will correlate mathmatically to a specific radius.

Does this radius correlate to a physical feature of the black hole such as a surface? Does this radius fall between the radius of the physical substance and the Schwarzschild radius? Is it the largest, intermediate, or the smallest of these?

**Tags:** black-hole

> **Comments:**
> **username_1**: Black holes are characterised by an angular momentum, not an angular velocity (which is unknown/undefined).  
> **username_2**: A black hole doesn't *have* a physical surface, but it has one or more horizons. See https://en.wikipedia.org/wiki/Rotating_black_hole  
> **username_3**: @RobJeffries There *is* an angular velocity associated to a black hole $\omega_{H} =\frac{a}{2M r_{+}}$, which is often interpreted as the angular velocity of the horizon. Specifying $M$ and $\omega_H$ is of course full equivalent to specifying $M$ and $J$ or $M$ and $a$  

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