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I just heard something I have heard a million times before: "The shape of a spinning black hole is an oblate spheroid." This time, it hit me as odd that a black hole's shape changes with a spin. The way I have always understood this effect, the spinning of an astronomical object causing the sphere to bulge at its equator, I have understood it to be the angular momentum of the outer parts of the body working against gravity to cause the equatorial bulge. To me, this makes perfect sense for virtually every stellar body, except black holes, because black holes' gravity are pretty much "infinite", so how does the rotation of black hole, work against its gravity to bulge at the equator? Is there there something I am missing? The only thing I can think about is mass closer to the equator has more energy because it's traveling faster, but I'm not sure the concept of movement through space or speed of that material has any bearing on the mechanics of a black hole.

So, the question is, why are black holes oblate? Obviously the answer is because it spins, but how does this spin resist the massive inward pull of gravity to cause the bulge? Or am I misunderstanding something fundamental here?

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  • $\begingroup$ The event horizon is just a mathematical surface. It isn't composed of a physical substance. $\endgroup$
    – PM 2Ring
    Commented May 3 at 2:25

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By your logic that the gravity is infinite, you could equally ask how can a non-spinning black hole be a sphere rather than a point?

The answer is that the surfaces you are discussing are not the shapes occupied by matter, but mathematical descriptions of important surfaces defined by the spacetime of these entities.

In the case of a spherically symmetric, static mass, these surfaces are naturally spherical and a function only of the mass of the black hole.

In the case of a spinning (Kerr) black hole, the spherical symmetry is broken and the surfaces are spheroids. The fact that the mass has disappeared behind an event horizon cannot disguise the fact that it also has angular momentum and the surfaces are functions of both the mass and spin of the black hole, with the asymmetry being governed by the ratio of the spin angular momentum and mass.

There are several important surfaces in the case of Kerr black holes that are described here.

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