Is a black hole a perfect sphere?

  • I know all about how black holes form and why their gravity is so strong. However, is the gravity equally powerful in all directions? Will the event horizon appear as a perfect sphere?

    One can not ***directly*** see the event horizon, but one can see radiation from the infall of material, which might give visual cues to the event horizon. The event horizon would ***be*** a perfect sphere, but it would not ***appear*** as a perfect sphere because of gravitational lensing. If you were stationary or moving directly towards or away from the black hole, it would appear as a perfect circle that was apparently larger than the actual black hole, also because of lensing.

    @Aabaakawad What about a spinning black hole? Surely that would have some effect on the event horizon, maybe making it more oval-shaped?

    you might think that because you are imagining the mass of the black hole to be distributed throughout the volume within the event horizon. It is not. All the mass is either concentrated into a point in the center of the black (a singularity), or an ultracompact object being held up from being a singularity by forces not yet known, (not likely, but who knows). Yes, a singularity can have spin, and I do not know how that works. It can also have an electric charge and/or a magnetic moment.

  • You cannot see the event horizon. That being said:

    A non-rotating black hole, free of external influences, has perfect spherical symmetry. All its properties are exactly the same in any direction, period. This is the Schwarzschild metric.

    Even if it is electrically charged, if it's non-rotating, and free of external influences, it is still perfectly spherically symmetrical - this is the Reissner–Nordström metric.

    As soon as it begins rotating, the black hole is no longer in perfect spherical symmetry. It acquires an ergosphere, which is shaped like a flattened ball. This is the Kerr metric.

    Kerr metric with ergosphere

    Significant external factors can distort the shape of the event horizon. Two black holes merging will go through a process where spherical symmetry is lost temporarily:

    Technically, anything near a BH would slightly distort the metric of spacetime, but in practice this would not be easily measurable unless the external factor is very large (another very massive object). So for most practical purposes, the metrics described above would apply.

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Content dated before 7/24/2021 11:53 AM