
Black holes spin almost at the speed of light - chupa-chups
https://www.forbes.com/sites/startswithabang/2019/08/01/this-is-why-black-holes-must-spin-at-almost-the-speed-of-light/
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ncmncm
This seemed obvious to me, but not from the "skater" analogy.

Almost every atom entering the event horizon goes in tangentially, because the
target is so small, falling from effectively infinite distance, which means it
should be going only infinitesimally less than lightspeed at that moment.

The only consumers of kinetic energy would be other particles that happen to
enter orbit at a different angle, and collide; and electromagnetic
interactions, producing e.g. AGN galactic jets. How much of the kinetic energy
gets diverted into those?

I see assertions that the singularity is toroidal, so the actual center of the
black hole is not itself singular. Can we posit a fully toroidal event
horizon, with a hole through it? Might we expect almost all black holes to be
that way?

It's not hard to imagine a black hole spinning one way to get an accretion
disk going the opposite way, so I would expect to find the occasional not-
spinning-much hole.

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gitrebase
What does it mean for a black hole to spin at the speed of light? Which point
on it spins at that speed?

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zaarn
Essentially, when spinning a black hole doesn't have a point at it's center
but a ring (a point can't rotate).

Theoretically a black hole can spin faster than the speed of light since the
event horizon is just space, not matter, and space can certainly exceed the
speed of light (see expansion of the universe).

IIRC from relevant literature, the event horizon shrinks with increasing speed
until it vanishes once the speed of light is exceeded and leaves behind a
naked singularity; a black hole with no event horizon.

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emilfihlman
What is a black hole with no event horizon? Does that mean that once crossing,
you are instantly subject to a great gravity? So there's a gravity gradient?
How is that possible?

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zaarn
It's a naked singularity. Light can escape from the center ring of the
singularity and you could possibly observe it. What exactly it is we don't
know, our physics just break down that deep into the gravity well. It could
just be a barely visible ring that saws your spaceship in half if you fly
through or a wormhole or possibly anything really.

