It's the https://en.wikipedia.org/wiki/No-hair_theorem , but it only applies after a while, not instantly.

Your disk will emit a lot of gravitational on electromagnetic radiation, and after a while it will be a nice sphere. (Unless it's rotating and it will be a nice somewhat-elipsoidal ball.)

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> and then an electron popped into existence due to quantum randomness

I feel there is a huge can of worms of technical problems in this sentence that nobody know how to solve for now. Just in case replace the quantum randomness with a moron with a broken CRT used as an electron cannon.

> then it would become a sphere instantly

Event horizons are non-physical. Better to think of it as "then a spherical event horizon would become apparent." When the mass within a given black-hole-shaped volume (spherical for non-rotating mass) is "one electron short" of being a black hole, then one can define a surface in the shape of the (future) black hole where the escape velocity is /just/ below the speed of light. In practice, all light emitted within that volume will already be captured by the mass, unless it's perfectly perpendicular to the (future) event horizon. When that extra electron is added, it becomes true that the escape velocity at that same surface is now the speed of light -- the definition of event horizon. But nothing needs to "form" to make this true.

That electron would take an infinite amount of time to reach the edge, since time dilates to infinity with gravity that strong.

> a sphere instantly

The concept of instantly doesn't work with time dilation like this. What you see will be different depending on if you are also falling in, or if you are far away.

Is that intended to be a correction? (I don't think the original statement needs correcting, other than by replacing "universe" with "observable universe" in both places.)

The event horizons.

In cosmological terms, what is barely not touching? Is that distance measured in meters, kilometers, AUs, lightyears, parsecs?

Maybe I should've linked my toy simulator in my initial comment.

https://cybersystems.dev/gtc/gtc.html

The events horizon.

Or in other words, black holes mergers conserve their total radius, not volume as one would get with normal matter.

I think this is over their lifetime, not when they merge?

For normal matter inspiraling, yes, but a black hole which is falling into a black hole doesn't get to glow in gamma rays to try to escape :) they can only lose mass/energy by making splashes in spacetime itself (or hawking radiation)

I think I'm going to answer my own question by saying both momentum and energy are conserved. The momentum of the entire system was zero before and after the collision. Energy must also be conserved, and since the final object is at rest, all the kinetic energy gets converted into rest mass energy, minus what is radiated away as gravitational waves.

My hunch is they would briefly pancake and much of the mass/energy contribution from their initial velocities would dissipate as incredibly high amplitude gravitational waves from the ring-down.

It would create a universe, obviously. First all the mass would attempt being squished at a singularity. WHILE the squishing continues, the first-in-line stuff would've already started to explode back-out inside the event horizon. From the inside viewpoint, this looks like the big bang. Once all the mass from the two black holes collide and loose momentum, the inside-universe no longer expands as fast. Things wobble a bit as all this happens, creating tangles and non-homogeneity. Could be caused by initial Planck-scale uncertainties even when having a perfect head-on collision.

When you say cracking open a black hole do you mean cracking the event horizon to form a naked singularity?

Hawking radiation occurs because black holes are sticky ‼

estimated distance of 2.2 Gpc per https://en.wikipedia.org/wiki/GW231123

The math seems to suggest R=a, or simply the spin in terms of length. It's certainly an oversimplification, as the answer will depend on the choice of metric.

Here's the best resources I've been able to find on the question. Roy Kerr himself responded to the Quora question:

> There is no Newtonian singularity at the Center of the earth and there is no singularity inside a rotating black hole. The ring singularity is imaginary. It only exists in my solution because it contains no actual matter. When a star collapses into a black hole it keeps shrinking until the centrifugal force stabilizes it. The event shell forms between the star and the outside. In 57 years no one has actually proved that a singularity forms inside, and that includes Penrose. instead, he proved that there is a light ray of finite affine length. This follows from the “hairy ball theorem”.

The stack overflow answer seems to describe the problem in terms I can better understand:

> It seems unlikely to me that you're going to be able to formulate a notion of diameter that makes sense here. Putting aside all questions of the metric's misbehavior at the ring singularity, there is the question of what spacelike path you want to integrate along. For the notion of a diameter to make sense, there would have to be some preferred path. Outside the horizon of a Schwarzschild black hole, we have a preferred stationary observer at any given point, and therefore there is a preferred radial direction that is orthogonal to that observer's world-line. But this doesn't work here.

https://physics.stackexchange.com/questions/471419/metric-di...

https://www.quora.com/What-is-the-typical-diameter-roughly-o...