[TMEHpodcast]

No matter how chaotic the merger looks, the event horizon must asymptotically become either spherical (Schwarzschild) or oblate (Kerr). The mass distribution inside doesn’t change this, general relativity doesn’t allow static “lumpy” horizons.

It’s wild how much happens in those milliseconds though. Numerical relativity papers like the one you shared from arxiv.org show the horizon “sloshing” before it stabilizes.

[geysersam]

When water sloshes it ejects small droplets. Can the event horizon eject black hole droplets during a violent merger event?

[Thiez]

It cannot. The event horizon by definition prevents mass and energy from leaving (ignoring the exception of Hawking radiation here). I'm assuming your "black hole droplet" would be a tiny black hole? But if you could remove a little chunk from the black hole then you've effectively taken mass out of it, which is impossible.

It is even the case that once two black holes have overlapping event horizons (so they "touch" in a way) they can't stop touching. So two black holes can zip past one another at a small distance, but if they high-five they can't stop merging.

[pavel_lishin]

Is it even sensible to talk about a "mass distribution" inside of an event horizon?

[kadoban]

Sure, especially consider if singularities are not real. Then what's inside the event horizon is just some bunch of unknown material in some actual shape. Why wouldn't it be?

If singularities are real...same thing but more boring answer maybe? (the distribution just being: in the center).

[lisper]

> Why wouldn't it be?

Because the whole concept of "shape" assumes properties of space that might not apply inside an event horizon?

[db48x]

There’s no reason to expect that the properties of space are different inside the event horizon than outside. Of course the direction of time turns sharply as you go inside, but otherwise space is just space.

You only get an asymmetric black hole during the milliseconds of a merger. And that asymmetry is entirely due to the mass distribution inside the black hole. The black hole only becomes spherical again once the singularities have merged. Or in the more common case of rotating black holes, they only become properly oblate again once their ringularities have merged. Either way it happens quite quickly.

[lisper]

> the direction of time turns sharply as you go inside

Yeah, that's what I meant. It's hard for me to reconcile the concepts of "the direction of time turns sharply" with "space is just space".

[fc417fc802]

I am certainly no physicist but I remember coming across academic papers in the past speculating about exactly your question. I recall one theorized about singularities being hollow with all of the mass (err was it space? spacetime?) compacted down into 2 dimensions on a shell at the surface (at least IIUC, which I probably didn't).

I think that concept might fit with the infinite time dilation preventing a merger from ever actually occurring? I'd be curious how that might differ for matter that's already inside when the critical mass is reached. (I'd also be curious to know all the creative and wacky ways in which I got the above completely wrong given that's just about inevitable.)

[db48x]

Mass curves space. All mass curves space all the time. You are bending the fabric of spacetime even now! Don’t try to deny it!

What does curvature mean? It means that the direction of time’s arrow is different in different places. To an observer outside of a large gravitational field, events inside the field appear to move more slowly than they would have outside of it. Black holes merely take this to an extreme. To an observer far from a black hole, a clock entering the black hole appears to slow down and finally _stop_ as it crosses the event horizon¹. But simultaneously an observer traveling with the clock observes something different. They see everything outside the black hole slow down and stop instead, while they continue to coast smoothly along. They notice nothing strange at the horizon itself; it is simply empty space with weird visuals in the distance.

This almost seems like a paradox, since the two observers each believe that the other’s clock has stopped. The reason why it’s not a paradox is that the space around the black hole is strongly curved, so strongly that the axis of time swaps place with one direction of space. At the horizon the axis of time flips over and points down into the black hole. The distant observer sees time stop because time is now edge–on, as it were. The observer falling into the black hole notices nothing weird near themselves, because both time and space still exist. Only the images of distant objects show any evidence of curvature. But the falling observer is doomed, for their own time axis now points at the singularity. Their timeline now ends abruptly, while the timeline of the distant observer extends potentially a vigintillion years.

For some edutainment on the subject, I recommend The Science Asylum. He’s done a bunch of videos on gravity and relativity, but here are two in particular:

  * Explaining Gravity Using Relativistic Time Dilation <https://www.youtube.com/watch?v=F5PfjsPdBzg&list=PLOVL_fPox2K83_36YgnGisn4rxNvgq1iR&index=7>
  * Why Can't You Escape a Black Hole? <https://www.youtube.com/watch?v=yPQUtuTraxs&list=PLOVL_fPox2K-zpTeryROTkmzzsMssSMWp&index=6>

¹ There are other effects too. The image of the clock _lingers_ on the horizon forever, since for it time has apparently stopped. But the redshift increases to infinity too, as the gravitational well becomes steeper, so no matter what wavelengths we observe in the image of the clock fades away beyond sight. Worse, the tidal forces caused by a real stellar–mass black hole will tear apart solid objects into a stream of plasma, even small objects. So the hypothetical black hole in our thought experiment must be very large indeed, to minimize the tidal forces enough that the clock survives the trip to the horizon intact and functional. And it can't be rotating either, since the rotation causes its own weirdness. This is the spherical cow of black holes.

[r0uv3n]

Eh, space inside or outside the horizon is only different in so far as to whether it can reach our timelike infinity. Locally you cannot even tell where any horizon might be (just look at a small patch of a Penrose diagram near a horizon), they are very much something related to global properties of the spacetime. In particular it's not problematic to talk about some extended volume in spacetime occupied by mass, as long as the divergence of the stress energy tensor is 0.

The point where our notions of geometry would break down would be near the singularity, not near the horizon, and we don't even know if a volume enclosed by a horizon (i.e. anything you might call a black hole) necessarily has a singularity inside, it's just that our simple mathematical models all assume one.

[mr_toad]

Because having some observational knowledge of the inside is impossible, in a sense it doesn’t matter.