LIGO detects most massive black hole merger to date

What happens when black holes collide? Does one black hole “consume” the other? Do they become a larger black hole? Does it get more dense or just larger?

What happens inside cannot be known.

As I understand it, black holes are defined by three quantities: mass, spin, and charge.

I'm assuming that these quantities will be additive post-merger.

Edit: "The black holes appear to be spinning very rapidly—near the limit allowed by Einstein's theory of general relativity."

Perhaps the additive spin becomes asymptotic. Alternately, the gravitational waves might have departed with the energy of the excess spin.

My basic understanding is that they combine, basically you just add the masses together. That increased mass then means more gravity, so the event horizon is pushed further out.

They become a more massive one. The volume of a black hole (assuming you're measuring at the event horizon) is determined only by its mass, so the final density is the same as you'd get for any other black hole of that mass regardless of how it came to be.

I don't know how to address the "consume" question. If you were pulling on a piece of fabric and two tears in it grew until they met each other to become one tear... would you say that the larger one consumed the smaller?

They become a larger black hole, mostly conserving mass, minus a few percent to gravitational waves. However, their mass is proportional to their radius, not volume, so it gets LESS dense. If you laid out a bunch of black holes in a line, just barely not touching, and let them merge, suddenly, the whole sphere of space enclosing the line becomes black hole. It also turns out that a black hole with the mass of the universe would have a volume about the size of the universe.

> turns out that a black hole with the mass of the universe would have a volume about the size of the universe

Mass and energy.

>> just barely not touching

Which part of them is barely not touching?

The event horizons.

I think the GP meant “merge”

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.)

Up until the universe was around a few billion years old, its Schwarzchild radius would have been larger than even the co-moving (not just observable) universe's radius, but the initial momentum from the big bang was high enough to prevent collapse.

> The volume of a black hole (assuming you're measuring at the event horizon) is determined only by its mass, so the final density is the same as you'd get for any other black hole of that mass regardless of how it came to be.

Wait, really? So if you had a super massive disk that was just 1 electron away from having enough mass to become a black hole... and then an electron popped into existence due to quantum randomness... then it would become a sphere instantly? Wouldn't that violate the speed of light or something?

> minus a few percent to gravitational waves

They actually convert up to 42% of their mass into energy, mostly radiation

https://youtu.be/t-O-Qdh7VvQ

> the "consume" question

My guess is that in some popular depictions black holes are like holes, and things fall in the holes, and even a small black hole can possible fall inside a bigger hole.

A better image is too drops of water on a glass, add some black ink for bonus realism. They merge into a bigger drop. Except, obviously black holes are not filled with water. And the "average density" of the new black hole is smaller then the "average density" of both original black holes, unlike the density of water drops on a glass. So don't take this image too literaly.

(There are some problems to define the "density" of a black hole, but let's ignore all of them.)

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)

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.)

---

> 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.

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

Is that true though?

Can't we generalize to say that we observe that black holes have a similar density (which is to say, proportion of mass to volume) any sample of the observable universe sufficiently large as to be roughly uniform?

In other words, doesn't this observation scale both down (to parts of the universe) and up (beyond the cosmological horizon, presuming that the rough uniformity in density persists), at least for any universe measured in euclidian terms?

It's very possible that I'm wrong here, and I'd love to be corrected.

...I also think we have to acknowledge that "similarly" is doing a fair bit of work here, as we're not accounting for rate of expansion - is that correct?

> 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.

My understanding is they just spiral into each other forever.

From our point of view nothing can actually fall into a black hole, instead it time dilates into nothing. "It is true that objects that encounter the event horizon of a black hole would appear “frozen” in time"[1]

So we would never actually see the black holes merge. In fact I'm not clear how a black hole can even form in the first place, since it would take an infinite amount of time to do so (again, from our POV).

(And yes, I know that from the POV of the falling object, they just fall in like normal. But that doesn't help us, because we'll never see it.)

[1] https://public.nrao.edu/ask/does-an-observer-see-objects-fro...

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.

> and after a while it will be a nice sphere

Time doesn't exist for black holes, so "after a while" is not something you can say about them.

That sounds suspiciously like "they were inside a region with enough mass to form an event horizon, but they escaped because they had enough momentum", which in turn sounds like "we can escape from inside an event horizon if we just move fast enough". Can you explain how that's not what you're saying?

I wish I had a straightforward answer to that. I'm sure the answer is some combination of cosmic inflation and dark energy, but by all means it appears the early universe either narrowly escaped, or simply is a black hole, that singularities are a flawed concept, that nothing is escaping the universe, and we are all stuck moving forward in time, and that the infinite future is the singularity.

In terms of creating a row of black holes where the space between each black hole is small relative to the size of the event horizon of each.

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

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

This is true, but it's not an observable distinction. It's true that in some sense those two black holes "haven't yet collided", but at this point they're well past the point of last observability and have now red shifted and time dilated into the invisible background. All the interesting stuff happens before that.

The events horizon.

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

BTW, the php in /chat2 seems to be broken, if you didn't know already. Great simulation, too.

I don't have an answer either. But in my amateur opinion, of the available options, I lean toward "is a black hole". If all the mass we can see adds up to a black hole the size of what we can seen, then if you add all the stuff outside the light cone, it should add up to enough mass to make a black hole radius that includes the distance out to there.

But that leaves us with black holes forming inside a black hole, which I have absolutely no idea what to do with.

Mass alone doesn’t do it. You need energy, namely the CMB, to push the observable universe close to its Schwarzschild limits.

Simulations linked in others comment:

https://www.youtube.com/watch?v=Tr1zDVbSjTM

https://www.youtube.com/watch?v=1agm33iEAuo

Thanks, I was kinda curious why it wasn't full of spam. it's running on a super neglected rpi, really need to wipe it and spend a month refreshing myself on basic webhosting stuff

Long before your disc neared the mass where it would form an event horizon, the matter it's made of would collapse into neutron star material, which would form a sphere.

Perhaps if it were exceptionally wide the whole disc wouldn't collapse. Maybe only the parts near it's center. In that case you'd end up with a large ring around a neutron star. Add a bit more mass and maybe it's now a ring around a black hole. The gravity of the ring might distort the event horizon in some way, I'm not sure quite how, but probably its possible to get a non-spherical hole in situations where the objects distorting the shape are still in the universe.

But as for the matter lost into the hole, it's gone. If the hole were to retain some shape based on what's "inside" of it, that would be the kind of information leak that the laws of physics do not permit.

The second one is much more correct, notice how it freezes before they actually merge because they time dilate out to infinity.

The analogy I like goes something like this. Imagine you're paddling in a canoe on the river. You approach a waterfall. If you do nothing you'll get consumed by the waterfall. So you try to paddle away from the waterfall, but as you get closer to the edge of the waterfall, the current gets stronger.

The event horizon is the imaginary line across the river which once passed, even if you paddle as quickly as you can, you won't be able to get away from the waterfall. Once you pass that line, you're bound to reach the waterfall eventually.

Now, thanks to Maxwell and Einstein, we know there's a maximum speed that anyone can paddle, the speed of light, and so we define the event horizon to be relative to this speed.

You can calculate the event horizon for just about anything. The main difference between a black hole and everything else, is that for a black hole the event horizon is larger than the object itself.

For example, the event horizon of a neutron star with a mass of 1.4 solar masses and a radius of 10km is about 4.1 km, well inside the neutron star. Thus you don't get the "black hole effect", since once you pass the surface of the neutron star the matter above you pulls you away from the center.

The river analogy is actually not far off what they try to use as an analog for testing black hole predictions, effectively a large water tank with a drain hole. Sixty Symbols did a video on this way back[1], and this thesis[2] goes into the details. Some are going beyond water using liquid helium to simulate quantum black holes this way[3].

[1]: https://www.youtube.com/watch?v=kOnoYQchHFw

[2]: https://arxiv.org/abs/2009.02133

[3]: https://pirsa.org/25010083

If they actually froze just before the merge, they will be peanut shaped like in the video.

I'm not sure if we can measure the shape of black holes, but I'm sure everyone think they are spheres with a slight deformation due to rotation.

The algorithm choose this for me https://www.youtube.com/watch?v=fKgQYOlpxmo

Ohh, I see, you mean "mass" should have been "mass and energy" rather than e.g. that (mass,volume) should have been replaced by (mass,energy) or something.

I confess I just ... take it for granted in this kind of context that "mass" or "energy" or "mass+energy" all mean the same thing. Someone who wants to refer just to the total amount of matter will say something like "the total mass of the matter in the universe".

It's commonplace for physicists to write just "mass" when talking about this sort of thing. E.g.,

P T Landsberg, "Mass scales and the cosmological coincidences", Annalen der Physik, https://onlinelibrary.wiley.com/doi/10.1002/andp.19844960203:

"Theories involving the parameters h, c, G, H (in a usual notation) are considered. A huge ratio of 10^120 of the mass of the universe (m_u) to the smallest determinable mass m_0 in the period since the big bang occurs in such theories."

(Not cherry-picked; I went to the Wikipedia article on "Black hole cosmology", noted that it just says "mass" rather than "mass-energy" or whatever, and followed the link in the attached footnote. Also, so far as I know, not crankery; Landsberg was an eminent physicist.)

"The volume of a black hole" and how you define it?