
Astronomers claim to have seen the merger of two black holes within a third - Hooke
https://www.nytimes.com/2020/06/25/science/black-hole-collision-ligo.html
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fjfaase
It is not about three black holes colliding at the same moment, but about a
three massive black holes in three colliding galaxies that one day, in the far
future, probably will collide and about a possible effect that two black holes
at a close distance might come together sooner in the presence of a third
black hole in the vicinity. So, not even discussing the possibility that they
may merge at the same moment.

~~~
pmiller2
> ...at the same moment.

As an aside, this is a rather funny phrase to be using about objects so
massive that time and space actually switch roles inside their event horizons
compared to the normal universe.

I don't feel like I can explain it very well, but here is a video that does
so:
[https://www.youtube.com/watch?v=KePNhUJ2reI](https://www.youtube.com/watch?v=KePNhUJ2reI)
(Skip to about 5:15 for the good stuff.)

~~~
pdonis
_> time and space actually switch roles inside their event horizons_

No, they don't. This is a common misconception based on a mathematical
artifact of one particular coordinate system.

 _> here is a video that does so_

This video is actually quite misleading, because in the diagram it shows of a
black hole at around 7 minutes ish, it is using coordinates in which the
artifact I mentioned above does _not_ occur, but then mislabels the
coordinates in the part of the diagram inside the horizon to make it seem as
if it does.

~~~
pmiller2
My understanding of the explanation and the video was that they don't
literally switch roles, in the sense that you could then travel backward in
time, or couldn't alter your position with respect to the singularity. It's
more that, in terms of light cones, inside the event horizon, you can move in
space in such a way encountering photons from the past, which is somewhat
_like_ travelling back in time. So, in a sense, the "arrow of time" doesn't
work inside the black hole. But, since all world lines do eventually end at
the singularity, which basically means that no matter what you do, you're
always moving toward it, space becomes time-like in the sense that there's
then an "arrow of space," analogous to the "arrow of time."

I found the mathematical explanation of sign flipping to be fairly easy to
understand. One thing in particular that equation makes clear is that at the
instant you cross the event horizon, whatever metric you're using is going to
become degenerate. That makes some sense, because you're literally crossing
over into a different realm there, but, it doesn't seem physical to me, unless
it's not actually physically possible for matter to literally fall _into_ a
black hole; rather that matter we perceive as falling into it gets smeared out
across the event horizon, its information being "painted on" to the event
horizon. (This reminds me of the "black hole information paradox," but I'm not
sure exactly how it all plays into it. For small enough holes and small enough
particles, I would guess there is some interaction between the particle
approaching the event horizon and Hawking radiation pushing back on it, kind
of like solar wind does. But, that's pure speculation.)

Physically, the thing that made the most sense to me was that of the 4
spacetime dimensions, outside of a black hole, the one we have designated as
"time" is so designated because it doesn't behave like the other 3, because we
can only travel in one direction along it. Inside the event horizon, it's
clear that one spatial dimension corresponding to the vector extending outward
from the singularity to your position acquires this property.

Because they only did the example of a Schwartzchild black hole, I'm sure this
all goes out the window in case of a rotating black hole, in which you no
longer have a point singularity, but a ring singularity [0].

Since I have never specifically studied GR, all my actual black hole knowledge
comes from intuition derived from a differential topology course. But, that
intuition is telling me that something similar happens in this general case,
except that space never actually becomes time-like. World lines do all still
end at some point on the singularity, and you can still get weirdness
encountering photons you might think you couldn't, so, my physics layman's
intuition is telling me what you have is basically all 4 dimensions acquiring
a similar character. That is, spatial dimensions start becoming somewhat time-
like, and time starts becoming somewhat space-like.

Differential topology gave me a really crude understanding of what goes on
here, but, I do understand that black holes are fundamentally simple objects.
They only have three properties: mass, charge, and angular momentum; so, you
should be able to describe them fully from any reference frame outside the
event horizon using those 3 variables plus the spatiotemporal location of the
hole.

At that point, if you have a universe with nothing in it but your one, lonely
black hole, you have a static universe. If you introduce matter, you could
theoretically model interactions between all the particles of matter in your
toy universe, and the black hole. If you constrain the number of particles
enough, you should be able to at least approximate how this idealized universe
evolves.

Is this a fair interpretation, or am I way off base?

~~~
pdonis
_> black holes are fundamentally simple objects. They only have three
properties: mass, charge, and angular momentum; so, you should be able to
describe them fully from any reference frame outside the event horizon using
those 3 variables plus the spatiotemporal location of the hole._

Yes, this is correct.

 _> At that point, if you have a universe with nothing in it but your one,
lonely black hole, you have a static universe._

Yes. (More precisely, it's static outside the hole's horizon.)

 _> If you introduce matter, you could theoretically model interactions
between all the particles of matter in your toy universe, and the black hole.
If you constrain the number of particles enough, you should be able to at
least approximate how this idealized universe evolves._

Approximate, yes. But there are no exact solutions known for such scenarios in
GR.

(Also, a minor point, GR models matter as continuous--"point particles" don't
really exist in GR except as "test objects" that can move within the spacetime
geometry but do not act as sources for the curvature of the geometry.
Continuous matter is described by the stress-energy tensor.)

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mabbo
> an invisible collision of invisible objects — black holes — had become
> briefly visible

> it would mark the first time that colliding black holes have produced light
> as well as gravitational waves

Can someone with a better understanding of astrophysics dumb this down for me
a bit? Black holes, by definition, don't let light escape. That's their
_thing_. So how did this collision produce light visible outside of them? I
mean, are we talking about a naked singularity? (Someone let Joseph Cooper
know).

Or is it just that something nearby the collision but outside of the event
horizon started producing large amounts of light?

~~~
sandworm101
Black holes have gravity. That gravity means things orbit them like a star.
(Dust, rock, gas, bits of planets etc.) When two black holes merge that
orbiting stuff collides really really fast. The light comes from that
collision and subsequent heating, not from the actual black holes.

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chrispeel
The word "claim" seems unduly negative. Why not use "report"? For example
"Astronomers report merger ..." Also, the title also seems to imply that the
two black holes are within a third black hole; wtf? I suggest as title
"Astronomers report merger of two black holes as the pair orbited a third
black hole".

Of course this is far from the title and focus of the paper, which is about
the EM counterpart...

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mesozoic
Either the title is incorrect or they're making the much greater claim that
they've retrieved information that escaped a black hole.

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k0t0n0
Wondering what is so special about black holes?

