Merger of two black holes (video)

[jack action]

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Text with the video:

The LIGO interferometer detected the merger of two black holes, each several dozen times the mass of our Sun. The video is very slowed down, because in reality everything only lasts a third of a second. To understand:

• The visible black regions are the “shadows” of black holes, from which no light escapes.
• The background (stars, gas, dust) is distorted by the curvature of space-time. This is the phenomenon of “gravitational lens”: light rays follow curved trajectories.
• As black holes merge, the warping of space-time changes the light patterns dramatically.
• The luminous ring (known as the “Einstein ring”) is formed by the light of a few stars directly behind. Light rays are “stretched” and “coiled” by the gigantic gravity of black holes. The black hole is the end of times.

Salukes

PS- Simulation carried out using Simulating eXtreme Spacetimes (SXS) Project: www.black-holes.org.

 

[Halc]

Excellent and informative video. I've seen similar simulations of the PoV of an observer falling into a very large black hole, the very interesting thing being that there is nothing weird that occurs at the event horizon. If the narrator doesn't tell you when it happens, you'd never know.

The visible black regions are the “shadows” of black holes, from which no light escapes.

So the first question I have involves this shadow. The black holes I think would be considerably smaller than these shadows depicted in the video. I'll use ray-tracing terminology (which is probably how the video was produced) but in reality light moves the other way:

Light moving towards the black hole will be absorbed. Light that crosses the photon sphere will be absorbed, so the shadow is at minimum 1.5 times 1r, the Schwarzschild radius. But the shadow is larger than even that, perhaps nearly 1.8r (wild guess), since any light heading that way will be bent enough as it approaches that it will cross that photon sphere.


2nd question:
Right at the end of the video, the merger is complete. Almost all of the background settles down and the stars stay put, except for a ring nearly at the bottom of the frame (top too, but it's out of the frame). The stars there continue to shift uncomfortably. There is still changes to the shape of spacetime going on at this time, and for some reason the changes result in chaotic light trajectories just at that radius. What's going on there?

 

[Ibix]

Light moving towards the black hole will be absorbed. Light that crosses the photon sphere will be absorbed, so the shadow is at minimum 1.5 times 1r, the Schwarzschild radius. But the shadow is larger than even that, perhaps nearly 1.8r (wild guess), since any light heading that way will be bent enough as it approaches that it will cross that photon sphere.

$\sqrt{27/4}R_S$, according to this post. (Edit: although that's for a Schwarzschild black hole, and these will be approximately Kerr.)

Right at the end of the video

Does anyone know how to view the video on mobile? All I see is the Facebook SDK moaning about tracking protection.

 

[jack action]

Does anyone know how to view the video on mobile? All I see is the Facebook SDK moaning about tracking protection.

www.facebook.com/reel/959491922268900

 

[Ibix]

What's going on there?

Not sure what that ringing is. My first thought is an artefact, perhaps if they used a simpler model of spacetime outside some radius. You might see funny dynamic effects at the join. Will have a think.

 

[Halc]

Not sure what that ringing is. My first thought is an artefact, perhaps if they used a simpler model of spacetime outside some radius. You might see funny dynamic effects at the join. Will have a think.

Not a model change it seems. The ring (sort of elliptical at first) is there the whole time and I think the merger was placed off-center deliberately to show a portion of it. The stars appear to move in opposite directions on either side of it. The stuff right at that 'ring' appears to move the fastest, counterclockwise if not just swinging wildly back and forth.
Something is visually singular there, like a boundary where the curvature inside reflects light back (letting you see stars in the opposite direction) and those outside are stars in the distance. The reflection ones would undulate the other way perhaps, being a mirror image of sorts. Right at the boundary is chaotic: a tiny change in the field directs light from a completely different direction.

Just guessing here. The action after the merge implies that it takes time still for the field to settle down. The 3D model of a merger (2D coordinate space, 1D coordinate time) is likened to a pair of trousers with twisted legs, all one event horizon that looks like two to an observer before the merge, but it's actually the same horizon, just as the EH of Sgr-A is shared with that of the BH in Andromeda. Anyway, point is, the trousers is not round immediately after the merge, and perhaps only approaches roundness after a time. The new BH of course is going to have a lot of angular momentum after such a merge.

 

[jedishrfu]

One intriguing fact not mentioned but presented in Kip Thorne's book, The Science of Interstellar, is that light not only bends when it travels around a black hole, but it may also complete numerous orbits before being ejected into space.

 

[vela]

I think this is the same video on YouTube for those who prefer to avoid interacting with Facebook.