
LIGO Black hole merger may have optical confirmation - throwaway2048
https://www.caltech.edu/about/news/black-hole-collision-may-have-exploded-light
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wcoenen
> _Once the black holes merge, the new, now-larger black hole experiences a
> kick that sends it off in a random direction, and it plows through the gas
> in the disk_

Hm, I would expect the merged object to move in the same way as the center of
mass of the unmerged objects. Where does the extra momentum for this "kick"
come from? Is there mass from the surrounding disk thrown in the other
direction? Or maybe some relativistic effect?

edit: apparently gravitational waves can carry away linear momentum, and
that's where the kick comes from[1].

[1][https://astrobites.org/2018/03/08/recoil-detectives-
searchin...](https://astrobites.org/2018/03/08/recoil-detectives-searching-
for-black-hole-kicks-using-gravitational-waves/)

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number6
How strange that "Seeing is believing" still holds even with gravitational
waves, which aren't electromagnetic phenomena. Human nature :)

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willis936
Two point verification always has value. It’s how every instrument we’ve ever
made is calibrated.

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number6
Can gravitational waves be verified by optical signals? I thought the whole
point of LIGO is that it's a method not relying on electromagnetic signals

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willis936
Yes. In fact that this is the exact (very exciting and somewhat recent)
experiment that proved that if gravity does propagate in higher dimensions,
then the dimensions must be very thin.

[https://youtu.be/3HYw6vPR9qU](https://youtu.be/3HYw6vPR9qU)

Edit: To be clear: the experiment I’m referring to was the observation of two
neutron stars merging. Since there was no event horizon prior to the merger,
we’d expect a lot of EM energy to be radiated away. This new observation is
neat because we’d expect much less EM energy out of a black hole merger.

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cletus
I don’t understand the surprise or confusion here. Yes, light can’t escape
black holes. But black holes are _systems_ And they typically would have
accretion disks. Those very much could emit light (and no doubt a ton of
X-rays and/or gamma rays) when the black holes “collide” (as violent and
energetic as these events are, I think “merge” is still probably more
accurate).

The magnitude of these events is simply mind-boggling. I think the first such
LIGO detection has an estimated 3-5 _Solar masses_ converted to energy.

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volodymyrs
the surprise is that to make an accretion disk which would make any detectable
emission, there had to be a lot of matter around a black hole.

the accretion disks are commonly seen around binaries consisting of a stellar
mass black hole and a regular star. stars feeds the disk in this case.

another possibility is to have a supermassive back hole in the galaxy center,
which feeds on the surrounding gas, making much larger and slowed disk.

what has never been seen before a stellar mass black hole (or pair of black
holes) feeding on the interstellar medium. This should happen of course, but
the expected luminosity is way below anything detectable.

binary black hole, i.e. a system without any addition mass to accrete, would
have to be in some very unusually dense medium, and the medium would need to
be disrupted by merger.

interestingly, such medium can occur around another back hole - a supermassive
one.

