LIGO Black hole merger may have optical confirmation | Hacker News Reader

LIGO Black hole merger may have optical confirmation

caltech.edu

93 pointsthrowaway204810 comments

> 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...

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

The LIGO concept was thought to be impossible for decades because of how small the signals are, and some would say it's still incredible that it works at all.

Whereas we've had a lot of success with optical detection and they're inexpensive experiments.

Also, there's a few naysayers continually making public comments, so it's good to have something else to point to for confirmation.

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

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

Depending on the source, we may or may not expect photon's to accompany the gravity waves. The most note worthy one so far was a gravity wave signal that we associate with a neutron star merger[1] that we also managed to catch a lot of photon signals which also matched what we think should come out of such a collision. Most black hole mergers that I know of would be dark. I imagine if they had a large accretion disks associated with them they might generate some light as well as gravity waves? But I'm not sure, I get most of my astronomy over lunch with colleagues.

[1] https://www.sciencemag.org/news/2017/10/merging-neutron-star...

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

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.

I recommend this video of the press conference from the neutron star merger discovery.

https://www.youtube.com/watch?v=mtLPKYl4AHs

Its an amazing example of collaboration where it stars with gravitational waves, that helped locate it and quickly half the world of astronomy was looking at it on every wave length. Its an unbelievable example of cooperation and using different observatories to look at the same event.

It also shows how LIGO can be used to find events, and then others can point there to get more information.

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.

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.

Reply on news.ycombinator.com