Apparently from 1 AU you might just be able to hear it! Even that close the stretching and squishing would be on the order of micrometers but the eardrum might be able to pick it up. The frequency would be comfortably in the audible range as well. [1]
Huh. IIRC, black hole mergers are also a likely source of short gamma ray bursts. So I suspect at 1 AU, one would need a lot of shielding to not get fried by the gamma rays.
EDIT: What I meant to say was: It would probably be quite awesome to experience such an event up close, provided it is possible to do so safely. ;-)
I believe that black hole mergers are dark in the entire electromagnetic (EM) spectrum. This is why the recent binary neutron star merger was so groundbreaking -- it emitted both gravitational waves and a broad spectrum of EM waves.
Indeed, though, it would be awesome to witness the collision and resulting gwaves personally, if such a thing was possible :)
Nah I think there are some effects in the visible range. You probably have some kind of accretion disk that gets shaken up by the merger, there might also be some effects due to Hawking radiation
But they're probably only visible from close range (like the 1AU mentioned above) and are too faint to be seen from 1Bi light years like the event that they captured
AFAIK, only supermassive black holes at the center of galaxies have disks of material that is falling inward (and emitting significant amounts of light in the process). Even then, they only actively feed in that way for a short period of time -- I think something like 10k years.
All of the LIGO observations have been of more basic stellar mass black holes merging together.
> AFAIK, only supermassive black holes at the center of galaxies have disks of material that is falling inward
Stellar binaries are extremely common, and there is a reasonably large supply of binarys where one star has become a compact object. Their companion stars often drop lots of matter onto them, resulting in a reasonable supply of black holes. Diskoseismologists and others working on Swift have catalogued hundreds of stellar mass black hole accretion disks.
Swift also spotted ASASSN-14li which was a star being shredded by an SMBH and forming an early accretion structure. The event has been followed up by other observatories (notably Chandra and the European very long baseline interferometry network). ASASSN-14li is an easy google search term (the trick is knowing the term in the first place :-) ), hopefully you will enjoy some of the hits. :-)
Ah, interesting. That makes a lot of sense. Would it be correct to say that if both objects in a binary pair are SMBHs, they would very likely not have an accretion disk, as the companion would be unable to send over any material?
That doesn't follow from the fact that black holes are "black". For example, black holes can emit a ton of radiation from accretion disk as matter accelerates and falls inwards, radiating huge amounts of energy [0]. Two naked black holes merging probably wouldn't emit much (if any) EM radiation, but if they had very active accretion disks, then it's certainly possible there would be a ton of EM activity.
Yes. I had not thought of that. But if either of the black holes (or both) had an accretion disk at the time of the collision, they would have been "visible" before.
I am not sure, though, if regular (stellar-mass) black holes with an accretion disk emit enough EM radiation to be visible at such distances, or if it would become lost among the radiation emitted by the rest of the galaxy.
(A merger between two supermassive black holes with active accretion disks must be a spectacular sight even from a safe distance.)
Gamma-ray bursts were believed to be due to mergers of two neutron stars, not black holes. LIGO's observation of a NS-NS merger coincident with a gamma-ray burst has strongly confirmed this picture.
I see. In the back of my mind there was still the possibility that it could have been created by two black holes merging or a neutron star falling into a black hole.
... I wonder, if a neutron star and a black hole merge, does the neutron star get shredded early enough to form an accretion disk, or does it just disappear like a marble falling into a hole?
For some reason my imagination is quite lively today, these questions just keep popping up in my head like banner ads. ;-)
Kinda. In essence you could turn yourself (in a space suit with a thruster pack, for example) into a human Cavendish experiment, with inspiralling stellar black holes as the suspended weights, the red "m"s in this diagram:
You'd effectively turn yourself into one of the grey "M"s and record the tugs and jolts you feel as you attempt to keep stationary (with respect to distant stars) above the inspirallers.
If you try to keep a fixed orientation and appparent (to you) distance between your navel and a distant galaxy, you will be pretty busy with your rocket pack if you are fairly close to the rotating system (the period of the tug you feel is driven by the orbital period, which in turn determines the frequency of gravitational waves).
Other observers are generally unlikely to agree with you about your navel-to-galaxy distance and orientation among other things (e.g. close in you may have a unique idea of the orbital period for sufficiently massive black holes), but General Relativity lets one be solipsistic if one wants. :-)
Now continue to imagine the red "m"s as black holes and the point at which the torsion wire connects the bar approximately corresponds to the centre-of-mass of the system. That's not quite right, but you can imagine that there is an invisibly thin bar -- or better still a slowly contracting spring -- connecting the two black holes, and that an imaginary torsion wire or pole could be kept perpendicular to that connection, and that you could float at the point the torsion wire connects to the bar. Your jetpack would not be very busy in that case, at least not until the black holes were almost in contact.
Finally, there's a gotcha here. The linearized gravity formalism that is used to study gravitational waves is only reliable (or even sensible) in the far field, which is no closer than some tens of wavelengths from the rotating system. The gravitational radiation (strictly speaking, the change in the metric under a particular splitting of spacetime into 3+1 space and time) propagates as a massless wave, so goes at the speed of light. So unfortunately near the end of the inspiral, if you are close enough to notice a relatively high frequency periodic tug, you also are also very likely in the near field limit, and have to do some exceptionally tricky solutions of the full field equations with all their glorious hyperbolic-elliptic nonlinearities in order to make robust predictions about your experience.
(Lots of theorists would love you to jot down your observations in great detail, though; we can figure out an approximate solution if you ever return. :-) )
EDIT: What I meant to say was: It would probably be quite awesome to experience such an event up close, provided it is possible to do so safely. ;-)