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When Black Holes Collide

Explanation: What happens when two black holes collide? This extreme scenario likely occurs in the centers of some merging galaxies and multiple star systems. The featured video shows a computer animation of the final stages of such a merger, while highlighting the gravitational lensing effects that would appear on a background starfield. The black regions indicate the event horizons of the dynamic duo, while a surrounding ring of shifting background stars indicates the position of their combined Einstein ring. All background stars not only have images visible outside of this Einstein ring, but also have one or more companion images visible on the inside. Eventually the two black holes coalesce. The end stages of such a merger may provide a strong and predictable blast of gravitational radiation, a much sought after form of radiation different than light that has never yet been directly observed.

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If our understanding of how gravity operates is not yet complete, nay still in its infancy even,
is searching for 'gravitational radiation' a search for something that doesn't exist ?

If it were that simple, we should be able to detect such things right under our very feet ??
Or under the proverbial apple on an apple tree....

A study of a few years ago of Jupiter showed that gravitational effects travel at c. Near a black hole c is approaching 0 to an outside observer so any gravitational radiation caused by the coalescing will be observed at the same time as the event horizon - ie never.

I was surprized to see the Einstein ring keeping turning even after the complete absorption of the smaller black hole. I would tend to think that, given that (as madtom pointed out) gravitational information travels at c, once no perturbation is noticeable in the remaining event horizon (i.e the smaller black hole entirely went past it), the gravitational lens would be static again.
Otherwise, wouldn't that mean that one can have an image of what is happening behind the horizon of events by looking closely at the gravitational lens (and in particular at the Einstein ring)?

Madtom: as far as I understand, relativity indicates that c is constant, regardless of the observer. At the event horizon, that's not c that plumets, but rather c being lower than the escape velocity of the black hole. I'd say that on an external observer's point of view, if information is sent outside the event horizon, it travels at c (about 299 792 000 m/s). The size of a meter may appear different to an external observer than to the observer that is near the event horizon, but that wouldn't change the value of c to anyone.
But again, maybe I'm wrong.

Scary all encompassing malefic yet so elegant

What would the actual time be for that to happen in real time?

RocketRon wrote:If our understanding of how gravity operates is not yet complete, nay still in its infancy even,
is searching for 'gravitational radiation' a search for something that doesn't exist ?

While it's likely our understanding of gravity is not complete, it is hardly in its infancy. Our understanding is deep and almost certainly close to complete. Our theory has been tested on numerous fronts and holds up extremely well. It is this theory that predicts gravitational waves, and the existence of those waves has been confirmed indirectly through other tests of gravitational theory. You would be hard pressed to find many physicists who harbor much doubt that gravitational waves exist or that they won't soon be observed.

If it were that simple, we should be able to detect such things right under our very feet ?

Because the technology required to make such a detection is complex and only recently developed. Just because a phenomenon is ubiquitous doesn't guarantee it is easy to directly observe.

azman45 wrote:What would the actual time be for that to happen in real time?

I believe that we should be capable of detecting waves from the final orbits over a few minutes, with the actual merger taking just a fraction of a second.

madtom1999 wrote:Near a black hole c is approaching 0 to an outside observer so any gravitational radiation caused by the coalescing will be observed at the same time as the event horizon - ie never.

The local speed of light is always c. What does change is the wavelength of the emitted light. The closer the emitted light is to the event horizon, the more energy it needs to spend to escape the gravitational pull. As a result, the wavelength get shifted towards red. Also, to an outside observer it appears that time runs more slowly closer to the event horizon.

RocketRon wrote:[...] is searching for 'gravitational radiation' a search for something that doesn't exist ?

While gravitational waves have not been detected directly, their is strong indirect evidence for their existence. The orbits of binary pulsars decay exactly as predicted by general relativity. See here for more.

a black hole must be the big bang in reverse

It is amazing to see something like this simulated! Wouldn't a merger of two (or maybe more?!) black holes generate a lot of x-rays?

star surfer wrote:Wouldn't a merger of two (or maybe more?!) black holes generate a lot of x-rays?

The merger itself not. But black holes are usually surrounded by matter that heats up due to collisions as it circles the black hole. This matter is a powerful source of powerful x-ray and gamma radiation.

SergeiPoulp wrote:I was surprized to see the Einstein ring keeping turning even after the complete absorption of the smaller black hole. I would tend to think that, given that (as madtom pointed out) gravitational information travels at c, once no perturbation is noticeable in the remaining event horizon (i.e the smaller black hole entirely went past it), the gravitational lens would be static again.

I am not sure, but given that gravitational waves travel at the speed of light, the information that the black holes have merged still takes some time to reach the outer parts. Maybe this is why the rings keeps turning after we see the merger as complete.

So it is still a singularity surrounded by a lot of stuff still taking up space ?

Isn't it claimed that black holes are infinitesmially small points? If so how can they actually merge being zero radius? Maybe a merge just means they fall within their event horizons.

ta152h0 wrote:So it is still a singularity surrounded by a lot of stuff still taking up space ?

It's still an ordinary black hole. No different from its parents, except that it has their combined mass (less some loss to gravitational radiation). So like any black hole, it is a (presumed) singularity surrounded by a poorly understood region bounded by an event horizon.

Huh, like two drops of oil in water....


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Among the galaxies observed to have a large central mass, and hence thought to have a supermassive central black hole, what would be the largest mass and the largest radius for an event horizon? What would be the radius of its Einstein ring?

A feature of the simulation (which is no doubt done to improve the rendering of what interested the simulators), is that there is nothing giving off light in between the holes and the observer. In general, such light sources are going to make it impossible to ever observe what was rendered here. I'm wondering about that. I believe the view held by the scientific community is that there are lots of black holes in the universe, but we have never had a good chance to view one. The Kepler mission had no trouble finding lots of planets transiting in front of their stars. Have we never seen a black hole transiting a star?

MarkBour wrote:Among the galaxies observed to have a large central mass, and hence thought to have a supermassive central black hole, what would be the largest mass and the largest radius for an event horizon? What would be the radius of its Einstein ring?

A feature of the simulation (which is no doubt done to improve the rendering of what interested the simulators), is that there is nothing giving off light in between the holes and the observer. In general, such light sources are going to make it impossible to ever observe what was rendered here. I'm wondering about that. I believe the view held by the scientific community is that there are lots of black holes in the universe, but we have never had a good chance to view one. The Kepler mission had no trouble finding lots of planets transiting in front of their stars. Have we never seen a black hole transiting a star?

A black hole is only a few kilometers across. That's way too small for a transit to be detected photometrically with our current technology.

Could someone tell me what that extremely loud noise was at the end of the video?

What I found rather interesting were the stages that the smaller black hole went through in it's death spiral. At a couple points it seemed to temporarily split into two separate black holes. Trying to mind experiment this but I am falling short of the manifestation of a third gravity well.

OB1Kubota wrote:Could someone tell me what that extremely loud noise was at the end of the video?

I didn't hear anything at all.

who said science is not funny ?

RocketRon wrote:
If our understanding of how gravity operates is not yet complete, nay still in its infancy even,
is searching for 'gravitational radiation' a search for something that doesn't exist ?

Chris Peterson wrote:
While it's likely our understanding of gravity is not complete, it is hardly in its infancy. Our understanding is deep and almost certainly close to complete. Our theory has been tested on numerous fronts and holds up extremely well. It is this theory that predicts gravitational waves, and the existence of those waves has been confirmed indirectly through other tests of gravitational theory. You would be hard pressed to find many physicists who harbor much doubt that gravitational waves exist or that they won't soon be observed.

If the recent realizations of the existence of dark matter and dark energy and the acceleration of the universe are anything to go by,
then a full understanding of HOW gravity operates is only very much in its infancy ?!

If we asked a conference of physicists and astronomers if gravity was a push or a pull, what split of the audience would we see. ?

If we go to the science bookstore and ask for a book on the finer details of precisely how gravity operates,
what choice of books do we have ? What science papers or journals spell it out in precise detail. ??

A Mr Einstein gave some very advanced (for the time) explanations of the 'what', but the 'how' is as yet still totally unexplained.
Still very much in its infancy ...