What happens in the area between black holes before they collide

[tflahive]

TL;DR

The area between two black holes before they collide must be under a great deal of stress. That stress change should be measurable by the effect of radiation or light coming through that area.

Recently there have been a lot of studies of black holes colliding and the gravitational waves that they produce. My question is: What is the effect on the space between the two black holes before they collide. The stress must be extraordinary. That stress should be measurable by radiation, or light coming through that area.

 

[PeterDonis]

What is the effect on the space between the two black holes before they collide.

It's empty space, not very different from the empty space close to the horizon of a single black hole.

The stress must be extraordinary.

What stress are you talking about? What kind of stress do you think empty space can have?

 

[Ibix]

You could certainly calculate the expected path of light from a source beyond the black holes passing between them. It would have to be a numerical calculation, as no analytical solution is known for colliding black holes.

However, I don't think there's an awful lot of point. We aren't aware of any nearby black hole pairs in the late stages of an inspiral, so there's no way to test the results. The ones we've seen with LIGO are too far away, and we only find out about them as they happen - so no time to get a telescope pointed even if it could resolve anything. And, as Peter implies, I don't think anything particularly extraordinary would be expected to happen - light paths will be complicated, but no more than that. We could be wrong about that, I suppose, but the observational difficulties mean we probably can't test this particular scenario anyway.

 

[PeroK]

Summary:: The area between two black holes before they collide must be under a great deal of stress. That stress change should be measurable by the effect of radiation or light coming through that area.

Recently there have been a lot of studies of black holes colliding and the gravitational waves that they produce. My question is: What is the effect on the space between the two black holes before they collide. The stress must be extraordinary. That stress should be measurable by radiation, or light coming through that area.

Despite what a lot of popular science sources suggest, there is no fabric of space to be put under stress. Space itself is vacuum.

 

[Vanadium 50]

Right now you are between two black holes. Feel anything unusual?

 

[Lord Crc]

Right now you are between two black holes. Feel anything unusual?

The black holes I'm between are not about to collide anytime soon, how about you?

 

[Vanadium 50]

How soon is soon?

 

[256bits]

What is the effect on the space between the two black holes before they collide.

Wondering what type of black hole(s)?
Super massive black hole with a baseball size hole.
Not answering, just wondering.
If a proton happened by chance to be at the point of merger, which hole would it fall into first?

 

[Ibix]

Wondering what type of black hole(s)?
Super massive black hole with a baseball size hole.

As long as GR holds up, it doesn't make any difference. There's nothing unusual about spacetime near black holes aside from the degree of curvature. For a sufficiently small (microscopic) hole I think we do eventually have to worry about quantum gravity, but since we don't have a working theory of quantum gravity we can't say much about that.

If a proton happened by chance to be at the point of merger, which hole would it fall into first?

Once the holes touch they are one hole, so this question isn't really answerable. Black holes are vacuum solutions, so you can't really imagine painting one red and one blue and describing parts of the combined hole as having come from the red one or the blue one.

 

[256bits]

Once the holes touch they are one hole, so this question isn't really answerable. Black holes are vacuum solutions, so you can't really imagine painting one red and one blue and describing parts of the combined hole as having come from the red one or the blue one.

So no spagetification of one black hole to another.
Sorry. Had to throw that in.
It is one of the numerous questions of black holes.
Thanks.

 

[PeterDonis]

There's nothing unusual about spacetime near black holes aside from the degree of curvature.

And even that can be small if the holes are large enough.

 

[Ibix]

So no spagetification of one black hole to another.

Well, I'd expect each horizon to be deformed towards the other. It's just that once they touch you don't have two separate horizons, and they aren't made of matter so it's a bit difficult to find anything you can point to and say "this used to be part of the small/large hole" in any way that isn't completely arbitrary.

 

[PeterDonis]

Moderator's note: Thread level changed to "I".

 

[Dale]

The tidal forces would be quite high for ordinary black hole mergers in the range we have detected so far. That isn’t exactly an effect on space, but certainly on any matter in the area.

 

[timmdeeg]

Well, I'd expect each horizon to be deformed towards the other. It's just that once they touch you don't have two separate horizons, and they aren't made of matter so it's a bit difficult to find anything you can point to and say "this used to be part of the small/large hole" in any way that isn't completely arbitrary.

I think in the moment they touch the resulting black hole is highly deformed, is radiating violently gravitational waves away and that then there exists only one singularity compared to the moment before.

 

[PeterDonis]

in the moment they touch the resulting black hole is highly deformed

In the sense that the intersection of the horizon with a surface of constant "time" in the coordinates usually used is not spherical, but shaped more like a dumbbell, yes.

is radiating violently gravitational waves

Yes.

and that then there exists only one singularity compared to the moment before

No. There is only one singularity. As far as spacetime is concerned, there is only one horizon too; it is just shaped like a pair of trousers instead of like a cylinder (speaking heuristically). What we normally describe as two black holes merging is just the legs of the trousers coming together; but in spacetime, the trousers are still one surface, not two.

 

[Dale]

it is just shaped like a pair of trousers instead of like a cylinder (speaking heuristically). What we normally describe as two black holes merging is just the legs of the trousers coming together

With the legs of the trousers being tightly twisted (but not touching each other) near the crotch.

 

[PeterDonis]

With the legs of the trousers being tightly twisted near the crotch.

Yes.

 

[timmdeeg]

No. There is only one singularity. As far as spacetime is concerned, there is only one horizon too; it is just shaped like a pair of trousers instead of like a cylinder (speaking heuristically). What we normally describe as two black holes merging is just the legs of the trousers coming together; but in spacetime, the trousers are still one surface, not two.

My remark refers to the moment "before they touch". Do you describe this moment?

 

[PeterDonis]

My remark refers to the moment "before they touch". Do you describe this moment?

I described the 4-dimensional spacetime geometry. There are no "moments" in what I described; I described the global properties of the geometry. The fact that there is only one horizon and one singularity is a global property of the geometry.

 

[Ibix]

I described the 4-dimensional spacetime geometry. There are no "moments" in what I described; I described the global properties of the geometry. The fact that there is only one horizon and one singularity is a global property of the geometry.

Can one draw a Penrose diagram, or something analogous to a Kruskal diagram, for colliding black holes?

 

[timmdeeg]

I described the 4-dimensional spacetime geometry. There are no "moments" in what I described; I described the global properties of the geometry. The fact that there is only one horizon and one singularity is a global property of the geometry.

I have been interpreting the trousers such that there is this "moment" and that there exist two separate black holes before this moment each with its singularity. But this is a misunderstanding if I understand you correctly.

 

[Ibix]

Can one draw a Penrose diagram, or something analogous to a Kruskal diagram, for colliding black holes?

Answering my own question, @bcrowell thinks not - https://physics.stackexchange.com/questions/445393/penrose-diagram-for-two-black-holes.

 

[PeterDonis]

Can one draw a Penrose diagram, or something analogous to a Kruskal diagram, for colliding black holes?

I have not seen one. The problem is that these diagrams require spherical symmetry so that two of the four dimensions can be suppressed. A spacetime with two black holes in it is not spherically symmetric. It is not even axially symmetric; in the general case it has no Killing vector fields at all, at least not globally (there may be approximate axial and timelike KVFs in the far future).

 

[PeterDonis]

I have been interpreting the trousers such that there is this "moment" and that there exist two separate black holes before this moment each with its singularity. But this is a misunderstanding if I understand you correctly.

It's not a misunderstanding so much as a use of vague ordinary language. What does "two black holes" mean?

If it means "the intersection of the horizon with a surface of constant time in my chosen coordinates is two disjoint 2-surfaces for times before some particular time in my coordinates", then in that sense, you can say there are two black holes before a certain time.

But if it means "there are two disjoint regions of spacetime which can't send light signals to infinity", then in that sense it is simply false; there is only one such region, and it is the one I described, shaped like a pair of trousers.

If you are looking at singularities, the only meaning of "black hole" that is relevant is the second one--singularities are "attached" to regions of spacetime, not coordinate choices. So there is no valid viewpoint according to which there are two singularities before some particular time and one after. There is just one.

 

[timmdeeg]

If you are looking at singularities, the only meaning of "black hole" that is relevant is the second one--singularities are "attached" to regions of spacetime, not coordinate choices. So there is no valid viewpoint according to which there are two singularities before some particular time and one after. There is just one.

Thanks that gave me a new insight.

 

[kimbyd]

Summary:: The area between two black holes before they collide must be under a great deal of stress. That stress change should be measurable by the effect of radiation or light coming through that area.

Recently there have been a lot of studies of black holes colliding and the gravitational waves that they produce. My question is: What is the effect on the space between the two black holes before they collide. The stress must be extraordinary. That stress should be measurable by radiation, or light coming through that area.

If the two black holes are colliding without matter nearby, no, there won't be any visible radiation at all. However, if there is matter there, it's definitely conceivable that there will be some interesting effects present. My thought, however, is that the space between two black holes would be less curved rather than more, because the gravity of the two black holes would partially cancel in that regime.

It definitely is an interesting question to me what would happen if two black holes with active accretion disks were to collide, and the answer is sure to be rather difficult to determine in precise terms due to the complexity of the systems. But interesting nonetheless.

 

[PeterDonis]

My thought, however, is that the space between two black holes would be less curved rather than more, because the gravity of the two black holes would partially cancel in that regime.

The "acceleration due to gravity" due to the two holes might partially cancel, but that is not spacetime curvature. Spacetime curvature will, if anything, be larger in between two black holes than it would be due to either hole individually. But really spacetime curvature is a tensor and doesn't have a single "size".

 

[timmdeeg]

The "acceleration due to gravity" due to the two holes might partially cancel, but that is not spacetime curvature. Spacetime curvature will, if anything, be larger in between two black holes than it would be due to either hole individually. But really spacetime curvature is a tensor and doesn't have a single "size".

As the SET is identical zero in this scenario do we talk about Weyl curvature then? And would a ball of test particles increasingly be distorted such that the long axis of the ellipsoid coincides with the straight line connecting the two black holes?

 

[PeterDonis]

As the SET is identical zero in this scenario do we talk about Weyl curvature then?

Yes.

would a ball of test particles increasingly be distorted such that the long axis of the ellipsoid coincides with the straight line connecting the two black holes?

To a first approximation, yes, at least for the case where the holes were large enough and far enough apart that linearly superposing the curvatures from the two holes was a good first approximation. But no exact solution is known for this case, and the EFE is nonlinear so as the holes get closer the actual curvature is more and more poorly approximated by a linear superposition of the individual solutions for each hole.