Can we see 2 black holes orbiting each other?

[HansH]

TL;DR Summary

as I understand time stops at the event horizon of a black hole for the far away observer; so can we actually see 2 black holes orbiting eachother?

as I understand time stops at the event horizon of a black hole for the far away observer; so can we actually see 2 black holes orbiting eachother? I also understood that the singularity is a moment in future. so what happens when 2 black holes orbit eachother with the singularites? from the outside world you could point to a kind of geometrical position where each singularity woud be so this would mean that also the 2 singularities would orbit around eachother. but for the actual singularity I also understood tat you cannot speak of a position as function of time. So how can that orbiting then be understood from both outside and inside the black hole?

 

[Ibix]

"Time stops at the event horizon" is about as meaningful as "distance stops at the North Pole", and is based on the same failure to distinguish between "coordinates do strange things there" and "there's actually something unusual there". I'd strongly suggest forgetting you ever heard it.

You can't actually see black holes, of course, because they don't emit light. But you can see hot gas falling in and see them occluding stars.

The singularities in black holes are not points. They are more like points in time and they lie in the future if everywhere inside the black hole. Thus when two black holes merge there only ever was one singularity, it just has an event horizon that looks like a pair of trousers with the legs twisted round each other. It only looks like there are two black holes, rather than one with a complicated event horizon if you forgot about the timelike extent of the horizons.

 

[HansH]

That sounds stil strange to me for the following reason:
If there only ever was one singularity then it was always already known that they would merge. But how can that be? because it could well be that the 2 black holes originally were stars and even before that, theys could have been gas clouds so there was no singularity then. so if that is true it sounds to me that the whole universe is really a movie that cannot be influenced as it should be on beforehand clear that some combination of 2 gascloulds will end on one merged black hole with 1 singularity?

 

[phinds]

He said

when two black holes merge there only ever was one singularity

He's not talking about pre-merger

 

[PeterDonis]

as I understand time stops at the event horizon of a black hole for the far away observer

You understand incorrectly. A correct understanding would be that we, as far away observers, cannot see light emitted at or inside the horizon. But that's not the same as "time stops".

can we actually see 2 black holes orbiting eachother?

We can't see light coming from inside their horizons. But we can see plenty of optical effects (like blocking out or lensing of light from stars that are behind the holes) that tell us that there are 2 black holes orbiting each other.

I also understood that the singularity is a moment in future.

In the future of all events inside the horizon.

what happens when 2 black holes orbit eachother with the singularites?

There is only one singularity, as @Ibix said. The spacetime "shape" of the horizon in a black hole merger is like a pair of trousers: the singularity is at the top, at the "waist" of the trousers.

from the outside world you could point to a kind of geometrical position where each singularity woud be

No. The singularity is a moment of time, not a place in space.

so this would mean that also the 2 singularities would orbit around eachother.

No. See above. There is only one singularity, and it doesn't orbit anything.

If there only ever was one singularity then it was always already known that they would merge.

Yes. The event horizon is a globally defined surface in spacetime: in order to know exactly where it is, you have to know the entire future of the spacetime. That includes knowing about all mergers of black holes.

there was no singularity then.

You can't think of the singularity that way; it is a moment of time, not a place in space. Today is Saturday; if I think of last Monday, there was no Saturday "then", but that doesn't mean there isn't one now.

 

[Ibix]

If there only ever was one singularity then it was always already known that they would merge.

In GR the universe is deterministic, so what happens in the future is completely determined by the state of the universe now. So singularities are hardly the only problem for you if you don't like this - literally every event in the entire universe was predetermined.

The other point to bear in mind is that the singularity is not a place in space - it's a moment in your future if you cross the horizon. The merger is also in the future, and since the singularity is inside the event horizon it's in the future of both observers falling in to a black hole(s) before and after the merger.

He's not talking about pre-merger

Actually, I sort of was. The singularity should be one continuous surface (for want of a better term).

 

[PeterDonis]

He's not talking about pre-merger

If a merger happens at all, there is always just one singularity; the singularity is a moment of time, to the future of all events inside the horizon, so it doesn't make sense to talk about "when" it is formed, any more than it makes sense to talk about "when" next Friday is formed.

 

[HansH]

If a merger happens at all, there is always just one singularity; the singularity is a moment of time, to the future of all events inside the horizon, so it doesn't make sense to talk about "when" it is formed, any more than it makes sense to talk about "when" next Friday is formed.

as I already described: if you start with 2 dustclouds later forming 2 stars and later forming 2 blackholes and later making these 2 blackoles merge then according to your reasoning: 'there is always just one singularity; the singularity is a moment of time, to the future of all events inside the horizon'

Then the only conclusion I can draw from that is that from the moment the 2 horizons were formed there is 1 singularity. but before there was a horizon there was no singularity. so Istill cannot follow your reasoning.

 

[PeterDonis]

as I already described: if you start with 2 dustclouds later forming 2 stars and later forming 2 blackholes and later making these 2 blackoles merge then according to your reasoning: 'there is always just one singularity; the singularity is a moment of time, to the future of all events inside the horizon'

Yes, that's correct.

the only conclusion I can draw from that is that from the moment the 2 horizons were formed there is 1 singularity

No. You are failing to understand what "the singularity is a moment of time" means. It means it is not a place in space and it makes no sense to talk about when it was formed, any more than it makes sense to talk about when the moment of time "noon next Friday" was formed.

 

[Ibix]

Then the only conclusion I can draw from that is that from the moment the 2 horizons were formed there is 1 singularity. but before there was a horizon there was no singularity. so Istill cannot follow your reasoning.

Whether the singularity exists yet is (from many events) a matter of choice, even in a single black hole. It is never in your past lightcone, only ever in your future lightcone and/or in the spacelike separated from you parts of spacetime. Attempting to reason about "when it forms" will only lead to madness.

The only straightforward answer is that it doesn't exist yet (rather like next Monday) but it will when you get there - only at that point you die, unlike Monday morning (I hope). So there will be one singularity, wherever you hit it. But that's adopting a set of coordinates, and other choices are possible. It may be possible to find a foliation such that early-time spacelike surfaces intersect the singularity in two places and later ones intersect it in one place. I don't know. But even if that is the case, it's like looking at the ankles of your trousers and pretending that they are separate garments because you refuse to look at the crotch. They aren't separate and you can't make them separate by ignoring the place where they join.

 

[HansH]

The reasoning in this topic makes me doubt if you assume nature concerning black holes has to meet the rules of causality. because looking to causality there is first the dust cloud and no singularity. so what next ?

 

[HansH]

does nature know from the fact that there is a singularity that the 2 dustclouds have to end up in that way?

 

[PeterDonis]

The reasoning in this topic makes me doubt if you assume nature concerning black holes has to meet the rules of causality.

Not at all. Causality works just fine in a black hole spacetime.

looking to causality there is first the dust cloud

Yes.

and no singularity

You continue to fail to realize what "the singularity is a moment of time" means.

Whenever you want to say "the singularity", substitute "noon next Friday" and see if what you are saying still makes sense. So you would say, for example: "looking to causality there is first the dust cloud and no noon next Friday". Does that make sense?

 

[Ibix]

does nature know from the fact that there is a singularity that the 2 dustclouds have to end up in that way?

In a universe governed by GR, yes. It's deterministic.

 

[PeterDonis]

does nature know from the fact that there is a singularity that the 2 dustclouds have to end up in that way?

As @Ibix has already pointed out, GR is deterministic, so specifying the initial conditions is sufficient to specify the entire spacetime, including any singularities.

However, that still doesn't mean the singularity is a place in space. It's not. It's a moment of time, and you have to take that into account in every statement you make. If you don't do that, you will be reasoning incorrectly.

 

[HansH]

In a universe governed by GR, yes. It's deterministic.

Then I really start to doubt if GR can be correct. because I can in theory decide to remove both dustclouds before they can end up in 2 black holes. so that would mean the there is no free will or statistical influence in nature and everything is already predetermined. Of course that could well be the case.

 

[HansH]

You continue to fail to realize what "the singularity is a moment of time" means.

I think I understand. I assume it means that the singularity is a moment in time that is always in the future because you can never reach it even not if you are falling into a black hole.

 

[HansH]

However, that still doesn't mean the singularity is a place in space. It's not. It's a moment of time,

of course. however the gravitational field outside the black hole suggests 'a centre of gravity' that would be at a position in the middle of a sphere with the mass of the black hole but so much spread out that is is not a black hole. and this centre of gravity can move in the same way as if the 2 of these masses orbit eachother. so what is it that makes this 'centre of gravity' behave like a physical point where the mass seems to be concentrated?

 

[Ibix]

I think I understand. I assume it means that the singularity is a moment in time that is always in the future because you can never reach it even not if you are falling into a black hole.

Oh, you can reach it. You die there, though, and do not go anywhere else.

Then I really start to doubt if GR can be correct. because I can in theory decide to remove both dustclouds before they can end up in 2 black holes. so that would mean the there is no free will or statistical influence in nature and everything is already predetermined.

Yes. That's what a deterministic theory means. It's hardly unique to GR nor to black holes.

It's worth noting that if you see two black holes that will eventually collide, four dimensionally you are only looking at one black hole. If they won't collide you are looking at two. If you don't have precise measurements you may not be able to tell which it is.

But the same is true of stars. Four dimensionally, if they are going to collide you are just looking at one star. If they collide then you can define a line in spacetime from the interior of one star to the interior of the other without leaving the star by extending it into the future to the merged star, then back into the past through the other star. Here's a rough sketch of a space-time diagram for a head-on collision:

A horizontal line in the diagram represents a 1d section of space at one time (for some not-very-well-defined notion of time). At the bottom of the diagram a section passes through two separate stars (yellow region); as you go up the diagram the space between them gets narrower and narrower until they merge, and at the top you have one large star. But looking at the whole thing, there's only one region of spacetime that is stellar interior. So in a sense there's only one star.

You could draw something similar for merging black holes. But since the event horizon is a structure that only really makes sense to think about in four dimensional terms (it can't be defined without it), you can't really look at the diagram except as a whole - so you always see one black hole with a complicated surface.

 

[jbriggs444]

because I can in theory decide to remove both dustclouds before they can end up in 2 black holes. so that would mean the there is no free will or statistical influence in nature and everything is already predetermined.

A discussion of free will versus predestination may be outside the permitted scope for this thread.

In my view, "free will" is the way we as human beings model the fact that our predictive models and our detailed knowledge of exact initial conditions are necessarily insufficient to make definite and timely predictions for our own future behavior. There is a Goedelian type contradiction lurking that can assure us of this.

 

[HansH]

A discussion of free will versus predestination may be outside the permitted scope for this thread.

I think it is just an essential part of the discussion. looking to the yellow picture above one could discuss about which point at the vertical axis represents the point of no return after which things get per definition deterministic. What I mean is that before a black hole is formed (we start with 2 dust clouds with increasing time) we could in theory influence things to prevent the black hole from forming. But once formed I assume nothing can be done to influence its future. so then it will end up in 1 singularity 'if you see two black holes that will eventually collide, four dimensionally you are only looking at one black hole. If they won't collide you are looking at two. ' so that is deterministic. (perhaps free will is not hte proper description but what I meant is that with free will I could blow the 2 dustclowds away so they never can form a star or a black hole))

 

[PeterDonis]

Then I really start to doubt if GR can be correct.

This will get you nowhere. GR is not a complete theory--it only describes gravity/spacetime geometry, and it doesn't include quantum mechanics. But within its domain it is correct. And within this thread, since it is in the relativity forum, GR is the theory we use to make predictions. If you want to talk about things like Hawking radiation or quantum gravity, those are out of scope in this subforum; they belong in either the Quantum Physics forum, or more likely the Beyond the Standard Models forum. And since we have no accepted theory of quantum gravity, such discussions cannot be resolved.

can in theory decide to remove both dustclouds before they can end up in 2 black holes

That would change the initial conditions (because you can't just magically "remove" the dustclouds--something else needs to be present that does that, and that something else has to be included in the initial conditions), which would change the overall spacetime. It does not in any way contradict what GR predicts with the original initial conditions.

that would mean the there is no free will

The question of "free will" is not a question of physics, it's a question of philosophy, and possibly something like cognitive science. It is out of bounds in this thread and this forum.

or statistical influence in nature and everything is already predetermined

That is what "deterministic theory" means, yes.

 

[PeterDonis]

the gravitational field outside the black hole suggests 'a centre of gravity'

Perhaps, but when you actually do the math in GR, you find that there is no such thing for a black hole. So this "suggestion" needs to be ignored.

what is it that makes this 'centre of gravity' behave like a physical point where the mass seems to be concentrated?

Nothing, because it doesn't; there is no such point. The singularity is not a place in space. It is a moment of time. I am putting that in bold because you still are not grasping the implications. You need to read that bolded statement again and again until it sinks in. And you should not post any further until you have done that, because you are continuing to reason based on false premises, and that doesn't work. And continuing to repeat the same misconceptions even after they have been corrected will end up getting you a warning and getting your thread closed.

 

[PeterDonis]

which point at the vertical axis represents the point of no return after which things get per definition deterministic.

Things are always deterministic in GR. There is no point where they "start" being deterministic.

before a black hole is formed (we start with 2 dust clouds with increasing time) we could in theory influence things

Only if whatever "influence" you are talking about is already included in the initial conditions. You can't start with one set of initial conditions, and then magically add some "influence" later that isn't included in them. That is not how a deterministic theory works.

 

[Vanadium 50]

Then I really start to doubt if GR can be correct....that would mean the there is no free will

That's a statement that a theory is wrong because you don't like it. You are free to believe that, but you cannot call that science.

 

[Nugatory]

Then I really start to doubt if GR can be correct. because I can in theory decide to remove both dustclouds before they can end up in 2 black holes. so that would mean the there is no free will or statistical influence in nature and everything is already predetermined.

The exact same argument can be applied to the rather more mundane statement that if we toss a rock into the air it will fall back to the earth. In both cases the laws of physics say that something will happen, the only difference is that the tossed stone is more familiar to us so we don't worry about the implications.

Of course that could well be the case

It could be, but as no conceivable experiment could test that proposition it is not physics or any other empirical science, but rather philosophy (and off-limits in this forum). This is a long standing problem - consider Laplace's demon.

 

[HansH]

The question of "free will" is not a question of physics, it's a question of philosophy, and possibly something like cognitive science. It is out of bounds in this thread and this forum.

forget about what I said about free will because that drives the discussion into the wrong direction. I mean deteministic instead as alteady mentioned earlier. so it is indeed about initial conditions and the fact that GR seems to be deterministic when the initial conditions are given. so at the moment the initial conditions of 2 gasclouds are given in the example it means that the 'singularity as being a future moment in time with a merger of 2 black holes in between' is a predetermined result.
my question is: do you agree if I change the set of initial conditions that the result could be that we get a different future moment in time witout a singularity?

 

[Ibix]

do you agree if I change the set of initial conditions that the result could be that we get a different future moment in time witout a singularity?

Sure. You might end up with four stars that are too small to collapse into black holes. Or you might get two stars that both collapse into black holes that don't merge.

The thing is, how do you change the initial conditions? In a mathematical model you can do it by fiat, but in reality you need to move matter and energy around. But the only available stuff is already in the world and, if the world is deterministic, the "change" is already baked in.

 

[HansH]

That's a statement that a theory is wrong because you don't like it. You are free to believe that, but you cannot call that science.

I never said I dont like it. The point is that GR is deterministic, so then it means that from the start of the universe onwards the end result was already determined and also all stages in between. This means causality is maintained because every next state is fully determined by a previous state. But we don't have any evidence that this is indeed how the universe works. so then I think there are only 2 conclusions possible:
1) GR is valid and the universe is predetermined and causal , so from the start of the universe onwards the end result was already determined.
2) GR is not valid and the universe is not predetermined.
regarding point 2 then the conclusion is that if I change the initial conditions while future outcome is already determined based on given initial conditions, fuuture outcome will change when I change the initial conditions. so if at t=t1 we have 2 dustclouds resulting in 1 singularity while I change the initial conditions at t=t2 then at t=t2 also the singularity is removed. and that is probably not in line with GR so then GR cannot be fully following how the universe works.

But it seems we are not allowed to discuss about option 2 because it is outside the scope of this part of the forum. I believe science is not limited to thinking in predetermined boxes however. because if you want to make steps you should think outside the box.

 

[Ibix]

1) GR is valid and the universe is predetermined and causal , so from the start of the universe onwards the end result was already determined.
2) GR is not valid and the universe is not predetermined.

Or 3) GR is not valid and the universe is predetermined (it obeys some other deterministic theory to which GR is merely an approximation).

 

[Ibix]

so if at t=t1 we have 2 dustclouds resulting in 1 singularity while I change the initial conditions at t=t2 then at t=t2 also the singularity is removed.

Again, how are you going to make a change when you only have tools that obey deterministic laws? The behaviour of the stuff you plan to use was already determined at t1, so the outcome was never going to be what you (erroneously) predicted at t1.

 

[HansH]

The thing is, how do you change the initial conditions?

in reality you need to move matter and energy around. But the only available stuff is already in the world and, if the world is deterministic, the "change" is already baked in.

at a small scale I assume that is what we are doing constantly. for example when I drive in a car I constantly move matter and energy around. so the fact that we cannot do that at a large scale is probaly not because it is not possible but because we as humans simply don't have sufficient options available. in theory we could make a large hydrogen bom and blow away a dustcloud I assume.

Or suppose there is a collapsed star that is just 1 kg away from further collapsing into a black hole. then I could send a rocket of at least 1kg to that structure making it become a black hole. so then the final situation of ending up in a singularity of not depends on my decision at t=t1 to change the initial conditions of the collapsed star by sending a rocket to that collapsed star.

 

[Ibix]

at a small scale I assume that is what we are doing constantly. for example when I drive in a car I constantly move matter and energy around.

Sure. And if you do that in a deterministic universe, that's always what was going to happen, whatever you thought before you did it.

in theory we could make a large hydrogen bom and blow away a dustcloud I assume.

Sure. And if you do that in a deterministic universe, that's always what was going to happen, whatever you thought before you did it.

I could send a rocket of at least 1kg to that structure making it become a black hole. so then the final situation of ending up in a singularity of not depends on my decision at t=t1 to change the initial conditions of the collapsed star by sending a rocket to that collapsed star.

Sure. And if you do that in a deterministic universe, that's always what was going to happen, whatever you thought before you did it.

 

[Ibix]

To put it another way, your mental model that doesn't include you sending a rocket to add the 1kg is not an accurate model. You didn't include your rocket in it. So you didn't change the initial conditions - you simply had the wrong ones.

 

[HansH]

To put it another way, your mental model that doesn't include you sending a rocket to add the 1kg is not an accurate model. You didn't include your rocket in it. So you didn't change the initial conditions - you simply had the wrong ones.

don't you think initial conditions also include mass being at a specific location at a specific time. if the rocket is here with me on earth it is too far away from the collapsed star to make it ever a black hole. but If I decide to send my rocket to that collapsed star it will for sure pass the critical limit I suppose. so keeping the rochet on earth or sending it to the star are 2 different initial conditions because the lead to a different result.