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Suppose we have two black holes moving on a path of direct frontal collision. Is it correct that from far away, due to time dilation, we can never "detect" that the two black holes merging or colliding? Would we perceive the relative motion of the two black holes slow down before the two event horizons touch? if that is the case:

question: Will a planet orbiting the system from far away feel a uniform distribution of mass located at the center or that of two blobs next to each other?

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    Related: Can black holes actually merge? – ACuriousMind Sep 06 '15 at 21:58
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    @ACuriousMind the answer has not been clearly answered from my point of view, so my plan is to offer a bounty in two days. –  Sep 06 '15 at 22:06
  • You can't actually "see" a black hole. All you can see are the effects on light and matter. When two black holes merge the theory predicts what you would see and some people have made videos of that. – CuriousOne Sep 06 '15 at 22:10
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    @CuriousOne that is why I asked what is the gravitational signature, my use of "see" was unfortunate –  Sep 06 '15 at 22:12
  • In my opinion (and that may be close to a layman's), the size of a black hole may be defined from the boundary of its event horizon. Two black holes can be said to be merged if their event horizons start to overlap. From this point of view, you can see black holes getting merged. You can not see anything inside the event horizon hence you do not know if the mass is distributed or centered in a black hole, everything we calculate about them is just our perception (I guess). – hsinghal Jun 10 '16 at 19:00

2 Answers2

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Is it correct that from far away, due to time dilation, we can never see the two black holes merging or colliding?

Correct. By definition you never see a horizon unless you cross it.

Would we perceive the relative motion of the two black holes slow down before the two event horizons touch?

Yes

will a planet orbiting the system from far away feel a uniform distribution of mass located at the center or that of two blobs next to each other?

It will feel more and more like that as time goes on. Of course, that isn't becsuse you feel mass that is far away. It is because the spacetime out near you evolves to be more an more like that. More and more like the kind of curvature that in the Newtonian weak field limit looks more and more like the effect of a large uniform distribution of mass.

It is confusing to me how you will perceive (based on electromagnetic radiation of increasing wavelength) that the two holes freeze before merging, and at the same time have a gravitational signature that looks like they fused

The gravitational signature never looks like they fused. In fact unless those black holes have been around forever we can still see the infalling material that made each of them. And in fact the infalling matter on the north side of one and the south side of the other one could decide at any moment to lose their courage and take off. And then the matter next to that could take off. And so on. The definition of the horizon is the events where they waited too long. We never see that by definition. So everything we see is events prior to that. So we see events prior to the too late.

So we always see matter where it isn't too late. So everything we see could come back out at us. Maybe it has to wait to get really symmetrically distributed before they can collectively push off and not leave things behind.Everything you see that looks similar to a black hole could still rip itself apart before it actually forms.

So it most definitely doesn't ever look fully fused. It just looks closer to merged. The electromagnetic appearance is a consequence of the time dilation, it doesn't cause it. In fact the black hole appears less massive over time as the light comes away from it.

Again, you can feel it from the curvature outside, you don't have to look electromagnetically. You could send neutral particles to orbit it in different planes and note the period to circumference and notice the ratio settles down in a way that doesn't depend on the plane you select.

Timaeus
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  • It is confusing to me how you will perceive (based on electromagnetic radiation of increasing wavelength) that the two holes freeze before merging, and at the same time have a gravitational signature that looks like they fused –  Sep 06 '15 at 22:22
  • @brucesmitherson I don't know why you would think frozen looks so different than merged, though since merged doesn't look like anything because we never see it I'm not even sure how to speculate about what confuses you. But I edited anyway. If you describe a confusion with sufficient detail I can try to address how to think about the issue correctly. – Timaeus Sep 06 '15 at 22:53
  • Thanks Timaeus. Freezing to me means I should perceive a two blob distribution of mass, merged means to me a single black hole and a single blob of mass. –  Sep 06 '15 at 22:57
  • @brucesmitherson You don't perceive blobs of mass you perceive the effects of curvature. And besides two spheres can become pancake like on each side so that they look similar to two half spheres getting closer together. – Timaeus Sep 06 '15 at 23:02
  • I think my confusion is more basic: it amounts on how can an observer far away explain the gamma ray burst that is emitted when the black holes merges if from his point of view they never actually merge. –  Sep 06 '15 at 23:05
  • @brucesmitherson Did someone say that you see them merge? You don't. Everything you see is by definition from outside the horizon so from before any horizon forms (if it ever forms). If there already is a horizon (for instance if there always was a black hole) and it is rotating you can still extract gravitational radiation and energy and such from it. But for things that look similar to black holes that formed from collapsing matter you can get radiation from outside it. – Timaeus Sep 06 '15 at 23:08
  • You can even see the matter from the exact dead center of each dense surface. Imagine a 4d spacetime with a bunch of $t=const$ surfaces layered on top of each other. Then imagine someone took the $t=\infty$ surface and pushed it down from above with a finger and so all the $t=t_{big}$ surfaces come from way up high and angle down really far so they can slip under the finger pushing down. There isn't anything beyond the $t=\infty$ surface as far as the external observer knows. And indeed unless that finger pushes all the way down then there isn't a hole in the external spacetime. – Timaeus Sep 06 '15 at 23:15
  • And instead each layer of constant time just slides down beneath the finger. So you always see something in that region. – Timaeus Sep 06 '15 at 23:16
  • @Timaeus I wish you would comment on this Question. Suppose I am watching two such events (with the critical impact parameter +/- 1mm) against checkerboard backgrounds so I can see the distortion of light passing them. What would I see? – Keith McClary Sep 07 '15 at 03:21
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Luckily black holes emit Hawking radiation, so in some sense we can see them.

So first there are two glowing balls approaching each other, after some time there will be one glowing ball that is twice as large as one of the initial glowing balls. (I'm considering black holes of same size)

And during the collision the balls distort to other shapes and shake violently emitting gravity waves.

All that happens quite fast when observed from far away. From closer observation point it happens even faster.

Just so that things don't become too absurd, all kinds of measuring instruments, like eyes and gravimeters, should agree what is happening during the merging.

stuffu
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