If we cannot observe anything fall into a black hole as an external observer, how can we observe a black hole merger?
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2$\begingroup$ Excellent question! Perhaps it makes sense to slightly adjust the wording: "If, in the coordinate system of an external observer, the falling of an object into a black hole is not visible. Then how, in the same coordinate system, can the contact and merging of black holes be observed?" Also the tag "General Relativity" is recommended, as there are other theories that do not include an event horizon. $\endgroup$– ImyafCommented Oct 22 at 10:46
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$\begingroup$ Basically for the same reasons we see a neutron star collapse into a black hole.Dont ask me how tho I barely remember anything from the GR class. $\endgroup$– Root GrovesCommented Oct 22 at 10:59
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$\begingroup$ This question is similar to: So Black Holes Actually Merge! In 1/5th of a Second - How?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. $\endgroup$– John RennieCommented Oct 22 at 11:49
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$\begingroup$ “we cannot observe anything fall into a black hole” - This is a popular myth. The radius of a black hole (the radial distance between the horizon and the origin measured in meters) is zero, as follows directly from the Schwarzschild metric. Therefore, once objects are asymptotically at the horizon, space wise they are at the same place where the singularity happens in the future. “Crossing the horizon” is another side of this myth. Nothing crosses the horizon since its radius is already zero. Instead the area of the horizon shrinks to zero in the future while all matter stays at the horizon. $\endgroup$– safesphereCommented Oct 23 at 7:19
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$\begingroup$ @imyaf Thanks I will edit it $\endgroup$– SCIENCEIUMCommented Nov 14 at 9:51
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1 Answer
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It is true that we cannot observe anything beyond the event horizon of a black hole. However, we can certainly observe matter that is in the process of falling into a black hole, but is still outside of the event horizon. This matter forms an accretion disk around the black hole, and we can see light and shorter wavelength radiation emitted by the matter in the accretion disk.
Observations of black hole mergers come from detecting the gravitational waves from the merger (and from its immediate aftermath, known as "ringdown") rather than observing electromagnetic radiation. So what is detected is minute changes in the structure of space-time itself. I don't think anyone has (so far) observed a black hole merger in visible light, although gamma ray bursts that seem to be associated with black hole mergers detected by gravitational wave observatories have been detected.
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$\begingroup$ I don't think your first paragraph addresses the point of the question. Indeed, OP has certainly in mind a radially infalling matter, which indeed does not enter the black hole for the external observer. In your second paragraph, you did not mention the ringdown phase of a black hole merger, which seems to be OP point. $\endgroup$ Commented Oct 22 at 11:31
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$\begingroup$ @JeanbaptisteRoux I have added a mention of ringdown to my second paragraph. I don't see what radially infalling matter has to do with observing black hole mergers, since we know that the black holes mergers that we can detect do not arise from a head on collision but rather as a result of a rapidly decaying mutual orbit. $\endgroup$ Commented Oct 22 at 11:44
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1$\begingroup$ This was just what I understood of the question: since (radially) infalling matter never crosses the event horizon for the faraway observer, how is it possible to observe two black holes merge? (hence one would "cross" the horizon of the other and vice-versa). I think the answer would boil down to simply "a black hole is a region in space-time, not 'matter', and thus these two regions can merge in a finite time for the faraway observer". I also think that in this form, the question has already been answered on PSE. $\endgroup$ Commented Oct 22 at 12:05
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$\begingroup$ Thanks a lot appreciate it @gandalf61 . Can we detect infalling matter crossing the horizon in a similiar way provided the mass is large enough ? $\endgroup$ Commented Nov 14 at 9:56
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