A BH binary is a system of two black holes orbiting each other in close orbit.


Now on this wiki page, there is a simulation of the merging black holes, but it is not black in the middle, between the two black holes. This could give you an idea, that the middle is an area where light can still escape (pass through).

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I have read this question:

Can we exit the event horizon of merging black holes?

where Bob Bee says:

No. When they merge their horizons will change shape, and eventually become the static or stationary shape of a BH horizon. Nothing inside either horizon while this is happening can escape. At all times the timelike curves stay inside, and the deformed horizons are where the lightlike curves end up. In each, and after they merge.

So when the two event horizons start touching each other(joining), there is a point where the individual event horizons open up, and there might be some light right where that happens, my question is, what happens to that light? Based on the answer, it should not be able to escape, but if not, then which singularity will it start moving towards?

As I understand, the event horizon is the area, where the extreme curvature just reaches the speed of light, meaning that the curvature is so, that it causes the escape velocity to be exactly $c$.

Now when the two event horizons meat, or join, the escape velocity should be c in both directions, towards both singularities? Which way will light move?


  1. What happens to light in the middle of a BH binary?
  • $\begingroup$ when the two event horizons start touching each other(joining), there is a point where the individual event horizons open up What does that mean? $\endgroup$
    – G. Smith
    Commented Mar 30, 2020 at 18:43
  • $\begingroup$ @G.Smith the two event horizons are a boundary to the outside world. No information can leak. When two such join, they have to open (into each other) up to join. As the answer says, this does not mean they open up to the outside world, just open up into each other, meaning they can leak information into each other. As they do that, they might incorporate more light/matter from the place in space where this joining happens. Which direction does light/matter go from there, towards which singularity? $\endgroup$ Commented Mar 30, 2020 at 18:47
  • $\begingroup$ You asked what happens to light in the middle of a binary, and made clear that you were talking about the region between the horizons, not within either one. Your last comment is inconsistent with your question. $\endgroup$
    – G. Smith
    Commented Mar 30, 2020 at 19:10
  • $\begingroup$ @G.Smith in the middle (first outside both EH), there is light/matter, and that light/matter could be incorporated as the two EH join, thus those light/matter particles could find themselves inside both EH. Right at the edge of the two EH, where the two EH touch/join, there can be light/matter. Though that light/matter is first outside both EH, they could find them selves inside both EH, as the two EH join. $\endgroup$ Commented Mar 30, 2020 at 20:02
  • $\begingroup$ @G.Smith you are correct I am not asking about light/matter that was originally inside either EH. I am asking about particles at the EH, where the two EH touch/join. $\endgroup$ Commented Mar 30, 2020 at 20:06

2 Answers 2


What happens to light in the middle of a BH binary?

Before two Black Holes merge the trajectory of the light between them is given by the spacetime geometry of the system. The light either escapes or "falls" into one of the two black Holes. The bizarre case that the position of the photon coincides with the position where the two Black Holes (their horizons) "touch" each other would mean that this photon being now inside the newly formed Black Hole moves towards its singularity.

  • $\begingroup$ correct, but the new BH has two singularities. Which one will it move towards? $\endgroup$ Commented Apr 1, 2020 at 16:14
  • $\begingroup$ The two singularities move towards each other. And the interior of a black hole is purely speculative anyways - anything below the event horizon is unobservable by observers outside of it. $\endgroup$ Commented Apr 1, 2020 at 16:31
  • $\begingroup$ This is correct, even after the Black Hole merger being highly deformed there exists one singularity within one event horizon. And in addition to what @safesphere said the singularity is not part of the manifold. $\endgroup$
    – timm
    Commented Apr 2, 2020 at 17:55

(My answer is based on unpublished work and should be considered accordingly.) Inside BH the coordinates change and the (3) spatial ones become (3) temporal while the (1) temporal one becomes (1) spatial. Whatever enters BH's horizon stops to "move" (in any conceptual sense) but it takes its "position" within BH {three temporal-coordinates, that represent the "time" of merging for the outsiders(one value) or the space-position for the insiders(one value too), and one spatial-value, that represent the three position-coordinates for the outsiders (three values) or the temporal-position for the insiders (three values too); motion looses its meaning}.


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