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When I see an animation of two colliding black holes anywhere, it looks like two merging beads of water. But why would the event horizon form a bridge during collision? There is no material flowing in-between. Wouldn't the event horizons "push" each-other away as they merge?(the effect not the black hole itself) For what I understand the gravity would poorly, but surely, be canceled out. Wouldn't this "push" only disappear from view as they got significantly closer, where it still exists but is trapped in a much larger one?

Just to make sure, as I see the similar questions: I don't mean what happens as they collide, but rather why isn't this portrayed if it does this effect at all?

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    $\begingroup$ Related: physics.stackexchange.com/questions/18769/… $\endgroup$
    – zephyr
    Commented Mar 29, 2017 at 15:17
  • $\begingroup$ Maybe you can get somewhere reading about contact binaries. Obviously they're not black holes, but the shape of the stars as they merge together is governed by the same laws of gravity as black holes, and they form similar shapes. This might be an easier topic to search for that will still answer your question. $\endgroup$
    – Cody
    Commented Mar 29, 2017 at 17:41
  • $\begingroup$ But the same problems arises; contact binaries have material transfer through a "bridge" between the two objects. Black holes, having "infinite" density, would have no such transfer. So the collision would have different dynamics because of this. $\endgroup$
    – Terran
    Commented Mar 29, 2017 at 18:25
  • $\begingroup$ My guess is they're showing a transient shape of the horizons, analogous to the shape of EM fields as two magnets or charged particles come together $\endgroup$
    – Carl Witthoft
    Commented Mar 29, 2017 at 19:17

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The event horizon is not a physical structure. Rather it is the boundary of a region of spacetime from which no information can escape. If you were (unfortunate enough to be) between two black holes, you may be in a position in which there is no net force, and yet you could not travel from your location to the point distant from the two black holes, no matter how fast you were able to move.

If you were in such a position, you would be inside the event horizon of the two black holes.

Finding the event horizon means solving the Field equations of General relativity. Exact solutions are known in several situations: A single mass concentrated at a point, a mass with a charge and a rotating mass. If there are two masses, then the Field equations have no exact solution, but can be solved numerically (with a fast enough supercomputer) It is the results of these calculations that you are seeing.

So, the shape you see is the event horizon, The event horizon of two nearby black holes can be calculated by a computer. Since you may be trapped between two black holes the event horizon will be deformed towards another black hole's event horizon, giving the "teardrop-> bridge->merger" appearance of the final moments of two back holes.

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  • $\begingroup$ Any ref's to articles or papers which discuss this analysis? $\endgroup$
    – Carl Witthoft
    Commented Mar 30, 2017 at 12:33
  • $\begingroup$ Im going to consider this the answer since it makes sense to me now but this also creates a new question, does it do what I just said to a single point on the event horizon? (It would, mathematically, have no dimensions to this but the universe sometimes deviates from the infinite, a.k.a. quantization) $\endgroup$
    – Terran
    Commented Apr 21, 2017 at 22:12

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