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Will the event horizons of a two black holes be perturbed or bent before a collision? What will the shape of the event horizon appear to be immediately after first contact?

David Z
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bill
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  • There is no contact. – Ron Maimon Dec 26 '11 at 03:12
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    @Ron, that may be technically true, but at some point the system shifts from having two distinct event horizons to having a single event horizon. That point could reasonably be referred to as "contact" IMO. – Harry Johnston Dec 26 '11 at 03:58
  • @Harry Johnston: The way it happens for a small black hole going into a big one is that the big one gets a little bigger nonlocally a little before the little one hits, and the little one smears over the big horizon. There is no contact. – Ron Maimon Dec 26 '11 at 10:24
  • @Ron, I suspect the OP is mainly interested in the case of two black holes of similar size. – Harry Johnston Dec 26 '11 at 20:05
  • @Harry: I agree, but there it is problematic, I don't know how it works, whether one grows, or whether you can consistently view either growing, etc. If I knew, I would give an answer. The only thing I know is that you can't collide horizons, because nothing ever gets to a horizon, time stops there. You can only effectively collide horizons in a coarse-grained picture (which is more physical anyway). – Ron Maimon Dec 27 '11 at 04:17
  • @RonMaimon: No! "Nothing ever gets to the horizon" is just a misunderstanding on your part. It's wrong because it applies to objects and light, etc. An event horizon is not an object, just a boundary in space. It's not that different from this "paradox": point a laser pointer really far away, say at the moon. Then change the direction really fast. It's not hard to show the dot can go faster than the speed of light. Impossible, right? No! The dot is not a "thing". – ThePopMachine Jan 15 '12 at 16:25
  • @ThePopMachine: You are totally wrong. The horizon is physical, and it is defined as the boundary of communication with infinity. It is as physical as the notion of "far away", which is always perfect in model black holes. The event horizon is an object, with viscosity and electric resistance, and its physicality is not changed by the fact that things can fall through (from their own point of view). This is Susskind's black hole complementarity (and 'tHooft's holography, and Thorne and Co's Membrane paradigm, and all modern AdS/CFT), and it obsoletes all previous thoughts about black holes. – Ron Maimon Jan 15 '12 at 17:41
  • @RonMaimon: This is so ridiculous. The fact that EH is defined a certain way or that it exhibits certain properties that you might also attribute to 'objects' doesn't make it an object. The fact that the blackhole as a whole has certain properties that are indistinguishable beyond the event horizon doesn't make the event horizon itself an 'object'.

    Like Harry said, at a certain point in time there are two horizons and at some later time there are one. If you don't think EHs can merge (i.e. contact) then how do you think supermassive blackhole formation at the centers of galaxies works?

     – ThePopMachine Jan 15 '12 at 21:01
  • @ThePopMachine: It is well known that both horizons smear out over the new horizons, by smooshing in thin layers, like kneading dough over and over, without ever touching. This is what makes this question interesting--- the two horizons asymptotically become one, by mixing up in a horrendously complicated way which only looks like a merger in a coarse graining. This is not my insight, it is due to either Gibbons or Horowitz or Pope, or Page, I can't remember exactly who, who used this non-meeting property to claim (incorrectly) that the Gregory Laflamme transition never happens. – Ron Maimon Jan 16 '12 at 08:04
  • @RonMaimon: I understand the principle of what you are saying. But this is a purely mathematical phenomenon. For example, if you have two identically sized blackholes on a direct collision course, it should be obvious that there is planar region between them where the forces/curvature balance and you could sneak by (i.e. the EHs actually flatten away from each other due to the gravity pulling away in the opposite direction). But asymptotically this would only apply to an infinitely thin plane and eventually the force toward axis between the blackholes is sufficient... – ThePopMachine Jan 16 '12 at 18:10
  • ... that nothing can escape away from the axis anyways. So there is a point where they are merged from the external point of view.

    I understand your point about "horrendously complicated" "smooshing", but if this persists then it is not true that blackholes "have no hair". At a certain point any such features must be so small and so thoroughly mixed that they cease to exist.

     – ThePopMachine Jan 16 '12 at 18:16
  • @ThePopMachine: Yes, this is accurate. The coarse graining is physical, and within string theory or any other quantum theory of gravity (or even in numerical relativity) the merger will complete in a finite time. But the dynamics are complicated, and hard to visualize. It will take a real simulation to answer this question fully. – Ron Maimon Jan 16 '12 at 18:25
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    @RonMaimon: Look at that. We both started telling each other we don't know what we're talking about and in the end we agree. Maybe there is hope for the world. :) – ThePopMachine Jan 16 '12 at 18:31
  • @ThePopMachine: Physics is objective. If the same holds on the Christianity stackexchange, then I would have hope for the world. I didn't change my mind about anything, and it seems that neither did you, we just found out we were talking about the same thing from slightly different perspective, which often happens. If somebody actually changes their mind, then that's a reason to cheer, but don't hold your breath. People are usually far too stupid to do that. – Ron Maimon Jan 17 '12 at 02:55

2 Answers2

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Yes,

Like del piero says, the gravitational field of each one affects the shape of the other, so it's similar to the "metaballs" used in computer graphics.

The exact shape will depend on the geometry, but it usually looks like two fingers reaching out to meet, like in Michelangelo's "The Creation of Adam". You can really see it in the first & last videos here, but there are some good videos of death spirals in between.

Note that the pulsing at the end of the first video isn't anything real; it's caused by the change in magnitude of the color scale shown in the bottom right corner.

There are also a couple of decent still frames (head on collisions of equally massed holes) on these two pages: 1,2.

This one has a shot of asymmetrically massed black holes, just after they join (with the original event horizons drawn underneath):

mdaoust
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The apparent horizon is only affected to the point that the curvature around one black hole is modified by the gravitational field of the other.

The event horizon as defined by the boundary of the causal past of future conformal infinity gets modified in advance of significant changes in curvature, but this precisely what you would expect from a global teleological definition. See Hawking and Penrose for more details.

del piero
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