# Intersection black holes

1. Aug 1, 2010

### BkBkBk

Would it be possible for something to pass through 2 event horizons,falling into both black holes?
also,if it can,what would happen if the objecct in question (lets say a proton) was at a point where the forces acting on it were equal in every direction?

say being at the centre of these two circles?(even though the forces wouldnt be equal at this point.

http://www.csnmail.net/~stjoan5/venn2.1.gif

would the proton quickly expand into a quark-gluon plasma?or would the gravitational/strong forces keep it confined?

2. Aug 1, 2010

### bcrowell

Staff Emeritus
An event horizon isn't a physical piece of a physical object. An event horizon is a property of a spacetime as a whole. When you have a spacetime that becomes flat at infinity, and there's a set of events that can't be causally connected to future events at infinity, then the event horizon is the boundary of that set. So by definition, it's not possible to have two event horizons that intersect, as in your drawing. Your pair of black holes will have a single event horizon, roughly corresponding the outer, dog-bone-shaped outline of your drawing.

In a frame of reference tied to the center of mass of the black hole system, the gravitational force on your proton is zero by symmetry, so it will produce no effect at all. If you pick a different frame of reference, accelerating relative to the first, then you can make the force on the proton be anything you like. This is always true in GR; the gravitational field can be whatever you like, depending on your frame of reference. This shows that the gravitational field, or gravitational forces, are not particularly useful concept in GR.

What could have a dramatic effect on the proton would be a very high tidal force (high Ricci curvature). For, say, solar-mass black holes, the tidal forces near the event horizon are far too small to have such an effect on the proton. The proton would not be ripped apart until it was very, very close to one of the singularities.

3. Aug 1, 2010

### BkBkBk

i get that its not an object in a physics sense,and that its a mathematical limit(although i have read somewhere that theres a possibility that there could be a sphere of photons surrounding the event horizon as long as the black hole didnt get more massive,just because their incoming angle and speed was just right that they took orbit)

ok it makes sense now i think about it as limit,thank you.

4. Aug 1, 2010

### Nabeshin

A probably unrelated note: The above configuration of event horizons never arises. What happens is the event horizons slowly deform until they finally connect through a small "bridge" which then expands into one uniform peanut-like shape, and finally settles down to a sphere.

I suppose it is slightly relevant because the concept of "passing through two event horizons" no longer works, as there is just one unified horizon.

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