Question

Is the "event horizon" of a black hole potentially violable? Black holes are commonly described as being unidirectional (matter / energy goes in, but doesn't come out), but is the event horizon of a black hole potentially deformable? That is, in the case of two black holes colliding (let's say), is there not a point when the effective gravitational field between the two black holes "cancel out", thereby causing a potential "escape path" for light traveling perpendicular to the axis the two black holes are on?

In a more complicated case, consider two black holes of similar mass orbiting each other in a stable configuration; since no matter or energy actually pass the event horizon of a black hole, would the area between the black holes be a potential zone for escape of energy? (I'm assuming minimal frame dragging effects here.) And since the black holes would need to rotate at a very high rate of speed to avoid actually colliding, would that not restrict the emission of energy to be along the axis of rotation?

A bit of clarification here; the primary question here is whether or not the gravitational force vectors of the black holes can properly cancel each other out at an appropriate point between them. If that's the case, then (in a static system) it can be easily determined that there is a plane through that point perpendicular to the axis along which the two black holes exist where the gravitational force vectors are deeply reduced (not zero, of course, except at the precise point where the masses perfectly balance; however, reduced significantly based upon the inverse distance from said axis).

In an (again static) two black hole system, orbiting material could enter a "figure eight" orbit; much of this material would pass through this "nodal point" (or very nearby it). The high speeds and volumes of the material would indicate a large number of collisions; some resultant (high-energy) ejecta from these collisions would result in some (small) quantity of ejecta traveling along the plane of reduced gravitational force. Now consider the (more dynamically stable) case of stably orbiting black holes; the plane of reduced gravitational force is itself rotating (about the axis of rotation of the black holes); this turns the "escape plane" (not entirely just a plane, it having some (small) quantity of thickness) into a line (again, not entirely just a line, but a very thin cone).

The primary point is that that the cancellation of force vectors effectively makes for a "dimple" in the event horizon of each of the black holes; that is, the existence of another strong gravitational attractor in close proximity deforms the event horizon of a black hole.

The question is, is there anything which invalidates this analysis? As mentioned before, frame dragging is a concern; is there anything else which would be a concern? (I suspect the frame dragging concern would actually be resolved by the mutual forced frame dragging, i.e. the holes' spins would "synchronize" because of the mutual frame drag effect.)

If this is a possibility, then does this not imply another means of black hole mass "evaporation" other than Hawking radiation? (And, in fact, wouldn't it imply the type of "polar jet" that has been observed?)

Answer 1

Well, by definition, an event horizon is a surface such that any event inside cannot affect an outside observer. (I think I've heard it defined as the boundary of a region in spacetime excluded from the causal past of future null infinity.) So the simple answer to your question has to be no, the event horizon cannot be "violated," otherwise it would not actually be an event horizon.

But I think you're getting at something interesting by considering the "cancellation" of gravitational fields between two black holes. I guess the question is, if you take an isolated black hole and combine it with another one to form a binary BH, are there any points that would be inside the event horizon of the single BH that are outside the horizon of the binary? That I'm not sure about, at least not without doing some analysis of spacetime around a black hole collision (and I don't think that's an analysis I would know how to do in detail). Besides, you'd have to define some physically meaningful procedure for identifying spacetime points around the original black hole with spacetime points around the binary.

I suspect that the answer to this is also "no," though; that is, every point that is inside the event horizon of a black hole cannot be placed outside an event horizon by the addition of additional matter.

Of course, if you had two orbiting black holes with a region between them that was not inside the event horizon, energy (in the form of light, for example) should certainly be able to escape from that region. Again, this stems from the definition of the event horizon: if light had no way to escape, then the region would be inside the horizon.