# Singularities in colliding black holes

• Jolb
In summary, when two black holes collide, their singularities merge the instant their event horizons touch. However, there is a lag between the event horizon touching and the singularities merging.
Jolb
When two black holes collide, do their singularities merge the instant their event horizons touch? Or is there a lag between the event horizons touching and the singularities merging when there are actually two singularities inside a single black hole?

Yes, there is a lag. Note that during the collision their horizons become distorted.
So yes, it takes additional time for them to collide but I doubt anybody can provide more information without numerical simulations.

Jolb said:
When two black holes collide, do their singularities merge the instant their event horizons touch? Or is there a lag between the event horizons touching and the singularities merging when there are actually two singularities inside a single black hole?

My intuition says pretty much the same thing as Dmitry67's. There has to be a lag.

However, I think the answer may be even more complicated. General relativity has a tendency to develop singularities spontaneously any time a sufficiently violent process occurs. For example, there's a broad research program being carried out by many different groups around the world to try to define cosmic censorship rigorously, and determine under what conditions it holds. A collision of two black holes is exactly the kind of violent, highly nonlinear process in GR that they have to worry about. So for instance, I could imagine that when two black holes collide you get more singularities than you started with, maybe some of them naked singularities -- I don't know. Someone who's an expert on this kind of thing could probably say more. I'm sure people have tried hard to model black hole mergers, since they're one of the best candidates for producing detectable gravitational waves. I would guess that collisions with a high degree of symmetry are the easiest to model, whereas highly asymmetric ones are probably much more difficult to say anything about.

This is an interesting question, actually... I think next time I get the chance I'll ask some of the folks at work (who numerically solve the E.Eqns and do these kind of black hole mergers) what happens to the actual singularities during a merger.

Nabeshin said:
This is an interesting question, actually... I think next time I get the chance I'll ask some of the folks at work (who numerically solve the E.Eqns and do these kind of black hole mergers) what happens to the actual singularities during a merger.

That would be great. If they can point us to a review article on arxiv or something, that would be very cool.

## 1. What is a singularity in the context of colliding black holes?

A singularity is a point in space where the gravitational pull becomes infinite and the laws of physics break down. In the context of colliding black holes, it refers to the point where the two black holes merge and their combined mass is concentrated into an infinitely small point.

## 2. How do black holes collide?

Black holes can collide when they are in close proximity to each other, either as a result of being in a binary system or due to the merging of two galaxies. As they get closer, their intense gravitational pull pulls them towards each other until they eventually merge into a single black hole.

## 3. What happens during a singularity in colliding black holes?

During a singularity in colliding black holes, the two black holes merge and their event horizons (the point of no return for anything entering a black hole) combine. This results in the formation of a new event horizon around the merged black hole.

## 4. Can we observe singularities in colliding black holes?

No, we cannot directly observe singularities in colliding black holes as they occur deep within the event horizon and are hidden from view. However, scientists can study the effects of these collisions, such as gravitational waves, to learn more about singularities and the behavior of black holes.

## 5. Are singularities in colliding black holes dangerous for Earth?

No, singularities in colliding black holes are not dangerous for Earth. These events occur millions of light years away and the gravitational waves they produce are too weak to affect us. Additionally, the gravitational pull of a singularity is only dangerous if one gets too close to it, but Earth is not at risk of colliding with a black hole.

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