# I What happens if two black holes collide

1. Nov 27, 2017

### Dalton Peters

I was reading up on the observation that occurred October of this year and started wondering what happens when two black holes collide do they merge to make one or do they collide as two separate objects kind of like two basketballs touching each other and also sort of a side question I've heard some people say that black holes have almost infinite mass does this mean that they might have almost infinite gravity as well (please excuse any ignorance that may be evident in this post)

2. Nov 27, 2017

### Lukeblackhill

@Dalton Peters , What happens when two black holes collide is better explain by Stephen Hawking's excellent work about the Event Horizon of black holes. Hawking discovered that when two black holes collide, they'll merge, and the new black hole formed will have the area of the even horizon (area after which nothing can flee from the black hole) bigger or the same size of the areas of the event horizon of the two black holes added together. The "infinite gravity" you refers to, comes from the fact that a black hole is formed when the density of an object gets so bigger that the curvature of space-time it creates is bigger in gravitational attraction than even light has speed to escape from. And considering light is the fastest thing in the universe, if light doesn't escape a black hole (after it passes by its event horizon), nothing escapes. I hope that might help.

3. Nov 27, 2017

### Dalton Peters

How does one calculate the event horizon

4. Nov 27, 2017

### PeroK

The people who say that a black hole has infinite mass are wrong. They have a finite mass and are formed when a very large star collapses in on itself. The gravity outside a black hole is, in fact, no different from the star from which they formed. The Sun is too small to become a black hole, but if hypothetically it did collapse into a black hole, then the orbit of the Earth would be unaffected.

But, as the black hole is much smaller than the star that formed it, you can get much closer to a black hole. So close that the gravity becomes so strong that even light cannot escape. In particular, the light from the collapsing star cannot escape.

There are several video simulations on line of two black holes colliding and merging.

5. Nov 28, 2017

### Dalton Peters

I have a sort of a side question as well since things can get much closer is there any black holes that have the gravity to rip apart an atom

6. Nov 28, 2017

### unusually_wrong

As far as I understand, yes. If you think about it this way, an atomic bomb works by splitting atomic nuclei. The amount of energy there is nothing compared to the gravitational force of a black hole. Also, mass does not become infinite at singularity, gravity does. The reason that light cannot escape is that the black hole pulls space-time toward it's singularity faster than light can travel.

7. Nov 29, 2017

### Lukeblackhill

It envolves the theory of General Relativity (what means a good deal of differential geometry) and concepts of topology, mainly those introduced by Sir Roger Penrose in the beginning of the 60's. I have a notion of it, but the calculations are quite complicated, more than my current level of understanding.

8. Nov 29, 2017

### PeroK

9. Nov 30, 2017

### Dalton Peters

I have one more side question photons are the fastest known thing right and they cant escape the event horizon is there anything out there that has more momentum than a photon and can escape the event horizon in theory?

10. Nov 30, 2017

### unusually_wrong

No, gravity is pulling spacetime to it's singularity faster than light can travel. Kind of the same way that the universe can expand faster than light. It's spacetime that is stretching.

11. Dec 4, 2017

### Lukeblackhill

@Dalton Peters, there's a theory which ascribes the existence of particles called Takions, which in theory has a mirror-sort of symmetry with the Lorent'z invariance. That means that Takions move faster than the speed of light and suffer the same effects einstein's predicted for normal particles, when it decreases its speed to move closer and closer to the speed of light. But that is merely a theory, and has nothing experimental in it, yet.

12. Dec 7, 2017

### ohwilleke

They are spelled Tachyons not "Takions" (even though it sounds the same phonetically - which is interesting and a bit unusual, usually on forums like this you get people with book knowledge who know how to spell words but not how to pronounce them, rather than visa versa).

13. Dec 11, 2017

### Lukeblackhill

@ohwilleke, it's not a mistake of spelling. In fact, I read about it in an article written by a Brazilian physicist, and that seems the way they write. I haven't read it in english, so I did not know how to write it.