# Can a near-by black hole affect the shape of another black hole.

1. Jan 20, 2010

### Edward Solomo

Can a "near-by" black hole affect the shape of another black hole.

Hello,

My name is Edward Solomon and I am curious to know what would happen in the following scenario.

Let us assume that we have two Black Holes, A and B. Both Black Holes have the same approximate mass and are not rotating. As they are non-rotating they are perfect spheres. If the black holes are on trajectories that will bring them "close" together (but not collide) can the shape of the event horizons be affected?

Allow me to put some numbers up. Lets say we have two black holes of about 10 solar masses each. Doing the math in my head that would give us two black holes at around 30km in radius.

Let us place the centers of the black holes 120 km apart. If the black holes still retain their perfect spherical shapes at this extremely close distance the surfaces of both (the event horizon) we be 60 km from one another.

However my intuition tells me that at this close distance they would not be perfectly spherical. The gravitational pull would "weaken" the other between the distance that separates them.

I believe that this would change the shape of the event horizon in a manner that would be analogous to a 2-D crescent moon shape. It would appear as a sphere with a dent in it, with the dent shaped like a parabolic cone.

Also if these black holes are not on a trajectory to collide they may even cause each other to rotate, with the angular acceleration increasing as the distance between the black holes decreases. This rotation would also affect the shape of the black holes.

Another curious thing to note is that if this parabolic dent could exist as I described, could the black holes be brought close enough to temporarily expose their singularities, without ever having to merge/collide with each other? If so, we may be able to replicate this in labs in the near future.

And the most interesting question, if one were to observe the formation and evolution of this parabolic dent, would the volume that previously occupied this dent be "pushed" even further into the black hole? If so, would this cause the rest of the surface to bulge? How would rotating surface react to such a parabolic dent?

If the volume is not pushed closer to the singularity, could any sort of energy/matter be expected to emerge from it, come back into existence to the observer?

If the black holes were rotating in a stable orbit around each other, could they be expected to eject enough energy/matter that would form a new center of mass at the gravitational midpoint between black holes A and B and merge all three masses into one black hole? An interesting affect of them rotating and orbiting around each other is that the "dent" would change position on the surface of both black holes, creating a sort of "volume storm."

I apologize in advance is this is a "stupid thread" and will refrain from future postings.

Last edited: Jan 20, 2010
2. Jan 20, 2010

### cmaurer1

Re: Can a "near-by" black hole affect the shape of another black hole.

Hmmm. I would not say that you should refrain but maybe gain a bit better understanding of black holes.

Do you realize how close 120km or 60km or 30 km is in terms of astrophysics? At those distances the black holes would be inside of each others event horizon's and well on their way to merging. Black holes themselves are tiny, think point charge to pin point size. The event horizon is what is so large and it grows the more mass that the black hole has. Most people think of the EH when they are thinking of a black hole but that is not what the real BH is.

Maybe read up on black holes a bit and then post a different question. And I don't mean wikipedia. lol

But to answer your question, a black hole will not change shape. The EH will expand when they two finally merge but the size of the BH itself shouldn't change that much. Have you every looked at a "picture" of a binary black hole merger? The gases swirling around the EH? They look pretty circular to me. lol Of course I am sure their is a paper out there somewhere dealing with the shape.

3. Jan 21, 2010

### bcrowell

Staff Emeritus
Re: Can a "near-by" black hole affect the shape of another black hole.

Hi, Edward Solomo,

Welcome to Physics Forums!

I think a lot of what you're saying is basically on the right track. General relativity is approximately linear in the weak-field limit, so you can make a rough analogy with nonrelativistic ideas like Roche lobes, etc. I think you're right that the event horizons will change shape. To first order, I think you can just add the potentials.

I'm sure plenty of people have put lots of effort into simulating black hole collisions, because they're of interest as candidates for producing gravitational waves that could be detected by LIGO et al.

Seems unlikely to me. Naked singularities are a topic of intense interest in current research, and if a process like this could produce a naked singularity, it seems likely to me that someone would already have found that out, and won the Nobel prize from it.

In general, it sounds to me like you have some very cool ideas, but if you want to get anywhere with these ideas, it's time to buckle down and study up on general relativity.

Regards,

Ben

4. Jan 21, 2010

### Edward Solomo

Re: Can a "near-by" black hole affect the shape of another black hole.

Hello and thank you for your response Ben. Based on both of your responses I searched the academic archives for any papers regarding binary black hole systems. All of the papers that I have found thus far are only concerned with the propagation of gravitation waves surrounding the system and none dealing with how the actual shapes of the black holes are being affected.

If you would happen to know a link or reference to this topic I would be very interested.

5. Jan 21, 2010

### Nabeshin

Re: Can a "near-by" black hole affect the shape of another black hole.

Edward,

I know that when two BH's are in the process of merging, the Event Horizon can deform itself to become an 8 shape, with the EH extending fully between the two black holes. This seems to suggest that the horizons themselves can indeed be altered by interactions between holes.

Cheers

6. Jan 21, 2010

### Edward Solomo

Re: Can a "near-by" black hole affect the shape of another black hole.

This is very interesting, it is possible there could be a "hollow space" between the two singularities, creating a small observable universe between them? If so, could this hollow space extend to the singularities themselves? This hollow space would of course be unobservable from the outside.

I do realize how close this is, I said "extremely close distance" in the original post. The problem assumes that the black holes are both moving at sufficient velocities on different trajectories that will allow them to escape collision. However I'm not sure how high the velocities would have to be for two objects of 10 solar masses to avoid collision by their gravitational pull with the centers of mass only 120km apart, the angle of approach would also affect the escape velocity, but that is not the point of this discussion, unless the "normal laws of physics" do not apply in this scenario. And when we're talking about black holes, 30km is a very normal size for a stellar black hole.

So, I'm asking how the tidal forces of a black hole affect the shape of another black hole when they come into close proximity.