what if two black holes should collide directly, and not merge by orbiting each other, but directly merge? If this were possible to occur, what would a signal to LIGO look like, and would the  energy of a direct merger be equivalent to a normal rotational merger energy release? Thanks.

It is impossible to give an exact answer without running simulations on a supercomputer. I have never heard of anyone modelling this case, presumably because it is so unlikely. 

I would guess there would be a narrow spike followed by a brief ringdown phase. The precise details would be affected by the relative orientation of their angular momentum 

I agree that is is very unlikely,  perhaps impossible unless arranged somehow. I was more interested in the merger efficiency, that is, how much mass loss would be given by such a theoretical merger vs. the standard orbiting one. I was hoping that if a numerical simulation had been done,  it would be reported here.    Thanks.

You could try contacting one of the LIGO team to ask if anyone has simulated this case. You could start here: https://stuver.blogspot.com/p/ask-question.html

Or here: https://cplberry.com/ 

The energy radiated would depend on relative mass difference, relative speed, the angular momentum of each and ... who knows what else. So you would probably only get an approximate answer, anyway. I assume less than the usual case, just because it happens much more quickly.

1 hour ago, hoola said:

I agree that is is very unlikely

On the other hand, it is a very big universe. So it could happen.

I would imagine the chirp ring down would simply have a faster rate. As far as the actual merging the total mass energy release should be the same.

Edit well it could be a bit more violent due to the momentum terms.

Edited September 6, 20196 yr by Mordred

2 hours ago, hoola said:

I agree that is is very unlikely,  perhaps impossible unless arranged somehow.

Unlikely, yes. Impossible, no.

Massive objects collide directly every day. There is nothing special about a black hole that would prevent such an occurrence. 

Could a simple experiment be done by watching two bubbles merge in water?

One with them in water that was spinning, and one with them in still water and measuring the difference. The effects would be tiny I imagine.

No wouldn't simulate any of the essential dynamics

yeah, bowling with black holes to get a perfect strike seems something an advanced species might do to see if the results matched prediction.

Who says we cannot simulate two BHs on a direct path merge. We are capable of doing such but takes more computing power than readily available equipment. 

Though I can do some first order calculations to ballpark it. (Beyond thinking of the relevant formulas off the top of my head lol)

Edited September 6, 20196 yr by Mordred