Thread: Black Hole Collisions

They certainly collide locally. But from the viewpoint of the external observer ... they are stuck in time.

Originally Posted by JFalz1024

I feel like the "centers" of the BH's would be pulled toward eachother and "collide" where the EH's stretch and meet eachother.

I'm with you, but simultaneously I'm missing something. What do you mean "stuck in time"?

Originally Posted by JFalz1024

I'm with you, but simultaneously I'm missing something. What do you mean "stuck in time"?

At the EH ... time is stopped from the refernce point of the distant observer. So if you are observing and event there ... like say a black hole collision, you would see it hit and get stuck in its initial point of collision. Since time is stopped ... in can not change ( by definition ), without external evens affecting it( more mass, another collision, hawking radiation ).

It is stuck in time. as time doesnt change ... change can not happen.

Originally Posted by tom

At the EH ... time is stopped from the refernce point of the distant observer. So if you are observing and event there ... like say a black hole collision, you would see it hit and get stuck in its initial point of collision. Since time is stopped ... in can not change ( by definition ), without external evens affecting it( more mass, another collision, hawking radiation ).

It is stuck in time. as time doesnt change ... change can not happen.

I see. So are we questioning perspective here? Such that if I was at the EH of one BH as another comes towards it, would it appear that once the EH's touch they stay that way since change cannot occur without time?

Makes sense. But wouldn't that only exist for the single moment in time where u were on the EH? It would appear that way while u were on the EH because time has stopped to that which is perceiving it as its being pulled in. Meanwhile just outside the EH the BH's would change after that moment surely because more mass has been added to the system, right?

I hope that wasn't too muddled. How does it sound?

Originally Posted by tom

They certainly collide locally. But from the viewpoint of the external observer ... they are stuck in time.

You have it backwards. From the viewpoint of the external observer, physical processes proceed normally. I found a couple of merging black hole simulationx on YouTube at Simulation of black holes collision - YouTube and Merging Black Holes [720p] - YouTube, but they seem mistaken to me. The event horizons should retreat from each other as the opposing gravitational forces cancel. Even after the black holes merge and the EH closes over both of them, there should be a line of normal space-time between the two orbiting singularities.

Originally Posted by Larry Caldwell

You have it backwards. From the viewpoint of the external observer, physical processes proceed normally. I found a couple of merging black hole simulationx on YouTube at Simulation of black holes collision - YouTube and Merging Black Holes [720p] - YouTube, but they seem mistaken to me. The event horizons should retreat from each other as the opposing gravitational forces cancel. Even after the black holes merge and the EH closes over both of them, there should be a line of normal space-time between the two orbiting singularities.

If im thinking of this correctly, the picture that comes to mind is two icecream cones stacked upside down on each other frozen in time (relative to us) with a gap of normal space-time between them? Also If they cancel out, what happens to all that energy?

no it would ONLY appear that way for the distant observer .... locally you are in the middle of a chaotic merge.

Originally Posted by JFalz1024

I see. So are we questioning perspective here? Such that if I was at the EH of one BH as another comes towards it, would it appear that once the EH's touch they stay that way since change cannot occur without time?

Makes sense. But wouldn't that only exist for the single moment in time where u were on the EH? It would appear that way while u were on the EH because time has stopped to that which is perceiving it as its being pulled in. Meanwhile just outside the EH the BH's would change after that moment surely because more mass has been added to the system, right?

I hope that wasn't too muddled. How does it sound?

This is one thing to remember ... there is a difference between gravitational potential energy( scalar ) and gravitational force ( vector ) between two BHs there is a point where there is no gravitational force ( as all gravitationa forces cancel themselves out ( like in the center of the earth ) ) ... however that point inbetween can be in a deep gravitational well ( gravitational potential gravity ). The EH is formed based on gravitational potential energy NOT based on Gravitational Force.

The thing is ... that to escape from the system ( the two black hole system ) you need to dig yourself out of that well so while the force can be a null vector it may take travelling at c to escape the system from the center.

Originally Posted by Larry Caldwell

You have it backwards. From the viewpoint of the external observer, physical processes proceed normally. I found a couple of merging black hole simulationx on YouTube at Simulation of black holes collision - YouTube and Merging Black Holes [720p] - YouTube, but they seem mistaken to me. The event horizons should retreat from each other as the opposing gravitational forces cancel. Even after the black holes merge and the EH closes over both of them, there should be a line of normal space-time between the two orbiting singularities.

They dont cancel out ... the gravitational potential energy increases at all points as they move close together ... at some point the EH will start to combine ... I believe this is at either 1.5 or 1.75 times the SR for two equally massive BHs

Originally Posted by AdrianX

If im thinking of this correctly, the picture that comes to mind is two icecream cones stacked upside down on each other frozen in time (relative to us) with a gap of normal space-time between them? Also If they cancel out, what happens to all that energy?

The event horizon is the product of gravitational gradient - when the orbital velocity becomes equal to the speed of light, and there is no longer an outward direction for a photon to take. It is not possible to move from a little inside the event horizon to a little outside the event horizon. Indeed, this is the cause of Hawking radiation, where spontaneous quantum pair production can result in one particle forming inside the event horizon and being trapped, while the other particle forms outside the event horizon and can escape.

While I can see your point about two black holes merging to become a larger black hole, it seems to me that the shape of the event horizons would change before the even horizons merged. If you consider a case where the gravitational gradient from opposing black holes cancels 50% of the gravitational force, the two event horizons would depress 1/4 of their radius. If you consider a particle moving through the center of that space, it would experience no gravitational force directly between the two event horizons, but would experience gravitational attraction from both black holes as it moved away from the center. On a tangential path, the gradient would be 2Gmsin(a)/r'^2 where m is the mass of each black hole, a is the angle between the center line between the black holes, and r'=rx/sin(a), where r is the initial distance between the center of the black holes and x is the distance traveled orthogonal to that line. This resolves to the function 2[sin(a)]^3/r'^2, which is always less than 1. If you want to break it down further, a=arctan(x/r). Most of the vectors cancel, as do all of the relativistic effects as long as you stay equidistant from the two black holes. This is a special case, but it demonstrates my point, I think.

Originally Posted by tom

This is one thing to remember ... there is a difference between gravitational potential energy( scalar ) and gravitational force ( vector ) between two BHs there is a point where there is no gravitational force ( as all gravitationa forces cancel themselves out ( like in the center of the earth ) ) ... however that point inbetween can be in a deep gravitational well ( gravitational potential gravity ). The EH is formed based on gravitational potential energy NOT based on Gravitational Force.

The thing is ... that to escape from the system ( the two black hole system ) you need to dig yourself out of that well so while the force can be a null vector it may take travelling at c to escape the system from the center.