PDA

View Full Version : General Relativity Question

chriscurtis
2007-Sep-02, 05:35 AM
If two black holes collide, what happens as the event horizons meet each other?

publius
2007-Sep-02, 05:52 AM
If two black holes collide, what happens as the event horizons meet each other?

Something very, very complicated. :lol:

I wish I could put into words just how complex mathematically that problem is. There is certainly no analytic solution for it. The Numerical Relativity group at the Max Planck institute does problems like this numerically. The equations get so complex that it stretches numerical techniques to the breaking point. They use these parallel processor farms with hundreds of processors working for ages. IIRC, it took months of CPU time just to get the last orbit before merger.

But conceptually what happens is the two horizons "merge" and the end result is a bigger black hole with a larger horizon. Massive amounts of gravitational radiation is emitted. I've read the peak instantaneous power of that radiation exceeds the entire output power of all the stars in the observable universe.

And, that radiation is not isotropic necessarily, and the center of mass can "push off" against its own radiation, and kick the merged result off linearly. "Radiation recoil". This kick can well eject black holes from galaxies, and there is a lot of work being done looking at that.

-Richard

astromark
2007-Sep-02, 11:09 AM
Something very, very complicated. ..yes Publius. I agree with all you have said. The collision of two super massive black holes could be a Galaxy destroying event., or not. I would like to add that according to a paper that Carl Sagan had quoted from " It would be possible for two massive black holes to simply slide into each other, creating a much stronger gravity well and a very powerful gravity shock wave. Star clusters and galaxies could be consumed by the ever growing force...."
I suspect that a more Violante impact event as Publius has outlined would be far more spectacular spectacle. Lets just confirm that if we ever witness such a event that it is a long, long way away.

Tim Thompson
2007-Sep-03, 09:46 PM
But conceptually what happens is the two horizons "merge" and the end result is a bigger black hole with a larger horizon.
When black holes merge, the surface area of the event horizon of the final black hole is always larger than the sum of the surface areas of the two (or more) individual black holes. This is a formulation of the second law of thermodynamics that is peculiar to black holes, where the entropy of a black hole is proportional to the surface area. The Black Hole Thermodynamics page (http://en.wikipedia.org/wiki/Black_hole_entropy) from Wikipedia looks to give pretty good description of black hole entropy.

John Mendenhall
2007-Sep-04, 05:05 PM
The Black Hole Thermodynamics page (http://en.wikipedia.org/wiki/Black_hole_entropy) from Wikipedia looks to give pretty good description of black hole entropy.

Best discussions and further links that I've ever read. Thank you, Tim!