Expert: Philip A. Stahl - 3/4/2009

Question
I have heard that black holes have the power to bend and rip space and time. Is
this possible?
Also, what happens when black holes collide?

Answer
Hello,

In actual fact, if one studies a space-time diagram (e.g. Kerr-type) for a black hole. it is not evident anything is being "ripped". One has to be aware that the terms sometimes used in popularizations of black holes do not literally translate to what they can do.

Having said that, if there was an inherent power to "rip" space-time it would reside more in what we call "naked singularities". A singularity itself represents the very center of the black hole where all its mass is concentrated. This yields an infinite density and (at that POINT) an infinite curvature of space-time. A "naked singularity" represents a singularity detached from the surrounding event horizon that usually is a part of normal black holes. In normal cases, the event horizon is what prevents light from escaping the black hole ...to enable direct observation. We have to go by indirect data instead (e.g. x -radiation emitted from a nearby star whose outer gas shell is being pulled into the event horizon of a neighboring black hole)

You can read more about these objects in another answer I gave on this forum:

http://en.allexperts.com/q/Astrophysics-3368/2009/1/infinity.htm

and more in general here:

http://en.wikipedia.org/wiki/Naked_singularity



As to what occurs when two black holes collide, assuming that they coalesce, the surface area of the final object will exceed the sum of the surface areas of the two separate surface areas. This is also referred to, e.g. by Hawking, as "the second law of black hole dynamics". (And it is analogous as I show below, to the 2nd law of thermodynamics, e.g. which asserts the tendency for entropy or disorder to increase within closed systems)

Since the entropy of a b. hole is proportional to the surface area, this means its entropy increases on collision (and assuming a follow-up coalescence. Obviously if there is no coalescence and the collision is just "grazing" we'd not expect the same increase in surface area or entropy)

What happens is that there is also an upper limit placed on the total amount of gravitational radiation that can be emitted in the collision.

Since gravitational radiation will be conveyed via gravitational waves (of order wavelength ~ 10^-14 m) then it appears that the increased surface area-entropy is probably the agent that limits the intensity of these waves.