MadSci Network: Astronomy

Re: Why is the event horizon of a black hole flattened?

Date: Sat Sep 29 14:57:46 2001
Posted By: Angelle Tanner, Grad student, Astronomy, UCLA
Area of science: Astronomy
ID: 1000527774.As

Wow. This question took some research on my part. It seems a lot of different references give different answers. Here is some information which may help clear things up:

Indeed, you are on the right track. Its the conservation of angular momentum which causes a rotating mass to flatten. The confusion among the various articles I have looked at involves what component of the black hole has the non-spherical or ellipsoidal shape. To provide a little more background, rotating black holes (or Kerr black hole) are a bit more complicated than non-rotating black holes. They have two event horizons---both an inner and an outer. They have a ring-shaped singularity as opposed to the point like singularity of a non-rotating black hole. Finally, they have a region outside the outer event horizon called the ergosphere.

Okay, what are all these mysterious pieces-parts? The event horizons have the same definition as the one for the non-rotating black hole. These are boundaries at which time appears to stand still to a distant observer. Like a non-rotation black hole, though, when you cross an event horizon, there is no hope of escape. Apparently, one horizon flips space and time one way while the other flips it back. Wacky stuff, if you ask me. The ergosphere is a region where it is impossible to stay still. In other words, even light rays are forced to orbit around the black hole when they enter the ergosphere. In contrast to the event horizons, one is able to enter and exit the ergosphere at will.

Now back to your question. According to one diagram I have found, all of these components are ellipsoidal in shape. Another diagram has the inner event horizon being spherical. My guess is that all components are probably ellipsoidal. Whew. I bet that raises more questions than answers but there it is.

For a diagram and some more explanation, check out Jillian's Guide to Black Holes.

Thanks for expanding my mind.
- Angelle Tanner

[It's also worth pointing out that when we say "rotating," what we really mean is that the black hole has angular momentum. Angular momentum is a quantity that describes an object's rotation, but it is more general than that. For instance, electrons have angular momenta. However, electrons appear to be point particles, with no spatial extent. Thus, they cannot be said to be "rotating." A more general way to think about a rotating black hole is to think not about the hole itself but its effects on spacetime around it. Einstein's general theory of relativity predicts that any object with angular momentum---the Earth, the Sun, a black hole---distorts the spacetime around it. Indeed, Gravity Probe B is an experiment designed to test this prediction using a spacecraft in Earth orbit. The idea is to see if the Earth's angular momentum distorts spacetime around the Earth. In the neighborhood of a Kerr black hole, these distortions would be even more severe. One may also want to check out The Hitch-Hiker's Guide to Black Holes for more information on Kerr black holes. Moderator]

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