Quote:
Originally Posted by Steve O
But a black hole is a singularity.
|
As
Cougar suggests, this is where you get hung up. The singularity is only part of the complex affair that we call a black hole. The singularity is just a point, or small collection of points, deep inside the black hole where our mathematical theory of physics fails, and we no longer have the ability to physically interpret the mathematics. But the black hole is in fact an extended region of spacetime surrounding the singularity. When we look at a black hole from far away the singularity is hidden from our view by the
event horizon. If you are going to say "a black hole is ..." something, it makes more sense to say it is an event horizon rather than a singularity, because the event horizon is something we can physically interact with. So now that we understand the extended nature of the black hole, we can more easily grasp the idea of rotation.
The size of the black hole is not too hard to get at. Just look up "
Schwarzschild radius" and you can get the mass dependent radius of a non rotating
Schwarzschild black hole, which is good enough for this discussion (rotating black holes are more complicated, but not significantly different in size). So a black hole with the same mass as Earth would only be about 8.8 millimeters in radius, not very impressive on the cosmic scale of things. Even a black hole as massive as the sun has a radius of only about 2.9 kilometers. Not much for size, but with the same mass, and therefore the same gravity, as our sun, it would gravitationally dominate the space around it just as our sun gravitationally dominates the solar system. The black hole at the center of our galaxy is about 3.7 million times the mass of the sun. For that we get a schwarzschild radius of about 10,000,000 kilometers, or about 0.07 astronomical units. That may still seem small, but anything that massive is gravitationally hard to ignore (see, i.e., the
UCLA Galactic Center Group webpage)
Now that we have all that figured out, the last thing to remember is that a black hole is not a solid object with a solid surface, the way we usually think of it. White dwarfs, neutron stars, even exotic quark stars and such are all still solid objects with solid surfaces. It's easy to understand rotation of a solid object. But in the case of a black hole, it is not a solid object that is doing the rotating, it is the region of space time that is the black hole that is doing the rotating. Whatever it was that became a black hole, maybe a star, or a whole lot of stars, must have had some angular momentum associated with it. That momentum must still be conserved, it can't just vanish. So the spacetime that is the black hole inherits the angular momentum of the progenitor, and the black hole rotates.
Finally, I will point out that solid surfaces & event horizons are observationally distinguishable, one from the other. Material that falls on a solid surface behaves differently from material that falls on an event horizon. This difference can be used to observationally infer the presence of an event horizon around a compact object, and therefore observationally distinguish between solid objects and black holes in candidate objects. This has been done, and it is fair to say that we have observationally segregated black holes from non black holes in a few cases (i.e.,
Remillard, et al., 2006;
Done & Gierlinski, 2003;
Paul, et al., 1998).