Posted on 07/12/2002 3:38:57 PM PDT by gcruse
I thought the black hole proper was literally a mathematical point. How can a point spin?
(Another dumb question from a guy who quit physics just before they got to the bizarre stuff.)
Gravitational collapse imparts spin, I would
think, from the conservation of angular velocity.
Much as pulling you arms in as you twirl on
your toes is supposed to speed you up,
the collapse into singularity would take
its spin with it. Being a point doesn't
mean never saying "I'm dizzy." But
the real experts should be along shortly. :)
Ah yes. Leaping Leprochauns. Aren't they predicted by string "theory"?
Something like the surprising amount of iron on an extremely redshifted quasar on another thread yesterday. That's a black hole, too.
Just trying to imagine matter being sucked toward the centerpoint after it has crossed the event horizon. I imagine it zips along pretty quickly. Whether or not it's still matter after crossing that boundary, I have NO idea. :-)
Aside from size, the are both black holes. The iron in this one is expected since it was a star, probably 2nd generation or later, of relatively recent creation and so it would be expected to have a lot of iron. The quasar would have been an entire galaxy created near the time of the Big Bang and shouldn't have had time to evolve so much iron.
Hey! It's weird out there!
Right, the event horizon definitely has a radius. But I've read that's not really the beginning of the black hole proper. It's just the solution to an arbitrary mathematical problem; namely, find the equation for the boundary beyond which you're not getting out. The black hole proper is at the very center, and is no larger than a mathematical point.
I particularly liked the frame dragging explanation, but there are a lot of other interesting comments here about it, too.
Thanks, everyone . . . I love these threads! :-) A lot of smart people hang out at FR, that's for sure.
A black hole ends in a singularity, i.e. a point where the known laws of nature break down. We don't really know much about them, it's really off the wall stuff, but perhaps the high gravity has some effect not on the black hole but on the area of space-time directly surrounding the black hole, giving it the effect of spinning.
However, it seems possible and within the laws of physics so far, to use a black hole to do some limited time travel.
Actually, it has a circumfrence and surface area. There's no way to measure the radius.
Hawking's theorem that 'a black hole has no hair' is interesting: it states that the only measurable quantities associated with one are its mass, angular momentum, and electric charge. In particular, there is no observable difference between a hole that was originally matter, one that was originally anti-matter, and one that was orginally just light or graviatational waves (assuming that's possible)
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