With a rotating black hole, what is rotating?
From: Ulmo (ulmo_at_cheerful.com)
Date: 06/29/04
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Date: Tue, 29 Jun 2004 21:32:54 +0000 (UTC)
With a rotating black hole, what is rotating? The event horizon is not
a physical object. It's just defined as the set of points such and
such distance from the singularity, so how can it be rotating? If you
defined the "effielsphere" as the set of points exactly 100 meters
from the top of the Effiel Tower, and then imagined that sphere as
"rotating", what would that mean? The event horizons are spherically
symmetric, and so are exactly the same whether they are "rotating" or
not. The spacetime curvature caused by the black hole is spherically
symmetric, so how would you know if it was "rotating"? The singularity
is just a mathematical point, and this can't be "rotating". If you had
a rotating physical object like a planet or a neutron star, there are
actual subatomic particles that are spinning around about the center,
but a black hole is not made of subatomic particles. If a physical
object like a planet or neutron star is rotating, you could imagine
that somehow spacetime is "attached" to the subatomic particles that
make it up, and thus somehow "dragged along", but a black hole is not
made of subatomic particles. It's just the singularity, which is a
mathematical point, surrounded by the event horizon, which is just
defined as such and such distance from the mathematical point. Also,
you could say the coordinate system is rotating, but the coordinate
system is purely a human invention, and if you wanted to, you could
set up a rotating coordinate system for stationary object.
David
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