Are black holes two-dimensional or three-dimensional?

On Fri, 7 Jul 2000, Eric wrote:

> Are blackholes 2 dimensional.
> (ie. laid out on rubber graphpaper and then place a weight in the center) as
> one person said
> or are they 3 dimensional spheres like a baseball with a region of compresed
> gravity out to say the size a red giant star where the event horizon starts.
> (ie. the event horizon is a sphere around the singularity, not a circle /
> donut that you could fly a space ship under to see what's underneath.)

Hello Eric,

Black holes are three-dimensional, and you may think of them as spherical
(our concepts of shape are not exactly valid near a black hole because its
gravity distorts space around it, and if the black hole is spinning, it
isn't exactly spherical.)

Many astronomers, including Stephen Hawking, use the analogy of a bowling
ball (or something heavy) placed on a rubber sheet to describe how gravity
can bend space. This is a two-diminsional model of a three-dimensional
concept. Our brains can understand curvature in two dimensions, but
curvature in three dimensions is much, much harder to grasp. I'll admit
that I cannot picture it!

Finally, the distance to which a black hole distorts space isn't the size
of a red giant star. The gravity of a black hole, just like the gravity
of our sun or any other object, can be felt to infinite distances. But
the "strange" effects of a black hole really only happen close to the
black hole's event horizon. The event horizon is 3 kilometers in radius
for every solar mass of material in the black hole, so a black hole with
three times the mass of the sun has a radius of 9 km. So, if we were to
replace the sun with a black hole of one solar mass, all the planets would
orbit exactly like they do now. (Whereas a red giant star can be as large
as Earth's orbit, or a few even larger than Jupiter's orbit!)

Thanks for writing!

Sincerely,
Kurtis Williams

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