If a super massiave black hole has the average density of water and is
made up of nothing but water would a air filled ball magically placed
on the surface float? why or why not please.

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Clarification of Question by bill22-ga on 16 Jan 2006 20:09 PST

I do not necessarily agree with that comment if we have a large enough
mass of just water will not light be unable to escape the mass of the
waters pull???

I assume that by average density you mean the total mass of the hole
divided by the volume of the event horizon. But the average isn't the
same as the actual density everywhere within. All the mass of the
black hole resides at the singularity, at the mass center. The event
horizon itself is just a radius from the singularity at which the
local escape velocity just infinitesimally exceeds the speed of light,
which is why it's black and escape-proof. So there's no surface there
made of matter for the ball to float upon. It'll just fall in.

Qed is correct.

In answer to your second question, if you started building a star made
of water it would collapse under its own gravity when it got to be a
bit over a solar mass in size. It would collapse to enormous
densities, forming a neutron star. It would be made of neutrons, not
water. If you added a bit more water it would collapse to form a black
hole with even higher "density".

Hi Bill, 

I believe that the problem lies in the fact that black holes aren't
made of matter as we know it, you are attempting to apply the logic of
the laws of physics that apply in our environment, to the environment
within a black hole. They are not the same, you can not have a black
hole with the average density of water. Anything that falls within the
'event horizon' of a black hole becomes part of the black hole (except
information according to Hawking but we wont go there...).

Hope that helps,

Eli

I agree with qed and kott.  However, here is a slightly different take
on the question.  The radius of a black hole with the "average
density" of water is about 400,000,000 km.  If the singularity was at
the sun, the event horizon would be between the orbits of mars and
jupiter.  Because this is such a gigantic black hole, the
gravitational field at the the event horizon is not that big, only
about 1000 times the field on the surface of the earth.  All the
'water' in your black hole would be at the singularity, because no
matter, including water, could withstand the pressure.  However, it is
easy to imagine a small, dense object that would produce a
gravitational field 1000 times as strong as that at the surface of the
earth, without the pesky singularity.  An air-filled ball would
certainly float in a tank of water in this gravitational field.

OK I think the comments have set me straight on this question. I will
have to post another question something to the effect " does light
ever escape our universe and if not is our universe a black hole if
our univesre is a black hole what is the singularity"