On Oct 11, 11:52*pm, Thomas Prufer prufer.pub...@mnet-
online.de.invalid wrote:

But wouldn't the stuff outside the shell move down, er, midpointwards, under the
influinece of gravity, and then continue on to the middle, i.e. the shell fill
up eventually?

I think all sort of things would happen. The shell of atmosphere
would most likely disperse pretty quickly as it wouldn't have anywhere
near enough gravity to sustain itself as an atmosphere. If it could
be magically held in place, things outside the shell would be drawn
toward the center of the sphere - but once inside the shell, they'd
float freely to the other side of the shell where they'd go flying
back out at the same speed they came in at (minus any frictional
losses they encountered passing through the shell.