Tim Cook <z99### [at] bellsouthnet> wrote:
> Alain wrote:
> > I have a BIG problem with Dyson spheres! Inside an hollow sphere, there
> > is NO gravity, if the sphere is built around a star, everything not held
> > in place will fall in the star. If you make it spin, all the air will
> > collect at the equator, untill the sphere collapses unto itself. The
> > equatorial part goes flying away and the poles plunging into the star.
>
> Why is there no gravity?  If you take a sun-sized star and build a
> sphere of Earths around it at 1 AU (dunno where you'd get that many
> Earths), does the now hollow sphere not have gravity on either its
> inside or outside surface?

A spherical shell exerts no gravity on anything within.  If you do the math
it turns out that no matter where you are within the shell, the gravity
from the material behind you exactly cancels the gravity from the material
in front of you.

Outside the shell, gravity works like you'd expect.

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