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.

Wasn't it Tim Cook who wrote:
>>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?

Mike Williams wrote:
> There's gravity on the outside, but not on the inside. It just so
> happens that the strong contributions from the small regions under your
> feet are exactly cancelled out by the weaker contributions from the much
> larger regions over your head. I don't think there's any simple way to
> demonstrate that, you have to calculate the integral like Newton did.

There's a good lot of discussion on Wikipedia. Search for Dyson Sphere 
and then go to the discussion page (tab at the top). Talks about the 
calculations et. al.

There'd be neutral gravity on the inside, but if you spin the sphere 
fast enough, then momentum-in-a-circular-restriction (aka centrifical 
force) will stick you to the sphere. Something like 32km/s (or 320 .. 
it's been a while) for a 1AU sphere.

(And the gravity on the outside would be horrendous, the gases from the 
sun would kill you, there's not enough matter to make it .. yada yada .. 
great concept for SciFi, and ray tracing, but not real life!)

Cheers!
Rick Measham

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