Wasn't it Tim Cook who 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?

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.

-- 
Mike Williams
Gentleman of Leisure

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