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A sphere contains all the mass on one side while the other side is empty of matter. Both sides are separated by a strong material, you name it, that will keep the sphere together. Given that the mass side is more massive than the sun or at least very massive, how would it behave in respect to gravity? Would it spin?

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The question is very unclear to me. Why would you expect it to spin? Is there some external field or does the question only concern it's self-interaction? – dmckee♦ Apr 21 '11 at 23:35
Agreed. This question is extremely confusing. – Ted Bunn Apr 21 '11 at 23:42
@Boundman: just FYI, the edit you submitted is the kind of thing that comments are intended for. (I think all users can comment on their own posts) You can always edit to question to make it more clear, that's how you get upvotes. – David Zaslavsky♦ Apr 22 '11 at 1:30
The question is clearly confusing, sorry everyone. Got confused by the 2 dimensional space-time analogy and took the sphere with half of it made of mass as something that could work as a fishing float or bobber. Sorry again for the stupid questionnnn – R. Zurschmitten Apr 22 '11 at 2:01

1 Answer

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If the sphere is rigid, meaning it cannot change size or shape at all, then gravity will have no effect. Specifically, the sphere will not rotate.

The distances between all parts of the sphere are fixed, so the gravitational potential, which depends only on these distances, cannot change. Therefore there is no way for the sphere to release any gravitational energy and so it cannot move.

If the sphere is not rigid, it may be able to move some, and will oscillate in its normal modes of oscillation. However, due to conservation of linear momentum, the sphere can't go anywhere, and due to conservation angular momentum, it cannot rotate.

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Why should gravity have anything at all to do with whether it will rotate? That should depend only on the initial conditions of formation and any external torques. – dbrane Apr 22 '11 at 9:48
@dbrane That was the point, yes. – Mark Eichenlaub Apr 22 '11 at 10:35

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