This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
In a universe with the same physical laws, but which is mostly water
with little bubbles in it, do the bubbles attract, repel, or what?
A bubble should produce a gravitational field that is the negative of
that produced by an equal volume of water in an empty universe. This is
because a point in space would be affected only by the water in the
symmetric image of the bubble with respect to that point. The effect
on another bubble in that field would be to attract that bubble since
it would be pushing the water around it away. Therefore, the bubbles