From a physics perspective, the many-world, as ridiculous as it sounds, is a more sound theory (believed to be correct by some top physicists) than negative gravity.

The explanation of why gravity is only positive is a complex one, but physicists believe it's almost axiomatic.

This doesn't mean that you can't have repulsive type of effects, similar to one of the contributions that's believed to make the universe expand more quickly than with the simple 4 fources (often called fifth force, maybe related to dark energy, and to the rate of the universe expansion acceleration).

PS. There are a number of Sci-Fi stories that use this idea, including a famous one from Asimov, in which if I recall correctly the normal gravitational force was working negatively when traveling in FTL speed. Obviously that seemed fishy but the book was cool (Nemesis).