As long as we're on the subject of fun things you can do with negative mass, I'd like to plug my novel "Warriors of the Hollow World." It's crammed with many (probably too many) ideas, characters, concepts, and craziness. One idea, however, was never fully explained--the "Hollow World" itself.
The "Hollow World," a.k.a. the Sphere, is an artificial habitat with 1000 times the living area of the Earth. The various peoples that inhabit it live on the inside. How is that possible?
At the center of the Sphere is the Source--a massive tetrahedral structure with fusion reactors at the points. Light beams out from these points illuminating half the interior surface at a time. Surrounding the Source is the Veil, a shell of moving negative mass objects which fly about, maintaining their positions and distances with small thrusters. The Veil has sufficient negative mass to provide a repulsive force on the outer positive mass shell on which the people of the Sphere live. This negative force makes it possible for them to live on the inside.
Thus, for the people of the Sphere, one gets to outer space by going down.
One of the interesting things about uniform shells of mass (be they positive or negative) is that there is no gravitational force inside the shell. Thus, in the region between the Veil and the Source, a starship can orbit, balancing centrifugal force against the gravitational pull of the central object. Ships in this region are unaffected by the Veil or the surface.
Between the Veil and the surface, orbits are impossible. Any ship flying in this region must constantly use its engines to keep from being pushed outward.
Outside the sphere there is virtually no gravitational force. Between the Source, Veil, and surface the net mass is nearly zero.
Part of the reason the people who built the Sphere constructed their world in this way was that they needed to hide from a powerful enemy. By keeping the mass of the entire artificial world nearly at zero, they (1) avoid any gravitational lensing that might reveal their position and (2) make it easy to move the habitat through space and find new and better places to hide.
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Hey! You Got Science in my Science Fiction (Essays)
Non-FictionShort essays on how to make your star drives, freeze rays, and time machines more "believable" along with some insights on what is and isn't possible. Most of these essays will probably focus on general relativity, as that's the field I've publishe...
