Randy introduces Jim to several ways in which people have theorized that gravity can be used to propel an object through space. The slingshot effect is the only proven method here, but people have found many ways that theoretically could induce propulsion taking advantage of non-commutative motions in space-time, negative inertia, artificially-induced gravitational dipoles, and creating bubbles in space-time. Show Notes: http://frontiers.physicsfm.com/25