Spacecraft already navigate our solar system using gravity wells as slingshots. The same basic principles operate in the intense gravity wells around black holes, which bend not only the paths of solid objects, but light itself. If a photon, or a light particle, enters a particular region in the vicinity of a black hole, it will do one partial circuit around the black hole and get flung back in exactly the same direction. Physicists call those regions "gravitational mirrors" and the photons they fling back "boomerang photons." Boomerang photons already move at the speed of light, so they don't pick...