An astronomer at Columbia University has a new guess about how hypothetical alien civilizations might be invisibly navigating our galaxy: Firing lasers at binary black holes (twin black holes that orbit each other).
The idea is a futuristic upgrade of a technique NASA has used for decades.
Right now, spacecraft already navigate our solar system using gravity wells as slingshots. The spacecraft itself enters orbit around a planet, flings itself as close as possible to a planet or moon to pick up speed, and then uses that added energy to travel even faster toward its next destination. In doing so, it saps away a tiny fraction of the planet’s momentum through space — though the effect is so minimal it’s pretty much impossible to notice.
The same basic principles operate in the 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.”