You might think of a laser as light forced into a single, directed beam, but scientists have recently discovered that if you fire a laser in one direction, the air itself can fire another right back. Using a 226nm UV pump laser, researchers at Princeton University managed to excite oxygen atoms to the point that they emit infrared light along the same channel as the original beam, except this time pointed back where it came from. Since the return beam's chemistry depends on the particles in the air to generate the return beam, the "backward laser" could potentially carry the signature of those particles back to the source and help identify them there. That seems to be the entire goal
