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 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, in fact -- the project, funded by an Office of Naval Research program on "Sciences Addressing Asymmetric Explosive Threats," hopes that such a laser can ID bombs from a distance by hunting for trace chemicals in the air. Sounds like the perfect addition to our terahertz specs, and one step closer to the tricorder of our dreams.