Can light itself cast a shadow? It may sound like a philosophical riddle, but researchers have found that under certain conditions, a laser beam can act like an opaque object and cast a shadow. The discovery challenges the traditional understanding of shadows and opens new possibilities for technologies that could use a laser beam to control another laser beam.

The technique works because the green laser (wavelength 532 nanometers) changes the way the crystal responds to some other wavelengths of light, including the 450 nm of the blue laser. Consequently, the crystal now serves to block the blue laser’s path, creating a shadow on a nearby screen.

Light shines; photons travel until they hit an object through which they cannot pass; this blockage creates a shadow, a little piece of darkness where the light has been impeded. Physicists, however,