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.

To ask if a beam of light can make a shadow sounds like a sort of Zen paradox, but physicists have demonstrated the possibility. They used a laser beam to block another laser beam, albeit with some crystalline assistance.

With the help of a ruby cube and two laser beams, researchers made one ray of light cast a shadow when illuminated by the other

When two beams of laser light are crossed in just the right way, the primary beam is unable to pass through the secondary – creating a shadowed line in the light that hits the opposite surface. "Our demonstration of a very counter-intuitive optical ...