Shadows, traditionally formed when objects block light, are a familiar part of daily life. However, researchers at Brookhaven National Laboratory have achieved something extraordinary—making light cast a shadow. Using clever manipulation of lasers, they demonstrated this seemingly impossible optical effect.
In everyday terms, shadows occur when light is obstructed by an object with mass, like a tree or a wall. Since photons, the particles of light, are massless, they shouldn’t block light or create shadows. But the team, led by Raphael Abrahao, found a way to defy expectations.
The experiment involved shining a high-powered green laser through a ruby crystal cube. A blue laser illuminated the ruby from the side. Remarkably, the green laser appeared to block some of the blue light, creating a shadow of its outline on a screen.
This effect meets the criteria for a shadow: it is visible to the naked eye, follows the contours of the green laser beam, and has a contrast similar to a tree’s shadow on a sunny day (22%). The science behind it lies in the interaction of light waves with the ruby’s atoms.
Where the green laser strikes the ruby, the crystal absorbs more blue light, forming quasiparticles called polaritons—hybrids of light and matter. These polaritons, with some mass, are technically responsible for the shadow. Yet, because polaritons are part-photon, the shadow fundamentally owes its existence to light itself.
This revolutionary study reframes how we see shadows and light. Although there aren’t any immediate practical uses for the discovery, it may spur improvements in imaging, fabrication, and lighting methods. More crucially, it contradicts long-held assumptions about light and its behavior, opening the door for future investigation into the unusual interplay of light and matter.