Laser Experiment Shows Light Casting its Own Shadow

A new experiment has demonstrated something seemingly impossible: light casting its own shadow. When a laser is manipulated in a specific way and illuminated side-on by another light source, it’s possible to create this unusual optical effect.

While we’re used to seeing shadows in everyday life, they appear as dark areas on a surface where an object blocks incoming light. The key word here is “object” – whether solid, liquid, or gas, shadows are usually created by objects with mass.

Overcoming the Mass Requirement of Shadows

This should rule out light, since photons have no mass. However, researchers at Brookhaven National Laboratory found a way to make light paradoxically cast its own shadow.

“The idea of laser light casting a shadow was previously thought impossible, as light usually passes through other light without interacting,” explained Raphael Abrahao, lead author of the study. “Our demonstration of this counterintuitive optical effect invites us to reconsider our understanding of shadows.”

To create this laser shadow, the team projected a high-powered green laser through a ruby cube, illuminating it side-on with a blue laser. In this setup, the green light blocks some of the blue light, projecting its outline on a screen.

Criteria for a True Shadow

The team explains that this projection meets certain criteria for a shadow: it’s visible to the naked eye, follows the contours of the surface it falls on, and reflects the position and shape of the green laser beam. It even reaches a maximum contrast of around 22% – comparable to a tree’s shadow on a sunny day.

The effect is due to intriguing optical physics. Where the green laser hits the ruby, it increases the amount of blue light the crystal absorbs. When this blue light hits the screen, it leaves a slightly darker outline in the shape and position of the green laser. This leads to an interesting question about the nature of the blocking.

Is It Truly the Light Casting the Shadow?

“The laser shadow effect requires the ruby to mediate the blockage, which raises the question of whether it’s the photons in the laser itself or the atoms in the ruby that block the illuminating light,” the team writes.

Light waves passing through a material interact with its atoms, creating quasiparticles called polaritons. Technically, it’s these polaritons – which do have mass – that are casting the shadow. However, polaritons are still partly made of photons, and the shadow wouldn’t be possible without these photons. The shadow also retains the appearance of the initial laser beam, meeting the conditions outlined by the researchers.

While immediate practical applications aren’t clear, the team suggests that this research could open up new possibilities in fields such as manufacturing, imaging, and illumination.

Read the original article on: New Atlas

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