A new experiment has demonstrated something that sounds physically impossible – light itself casting a shadow. It turns out that if you manipulate a laser just right, then hit it side-on with another light source, it’s possible to create this bizarre optical effect.
Shadows are so familiar in everyday life that it feels a little weird having to define them, but they appear as a darker area on a surface, where an object has blocked incoming light. The key word there is actually “object” – whether it’s a solid, liquid or gas, shadows are only known to be made by objects with mass.
That should rule out light, since photons are massless. But in the new study, researchers at Brookhaven National Laboratory have found a way to, paradoxically, make light cast its own shadow.
“Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting,” said Raphael Abrahao, lead author of the study. “Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow.”
To create this laser shadow show, the team shone a high-powered green laser through a cube made of ruby. This was then lit up side-on by a blue laser, so the green light ends up blocking some of the blue light, projecting its outline on a screen.
The team says this projection meets a few criteria of a shadow: namely, it can be seen by the naked eye, it follows the contours of the surface it falls on, and it follows the position and shape of the green laser beam. It even packs a maximum contrast of about 22% – equivalent to the shadow a tree casts on a sunny day, the team says.
The effect works thanks to some intriguing optical physics. Where the green laser hits the ruby, it increases the amount of blue light the crystal absorbs. When that blue light then hits the screen behind it, it leaves a slightly darker outline in the exact shape and position as the green laser. This explanation might bring up a potential semantic argument.
"The laser shadow effect requires a ruby to mediate this blockage, which raises the interesting question of whether the photons in the object laser themselves are blocking the illuminating light or rather it is the atoms in the ruby," the team writes.
Light waves passing through a material couple to its atoms, creating quasiparticles called polaritons. The team admits that technically it's these polaritons, which do have mass, that are casting the shadow. But on the other hand, polaritons are still half-photons, and the shadow couldn't be cast without those photons. It also still resembles the first laser beam, too, in all the ways the researchers outlined.
Regardless of where you stand, the experiment was mostly an exercise in investigating how to bring a hypothetical question to life – how can we make light cast a shadow? Real-world applications might not be immediately clear, but the team suggests the research could lead to new possibilities for fabrication, imaging and illumination.
The study was published in the journal Optica.
Source: Optica via Eurekalert
