Nothing can be fully transparent, physicists discover

The Guinness Book of World Records lists Ivan Smalyukh as the inventor of the most transparent material known. His lab at the University of Colorado Boulder has developed gels that could be used for window insulation by carefully tuning them until they bent light almost as little as air does, making them clearer than the glass itself.

His work and other recent advances might suggest that researchers are on a path toward developing a completely transparent substance. That dream was recently proved false, however, when a group of theoretical physicists found that no light-bending material can ever be fully transparent, owing to a quantum process that turns waves of light into waves of sound. The researchers argue that even the cleverest engineering will never be able to create a material that doesn’t lose some of the light that passes through it. But the loss can be very, very small.

Smalyukh’s gels already get pretty close. But they’re designed to be applied thinly: a foot-thick layer would transmit very little light.

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If you want to see farther, you’d have better luck peering through a fiber-optic cable, like those used to link the Internet across the ocean. Fiber optics carry light on bending paths for miles before they need to amplify signals for the next section of cable.

To do even better, you might turn to a stranger substance. Cool helium down to only a few degrees above absolute zero, and it becomes a kind of liquid known as a superfluid. In a superfluid, quantum mechanics coordinates the way different atoms move, letting the liquid flow with no friction at all. Liquid helium, in particular, is astonishingly clear, and although it absorbs x-rays, scientists have not been able to measure an effect on visible light.

Theoretical physicists Alberto Nicolis, Federico Piazza and Angelo Esposito have spent their careers thinking about strange quantum materials such as superfluid helium. Nicolis and Piazza were curious about what would happen to a material that was as transparent as possible. It would have to be cold, so that light didn’t get lost in the jiggling of already moving molecules, and inert, with no chemical reactions or electric currents that might absorb light as a source of energy. They expected that such a material would still absorb some light particles, called photons, through subtle quantum processes, and they set out to estimate how much light would be lost.

Nicolis and Piazza told this to Esposito, who realized he had made a similar estimate—not for light but for dark matter. He had been trying to figure out how to detect a hypothetical particle called a dark photon and realized it could potentially be found if it nudged an atomic nucleus in superfluid helium.

“In interacting with one of the nuclei, it gives it a kick,” Esposito says.

That kick would trigger a tiny vibration, a kind of quantum mechanical sound wave. As this happens again and again, the dark photons would lose energy. Esposito realized the same thing should happen to normal photons. Eventually the photons would lose so much energy that they would be undetectable.

That “eventually” can be a long time. The perfect superfluid the physicists imagined is so clear that light could travel across the diameter of Earth before it becomes noticeably fainter. But eventually the light would still fade. And while the researchers’ model only describes superfluids, they expect other materials to follow similar rules.

“I think it’s universal for all materials,” Esposito says. The group posted its findings as a preprint on the website arXiv.org on October 8.

“I find this work super interesting,” says Borna Salehian, a physicist at the Swiss Federal Institute of Technology Zurich, who was not involved with the project. “In theoretical physics, we look for fundamental limitations set by nature. This paper precisely gives one!”

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