Physicists are building a ‘mirror’ that reflects light in time

Would you like the famous echo joke “Who’s the Weasel King?” try it over a time-reflecting pit, the answer won’t be “Donkey!” but “Read! as if you were playing a record upside down. Incidentally, the physicists did not build a sink during their research, but a “mirror” that reflects electromagnetic waves – light – in time in the opposite direction. Or more precisely: a six-meter plate, covered with metal wicks and electronic switches.

“A normal mirror spins left and right, a phase conjugate mirror, as we call it in practice, reverses time,” says physicist Carlo Beenakker of Leiden University, who has not himself even participated in the research.

Although such techniques have been around for a while, they have so far been limited to very narrow wavelength ranges, he says. As if your mirror only works on a specific shade of a color. In their new research, the physicists achieve the same result for light in a much wider range of wavelengths, a stepping stone to the actual use of such mirrors in practice.

‘something magical’

“Don’t get me wrong: As exciting as time inversion might sound, it’s not a time machine or anything,” says Beenakker. At the same time, he describes what is possible with such mirrors as “something magical”.

The best known application is for use to eliminate light disturbances. In experiments you can then shine a beam of light through something cloudy, so that the light scatters. “A glass of milk, for example,” he says. By letting the light fall on a time-reversing mirror, you can then rid that light of scattering.

“People often say, if only I could rewind the movie, undo that mistake. Unfortunately, that’s not possible with humans, but light can do something comparable,” says Beenakker. In practice, this effect is useful, for example, for astronomical observations from Earth, when you need to free the light of the deep cosmos from disturbances in the Earth’s atmosphere.

Red turns green

Although the physicists performed their experiment with light that is not visible to the naked eye, the result of a time mirror on ordinary light would be bizarre. Because the frequency of this light changes with the technique used, you would see the colors reflected differently in the time-reversing mirror of these physicists: red would turn green, orange would turn blue, and yellow would appear purple.

The fact that it has now succeeded for the first time in providing time inversion for a wide range of wavelengths is due to the fact that the researchers made the mirror from metamaterials. These materials have such counterintuitive optical properties that they are also used in the development of “invisibility cloaks”, where you bend light around something so that you can no longer see an object.

According to the researchers, the ability to manipulate electromagnetic waves with metamaterials not only in their spatial motion but also in time can lead to more efficient wireless communication and improved radar technology in the long run, among other things.