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.

“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 himself hasn’t 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, physicists get the same for electromagnetic waves – light – in a much wider range of wavelengths, a stepping stone to the actual use of such mirrors in practice.

“Make no mistake: As exciting as time inversion might sound, it’s not a time machine or anything like that,” says Beenakker. At the same time, he describes what is possible with such mirrors as “something magical”.

The best known application is to use them to suppress 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 something similar can be done with light,” 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.

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-reversal mirror of these physicists: red would turn green, orange would turn blue, and yellow would turn purple.

The ability to manipulate electromagnetic waves in time as well as spatial motion could, in the long term, lead to more efficient wireless communication and improved radar technology, the researchers say.