Physicists are building a “mirror” that reflects light back in time

Do you like the well-known Echo joke “Who’s King Weasel?” Try it on a time-reflecting pit, the answer won’t be “donkey!” , but “Read!” , as if you were playing a disc in reverse.

“An ordinary mirror that rotates left and right, and a conjugate phase mirror, as we call it in practice, reverses time,” says physicist Carlo Benaker of Leiden University, who was not involved in the research.

Although such technologies have been around for some time, until now they have been limited to very narrow wavelength ranges, he says. As if your mirror only works on one very specific shade of color. In their new research, the physicists have achieved the same thing for electromagnetic waves — light — in a much wider range of wavelengths, a stepping stone to actually using these mirrors in practice.

“Make no mistake: As exciting as a time reversal may sound, this is not a time machine or anything like that,” says Benaker. At the same time, he describes what is possible with these mirrors as “something magical.”

The most famous application is their use to remove disturbances from light. In experiments, you can then shine a beam of light through a cloudy object, so that the light scatters. “A glass of milk, for example,” he says. By letting the light fall on a time-reflecting mirror, you can then eliminate the scattering of that light.

People often say: If only I could return the movie, undo this bug. Unfortunately, this isn’t possible with humans, but something similar can be done with light,” Benaker says. In practice, this effect is useful, for example, for astronomical observations from Earth, when you need to rid light from the deep universe of interference from the atmosphere. Earth’s atmosphere.

Although the physicists performed their experiment on light invisible to the naked eye, the result of the time mirror on ordinary light would be strange. Because the frequency of this light changes according to the technology used, you will see the colors reflected differently in these physicists’ time-reversing mirror: red will appear green, orange will appear blue and yellow will appear purple.

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