Scientists Created a Mind-Bending 'Mirror' That Reflects Time Backwards

Seeing our reflections staring back at us in the bathroom mirror, or hearing our voices echoed off the walls of a tunnel, aren’t exactly phenomena to write home about. But now scientists have shown that it’s possible to reflect something far less intuitive: time.

In a paper published in Nature Physics, a team from City University of New York has reflected part of an electromagnetic wave backwards in time. It’s the first time this mind-bending feat has been achieved with this particular type of signal, and their technique could eventually help engineers create computers that send super-fast signals through light. 

The kind of light or sound reflections that we’re used to, also known as spatial reflections, happen when a wave meets a roadblock of sorts—some physical surface that it can’t pass through. Instead of continuing on its merry way, the wave bounces back. “Light bounces off a mirror because the impedance of the mirror material is very different to air, so the waves that hit the mirror have to go back, they can’t enter the mirror,” study co-author Andrea Alù told Motherboard. 

Time reflection is similar, said Alù, except that instead of a physical roadblock, a time mirror works by creating an abrupt change in time. “The larger the contrast, the stronger the time reflection will be,” he said. 

In their study, Alù and his team built a time “mirror” by creating a material capable of literally bending space and time—a so-called metamaterial. The material looks like a large plastic board covered in a long strip of metal weaving back and forth. The metal is loaded with a dense collection of switches that can be flipped on and off faster than the frequency of the incoming wave. This abrupt switching is what creates the mirror. 

To confirm that it worked, researchers sent a short electromagnetic signal with an uneven shape (that is, some of the bumps in the wave were higher than others)  into the metamaterial. They could tell the wave had been time reflected because it came out backwards and stretched out – the two hallmarks of time reflection.

Picture it like this: If you were to look at your face in a time mirror, you’d see the back of your head instead of the front. Plus, because the wave is being stretched out, colors would change too, since color is basically how fast its signals oscillate in time. Your red t-shirt would look green, your orange juice would turn blue and your yellowish hair would become violet.

This isn’t the first time scientists have flipped time, and the idea itself dates back some six decades. “It’s actually a very common operation. Time reflection is actually the key to many technologies,” said Alù. For example, it’s possible to clean up radio communication signals digitally by taking the signal, storing it in memory, processing it to reverse it and spitting it back out again. This cancels out some of the distortion that happens when a signal is sent. Researchers have also been able to reverse waves with very specific frequencies before. 

The latest experiment is different because researchers were able to time-reverse a wave with a lot of different frequencies in it, as opposed to just a single one, and did so without using a computer. “Our experiment shows that you could do this with any frequency essentially,” said Alù.

The study means researchers are one tiny step closer to creating optical computers—systems that use light made by lasers to process and store data—or other tech that harnesses light. To be able to time reflect light waves, which have a much higher frequency, you’d need much faster switches, said Alù. “An important next step is to demonstrate we can reflect time for higher frequencies. Typically switching technology starts breaking down as you go into the terahertz frequencies,” explained Alù. Their next experiments will be in the hundreds of gigahertz frequency range, still more than a thousand times off from the 400 plus terahertz of visible light. 

Eran Lustig, a postdoctoral scholar in electrical engineering at Stanford University, who wasn’t involved in the study, told Motherboard in an email that currently our ability to change the properties of light are limited, which means our ability to build tech relying on light is also limited. 

“Breaking this paradigm will enable much greater freedom in manipulating light in communication, computation and more,” he said. “Besides lasers, this demonstration of a time reflection is a step forward in our ability to modulate materials in time,” Lustig said.