‘Time mirrors’ are actually real: physicists confirmed a 65-year-old theory in a new study

In a bold study, researchers have pulled off something that sounds like a script from a sci-fi movie. In their research, scientists from the Advanced Science Research Center at The City University of New York (CUNY) have shown that part of a wave can behave as if it's going back in time.

Their mind-bending experiment challenges what we thought we knew about time and also suggests that our understanding might not have been the full truth all along. Here’s how they did it and what it means for our understanding of the universe.

To understand a time mirror, we first have to understand how regular mirrors work. A regular mirror sits in just one place and bounces a wave back when it hits the surface. In the case of a time mirror, though, the material the wave is traveling through suddenly changes everywhere at once. This change acts like a wall in time rather than in space.

Explaining this, the researchers note in their published paper, “Time reflection is a uniform inversion of the temporal evolution of a signal, which arises when an abrupt change in the properties of the host material occurs uniformly in space.”

To prove this, researcher Hady Moussa and his team at CUNY's Advanced Science Research Center built a strip of metal that was wired with fast electronic switches and capacitor banks. This created what we know as a metamaterial. 

After this, the team flipped the switches almost instantly, in under 3 nanoseconds. This made the entire strip undergo an almost simultaneous change in its electrical properties. That sudden, uniform shift then created the temporal boundary that made it possible to split the wave in two.

In their study, nothing actually moved backward through time, and the signal itself never changed direction. Instead, the material changed instantaneously and created a new part of the wave that went in the opposite direction with a shift in its frequency.

The wave keeps the same momentum, so when the medium changes, its frequency has to shift to fit the new conditions. This gives researchers a clear marker to look for. The researchers noted that the signal reached nearly 90% of the ideal strength and then it faded out within roughly 12 nanoseconds.

Once they had successfully created one temporal boundary, the researchers added another shortly afterward. This gave them a setup much like a Fabry-Pérot cavity, where light is trapped between two mirrors and bounces back and forth.

Here, though, the two "mirrors" weren't in two places. They existed at two different moments in time. By changing the amount of time between the two switching events, they could control the spacing between the reflected pulses and shift the interference pattern in the frequency spectrum.

In a normal mirror or optical cavity, these changes would be controlled by physical distance. Here, the researchers were able to produce the same kind of effect simply by changing the timing. 

In 1958, physicist R. Morgenthaler first worked out the math behind this concept, and R. L. Fante extended it further in 1971. But they could not test the theory back then, as they did not have the hardware needed for it.