Walk through a maze of mirrors, you'll soon come face to face with yourself. Your nose meets your nose, your fingertips touch at their phantom twins, stopped abruptly by a boundary of glass.
Most of the time, a reflection needs no explanation. The collision of light with the mirror's surface is almost intuitive, its rays set on a new path through space with the same ease as a ball bouncing off a wall.
For over sixty years, however, physicists have considered a subtly different kind of reflection. One that occurs not through the three dimensions of space, but in time.
Now researchers from the City University of New York's Advanced Science Research Center (CUNY ASRC) have turned the theory of 'time reflections' into practice, providing the first experimental evidence of its manipulation across the electromagnetic spectrum.
"This has been really exciting to see, because of how long ago this counterintuitive phenomenon was predicted, and how different time-reflected waves behave compared to space-reflected ones," says physicist Andrea Alù, founding director of the CUNY ASRC Photonics Initiative.
Put aside thoughts of TARDIS-like technologies rewriting history. This kind of time reflection is even weirder. And, it seems, actually possible after all.
By the 1970s, it was becoming clear that there was an analog for spatial reflection in the time component of a quantum wave of light. Change the medium a wave is traveling through quickly enough, in just the right way, and the temporal component of the wave will change with it.
The effect of this reflection in time isn't going to rip a hole in reality. But It will shift the frequency of the wave, in ways technology could exploit across varied fields like imaging, analogue computing, and optical filtering.
Strangely, the 'echo' of altered frequency is also a reversal of the signal. If it was an echo of your voice counting one to ten, you'd hear each number spoken backwards, from ten back to one, in a chipmunk squeak.
Equivalents in acoustics and magnetism have been experimented with before, as has a limited investigation of narrow frequencies in electromagnetic temporal reflection using a computer setup.
Exploring the phenomenon on a less-constrained level would require uniform and sudden variations across the whole electromagnetic field of a material, something experimentalists assumed would demand too much energy to make work.
Until now, it seems.
"Using a sophisticated metamaterial design, we were able to realize the conditions to change the material's properties in time both abruptly and with a large contrast," says Alù.
The team shone a mix of frequencies through a purposefully designed metal strip roughly 6 meters in length, loaded with switches and capacitors. Triggered at the same moment, the capacitors unloaded their charge, swiftly altering the impedance of the metamaterial as the signal passed through.
This shock change created an echo in the broad range of light waves, demonstrating a reflection in their temporal properties.
Metamaterials are artificial constructs that have no equivalent in the natural world. Designed with unique properties that are tasked with a particular purpose, they have been made to suit different structural, acoustic, and optical needs.
Finding a metamaterial capable of time reflection provides engineers with a whole new tool for manipulating light.
"The exotic electromagnetic properties of metamaterials have so far been engineered by combining in smart ways many spatial interfaces," says physicist Shixiong Yin, one of the study's lead authors.
"Our experiment shows that it is possible to add time interfaces into the mix, extending the degrees of freedom to manipulate waves."
This research was published in Nature Physics.