Using an engineered metamaterial scientists in New York City were able to successfully observe time reflections for the first time.
The explanation of spatial reflections -- whether by light or by sound -- are pretty intuitive. Electromagnetic radiation in the form of light or sound waves hit a mirror or wall, respectively, and change course. This allows our eyes to see a reflection or echo of the original input. However, for more than 50 years, scientists have theorized that there's another kind of reflection in quantum mechanics known as time reflection.
This term might conjure up images of a nuclear-powered DeLorean or a particular police box (that's bigger on the inside), but that's not quite what scientists mean by the term. Instead, time reflections occur when the entire medium in which an electromagnetic wave travels suddenly changes course. This causes a portion of that wave to reverse and its frequency transforms into another one.
Because these time reflections require a uniform variation across an entire electromagnetic field, scientists assumed it would require too much energy to actually observe time reflections in action. But scientists from the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) in New York City successfully observed time reflections by sending broadband signals into a strip of metal filled with electronic switches that were connected to reservoir capacitors.
This allowed the researchers to trigger the switches at will, doubling impedance along the strip. This sudden change caused the signals to carry a successful time-reversed copy. The results were published in the journal Nature Physics.
"It is very difficult to change the properties of a medium quick enough, uniformly, and with enough contrast to time reflect electromagnetic signals because they oscillate very fast," Gengyu Xu, a coauthor and post-doc student at CUNY ASRC, said in a press statement. "Our idea was to avoid changing the properties of the host material, and instead create a metamaterial in which additional elements can be abruptly added or subtracted through fast switches."
This time reflection also behaves differently than spatial reflections. Because this time echo reflects that last part of the signal first, the researchers say that if you looked in a time mirror, you would see your back instead of your face. To translate the experience acoustically, it'd be like listening to a tape on rewind -- which is to say fast and high-pitched.
The shift in frequency, if it could be perceived by our eyes, would look like colors of light suddenly changing to another color, such as red switching to green. This strange counter-intuitive nature of time reflection is part of what has made studying the concept so difficult.
"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," corresponding author Andrea Alù, a physics professor and director of CUNY ASRC's Photonics Initiative, said in a statement.
The big question: Why have scientists worked toward recreating this theoretical time reflection in a laboratory? Well, more minute control of electromagnetic waves can vastly improve wireless communications and even lead to advancements in low-energy, wave-based computers.
In other words, it simply helps to know everything there is about electromagnetic waves -- both forward and backward.