Scientists astonished to see ‘time reflections’ in electromagnetic waves

Scientists have been stunned by seeing “time reflections” in electromagnetic waves – an astonishing phenomenon that was previously only hypothesised to exist.

The effect has been likened to looking in a mirror and seeing the reflection of your own back, or hearing an echo but with the sound reversed as when a tape is played backwards.

Scientists have long hypothesised that such an effect exists. But it was difficult for it to be observed because it requires changing the properties of a material quickly and substantially enough for them to happen.

Now, using a special kind of material, scientists say they have observed it happening. By sending signals through a strip of metal, and quickly triggering a change in the physical properties, researchers saw those signals reverse in time.

“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,” said Andrea Alù, Distinguished Professor of Physics at The City University of New York Graduate Center and an author on the new paper, in a statement.

“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.”

The reflections we are used to seeing are spatial reflections: electromagnetic light waves hit a mirrored surface, bounce off, and create the effect that allows us to use mirrors and more.

Those spatial reflections happen when a wave hits a specific boundary, such as a mirror. But time reflections are different in a variety of ways – for one, they occur when the whole medium the wave is travelling through changes, all at once.

When that happens, part of the wave is flipped in time, and its frequency changes. It was that effect that scientists hoped to see in the new study.

Researchers hope that the discovery could lead to a variety of practical applications, including wireless communications and new small, low-energy computers. Previously, it was thought that those applications would not be possible because it was too difficult to change the medium quickly enough.

“The key roadblock that prevented time reflections in previous studies was the belief that it would require large amounts of energy to create a temporal interface,” said Gengyu Xu, the paper’s co-first author and a postdoctoral researcher at Advanced Science Research Centre at CUNY, in a statement. “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.

“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.”

A paper describing the findings, ‘Observation of temporal reflection and broadband frequency translation at photonic time interfaces’, is published in Nature Physics.