manipulating time
Schematic drawing of a photon collision at a time interface. CREDIT: Anna Umana, Advanced Science Research Center at the CUNY Graduate Center.

Researchers Manipulating Time Cause First-Ever Successful Photon Collisions

Researchers have successfully forced electromagnetic (EM) waves that usually pass right through each other to collide head-on by manipulating time, made possible with the unique properties of metamaterials.

Inspired by the concept of using macro-scale waves like tsunamis or earthquakes to cancel each other out, the manipulation of time interfaces to cause these photons to collide instead of pass through each other could open up a wide range of engineering applications, including advances in telecommunications, optical computing, and even energy harvesting.

Is Using One Wave to Cancel Another Wave Possible?

Engineers studying the devastating effects of wave-driven catastrophes like tsunamis and earthquakes have often postulated the idea of using a counter-wave to stop them in their tracks. Unfortunately, various laws of physics have proven the idea impractical, if not downright impossible. Still, researchers studying wave interactions at the atomic and sub-atomic scale wondered if the idea might be possible in the quantum universe.

“While such an outcome (stopping a tsunami) is impossible in conventional wave physics, we knew it was possible in principle with a temporal metamaterial,” said Emanuele Galiffi, a postdoctoral fellow at Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) and the leading author on this latest study. “Our experiment allowed us to demonstrate this concept in action for electromagnetic waves.”

Manipulating Time Proved Key to Causing First-Ever Photon Collisions

To make the historic photon collision, the CUNY ASRC team first looked back at the work they had previously published on something called time reflections.

From a purely theoretical standpoint, time reflections are exactly what they sound like. Instead of a reflection of a light wave, like seeing oneself in the mirror, or a reflection of a sound wave, like hearing an echo, a time reflection is rooted in the idea that time moves like a wave, and given the right conditions, time could be reflected backward. At least on a quantum scale.

Although the idea had been discussed for over six decades, there was no known material that could change its overall state quickly enough to initiate these time reflections. Then earlier this year, the same CUNY ASRC team behind this latest breakthrough finally proved the existence of time reflections by using a metamaterial that had only recently been developed.

“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,” explained Andrea Alù, Distinguished Professor of Physics at The City University of New York Graduate Center and founding director of the CUNY ASRC Photonics Initiative at the time of the discovery. The result was the first observable evidence that time had been reflected backward.

Now, Professor Alù and his team have shown that manipulating time in a metamaterial can indeed cause photons to collide.

“Our work is building on a series of experiments that show how we can create metamaterials with unique properties that emerge from abrupt time variations of their electromagnetic properties,” said Alù. “These variations allow us to manipulate wave propagation in ways not seen in nature.”

This newest work, which was published in the journal Nature Physics, proved that the abrupt temporal changes in tailored metamaterials they had previously demonstrated, a.k.a. time interfaces or time reflections, could actually make electromagnetic waves collide. It’s something seen in macro-scale objects like pool balls colliding, but at the sub-atomic scale, it is something never witnessed before.

Also notable, the researchers said that by manipulating time, they were able to control the nature of the collision to determine if energy was lost or conserved. For example, two pool balls colliding conserve the vast majority of their energy, whereas two rubber balls colliding lose much of theirs.

“We were also able to control whether the waves exchanged, gained, or lost energy during these collisions,” said Alù.

Discovery Could Lead to Technological Advances in Numerous Areas

One interesting application proposed and actually demonstrated by the researchers involved using their method to shape electromagnetic pulses by colliding them with one another.

“This technique allows us to use an additional signal as a mold to sculpt a pulse that we are interested in structuring,” said Gengyu Xu, a postdoctoral fellow with Alù’s lab and co-leading author of the paper.  Xu says they have already shown this ability using radio waves and that they are now working to “realize this sculpting ability” at higher frequencies.

In their conclusion, the researchers state that the ability to collide EM waves with the goal of controlling wave propagation could also lead to breakthroughs in a number of areas that rely on such processes. These include advances in wireless communications, energy harvesting, imaging, and computing.

Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at, or email him directly at