Physicists have experimentally reversed electromagnetic waves in time, confirming a theoretical prediction that has stood for decades. The process, known as time reflection, causes a wave to retrace its path backward through time rather than space.

The experiment, published in Nature Physics led by Hady Moussa at the City University of New York’s Advanced Science Research Center (CUNY ASRC), produced the first clear and repeatable observation of this effect.

Rather than interfering with time itself, the reversal occurs due to a sudden shift in the physical conditions of the wave’s environment. When those conditions are controlled precisely, a portion of the wave reflects backward in time, forming a reversed copy of the original signal.

Creating a Time Interface

To achieve this, the research team designed a transmission-line metamaterial made from a metal strip embedded with high-speed electronic switches. These switches were connected to capacitor banks and allowed for near-instant changes in the material’s electromagnetic properties.

At a critical moment, the team triggered a rapid doubling of the material’s impedance, which refers to its resistance to electrical current. This created what the researchers describe as a temporal boundary. When the wave encountered this sudden shift, part of it reversed in time.

This reflection is fundamentally different from the spatial kind seen in mirrors. Here, the reversal occurs because of an engineered change in the material’s properties over time, not because the wave bounces off a surface.

The original research article explains how synchronised switching across the metamaterial was essential to achieve a uniform time interface. A supporting overview published by Earth.com details how programmable circuits delivered the energy burst required for the effect, confirming that the process can be accomplished using widely accessible technology.

Confirming a Long-Standing Theory

The idea of time reflection has existed in theoretical physics for more than half a century. Models suggested that when a wave experiences a sudden change in its medium, it could reflect in time rather than space. Until now, no experiment had demonstrated this effect in full.

One major challenge had been achieving the sharp, uniform temporal shift necessary to produce clean reflections. By coordinating the switching components precisely, the CUNY team created an environment where the effect could occur under repeatable conditions.

In addition to time reversal, the experiment also produced frequency translation, shifting the signal to a different point in the spectrum. This capability could lead to new tools in spectrum engineering, adaptive filters, and frequency-selective devices.

The findings build on concepts from research into time-varying photonic media and other theoretical work on spacetime metamaterials. Until now, those theories had lacked a firm experimental foundation in the electromagnetic domain.

Expanding Wave Control in the Time Domain

Researchers are now looking at how time reflection might be applied in practice. One direction involves using temporal cavities, where two time interfaces trap a signal and reflect it back and forth through time, creating novel interference effects.

The technology could also be adapted to manage different types of waves, including acoustic, mechanical, or spin-based systems. Improving the timing accuracy of the switching circuits remains a top priority, especially for higher-frequency implementations.

This work was developed in collaboration with the CUNY Graduate Center and the Advanced Science Research Center, both institutions with expertise in photonics, circuit design, and wave dynamics. The system relies on compact, programmable components that could be adapted for broader experimental use.

The concept also connects to evolving research in photonic devices that operate under dynamic material conditions. Such systems offer real-time control of energy and signal flow and could play a role in emerging quantum and optical technologies.

A Measurable Reversal, Not a Disruption of Time

The experiment does not imply a reversal of time itself. The reflected wave moves backward within the system due to engineered conditions, but time outside the system continues as normal. What has changed is how scientists can influence wave direction using temporal modulation.

This technique adds a powerful new tool for managing energy flow, improving wave control, and building reconfigurable systems. The ability to reflect a wave in time expands the range of possible behaviours that can be engineered into electromagnetic systems.

Researchers expect further studies to refine the switching process, improve wave fidelity, and explore whether temporal boundaries can be layered or combined with spatial interfaces. As hardware improves, new architectures for time-based computing and communications may emerge.