An experiment led by researchers at the City University of New York (CUNY) has demonstrated that electromagnetic waves can be reflected in time, not just space. Known as temporal reflection, the phenomenon creates a reversed copy of a signal that appears to travel backward, not through location, but across the timeline of a controlled system.
The results, published in Nature Physics , represent the first experimentally verified observation of a “time mirror”, a theoretical construct in wave physics dating back over 50 years. The development is viewed as a milestone in the study of time-varying photonic media, a field that until now lacked experimental clarity.
Physicists emphasize that this is not a reversal of time itself. The wave retraces its steps within a fixed system due to a controlled, instantaneous change in the wave’s environment. Time outside the system progresses normally; only the signal behaves as if reflected at a boundary in time.
A Mirror That Flips Time, Not Space
The experiment, led by Dr. Hady Moussa at the Advanced Science Research Center at CUNY Graduate Center, used a specially fabricated transmission-line metamaterial constructed from metallic strips embedded with high-speed electronic switches and capacitor arrays. At a precisely coordinated moment, researchers triggered an abrupt doubling of the material’s impedance, creating what they describe as a temporal boundary.
When an electromagnetic wave encountered this shift, a portion of it reflected backward in time, forming a reversed copy of the original waveform. The reversal was not observed as spatial reflection, but as a signal that appeared to move against the timeline of the system.
Synchronization was critical. The switches had to activate within nanoseconds across the entire device to maintain uniformity. According to Nature Physics, the team’s circuit simulations confirmed that the rise time of the switches was sufficiently fast to avoid degradation of the time-reflected signal.
The experiment also produced broadband frequency translation, with the reflected wave exhibiting a shift across the electromagnetic spectrum. This frequency change confirmed theoretical predictions and opens potential for frequency-selective device development.
Theory Meets Circuitry After 50 Years
The concept of time reflection has existed since at least the 1970s, when models proposed that sudden changes in the material properties of a wave’s medium could cause partial temporal reversal. Similar to how light reflects off a spatial boundary, a wave could reflect off a discontinuity in time.
Previous attempts to observe the effect failed due to challenges in producing a sharp and uniform temporal interface. Until 2025, no experiment had achieved the temporal impedance shift required to cleanly isolate a reversed signal. According to the study’s authors, the successful demonstration came from improvements in programmable circuits and tighter control over component synchronization.
The project was conducted in collaboration with multiple departments across the City University of New York, including electrical engineering teams at City College and theoretical physicists at the Graduate Center. Additional contributions came from Syracuse University and other members of the photonics community.
Co-author Andrea Alù, a leading researcher in spacetime metamaterials, has previously described time modulation as a “missing dimension” in traditional wave manipulation. In a statement, the team confirmed that the findings establish a working platform for experiments in dynamic wave control across multiple domains.
Precision Timing, Power Constraints, and Scaling Risks
Despite the successful demonstration, the experiment’s scalability remains in question. The system operates under strict timing conditions, and the switches require high-fidelity synchronization. Measurements also indicate that finite switching time reduces the amplitude of the reflected signal; further engineering will be required to sustain the effect at higher frequencies.
Researchers also note that while electromagnetic wave reversal has been confirmed, the method may not translate directly to other domains such as acoustics, spintronics, or gravitational systems, where different physical constraints apply.
Energy input remains a key factor. The system relies on coordinated capacitor discharge across multiple cells to trigger the impedance change. This process introduces heat and timing limitations that may complicate long-duration or real-time applications.
Moreover, the broader interpretation of time reflection raises unresolved questions in the physics community. If signals can be reversed temporally in engineered systems, researchers may need to revisit foundational assumptions about causality, signal entropy, and time-symmetric theories in electromagnetism.
There is no evidence that the effect has utility beyond contained systems; time itself remains unaffected outside the medium. Nevertheless, the possibility of chaining or layering multiple time interfaces has been raised as a potential research direction.
Time Control as a Tool, Not a Paradox
The study aligns with ongoing research into photonic time crystals, temporal cavities, and time-modulated frequency filters, all of which explore how engineered environments can reshape the behavior of light and electromagnetic signals. According to materials published by CUNY ASRC, the hardware used in the experiment can be miniaturized and incorporated into adaptive systems.
If time reflection can be harnessed at scale, future applications may include signal encryption, wave-based memory, and reconfigurable antennas capable of operating over dynamic frequency bands. The ability to isolate and reflect waveforms temporally could also enhance imaging systems, allowing selective reversal of background noise without affecting the target signal.
These developments coincide with international interest in nonreciprocal photonics, quantum metamaterials, and spacetime computing, areas where time symmetry is increasingly regarded as a design parameter rather than a constraint.