Two researchers say they have found a way for the energy peak of a wave pulse to reach a receiver ahead of the wave that carried it, and they insist the result leaves special relativity untouched.
That is to say, it does not violate Einstein’s famous rule that nothing can travel faster than the speed of light in a vacuum.
The mechanism in question is interference. A pulse, whether of sound or light, travels directly from source to receiver, while a second copy bounces off a flat surface before arriving. Where the two paths overlap, the waves reshape each other, creating a new combined peak. Under the right conditions, this combined peak will have arrived before the peak of the direct pulse would have arrived on its own.
Lessons from the deep
The study grew out of a puzzling problem in whale tracking. Researchers commonly track whales by recording their calls on multiple hydrophones and triangulating from the arrival times, but the method has long produced errors that nobody could explain.
In a 2024 paper, acoustician John L. Spiesberger of the University of Pennsylvania and oceanographer Eugene Terray of the Woods Hole Oceanographic Institution finally identified the culprit.
When a whale swims near the surface, part of its call reflects off the underside of the water and interferes with the direct signal. This interference pushes the detected peak later in time, making the sound appear to travel more slowly than it actually does.
Taking this further, Spiesberger and Terray later modeled pulse shapes closer to those found in the real conditions noted earlier and discovered the effect could run in reverse. In one simulation of sound waves moving underwater at 1,500 meters per second, the energy peaks appeared to travel at 1,694.5 and 2,782.5 meters per second, the latter nearly twice the speed of sound in the model.
Crucially, the pair argues that this is not just an underwater quirk and that the same wave-interference behavior should apply to light in a vacuum.
“We prove the speed of information is less than or equal to the speed of light in a vacuum, so the effect does not violate special relativity,” the duo wrote in their latest work published recently in the interdisciplinary journal Physical Review E.
To demonstrate this, they modeled the transmission of two signals, a 1 and a 0, identical until the moment the source commits to one. The point at which the receiver can reliably distinguish between them marks the arrival of new information, and in every case, that information traveled no faster than the direct path allowed.
Interference, nevertheless, did give the energy peak a modest boost, yet the researchers were unable to explain why.
It’s worth noting that, while these simulations provide a rigorous mathematical framework, the results remain entirely theoretical given that neither the acoustic nor the optical version of the experiment has been carried out in a laboratory setting to date.
Sources: Physical Review E, Journal of the Acoustical Society of America, arXiv, Science Alert, Related work (2025 predecessor)