A new phenomenon observed in sound waves in water has indicated a “gap” that causes the energy of a wave to move faster than the limit of the speed of light. This effect, however, does not allow the transmission of information above the barrier established by the special theory of relativity.
The discovery had its origins in studies focused on the propagation of sound in the oceans and, in theory, it could also manifest itself in electromagnetic waves in a vacuum.
This phenomenon arises from the interaction between waves that follow different paths. Under certain conditions, the combination of a wave traveling directly from source to receiver with another traveling along a reflected path can shift the peak energy of the resulting wave to a position that suggests its arrival at the destination before the peak of the direct wave.
In a simulation conducted by researchers, sound propagated underwater at 1,500 meters per second. Despite this, the energy peaks arising from the interference had apparent speeds of 1,694.5 m/s and 2,782.5 m/s. The maximum speed observed was almost twice the speed of sound used in the model.
Despite the apparent breaking of the limit, this does not indicate that something has actually exceeded the speed of light. “We proved that the speed of information is less than or equal to the speed of light in a vacuum, therefore, the effect does not violate special relativity,” said acoustician John L. Spiesberger, from the University of Pennsylvania (USA), and oceanographer Eugene Terray, from the Woods Hole Oceanographic Institution, in the study.
How Whale Watching Sparked Discovery
- The investigation has its roots in studies that aim to locate whales in the ocean;
- To determine a whale’s position, scientists can use multiple hydrophones, equipment capable of capturing underwater sounds. By comparing the moment the sound arrives at each sensor, it is possible to triangulate the animal’s location;
- The challenge arises because, in certain circumstances, this method can generate considerable errors;
- In a 2025 study, Spiesberger and his colleagues identified a phenomenon that could help clarify some of these disagreements.
When a whale is relatively close to the surface, some of the sound it emits goes directly to the hydrophone. Another portion may reach the water surface and be reflected before reaching the sensor.
At the meeting point of the two waves in the hydrophone, interference occurs. This combination changes the shape of the received wave and can modify the point of greatest signal intensity.
In previous work, the researchers noted that this interference could make the sound appear slower than the wave that followed the direct path between the whale and the sensor. However, when testing different types of sound pulses, Spiesberger realized that the same effect could happen in the opposite direction.
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The apparent early arrival of the wave peak
Imagine two waves that originate from the same source. One of them moves directly to the receiver, while the other travels a longer path, being reflected off a surface before reaching the same point. If the time difference between the two is adjusted in a specific way, the interference has the power to reshape the resulting pulse.
The result is intriguing: the energy peak of the wave that is combined can reach the receiver before the peak of the wave that only followed the direct path.
This behavior was proven in the simulations carried out.
With sound traveling at 1,500 meters per second in the water, the researchers observed energy spikes that appeared to travel at 1,694.5 m/s and 2,782.5 m/s. The second value represents a speed almost twice that of sound used in the model
However, there is a fundamental distinction between the movement of the energy spike and the speed at which information is transmitted.
Why the transmission of information follows the limit of light
In order to verify whether the phenomenon could actually enable faster-than-light communication, the scientists carried out a second simulation. They modeled the transmission of two distinct signals, represented by the numbers 1 and 0.
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Until a specific instant, called time zero, the two signals were identical. At that moment, the source switched to 1 or 0, and this modification began to propagate to the receiver through the direct and reflected paths.
The moment in which the receiver can confidently identify whether it received a 1 or 0 indicates the moment in which new information actually reached its destination.
The outcome was consistent with the theory of relativity: the information did not reach the receiver any faster than the signal that took the direct path.
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The interference managed to move the energy spike forward, creating the impression of a high speed, but it was not able to cause the receiver to receive new information before it had the time necessary to complete the direct path.
In other words, the phenomenon can generate a kind of apparently superluminal speed, but it does not allow the transmission of information above the speed of light in a vacuum.
They noted that the interference seemed to provide a slight increase in the speed of information transmission, although still below the limit established by relativity. They admit they are not sure about the exact reason for this behavior.
Study evaluates the occurrence of the effect in electromagnetic waves
The most speculative part of the study lies exactly here. The authors suggest that the same mechanism, which combines a direct path with a reflected path, could work not only with sound waves, but also with electromagnetic waves in a vacuum.
“Our conjecture that the direct + reflected path effect exists for electromagnetic waves provides a superluminal propagation mechanism different from microwave tunneling, quantum tunneling, and anomalous scattering,” the researchers wrote.
They emphasize, however, that phenomena of this nature can produce speeds greater than that of light for certain aspects of the wave, while the speed of information remains below the relativistic limit. For this reason, the next step is to try to observe the effect experimentally, whether using sound waves or light.
If the phenomenon is similarly confirmed for electromagnetic waves and yields the expected results, the researchers indicate that it could represent a particularly direct way of observing apparently superluminal propagation.
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“Classical physical effects sometimes find equivalents in quantum mechanics, and it may be worth considering whether the direct + reflected path has analogues in the quantum world,” Spiesberger and Terray concluded.
For now, the speed of light remains unattainable for information. What the research reveals is something more subtle: a way to make the energy peak of a wave demonstrate that it reached its destination more quickly, without the information itself achieving the same feat.