While tracking whale calls near Massachusetts, oceanographer John Spiesberger noticed that the software he was using was clocking speeds for whale calls greater than the speed limit for sound traveling in seawater. Spiesberger, of the University of Pennsylvania, initially thought there was something wrong with his program. After all, if the software wasn’t to blame, it would mean that those whale calls were âbreakingâ physics.

It turns out there wasnât anything wrong with his coding, nor was there any violation of physics: what he was seeing was even weirder than that. The exceptional speeds seemed to have come from a physical effect caused by the hydrophone receiving not only the direct sound wave but also its echo, reflected against the oceanâs surface. Combined, these signals make it appear as if the whale calls went supersonic, when in reality, the observed phenomenon is an effect of wave interference, an unexpected finding predicted by Einsteinâs theory of special relativity.

âMost of us donât hear a whale call and think, âWow, look at the special theory of relativity in action,ââ Spiesberger said, who recently published his findings with co-author Eugene Terray, from the Woods Hole Oceanographic Institute, in Physical Review E. In a University write-up on the research, Spiesberger added that heâd ânever guessed any connection existed.â

Stranger than fiction

Most physicists will come to the rare agreement that general and special relativity reflect some of Einsteinâs best, most consequential observations. Special relativity explains the relationship between space, time, mass, and energy (itâs also where the famous E=mc2 equation comes from). In a vacuum, the speed of light is the same for any observer. But this is relative to an observerâs reference frame. For instance, a whale and a human swimming are subjected to the same laws of physics, but because theyâre moving at different speeds, the two âframesâ experience time and space differently.

To give a more practical example, GPS devices depend on a network of satellites with atomic clocks to precisely keep track of locations. But these satellites whiz around Earthâs orbit at blinding speeds, which means theyâârelativeâ to the time we experienceâtick an extra 7 microseconds each day. So, the atomic clocks on satellites must subtract 7 microseconds daily to keep things consistent with the time we experience here on the surface.

Now back to the whales

According to Spiesberger, oceanographers depend on hydrophones to track whales. These devices allow scientists to find whales from 62 miles (100 kilometers) away underwater. When the hydrophone picks up far-traveling whale calls, scientists then âcan use them to pinpoint where an animal is by comparing when its sound reaches receivers spread across the seafloor,â Spiesberger explained.

But the sounds get mixed up when a whale is physically closer to the ocean surface. In the first place, a whale call isnât a singular, neat wave packet that bounces off the hydrophoneâs receiver. This means that the microphone picks up some of the sound, whereas other parts of the sound wave can travel elsewhere and reach the instrument slightly later.

When this inevitably happens, the two waves can interfere with each other and âadvance the arrival time of the combined signal instead of merely altering its strength,â according to the paperâs accompanying Synopsis column. In other words, to the observer, it appears as if the whale calls went supersonic.

In the eye of the beholder

But again, thatâs only relative to the observer. No physics laws were violated, it turns out, and as Spiesberger explains, what âappearsâ to speed up isnât the signal itself, but the âposition of the signalâs strongest peakâ that registered in the instrument. That said, the authors noted in the paper that thereâs still a need to experimentally verify that what they saw represents a novel instance of special relativity in underwater acoustics.

As such, the pair concluded its work by describing potential experiments. Spiesberger expressed his intentions to replicate the âacousticâ version of his observations; if that works, it might even be possible to replicate the effect with light instead of sound.