While tracking whale calls near Massachusetts, oceanographer John Spiesberger noticed that the software he was using recorded speeds of whale calls above the speed limit for sound traveling in seawater. Spiesberger, of the University of Pennsylvania, initially thought something was wrong with his program. After all, if the software wasn’t to blame, it would mean that those whale calls were “breaking” physics. It turned out that there was nothing wrong with his coding nor any violation of physics: what he was seeing was even stranger than that. The exceptional speeds seemed to come from a physical effect caused by the hydrophone receiving not only the direct sound wave but also its echo, reflected against the ocean surface. Combined, these signals make it appear as if whale songs are becoming supersonic, when in reality, the observed phenomenon is an effect of wave interference, an unexpected finding predicted by Einstein’s theory of special relativity. “Most of us don’t listen to a whale song and think, ‘Wow, look at the special theory of relativity in action,'” said Spiesberger, who recently published his findings with co-author Eugene Terray of the Woods Hole Oceanographic Institute in Physical Review E. In a University paper about the research, Spiesberger added that he had “never guessed there was any connection.” Stranger than fiction Most physicists will come to the rare agreement that general and special relativity reflect some of Einstein’s best and most momentous observations. Special relativity explains the relationship between space, time, mass and energy (also where the famous equation E=mc2 comes from). In a vacuum, the speed of light is the same for any observer. But this is relative to an observer’s frame of reference. For example, a whale and a swimming human are subject to the same laws of physics, but because they move at different speeds, the two “frames” experience time and space differently. To give a more practical example, GPS devices rely on a network of satellites with atomic clocks to accurately track locations. But these satellites orbit the Earth at blinding speeds, meaning that, “relative” to the time we experience, they clock in an extra 7 microseconds each day. So the satellites’ atomic clocks must subtract 7 microseconds daily to remain consistent with the time we experience here on the surface. Now back to the whales According to Spiesberger, oceanographers rely on hydrophones to track whales. These devices allow scientists to find whales up to 100 kilometers (62 miles) away underwater. When the hydrophone picks up the calls of whales traveling far away, scientists “can use them to identify where an animal is by comparing when its sound reaches receptors spread across the seafloor,” Spiesberger explained. Researchers at sea used an ocean receiver called TOSSIT to track the whales’ calls. Credit: University of Pennsylvania But the sounds blend together when a whale is physically closer to the ocean surface. First, a whale song is not a singular, ordered packet of waves bouncing off the hydrophone receiver. This means that the microphone picks up one part of the sound, while other parts of the sound wave can travel elsewhere and reach the instrument a little later. When this inevitably happens, the two waves can interfere with each other and “advance the arrival time of the combined signal rather than simply altering its strength,” according to the Synopsis column accompanying the article. In other words, to the observer, it appears as if the whale songs are becoming supersonic. In the eyes of the beholder But, again, that is only relative to the beholder. It turns out that no physical law was violated and, as Spiesberger explains, what “seems” to accelerate is not the signal itself, but the “position of the strongest peak of the signal” that was recorded on the instrument. That said, the authors noted in the paper that it still needs to be verified experimentally that what they saw represents a new example of special relativity in underwater acoustics. As such, the pair concluded their work by describing possible experiments. Spiesberger expressed his intentions to replicate the “acoustic” version of his observations; If that works, it might even be possible to replicate the effect with light instead of sound.
