Posted by A strange low-frequency sound produced by humpback whales is offering SETI researchers an unusual lesson about how difficult it can be to recognize an unfamiliar signal, even when it has been present for generations.
The sound, known as the “thrum,” has long been reported along Alaska’s coast, where people compared it to distant thunder or machinery. Researchers eventually linked it to humpback whales surfacing and exhaling. Its fundamental frequency is around 35-40 hertz, close to the lower limit of human hearing, and it can travel through the atmosphere over considerable distances. In a recent SETI Live discussion, researchers examined what this discovery could mean for the search for extraterrestrial intelligence. The central lesson is not that whales resemble alien civilizations, but that intelligence can produce signals through channels observers may not initially consider important.
Humpback whales communicate in multiple ways, including underwater songs, social calls, bubbles, body movements, tail slaps, and aerial sounds. Researchers have now identified roughly a dozen categories of above-water sounds, although only a few had previously been formally described. The function of the thrum remains uncertain: it may carry information, result from forceful exhalation, or serve another purpose. A STRIDE-funded SETI Institute project is also testing whether whale exhalation plumes might create measurable electromagnetic effects. Researchers use directional microphones, hydrophones, magnetic sensors, and a loop antenna to compare acoustic and possible electromagnetic signals. No electromagnetic effect has yet been detected. The experiments highlight another important parallel with SETI: unusual signals must first be separated from interference. In Alaska, the team found that phones, solar electronics, engines, and even movement on the research boat could contaminate measurements.
Similar challenges arise in searches for technosignatures, where instrumental and terrestrial interference must be eliminated before any anomaly can be considered significant. The broader implication is that SETI depends heavily on what researchers choose to measure. Radio waves, lasers, and other established technosignatures define much of today’s search space, but unfamiliar intelligence could produce signals through unexpected channels. The humpback “thrum” therefore serves as a useful reminder: a meaningful signal may exist long before scientists recognize that it deserves attention.
For more details, listen to the full SETI Live conversation.
