Thirty times louder than the background sky, the Wow signal has never been heard again

On August 15, 1977, the Big Ear radio telescope at Ohio State University picked up a burst of radio energy thirty times more intense than any background noise ever recorded at that frequency. Jerry Ehman, a volunteer reviewing the observatory’s tractor-feed printouts days later, saw the six-character sequence “6EQUJ5” jump off the page, circled it in red ballpoint and scrawled “Wow!” in the margin, giving the signal its lasting name. Nothing like it has been detected again from that patch of sky, in that band, in the decades since — not by Big Ear during the roughly 100 follow-up observations Ohio State ran at that declination, and not by any telescope pointed there afterward.

The signal arrived at 1,420 megahertz, the frequency at which neutral hydrogen naturally emits radiation and a band radio astronomers had long flagged as a plausible channel for an intentional broadcast, since hydrogen is the most common element in the universe and any civilization studying radio astronomy would likely know its frequency. Big Ear’s fixed, sky-scanning design meant the signal rose and fell over about 72 seconds as Earth’s rotation carried the telescope’s beam across the source, a pattern consistent with something fixed on the sky rather than a passing satellite or aircraft. That shape, combined with the signal’s narrow bandwidth and its unusual strength, is why the reading has outlasted almost every other candidate technosignature in the search for extraterrestrial intelligence.

A signal shaped like something real, not noise

Radio interference of terrestrial origin tends to spread across a wide range of frequencies or fluctuate erratically; the Wow! signal did neither. It held a narrow band and traced the smooth rise-and-fall curve expected of a genuine point source drifting through a directional antenna’s field of view. Ehman himself, interviewed decades later by the Ohio State University Radio Observatory, said he doubted an extraterrestrial origin partly because Big Ear returned to that patch of sky roughly a hundred more times afterward and never caught the signal again, and he speculated instead that an Earth-bound transmission had simply reflected off a piece of space debris. The absence of a repeat detection is what has kept the case open rather than closed: a satellite or comet would be expected to pass through the same patch of sky again, while a signal tied to a single transient astrophysical event would not.

A hydrogen cloud, not a civilization, in one new model

Abel Mendez, an astrobiologist at the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, led a 2024 reanalysis that proposes the signal came from a natural source rather than an artificial one. Mendez’s team combed through archival data from the Arecibo REDS survey — a project cataloging radio emissions from red dwarf stars — and found weaker signals with the same narrowband hydrogen-line signature scattered elsewhere in the sky. The paper argues that a sudden, powerful burst from an object such as a magnetar could trigger a process called superradiance in a nearby cloud of interstellar hydrogen gas, amplifying its faint natural glow into something briefly loud enough for Big Ear to register as the Wow! signal. It is, in effect, a proposal that the sky itself produced a one-time flash rather than that anyone sent a message.

Why one paper has not closed the case

Dr. Franck Marchis, director of Unistellar Citizen Science at the SETI Institute, and Dr. Lauren Sgro, an astronomer and outreach manager for the institute’s LaserSETI project, discussed Mendez’s findings on the SETI Institute’s YouTube channel and were careful to note what the paper does and does not establish. The SETI Institute published their assessment that the superradiance model offers a plausible mechanism, not a confirmed one, because the transient nature of the proposed trigger event makes it nearly impossible to verify after the fact. The Wow! signal’s defining feature — that it never happened again — is exactly what makes Mendez’s hypothesis so hard to test: a hydrogen cloud flare, like the original signal, would likely be a single, non-repeating event, so no follow-up observation can catch it in the act.

What would actually settle it

Marchis and Sgro pointed to the need for coordinated, multi-station observation as the real path forward, which is the premise behind LaserSETI, a project deploying a global network of detectors so that any future transient — whether a natural flare or an artificial signal — gets recorded simultaneously from multiple locations instead of by one telescope working alone. Mendez’s 2024 paper, posted alongside a follow-up analysis of archival Ohio SETI data, argues its case using signals it says resemble the Wow! event but never matches its full intensity. Until a comparably strong, comparably shaped signal turns up a second time from the same coordinates, researchers on both sides of the debate agree on one thing: the printout Ehman circled in 1977 remains the strongest candidate either camp has, and neither can yet prove what produced it.

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This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.