The Wow! Signal

August 15, 1977. The Big Ear radio telescope at Ohio State was running its nightly sky survey when something crossed its beam. The printout showed a narrowband signal at 1420 MHz — the hydrogen line, the frequency SETI researchers had flagged as the most logical channel for a deliberate broadcast — arriving at 30 times background intensity. It lasted 72 seconds, consistent with a fixed-beam telescope sweeping past a point source in the sky. Astronomer Jerry Ehman reviewed the data days later and wrote one word in the margin: "Wow!" Forty-eight years on, that annotation is still the most honest summary of where things stand.

What happened

The Big Ear printout shows the signal's signature as a sequence of alphanumeric intensity codes: 6EQUJ5. Each character represents the signal's strength in a given time interval; "U" — the peak — corresponds to roughly 30 times the background noise floor. The signal came from the direction of Sagittarius, near the hydrogen line at 1420.406 MHz. Duration and intensity both matched what you'd expect from a point source transiting a fixed radio beam.

The telescope used two feed horns, separated by about three minutes of sky-sweep time. A genuine point source should have appeared in both. The Wow! signal appeared in only one. Ehman noted this in his 1998 retrospective and called it the strangest feature of the record. It remains unexplained.

The evidence

The signal's characteristics — narrowband, near the hydrogen line, strong, brief — match the theoretical profile of an intentional extraterrestrial transmission almost exactly. That's not a conclusion; it's a description of why SETI researchers take it seriously.

What the record also shows: nobody has heard it again. Arecibo, the Allen Telescope Array, and a Very Large Array campaign by Robert Gray and Kunal Mooley have all searched the original coordinates without a comparable detection. The Arecibo and ATA follow-up campaigns spanning 1987 through 2017 add up to four decades of silence at substantially greater instrumental sensitivity than Big Ear had in 1977. A continuous astrophysical source at that location is effectively ruled out. A one-time transient — natural or otherwise — is not.

What the explanations don't explain

In 2017, astronomer Antonio Paris proposed a natural candidate: hydrogen clouds surrounding comets 266/P Christensen and P/2008 Y2 (Gibbs), which he argued were in the telescope's beam in August 1977. The hypothesis is testable and worth taking seriously on those grounds alone.

The problem is the numbers. Gray, Mooley, and others have argued that cometary hydrogen comae are too diffuse and too cool to produce the observed narrowband intensity. The signal was narrowband — cometary hydrogen emission isn't. Paris's hypothesis hasn't been adopted by the relevant community, though it hasn't been formally closed either. It's on the table. It's contested.

The single-beam detection is its own puzzle. If the source were a comet — or any extended natural emitter — it should have appeared in both horns. If it were a point source, same. One detection, no repeat: the data doesn't fit cleanly into any proposed explanation.

What's still open

The Wow! signal is the strongest unexplained candidate technosignature on record — not because the evidence confirms extraterrestrial origin, but because nothing else has explained it either. The comet hypothesis is contested. The single-beam anomaly is unresolved. The non-repetition rules out a persistent source but leaves a transient fully open.

Ehman himself was careful: he never claimed the signal was extraterrestrial. He said it had the right characteristics and that he didn't know what it was. That's still the most accurate summary of the case. One detection. Seventy-two seconds. No repeat. We don't know.