What Was the 1977 Wow! Signal? (Narrowband Radio Astronomy & Modern Hypotheses)

The August 15, 1977 Detection: Jerry Ehman and "6EQUJ5"

On the night of August 15, 1977, the Ohio State University "Big Ear" radio telescope was conducting a routine SETI (Search for Extraterrestrial Intelligence) sky survey near the constellation Sagittarius [1,2]. Several days later, astronomer Jerry R. Ehman was reviewing the tractor-feed computer printouts when he spotted an astonishing sequence of characters: 6EQUJ5 [1,2,3].

Ehman circled the six characters in red ink and scrawled "Wow!" in the margin—giving the event its historic name [1,2]. In the Big Ear’s printout system, numbers 1 to 9 represented signal-to-noise ratios, followed by letters A through Z (with "U" indicating a signal exceeding 30 standard deviations above background noise) [1,2,3]. It remains the strongest candidate signal for an artificial interstellar transmission ever recorded [1,3].

"Jerry Ehman circled the sequence 6EQUJ5 in red ink and wrote "Wow!" after discovering a radio signal 30 standard deviations above cosmic background noise."

The "Water Hole": Why 1420 MHz Was the Ideal Interstellar Channel

The signal possessed characteristics that precisely matched theoretical predictions formulated by physicists Giuseppe Cocconi and Philip Morrison in their 1959 Nature paper [1,4]. The transmission was detected at a frequency of 1420.455 MHz—the exact 21-centimeter emission line of neutral hydrogen ($H$), the most abundant element in the cosmos [1,4,5].

Because the hydrogen line sits within the quietest radio window in the electromagnetic spectrum (between hydrogen at 1420 MHz and hydroxyl ($OH$) at 1666 MHz, together spelling $H_2O$ or the "Water Hole"), astronomers had long postulated that any technological civilization attempting interstellar contact would broadcast on this universal frequency [1,4,5]. Moreover, the signal was extremely narrowband (< 10 kHz), a trait that cannot be produced by standard thermal cosmic phenomena like pulsars or quasars [1,3,5].

The 72-Second Bell Curve: The Signature of Deep Space

Crucially, the Big Ear was a fixed meridian telescope that relied on Earth’s rotation to sweep across the sky [1,2,3]. A stationary source in deep space would take exactly 72 seconds to transit through the telescope’s 10-arcminute antenna beam, producing a characteristic bell-shaped intensity curve [1,2].

The Wow! Signal rose for 36 seconds, peaked at "U" (30.5x noise), and fell symmetrically for 36 seconds, perfectly conforming to the point-source profile of a distant celestial object far beyond Earth’s atmosphere [1,2,3]. However, the Big Ear featured two horn feeds scanning three minutes apart; while the first feed registered the signal, the second feed three minutes later detected nothing, indicating the source either shut off or moved [1,2,3].

"The signal rose for 36 seconds and fell for 36 seconds, perfectly matching the 72-second transit of a fixed deep-space point source through the telescope beam."

Modern Astronomical Hypotheses: Comets vs. Natural Astrophysical Masers

Over the past 50 years, dozens of subsequent searches—using the Very Large Array (VLA), the Green Bank Telescope, and the Parkes Observatory—have monitored the exact celestial coordinates in Sagittarius without detecting a single repeat pulse [1,3,6].

In 2017, astronomer Antonio Paris proposed that the hydrogen coma of passing comets (266P/Christensen or P/2008 Y2) caused the signal, though radio astronomers widely dismissed this because comets emit diffuse, broadband radiation far weaker than the 30-sigma peak [1,6]. In 2024, the Arecibo WOW Project at the Planetary Habitability Laboratory (PHL) at UPR Arecibo published data proposing that the event could have been a rare, transient astrophysical hydrogen maser flare—where an intense flare from a magnetar or soft gamma repeater passed through a cold interstellar hydrogen cloud, stimulating an ultra-bright, narrowband 1420 MHz burst [1,7].