Science
The Wow! Signal
On August 15, 1977, a radio telescope picked up a 72-second signal from deep space. Astronomer Jerry Ehman circled it and wrote 'Wow!' It has never been explained.
Trace the connectionsWhy this rabbit hole matters
On August 15, 1977, astronomer Jerry Ehman was reviewing data from Ohio State University's Big Ear radio telescope when he noticed a 72-second burst of radio energy so unusual he circled it on the printout and wrote 'Wow!' in the margin. The signal matched the predicted characteristics of an extraterrestrial transmission: it was narrowband (around the 1420 MHz hydrogen line, the universal 'cosmic watering hole'), it appeared to come from the direction of Sagittarius, and its intensity far exceeded background noise. Despite dozens of follow-up observations over 45 years using the world's most powerful radio telescopes, the Wow! Signal has never been heard again.
The Big Ear telescope's output was printed as columns of alphanumeric characters representing signal intensity; letters after '9' indicated higher intensities. The Wow! Signal printed as '6EQUJ5' — a sequence that peaked at intensity 'U' (30 standard deviations above background noise) before fading. It lasted exactly 72 seconds, which matches the expected duration for a signal from a fixed point in space as Earth rotates — consistent with an extraterrestrial point source. The signal appeared in only one of Big Ear's two feed horns, suggesting a genuine point source rather than terrestrial interference. It came from near the Chi Sagittarii star group, 220 light-years away.
Connections to follow
SETI: The Search for Extraterrestrial Intelligence
SETI — the Search for Extraterrestrial Intelligence — has been the most optimistic scientific enterprise of the 20th century: a systematic attempt to detect signals from other civilizations. Founded on the logic of Frank Drake's famous equation (1961), SETI began with Project Ozma at Green Bank in 1960 and has continued through hundreds of programs including SETI@home (which distributed analysis to millions of home computers), the Allen Telescope Array, and Breakthrough Listen. Despite 60+ years and billions of data points, the Wow! Signal remains the only candidate ever to pass initial screening.
The Hydrogen Line: Universal Frequency of Contact
The 1420 MHz frequency (21-centimeter line) is the emission frequency of neutral hydrogen atoms — the most abundant element in the universe. SETI theorists Philip Morrison and Giuseppe Cocconi proposed in 1959 that any civilization wishing to make contact would choose this frequency, because any civilization capable of radio astronomy would know it, and because the 'cosmic watering hole' between 1420 MHz and 1666 MHz (hydroxyl line) is relatively free of natural background noise. The Wow! Signal's apparent centering on this frequency is one of its most compelling features — and the main reason it cannot be dismissed as terrestrial interference.
Why It Was Never Heard Again
The Wow! Signal's non-repetition is its central puzzle. Big Ear performed over 50 targeted follow-up observations in the months after the discovery; the Very Large Array and Breakthrough Listen have since added to the search — all without detecting a repeat. Several explanations have been proposed: the signal was a one-time deliberate transmission aimed at Earth; the source is intermittent and hasn't been observed at the right moment again; the transmitting civilization is moving and the signal has Doppler-shifted out of the monitored band; or the signal had a natural, non-intelligent origin. The 2016 comet hypothesis (Antonio Paris) proposed that hydrogen clouds around comet 266P/Christensen produced the signal — but the comet was not near those coordinates in 1977, and Jerry Ehman rejected the hypothesis.
Big Ear and the Human Story Behind the Discovery
The Big Ear radio telescope at Ohio State University ran one of the longest-duration SETI programs in history — scanning continuously from 1973 until 1997, when Ohio State demolished it to expand an adjacent golf course. Jerry Ehman was an unpaid volunteer scientist reviewing paper printouts three days after the signal was recorded when he circled '6EQUJ5' and wrote 'Wow!' in the margin — because he had no better word. The original annotated printout is now at the Ohio History Connection. Ehman maintained scientific caution throughout, repeatedly resisting over-interpretation: 'I wish it had occurred more than once' became his standard public statement. The gap between the possible significance of the discovery and the institutional response (demolition of the telescope) is one of science's more disquieting footnotes.
The 2016 Reply: Messaging Back to the Source Coordinates
On August 15, 2016 — the 39th anniversary of the Wow! Signal — the National Center for Space Studies of France, working with scientist Yvan Dutil, beamed a 10,000-word reply to the coordinates from which the original signal came. The reply was a compressed mathematical-linguistic message designed to be decodable by a civilization with radio astronomy capability. The target was the area of sky from which the Wow! Signal originated (a 2-degree region of Sagittarius). The reply was transmitted via the Arecibo Observatory. The effort was partly symbolic — the original signal's coordinates are uncertain enough that the reply may not reach its intended source — but it was also a serious scientific proposal: that if the Wow! was genuinely artificial, a reply in kind is the appropriate scientific response.
The Cometary Hypothesis: A Proposed Natural Explanation
In 2017, astronomer Antonio Paris published a paper proposing that the Wow! Signal was caused by hydrogen gas emissions from comets 266P/Christensen and 335P/Gibbs, both passing through the relevant region of sky in August 1977. The hydrogen line (1420.406 MHz) at which the Wow! Signal was detected is precisely the frequency at which neutral hydrogen emits — and comets contain abundant hydrogen in their tails. Paris argued that the signal's bandwidth, duration, and frequency were consistent with cometary emission and inconsistent with assumptions that required an artificial source. The hypothesis was subsequently criticized by SETI researchers: the original observers had access to comet tracking data and would have noticed a comet in the beam; the intensity of the signal exceeded what known comet hydrogen emission could produce at the calculated distances; and the signal was not repeated when the beam swept the same region in subsequent hours, inconsistent with a slowly moving comet.