A few evenings after 15 August 1977, an astronomer named Jerry Ehman sat at his kitchen table working through a stack of fan-fold computer printout. The paper carried the previous days' output of the Big Ear radio telescope: column after column of 1s, 2s and blanks, the ordinary murmur of the cosmos rendered as faint characters. Then, partway down one page, a vertical string leapt out at him: 6EQUJ5. A narrowband signal had risen from nothing to around thirty times the background hiss and sunk back to nothing, all within 72 seconds. Ehman ringed the six characters with a red pen and wrote a single word in the margin — 'Wow!' — unwittingly naming the most tantalising 72 seconds in the history of the search for extraterrestrial intelligence.
Big Ear, run by Ohio State University from a field in Delaware, Ohio, was an ungainly giant: a flat, tiltable aluminium reflector spread over an area bigger than three football pitches, staring at a fixed strip of sky while the Earth's rotation dragged the heavens past. By 1977 it had been re-tasked to what became the longest-running SETI programme in the world, with volunteers such as Ehman sifting the output for free. The telescope's geometry explains the magic number. Any point source in the sky took exactly 72 seconds to drift through Big Ear's beam, swelling and fading in a smooth, predictable curve — and the Wow! signal traced that curve precisely. That is the signature of something fixed among the stars, not a passing aircraft or a sparking power line. And because the telescope sampled that patch of sky for only those 72 seconds, the source could have been transmitting for hours, or for years.
The frequency deepened the intrigue. The printout's code ran from 0 to 9 and then A to Z for ever-higher intensities, so 'U' marked a signal roughly thirty times the background — far beyond anything the survey normally recorded. And it arrived at 1420.4556 MHz, a whisker from the 1420.40575 MHz emission line of neutral hydrogen, the most abundant element in the universe. In 1959 the physicists Giuseppe Cocconi and Philip Morrison had famously argued that any civilisation wishing to be noticed would broadcast near that natural, universal frequency. It is also a band protected by international agreement, in which terrestrial transmitters are forbidden — an awkward fact for anyone arguing that the Wow! signal was mere human interference.
The hunt for an encore
The signal came from the direction of Sagittarius, near the star group Chi Sagittarii, looking towards the crowded heart of the Milky Way. Big Ear listened through two feed horns aimed at patches of sky a few minutes apart, and the signal registered in only one — but the electronics did not record which, so there are two candidate positions, and nothing appeared when the second horn swept the same region minutes later. Ehman and his colleagues re-observed the spot for weeks and heard nothing. The astronomer Robert Gray turned the search for an encore into a decades-long personal quest, hunting with the META array and later with the Very Large Array in New Mexico — among the first times that instrument was pointed at a SETI target. Far more sensitive instruments have since swept the same patch of sky, for far longer than Big Ear ever could. Silence, every time.
Explanations have been proposed and, one by one, found wanting. Satellites and aircraft should not broadcast in the protected band, and would not mimic the 72-second drift curve of a fixed celestial source. A 2017 claim that a pair of passing comets wrapped in hydrogen gas were responsible was widely rejected by radio astronomers: comets are far too faint at that frequency, and were arguably not even in the beam. Ehman himself has remained scrupulously cautious for decades, resisting the alien conclusion and the tidy debunkings alike. History added its own strange garnish — Elvis Presley died the following day — and the telescope did not outlive the mystery: Big Ear was demolished in 1998 to make way for an expanding golf course.
Half a century on, the Wow! signal remains the strongest candidate the search has ever logged — a single, pristine, unrepeated data point: either an unknown natural phenomenon that has kept its secret for decades, or the one moment when somebody's transmitter and our antenna briefly faced each other across the galaxy. The universe said one word, and has held its silence ever since.
Quiz nuggets
- The Wow! signal was recorded on 15 August 1977 by Ohio State University's Big Ear radio telescope in Delaware, Ohio.
- Jerry Ehman circled the printout code 6EQUJ5 in red pen and wrote 'Wow!' beside it; the 'U' marked an intensity about thirty times the background.
- The signal lasted 72 seconds — exactly the time Big Ear's fixed beam took to sweep any point of sky — at 1420 MHz, the emission frequency of neutral hydrogen.
- It came from the direction of Sagittarius, near Chi Sagittarii, and has never been detected again despite decades of follow-up searches.
- Big Ear was demolished in 1998 to make way for a golf-course expansion.