WOW SIGNAL

Opening

On the night of 15 August 1977, Ohio State University's Big Ear radio telescope, listening for narrowband signals near the hydrogen line, recorded a burst that rose and fell over seventy-two seconds as the sky drifted through its beam. A few days later the volunteer astronomer Jerry Ehman was reading the computer printout by hand. In channel 2 he found the string 6EQUJ5, circled it in red pen, and wrote "Wow!" in the margin. That word became the name of the most famous candidate signal in the search for extraterrestrial intelligence.

The frequency sat almost exactly at 1420 megahertz, the natural radio frequency of hydrogen, which scientists had long suggested an alien civilisation might choose. The intensity peaked far above the background, and the shape was exactly what a fixed point in the sky should produce. Big Ear looked again. Other telescopes looked. The signal has never come back.

Timeline

15 August 1977, 22:16 Eastern timeBig Ear, near Delaware, Ohio, is on a routine search. The telescope does not move; the Earth's rotation sweeps its beam across the sky. For one seventy-two-second window, the intensity in a single narrow channel climbs through the observatory's alphanumeric code to "U", about thirty times the background noise, and falls again. Nobody is watching the printer at the time.

A few days laterEhman reviews the printouts, circles 6EQUJ5 and writes "Wow!"

Following monthsBig Ear points at the same patch of sky, in Sagittarius, dozens of times. Nothing repeats. Ehman later says, "We should have seen it again when we looked for it 50 times."

1987 to 1999The astronomer Robert Gray leads the most persistent follow-up searches: with the META array at Harvard in 1987 and 1989, with the Very Large Array in New Mexico in 1995 and 1996, and with a telescope in Tasmania in 1999. None finds anything.

1998Big Ear is dismantled to make way for a golf course.

2012For the 35th anniversary, the Arecibo telescope in Puerto Rico sends a reply of about 10,000 messages from the public towards three stars, none of them actually inside the region the signal came from.

2017A claim that passing comets caused the signal draws wide attention and is rejected by other astronomers.

2020 to 2022The amateur astronomer Alberto Caballero identifies likely Sun-like stars in the signal's region. A targeted search by Breakthrough Listen in May 2022 finds no sign of technology.

2024Abel Méndez and colleagues at the Planetary Habitability Laboratory publish a study based on archived Arecibo data proposing a natural cause.

Key figures

Jerry Ehman is the man who noticed. He was doing the unglamorous work of reading numbers. The red pen is why the case has a name. In 2019 he said he was convinced the signal "certainly has the potential" of being the first from extraterrestrial intelligence.

John Kraus built Big Ear and ran the observatory, and wrote the early technical accounts of the signal.

Robert Gray spent years trying to catch it again.

Abel Méndez led the 2024 study that offers the most serious natural explanation so far.

Physical and documentary trail

What survives is the Big Ear printout: the line 6EQUJ5, with its intensity rising and falling across the seventy-two-second window, and Ehman's circled "Wow!" on the same sheet. The frequency was later given as about 1420.4556 megahertz, within a whisker of the hydrogen line, and its width was less than 10 kilohertz, far narrower than most natural radio sources. Kraus and Ehman gave slightly different figures, both close to the hydrogen line.

Big Ear listened through two feed horns, side by side, and the signal appeared in only one of them. Because the record doesn't show which, the source could lie in either of two spots in Sagittarius, about 2.5 degrees south of the star Chi Sagittarii. What is missing is any second telescope that logged the same seventy-two seconds, any modulation that looks like a message, and any repeat.

Wow!

Things you might not know

• It lasted 72 seconds. That is exactly how long Big Ear's beam took to sweep past a fixed point in the sky.
• The code was a scale. The printout used the digits 1 to 9 and then letters for stronger signals, so "U" near the middle of 6EQUJ5 was very strong indeed.
• It came from only one of two horns. That is why its position in the sky has two possible answers.
• A 2024 study found faint cousins. Abel Méndez and colleagues searched archived Arecibo data and found similar narrowband signals near the hydrogen line, about a hundred times weaker and in several places.
• Their proposed cause is a cosmic flash. They suggest a strong transient source, such as a magnetar flare, briefly made a cold hydrogen cloud act like a natural maser.
• It was stronger than first thought. Reanalysing the Big Ear records, the same team found the 1977 signal was about four times stronger than previously believed.
• The data is due out for the anniversary. The team plans to archive and share all of Big Ear's data by 2027, the signal's 50th anniversary.

Competing explanations

An interstellar beacon or leakage. This is why the printout was so exciting: narrowband, at the hydrogen frequency, with the shape of a point source. It has never been heard again, which is a problem for a deliberate beacon and less of one for a brief leak or a sweeping transmitter.

A natural flare in a hydrogen cloud. The 2024 Arecibo study's explanation, which accounts for the frequency, the narrow width and the one-off nature of the burst. It has the best evidence behind it so far, but it is a proposal, not a confirmed observation of what Big Ear saw.

A comet. Offered in 2017 and widely rejected: the comets named were not where the explanation needed them to be, and comets are not known to emit this way.

Earth-side interference. Always the first mundane guess. The signal's perfect point-source shape and its frequency, in a band protected for astronomy, are what this reading has to explain away.

Later life of the case

Big Ear was dismantled in 1998, but the signal kept its fame, and the circled printout became one of the most recognisable images in the search for alien life. The 2012 Arecibo reply, the 2017 comet claim and the 2022 star search all kept it in the news. The 2024 study offers the most serious natural explanation so far. For the more recent arguments over objects from beyond the solar system, see interstellar visitors.

Finding

Big Ear recorded a seventy-two-second narrowband burst on 15 August 1977, Jerry Ehman circled 6EQUJ5 and wrote "Wow!" on the page, and the sky has not said it again. Something in that beam was loud, brief and shaped like a point passing through. A natural flare in a hydrogen cloud is now the strongest explanation, but nobody has caught one in the act at that strength.

What still will not resolve: a repeat from the same patch, or a second telescope that logged the same seventy-two seconds.