The Wow! Signal: A Message from the Stars?

The Story

In the 1970s Ohio State University's Big Ear radio telescope, run partly by volunteers, was scanning the sky for narrowband radio signals. A computer printed the results as columns of numbers and letters. According to astronomer Jerry R. Ehman's own account, a technician would deliver the printouts to his home every few days.

A few days after 15 August 1977, probably on 19 August, Ehman began going through the latest run. 'A few pages into the computer printout I was astonished to see the string (sequence) of numbers and characters "6EQUJ5" in channel 2,' he later wrote. He 'immediately recognized this as the pattern we would expect to see from a narrowband (i.e., narrow frequency band) radio source of small angular diameter in the sky.' In red pen he highlighted the six characters and wrote 'Wow!' in the margin. The signal had arrived, he calculated, at about 11:16 p.m. Eastern Daylight Time on 15 August.

'There is simply too little data to draw many conclusions,' wrote the astronomer who circled it.

It lay near 1420 MHz, the hydrogen line. Ehman checked the printout for a repeat on later days. It had not: 'it didn't,' he wrote. In 1977 the telescope had no way to track or quickly re-observe a source.

Ehman's report works through planets, asteroids, satellites, aircraft, spacecraft and ground transmitters. He concluded that since the terrestrial explanations were 'either ruled out or seem improbable,' an extraterrestrial intelligence 'might have sent the signal.' But he refused to go further: 'the origin of the Wow! signal is still an open question for me. There is simply too little data to draw many conclusions.'

In 2024 a team led by Abel Méndez posted a preprint based on archived Arecibo Observatory data from 2020. They found faint narrowband signals near the hydrogen line, 'similar to the Wow! Signal, although two-orders of magnitude less intense,' coming from small clouds of cold hydrogen. They hypothesized that the Wow! signal 'was caused by a sudden brightening of the hydrogen line in these clouds triggered by a strong transient radiation source, such as a magnetar flare or a soft gamma repeater.'

That idea remains a hypothesis. Nearly fifty years on, the signal has never been heard again, and its source is unknown.