In 1977, Ohio State's Big Ear radio telescope recorded a strong, narrow-band signal from the direction of Sagittarius. Astronomer Jerry Ehman circled the printout and wrote 'Wow!' It has never repeated despite many searches — and remains one of the most tantalizing single data points in the search for extraterrestrial intelligence.
The exciting interpretation is that the signal was an intentional transmission — a beacon from an extraterrestrial intelligence.
The signal sat near the 1420 MHz hydrogen line, a frequency long predicted as a likely interstellar 'hailing' channel.
Its intensity rose and fell exactly as expected for a fixed celestial source passing through the telescope's beam.
Despite many attempts, nothing has been detected from that spot again.
The signal is genuinely unexplained and has never recurred. Proposed natural sources — including a controversial comet hypothesis — remain disputed, and the single, non-repeating nature of the event makes any firm conclusion impossible.
Skeptics emphasize that one unrepeatable reading can't be confirmed as anything. Earthly interference or an unknown natural source can't be ruled out, and extraordinary claims need repeatable evidence.
A real, recorded, unexplained signal that never came back is the definition of an open file. It may forever sit as a single, maddening question mark.
The Wow! Signal is a cornerstone of SETI lore and has inspired music, documentaries, and at least one attempt to beam a reply.
- Jerry Ehman's account of the Wow! Signal
- Ohio State Big Ear / SETI archival materials
- Peer-reviewed papers debating natural explanations