The Wow! Signal

On August 15, 1977, astronomer Jerry Ehman was reviewing data from Ohio State University’s Big Ear radio telescope when he found something extraordinary. A 72-second burst of radio waves, originating from the direction of the constellation Sagittarius, displayed characteristics expected from an intelligent extraterrestrial source. Ehman circled the signal on the printout and wrote “Wow!” in the margin. Despite decades of searching, the signal has never been detected again.

The Big Ear Project

The Big Ear radio telescope was participating in SETI (Search for Extraterrestrial Intelligence), scanning the sky for narrowband radio signals that might indicate intelligent origin. Natural cosmic radio sources produce broadband signals across many frequencies; a narrowband signal concentrated on a single frequency would suggest technological origin.

The telescope scanned the sky as Earth rotated, recording data on computer printouts. Jerry Ehman, a volunteer researcher, was reviewing these printouts days after they were recorded.

The Signal

The signal Ehman found was remarkable. It was at 1420.4556 MHz, extremely close to the hydrogen line frequency of 1420.405 MHz—a frequency many astronomers believe intelligent civilizations might use because hydrogen is the most abundant element in the universe and this frequency is free of natural interference.

The signal’s intensity rose and fell over 72 seconds in a pattern exactly matching how a deep space signal would appear as the telescope swept past its source. The intensity reached 30 times the background noise level.

The alphanumeric sequence on the printout—6EQUJ5—represented the signal’s intensity over time, with “U” indicating the peak, the highest value possible in the recording system.

The Search for Explanation

Since 1977, the Big Ear telescope and others have repeatedly scanned the same region of sky. The signal has never been detected again. If it were a beacon intended to attract attention, one would expect it to repeat.

Various explanations have been proposed:

Terrestrial interference, such as a military or commercial transmission. However, the frequency is protected for radio astronomy, and the signal pattern matched a deep space source.

A reflecting satellite or space debris. Possible but would require an unusual set of coincidences.

Interstellar hydrogen clouds. Some natural phenomena could theoretically produce such a signal, though none observed have.

An extraterrestrial transmission, either a beacon that we happened to catch once, or a signal directed specifically at us that was not repeated.

Why It Matters

The Wow! signal remains the strongest candidate for a potential extraterrestrial transmission ever detected. It met the criteria SETI researchers had established: narrowband, at a logical frequency, from deep space, at significant intensity.

Its failure to repeat is frustrating but not definitive. A civilization broadcasting in multiple directions might not repeat toward any one target frequently. A one-time transmission would be detectable only by chance.

Assessment

The Wow! signal has never been explained. It was not an equipment error—the signal was real. It was not terrestrial in the conventional sense—the source was in the sky, moving as Earth rotated. It was not obviously natural—narrowband signals are extremely rare in nature.

For 72 seconds in August 1977, something sent a signal from the direction of Sagittarius. We do not know what, and despite our best efforts, we have never heard from it again. The mystery remains, written in Jerry Ehman’s amazed annotation: Wow!