The Wow! signal: seventy-two seconds, and nothing since

On 15 August 1977 the Big Ear radio telescope at Ohio State University recorded a burst of radio energy from the direction of Sagittarius. Reviewing the computer printout days later, the astronomer Jerry Ehman circled the sequence of characters representing its intensity and wrote "Wow!" in the margin.

Nearly fifty years later it remains the most interesting unexplained result in the history of the search for extraterrestrial intelligence, and the reasons are technical rather than romantic.

What was actually recorded

A narrowband signal close to 1420 MHz — the frequency at which neutral hydrogen radiates, and the frequency SETI researchers had argued for decades any deliberate transmission would use, because it is the most conspicuous landmark in the radio spectrum.

It lasted 72 seconds, which is exactly what a fixed telescope observing a point in the sky as the Earth turns would see from a source that stayed put. Its intensity rose and fell in the shape that beam pattern predicts.

It was strong — around thirty times the background — and narrowband. Natural astrophysical sources tend to be broadband; narrow bandwidth is characteristic of a transmitter.

Why it is not proof of anything

One observation, never repeated. The same patch of sky has been searched many times since with more sensitive instruments and nothing has been found.

Big Ear also had two feed horns, and a genuine sky source should have appeared in both a few minutes apart. Only one recorded it — which is odd, and which the surviving records do not fully resolve.

The explanations offered

Terrestrial interference is the obvious candidate and the hardest to exclude retrospectively. A reflection off space debris, or a transmitter briefly in the wrong place, would produce a one-off narrowband burst — but the frequency is protected internationally for radio astronomy, and the duration matches sidereal drift rather than a passing source.

A comet was proposed in 2017, on the grounds that hydrogen clouds around comets could emit near 1420 MHz. The proposal was examined and largely rejected: the comets in question were not in the right place, and comets do not produce narrowband emission of that kind.

More recently it has been suggested that a cloud of cold hydrogen could be stimulated into a brief maser-like emission by a sudden burst from a magnetar. That is a serious proposal and it is not confirmed.

What it actually taught SETI

That a single detection is worth very little, and that the field needed to be designed around repeatability from the start. Modern searches are built to re-observe candidates automatically within minutes, precisely so that a future Wow! signal can be confirmed or dismissed on the night rather than argued about for half a century.