The Wow! Signal

On the night of 15 August 1977, a radio telescope at the Ohio State University in the United States picked up an extraordinary signal from space. The telescope, nicknamed "Big Ear," was part of a programme called SETI — the Search for Extraterrestrial Intelligence — which had been systematically scanning the sky for unusual radio transmissions since the 1960s. The signal that arrived that night was unlike anything the programme had ever recorded.

The astronomer on duty, Jerry Ehman, was reviewing the computer printout of the telescope's data several days later when he noticed the anomaly. The signal appeared as a sequence of alphanumeric characters — the shorthand used by the programme to represent signal strength — and the sequence was so dramatically different from the background noise of space that Ehman circled it in red pen and wrote a single word in the margin: "Wow!"

That annotation gave the signal its name. The Wow! Signal, as it became known, remains to this day the strongest candidate for an artificial extraterrestrial radio transmission ever detected. It has never been satisfactorily explained, and despite decades of effort by radio astronomers around the world, it has never been detected again.

The signal lasted 72 seconds — the maximum duration for which Big Ear could observe any point in the sky as Earth rotated — and then it was gone. In those 72 seconds, it demonstrated several characteristics that made it deeply interesting to scientists. Understanding why requires a brief explanation of what SETI was looking for, and why this signal appeared to fit the profile so precisely.

To understand why the Wow! Signal was so remarkable, it helps to know what SETI scientists were looking for. The programme was based on a theoretical argument made in 1959 by physicists Giuseppe Cocconi and Philip Morrison, who proposed that any technologically advanced civilisation wishing to communicate across interstellar distances would most logically use radio waves — specifically at a frequency associated with hydrogen, the most abundant element in the universe. This frequency, known as the hydrogen line, sits at 1420 megahertz, and it was considered the most logical "cosmic watering hole" — a frequency that any intelligent species in the galaxy would recognise as meaningful.

The Wow! Signal arrived at almost exactly the hydrogen line frequency. This alone made it immediately interesting. But it had several other properties that distinguished it from natural sources of radio emission. It was narrowband — meaning it occupied an extremely narrow range of frequencies, which is characteristic of artificial transmissions and very unusual in natural phenomena. It was also powerful, registering at roughly 30 times the intensity of the background noise of space. And it appeared to come from a fixed point in space, consistent with a source far beyond our solar system.

Perhaps most intriguingly, the signal displayed a characteristic rise-and-fall pattern in intensity that was entirely consistent with the way Big Ear's beam would scan across a fixed point in space as the Earth rotated. In other words, it behaved exactly as a genuine signal from a distant, fixed cosmic source should behave — not like local interference, not like a satellite, and not like any known natural phenomenon.

The combination of these properties led many astronomers to conclude — cautiously — that the Wow! Signal was the most compelling candidate for an artificial extraterrestrial transmission ever detected. The scientific consensus has never gone further than that.

In the decades since 1977, astronomers have made numerous attempts to re-detect the Wow! Signal. The Ohio State University Big Ear telescope itself was demolished in 1997 to make way for a golf course — a fact that many in the SETI community found painful — but other, more powerful instruments have since been pointed at the region of sky from which the signal appeared to originate.

Despite extensive searches using some of the world's most sensitive radio telescopes, including the Arecibo Observatory in Puerto Rico before its collapse in 2020, and the FAST telescope in China — currently the world's largest radio dish — the signal has never been detected again. This non-repetition is both the most frustrating and the most significant feature of the case. A genuine transmission from an extraterrestrial civilisation might be expected to repeat, if it is a deliberate beacon, or to be detectable on further observation, if it is a continuous broadcast. The silence since 1977 remains unexplained.

In 2016, a team of researchers proposed a natural explanation for the signal. They suggested that it could have been caused by two comets passing through the field of view of the telescope at the time, their hydrogen clouds producing a brief, intense radio emission at the hydrogen line frequency. The comets in question had indeed been in the relevant region of the sky at the time, and the team published their findings in a peer-reviewed journal.

The cometary hypothesis was met with significant scepticism from many astronomers, however, who pointed out that comets had never been observed producing radio emissions of the intensity and narrowband characteristics of the Wow! Signal. The debate continues, and the most honest summary of the current scientific position remains what it has been for nearly fifty years: we do not know what caused the Wow! Signal.

The Wow! Signal occupies a unique position in the history of science and human culture. It is not proof of extraterrestrial intelligence — no serious scientist has ever claimed it is. But it is the closest thing to such proof that humanity has ever encountered, and its implications — both scientific and philosophical — are profound.

From a purely scientific perspective, the signal demonstrates that SETI is not a merely theoretical enterprise. Something unusual happened at Big Ear on 15 August 1977. Whatever its cause, it falls outside the range of phenomena that our current understanding can definitively explain. This matters because it means the search is real, the instruments are capable of detecting anomalies, and the universe appears to contain signals that do not behave like noise. The argument for continuing and expanding the search gains force from the existence of the Wow! Signal, even though — or perhaps precisely because — it remains unexplained.

Philosophically, the signal raises questions that go to the very heart of what it means to be human. We are a species that has existed for a cosmic eyeblink, on a small planet orbiting an unremarkable star in one of hundreds of billions of galaxies. The Fermi Paradox — the observation that a universe as old and large as ours should, statistically, be teeming with intelligent life, and yet we appear to have found no evidence of any — has no satisfying answer. The Wow! Signal does not resolve the paradox, but it opens a door.

For 72 seconds in 1977, something in deep space produced a signal that matched, in almost every measurable respect, what scientists had theorised an extraterrestrial transmission would look like. Whether it was the product of an alien civilisation, a comet, or something else entirely, those 72 seconds changed the way many scientists and thinkers understood the question of whether we are alone. That question remains open. The Wow! Signal reminds us that it is not merely philosophical — it is scientific, and it is real.