The Wow! signal

Scene · reconstructed from the paperworkPicture it. Ohio, the night of 15 August 1977. Ohio State University's radio telescope, Big Ear, is listening to the sky. Nobody is watching it. It is running unattended, as it often did, sweeping a strip of sky as the Earth turns and printing what it hears. At about 11:16 p.m. Eastern time, according to Jerry Ehman's own report[ehman-2007], it records something.

The signal sits at 1420.4556 MHz. That number matters. It is close to 1420 MHz, the natural frequency of neutral hydrogen. The signal is narrow, strong, and it rises and falls over 72 seconds[bigear-faq], exactly the time a fixed point in the sky takes to pass through the telescope's beam.

Big Ear had two feed horns, and so two beams looking at slightly different patches of sky. The signal appeared in only one of them[bigear-faq]. The observatory's notes add that no variation was detected within it and that no audio was recorded. What survives is the computer printout: columns of numbers and letters that encode signal strength.

Later Ehman, a volunteer on the project, went through the printout by hand. He found the sequence 6EQUJ5, an unusually strong rise and fall, circled it, and wrote one word in the margin: Wow! His 2007 report[ehman-2007] gives the full technical account of that sequence and of what it means.

No person saw anything that night. There was no flash, no sound, no witness at the telescope. The only witness is a machine, and the man who read its printout. That is unusual for this Archive, and in one way it is a strength: the record was made automatically, at the time, by an instrument working normally. The observatory's director in 1977, Bob Dixon, later put it plainly, as Astronomy Now[astronomynow-2017] quoted him: the telescope was working perfectly normally.

The weakness is the same fact turned around. One event, one beam, 72 seconds, no audio, no second look. The signal was never detected again, as the observatory FAQ[bigear-faq] and Ehman[ehman-2007] both record.

So the night of the Wow! signal is a single line on a printout. It is real, it is dated, it is measured. What it was is the question the rest of this file follows, and the answer depends heavily on how much one line can carry.

The paper trail for the Wow! signal is short, and most of it comes from the people who ran the telescope.

The core document is Ehman's 30th anniversary report[ehman-2007], drafted on 9 July 2007 and last revised in 2010. It is a technical account by the man who found the signal: the time, the 1420.4556 MHz frequency, the 6EQUJ5 code, the problem of the two horns, and the follow-up searches. It is hosted on the Big Ear memorial site run by NAAPO, the volunteer group that operated the telescope in its last years.

The second document is the observatory's own FAQ[bigear-faq] on the same site. It confirms the 72-second window, the single beam, the lack of variation and of audio, and the fate of the instrument. It has no page date, and the Archive marks it so.

Before the long report there was a newspaper. In September 1994 the Cleveland Plain Dealer[plain-dealer-1994] ran a feature on the signal and on Ehman. He was less convinced than many readers. Intelligent senders, he said, would have done it far more than once, and he suggested an Earth signal reflected off space debris. The memorial site reprints the article with the observatory's own bracketed notes.

By 2007 his view had shifted in tone, not in caution. In the report[ehman-2007] he finds terrestrial origins ruled out or improbable and says an extraterrestrial origin cannot be excluded. His summary line is the one the Archive keeps on the cover: there is simply too little data to draw many conclusions.

Then the instrument itself left the record. In early 1998 land developers destroyed the telescope to extend a golf course, as the FAQ[bigear-faq] records. That ended any chance of re-observing with the original equipment. Whatever Big Ear heard, no one can point Big Ear at it again.

The paper trail also includes searches that found nothing. Ehman's report describes follow-up observations of the same part of the sky by Big Ear. The exact count and dates are not settled in what we have opened, and the Archive lists that in its open questions.

The inventory is simple. A printout, a technical report by its finder, the observatory's notes, a 1994 newspaper interview, and a list of searches that came back empty. All of it points to one recorded event, carefully described, never repeated.

The Wow! signal has had two attempts to close it, forty years after the night, and they went very differently.

The first came in June 2017. Antonio Paris, of the Center for Planetary Science, published a paper[paris-2017] reporting 200 radio observations of comet 266/P Christensen. He reported that the comet emitted at 1420.25 MHz and concluded that the Wow! signal was likely a natural emission from a Solar System body. The paper appeared in the Journal of the Washington Academy of Sciences and attracted wide press attention.

The reply came within a day. Astronomy Now[astronomynow-2017] collected the objections. Bob Dixon, director of the Ohio State Radio Observatory in 1977, said the comet was in the wrong position. Comet specialist Alan Fitzsimmons and Seth Shostak said they knew of no comet emitting at 1420 MHz; Fitzsimmons called the conclusion rubbish. The observatory staff wrote a longer rebuttal on their own site, which the Archive could read but cannot link, because that site refuses https.

The second attempt is more interesting. In August 2024 Abel Méndez, Kevin Ortiz Ceballos and Jorge Zuluaga posted a preprint[mendez-2024] based on archived Arecibo data from 2020. They found narrowband signals near 1420 MHz that resemble the Wow! signal, only far weaker, and attributed them to small, cold hydrogen clouds. They propose that a sudden brightening of such a cloud, perhaps triggered by a magnetar flare, produced the 1977 signal.

The press account[physorg-2024] gives the hedges fairly. The clouds could be identified; the triggering source could not. The work was not yet peer-reviewed. Its sharpest line is about scale: the only difference between the signals seen at Arecibo and the Wow! signal is their brightness.

That is the turn. The comet was a single proposed source that the observatory itself rejected on position. The hydrogen cloud is a class of natural source, with data behind it, that does not yet name the event that would have made one cloud flare. The Archive's Other Side[mendez-2024] box gives it at full strength.

Our label stays Unexplained. The comet idea is rejected by the people who ran the telescope. The hydrogen idea is promising, but it is a preprint and leaves the trigger open. A label change to Explained needs independent confirmation, not a good headline.

What is still open in the Wow! file is a short list, and every item on it is checkable in principle.

First, publication. The Méndez preprint[mendez-2024] was revised in September 2024. Whether it has since passed peer review and appeared in a journal is on our to-verify list. If it has, the Archive will read the published version and update the analysis.

Second, the trigger. The hydrogen-cloud idea needs a source that briefly brightened one cloud enormously. The press account[physorg-2024] is clear that the source was not identified. A magnetar flare is a suggestion, not a finding. Until a candidate is named and checked, the explanation has a gap at its centre.

Third, the follow-up searches. Ehman's report[ehman-2007] describes Big Ear's later observations of the same part of the sky, and the memorial site gives a figure of just over a hundred observations at that and nearby declinations. The exact count and dates are still not found. That matters: the strength of never detected again depends on how often, and how hard, anyone looked.

Fourth, the observatory's own rebuttal of the comet paper. The Astronomy Now[astronomynow-2017] report covers the main points. The fuller staff text exists on the NAAPO site, but over plain http only, so the Archive cannot link it under its rules. A secure copy would let us cite it directly.

Fifth, the instrument. The telescope was destroyed in early 1998[bigear-faq]. No one can re-run the night. Any new answer has to come from other telescopes and from the archive data of others, as the Arecibo work did.

The honest position: Confirmed, a strong narrowband signal near 1420 MHz was recorded by Big Ear on 15 August 1977, in one beam, for about 72 seconds, and was never detected again. Rejected by the observatory, a comet source. Proposed with data but not settled, a flaring hydrogen cloud. Not shown, a message.

• Has the 2024 hydrogen-cloud paper been published and replicated?
• What could make one cold cloud flare that brightly?
• How many times did anyone look again, and when?

The folder stays open and the file stays a draft until one of those has a firm answer. One line on a printout, circled in pen, can carry a great deal of hope, and the Archive's job is to note exactly how much it can carry as evidence.