It is August 15th, 1977. A warm summer’s day at Ohio State University’s Big Ear radio telescope, in the countryside at the edge of the city of Delaware, Ohio. Other than the tweeting of birds and the flutter of butterflies, nobody is present.
Inside the control house – basically a shed – are some bulky computers and a printer producing a slow trickle of numbers: lots of 1s, 2s, 3s, indicating the detection of radio emission from space 1, 2 and 3 times the usual radio background. The occasional radio-emitting galaxy, or galactic hydrogen cloud. Nothing unusual. Nothing special.
All is quiet.
Then, all of a sudden, the printer bursts into life. The 1s and 2s are replaced by a flurry of higher numbers and then letters signifying radio signal strengths 10 or more times stronger than the background. The telescope detects this powerful radio blast for 72 seconds.
Days later, somebody – a technician, maybe a student – arrives at the telescope to collect the printouts and deliver them to Jerry Ehman, the Big Ear’s Project Scientist. He’s given a huge wad of perforated printer paper to sift through, looking for anything unusual. Finding the signal detected on August 15th, he’s amazed by what is the most powerful narrowband radio emission he has ever seen.
He circles the sequence of numbers and letters, writing ‘Wow!’ in the margin, and history is made.
Nobody knows what the Wow signal was. We do know that it was not a regular astrophysical object, such as a galaxy or a pulsar. Curious the frequency that it was detected at, 1,420 MHz, is the frequency emitted by neutral hydrogen atoms in space, but it is also the frequency that scientists hunting for alien life listen to. Their reasoning is that aliens will supposedly know that astronomers will already be listening to that frequency in their studies of galactic hydrogen and so should easily detect their signal – or so the theory goes. Yet there was no message attached to the signal. It was just a burst of raw radio energy.
If SETI had a mythology, then the Wow signal would be its number one myth. And while it has never been forgotten by the public, the academic side of SETI has, by and large, dismissed it, quite possibly because it hasn’t been seen to repeat, and therefore cannot be verified – the golden rule of a successful SETI (Search for Extraterrestrial Intelligence) detection.
One man, however, believed there was more to its story, and never gave up in his pursuit of the signal. His name was Robert Gray.
Bob Gray spent his youth building radios. Like most children of the Apollo age, he had a fascination for astronomy, and while studying at the University of Illinois Chicago for a master’s degree in urban planning, he’d spend as much time as possible in the astronomy department’s library keeping abreast with the latest developments in space. It was here that he came across an article about the Wow signal. Quickly, his mind was made up: he was going to search for the signal himself.
To do so, he needed a radio telescope. From a ‘hamfest’ – a kind of flea market for ham radio operators – he picked up a 12-foot dish previously used to relay telephone calls between microwave towers, plus a motorized base so that the dish could move to track the stars as they wheeled overhead. By 1983 the telescope was up and running, and his endeavors had got him noticed enough to be invited to a SETI workshop at the Green Bank radio observatory to give a presentation to the leaders in the field, including the formidable Barney Oliver, former head of R&D at Hewlett–Packard and one of the leaders of NASA’s SETI program.
Gray, though, once told me how he found this first foray into professional science to be a chastening one. His presentation didn’t go well.
“I was pretty green and came away a bit blistered,” he said. ”But I met many people in the field.”
Thirty-five years later, things had completely changed around. When professional astronomer David Kipping of Columbia University wanted to find out more about the Wow signal for a paper he was working on, the first person he reached out to was Gray.
“I became interested in the Wow signal and, as I dug around in it, obviously Robert’s name came up pretty quickly,” says Kipping. Gray became a co-author on Kipping’s paper. Yet tragedy was about to strike. It would be Gray’s final contribution to the puzzle of the Wow signal.
A Big Ear on the Heavens
First, let’s take a step back and really dig into what the Wow signal was. The Big Ear telescope didn’t look like your typical radio telescope. For starters, it didn’t have a dish. Rather, it was a huge plane of aluminum approximately 100 meters long and 30 meters wide, with a static reflector at one end and a tilting reflector at the other, which allowed the telescope to look over different declinations (up and down in the sky) but not in right ascension (left and right). The telescope was therefore limited to what was passing through that narrow strip of sky that it could see as the Earth turned. Radio waves would come down, bounce off the tilting reflector, and then be turned back by the static reflector, focusing at one of two feed horns – funnel-shaped boxes 1.5-meters apart in an east-west direction that housed the receivers. Radio signals would first be detected in one horn for 72 seconds, then there would be a gap in coverage until the radio-emitting object was then seen in the second horn a few minutes later for another 72 seconds.
That was the first weird thing about the Wow signal – it was only detected in one feed horn. Astronomers do not know which horn because the apparatus was not set up to distinguish between them – nobody was expecting sources to switch on and off between the horns!
The second weird thing was the sheer strength of the signal. The Big Ear’s receivers measured the Wow’s signal strength as 6EQUJ5, with the numbers 1 to 9 signifying signal strength 1 to 9 times the background, and then letters for 10 or more times the background level. At one point (signified by the letter U) the Wow signal reached a strength 30 times greater than the radio background. This was unprecedented.
The rise and fade of the signal is the result of the source of the signal moving into, across, and then out of the Big Ear’s field of view. Terrestrial interference doesn’t track with the stars – it would just be ever-present. So the source of the Wow signal had to be something in space. In 2015 Antonio Paris of St. Petersburg College claimed it was an emission from two comets, but this was dismissed by astronomers who study comets for their day job, and who pointed out that comets are unlikely to emit enough hydrogen to produce strong radio signals, and that even if they did, they should have been detectable in both feed horns.
Could it have been a satellite, perhaps? Possibly, but there are things wrong with that scenario too. A low-orbit satellite would pass through the field of view in less than a second. A high altitude, possibly geostationary satellite might do the trick, but none were in the vicinity at the time. What about a top-secret military satellite, kept off the books? Perhaps, but 1,420MHz is a protected frequency kept exclusively for astronomers around the world, so not only should satellites not be transmitting at that frequency, but as David Kipping rhetorically asks, “why would you send a secret communication on a channel that’s so heavily monitored?”
It’s these discrepancies in human-based explanations that led Bob Gray in his belief that the Wow signal just might have been of extraterrestrial origin. Despite his experience at the 1985 Green Bank workshop, there were some positives because that’s where Gray met Paul Horowitz, who led Harvard’s SETI program. Impressed by Gray, Horowitz made him an offer: to search for the Wow signal by graduating onto using professional radio telescopes.
Horowitz allowed Gray to use the famous META (Mega-channel ExtraTerrestrial Assay) experiment on the 26-meter radio telescope at Harvard’s Oak Ridge Observatory. META was able to scan across eight million radio channels simultaneously and was partly funded by a donation from Steven Spielberg. Gray then won time on the Very Large Array in Socorro, New Mexico, made famous as the backdrop for Jodie Foster in the movie version of Carl Sagan’s novel, Contact.
Neither META nor the VLA turned up any fresh evidence for the Wow signal, but their observations were useful in helping to constrain how often the signal might repeat. However, telescopes in the Northern Hemisphere are not best placed to chase the Wow signal, which was detected in the constellation of Sagittarius, lying low in the sky from the north. Telescopes in the Southern Hemisphere have a far better view, with that region of sky high overhead for up to 18 hours per day. So Gray’s next destination was Down Under.
But Gray couldn’t get time on Australia’s biggest radio telescope, Parkes, even with popular Australian astronomer Ray Norris supporting him. “It was just a military satellite,” countered the Australian astronomical authorities. So Norris instead pointed Gray in the direction of a former student of his, by the name of Simon Ellingsen, who operated the University of Tasmania’s 26-meter radio telescope at Mt Pleasant, just outside Hobart on the Australian island state of Tasmania.
Ellingsen picks up the story from here, telling Supercluster about his recollections of how he met Gray back in the early 2000s.
“It’s stretching my memory a bit, but Bob sent me an e-mail about the Wow signal,” he says. Although Ellingsen had never done SETI before, “I was interested in his hypothesis,” he explains. ”The Wow signal looks quite different from most RFI [radio frequency interference], which is what made it worth following up, and our antenna was able to collect to necessary data.”
Terrestrial interference eventually curtailed Ellingsen and Gray’s efforts to re-detect the Wow signal, but again the observations provided important constraints for the signal’s possible repeat cycle, establishing that whatever the cycle’s period was, it was likely longer than 14 hours.
Gray continued his search over the years, most recently with the team at the SETI Institute on their 42-dish Allen Telescope Array, which further constrained the repeat cycle to longer than 40 hours.
Knowing the repeat cycle can offer important clues. If a transmitter is on a planet, then we might expect it to flash our way in time with the planet’s rotation. While most planets might complete one revolution in less than 40 hours, not all will – Venus spins so slowly on its axis that one Venusian day lasts 243 Earth days, for example. So the fact that we haven’t seen the Wow signal repeat yet doesn’t necessarily mean that it isn’t on a planet. Alberto Caballero, a citizen scientist from Spain, has explored the Gaia star catalog that contains precise positions of millions of stars as measured by the European Space Agency’s Gaia satellite and found the most Sun-like star in the field of view at the time the Big Ear detected the Wow signal.
This star, named 2MASS 19281982-2640123, is an estimated 1,800 light years away. It’s not currently known if any exoplanets orbit it, but even if the Wow signal was a bonafide alien message, there’s nothing to say that it came from this star, or even that it came from a planet around any star at all – perhaps the beacon is located in deep space.
A Faint Glimmer
Yet, after decades of searching, no further evidence for the signal had been found, and most interested parties had moved on from it. As Kipping says, “I do get the feeling that the Wow signal is not considered as anything other than a curiosity at this point.”
Even Bob Gray was beginning to feel downhearted despite writing a book (‘The Elusive Wow’) on the subject. In 2019, Gray told me in an email that “the mounting absence of evidence (and funding) moderates my enthusiasm.” It seemed the end was drawing near in the search for the Wow signal.
Enter David Kipping.
As an exoplanet scientist, Kipping is the Principal Investigator at Columbia University’s Cool Worlds Lab, where astronomers study planets that are on longer orbits around their star. But Kipping’s interest in astronomy is fairly wide, and he was fresh from writing a paper about ‘Black Swans’ in astronomical data – how to statistically analyze one-off or nearly one-off events with small data sets – when his thoughts turned to the Wow signal. Kipping wanted to apply his statistical analysis techniques for black swans to the Wow signal, but he needed the original observing logs, which Gray had.
So, after years of Gray going to professional astronomers and trying to persuade them to work with him, things had come full circle, and professional astronomers were now coming to Gray and asking for his help.
Kipping’s idea was that the Wow signal may be stochastic in nature. Think of a bunch of radioactive isotopes. We know they have a half-life, and therefore we can ascribe a probability function to their decay, but we can’t predict exactly when they will decay – hence we describe them as stochastic. Kipping wondered if the same principle might apply to the Wow signal – that there’s a probability function that it will flash in our direction in a given timespan, but when exactly it appears is unpredictable.
There is actually a good reason why aliens might want to be so obtuse.
“It’s to avoid observing gaps,” says Kipping. Earth rotates once every 24 hours, so in the case of the Big Ear telescope (it was knocked down in 1997 to pave way for new homes and a golf course), it only saw the region of sky where the Wow signal was detected once per day. If the beacon were on another planet, then it would align with the Big Ear once per day only if it also had a 24-hour rotation period, which would seem like an unlikely coincidence. Otherwise, it would be sending its signal at a different time to when Big Ear was watching.
“If you really wanted to make sure someone detected [the Wow signal], you wouldn’t set it on a regular schedule,” says Kipping. Instead, you’d mix the transmission times so they cater to a variety of observing modes.
So Kipping and Gray modeled a stochastic beacon and the probability that the Big Ear could detect it. The results were surprisingly optimistic.
“We found that the probability is quite high,” says Kipping. “About 1 in 3 of the simulations match what Big Ear saw.”
In other words, the Big Ear had a 33% chance of detecting a stochastic beacon. However, when you factor in all the other subsequent observations – META, the VLA, the Hobart radio telescope in Tasmania, the Allen Telescope Array – and all their non-detections, the probability drops substantially.
“We found that when you fold in those other observations, the 33% drops to a 1.8% chance, so it becomes improbable, though not implausible, that it was a stochastic repeating beacon that has managed to slip through all these subsequent observations,” says Kipping.
Robert Gray died in December 2021, aged 73. Though he never got to see the finished product, his final contribution to the search for the Wow signal was the paper he began writing with Kipping, and it may have done just enough to reignite some new interest in the Wow signal, showing that there is still a chance that there is more to it than meets the eye. Now, the stage is set for somebody else to take on the task of determining whether this mysterious blast of radio waves was humanity’s first contact with extraterrestrial life or just some rogue human activity.
“We all love mysteries, and every so often an astronomer like myself comes along and takes a stab at it, and I was just pleased for Bob that his last paper actually had some kind of glimmer of hope that it was an extraterrestrial signal,” says Kipping. “It’s a faint glimmer of hope, but it’s something.”