By ROSS ANDERSEN
Late one night in the summer of 1977, a large radio telescope outside Delaware, Ohio intercepted a radio signal that seemed for a brief time like it might change the course of human history. The telescope was searching the sky on behalf of SETI, the Search for Extraterrestrial Intelligence, and the signal, though it lasted only seventy-two seconds, fit the profile of a message beamed from another world. Despite its potential import, several days went by before Jerry Ehman, a project scientist for SETI, noticed the data. He was flipping through the computer printouts generated by the telescope when he noticed a string of letters within a long sequence of low numbers—ones, twos, threes and fours. The low numbers represent background noise, the low hum of an ordinary signal. As the telescope swept across the sky, it momentarily landed on something quite extraordinary, causing the signal to surge and the computer to shift from numbers to letters and then keep climbing all the way up to “U,” which represented a signal thirty times higher than the background noise level. Seeing the consecutive letters, the mark of something strange or even alien, Ehman circled them in red ink and wrote “Wow!” thus christening the most famous and tantalizing signal of SETI’s short history: The “Wow!” signal.
Despite several decades of searching, by amateur and professional astronomers alike, the “Wow!” signal has never again been found. In his new book, The Elusive Wow, amateur astronomer Robert Gray tells the story of the “Wow!” signal, and of astronomy’s quest to solve the puzzle of its origin. It’s a story he is well-positioned to tell. That’s because Gray has been the “Wow!” signal’s most devoted seeker and chronicler, having traveled to the very ends of the earth in search of it. Gray has even co-authored several scientific articles about the “Wow!” signal, including a paper detailing his use of the Very Large Array Radio Observatory in New Mexico to search for it. I spoke with Gray about the “Wow!” signal, radio telescopes, and the economics of prospective extraterrestrial civilizations.
From a technical standpoint, what makes the “Wow!” signal so extraordinary?
Gray: The main thing is the profile of the signal, the way it rises and falls over about seventy-two seconds. When we point these big dish antennas up at the sky, and a radio source moves across them, they have a special signature, a kind of fingerprint. That fingerprint results from the “loudness” of the radio source slowly increasing, getting to a peak as the dish points straight at it, and then slowly decreasing as the object moves across the dish and past its beam of observation. In the case of the “Wow!” signal, the signal followed that curve perfectly. It looked exactly like a radio signal in the sky would look, and it’s pretty unlikely that anything else—like an airplane or satellite or what have you—would leave a special signature like that.
Also there’s not much doubt that the “Wow!” signal was a radio signal, rather than something from a natural source like a quasar. That’s because Ohio State was using a receiver with fifty channels, which is sort of like having fifty AM radios, each tuned to adjacent stations. With the “Wow!” there wasn’t any noise on any of the channels except for one, and that’s just not the way natural radio sources work. Natural radio sources diffuse static across all frequencies, rather than hitting at a single frequency. So it’s pretty clear that this was a radio signal and not a quasar or pulsar or some other natural radio source, of which there are millions. It was very narrow band, very concentrated, exactly like a radio station, or a broadcast, from another world would look.
The “Wow!” signal turned up very close to the frequency at which hydrogen glows. Why is that significant?
Gray: Well there’s a little history there. In the early sixties when people started thinking about the possibility of detecting extraterrestrial broadcasts with radio telescopes, one of the first frequencies suggested was the frequency that interstellar hydrogen glows at. At the time, it was one of the few interstellar emission lines that was known, and a lot of radio observatories had a receiver that could pick it up so it was especially convenient to look for broadcasts there. If you imagine that there are all of these radio astronomers around the universe looking at the stars with big antennas, which is what you need to pick up a signal from that far, chances are that they too would be listening at the frequency of hydrogen, because there is so much of it around. It’s the wave you can use to map the gas in galaxies, so it’s a natural “channel” for astronomers to look at. There weren’t a lot of frequencies that had that natural characteristic. So in the early decades of SETI, that’s the frequency that most people chose to listen at.
By the way, not everybody agrees with this strategy now. A lot of new emission lines have been found, and so the current best practice is to listen to millions of frequencies at a time so you don’t have to guess which one ET might favor. And that’s exactly what NASA’s SETI project tried to do, and that’s what the Allen Telescope Array at U.C. Berkeley is trying to do. But it just so happened that the Ohio State people were using the hydrogen strategy when they found this thing, and, it just so happens that the “Wow!” signal was fairly close to where Hydrogen was dwelling. So if you believe the magic frequency strategy, that extraterrestrials would necessarily broadcast in the Hydrogen frequency, then the “Wow!” signal sort of fits that.
Is it possible that the “Wow!” signal is somehow a computer glitch, or a signal from earth that was reflected off of space debris of some sort?
Gray: Of course it’s possible. It could have been any number of things. However, it almost certainly wasn’t a computer glitch, because it showed this rise and fall of intensity that’s just exactly what a radio source from the sky would look like. Also, the Ohio State radio telescope was cleverly rigged to filter out local stuff.
The only thing that conceivably could have made that special signature is a satellite of some sort at just the right distance, going just the right speed, in order to mimic a celestial object traversing the sky. So that’s a possibility, but it seems pretty unlikely for a number of reasons. First, it would have been seen by a lot of people. Ohio State would have seen it repeatedly, because satellites broadcast repeatedly. Secondly, if it was a secret satellite it would have been pretty stupid to broadcast at a frequency that radio astronomers across the world listen to.
For a long time, Jerry Ehman, who actually scribbled “Wow!” on the original computer printout, considered the possibility that it was a piece of space debris reflecting a signal from the earth back down into the antenna. But he no longer believes that to be the case. And I’m not saying that it definitely was an extraterrestrial broadcast; there’s no proof of that. The best way I can think to analogize this thing is to say that it was a tug on the cosmic fishing line. It doesn’t prove that you have a fish on the line, but it does suggest that you keep your line in the water at that spot.
Some have suggested that if the “Wow!” signal was alien in origin, then perhaps it sweeps around its home planet or star, the way light does from a lighthouse, which would explain why it hasn’t yet reappeared. Do you think that’s plausible?