For decades, Robert Gray has been trying to duplicate the most surprising and still-unexplained observation in the history of the search for extraterrestrial life.
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.
The Very Large Array in New Mexico
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?
That's my favorite theory. And it's just an idea of course. But when
you step back from all of this a little bit, you notice that almost all
searches for extraterrestrial intelligence have been surveys that look
at all of these different spots in the sky for just a few minutes at a
time. And the assumption such searches operate on is that there is a
beacon, or a broadcast of some sort, that is on all the time, and so all
you have to do is survey the sky and if it's there you'll find it. It's
the easiest method, and it's the right thing to do when you're first
But if you look at this in a
deeper way, and you calculate the kind of energy it would take to
operate a beacon that is on all the time, broadcasting in all
directions, strong enough so you could pick it up from many, many light
years away, the amount of power is enormous. It's in the range of
thousands and thousands of big power plants. We humans certainly
couldn't do something like that now. So to have a signal that's always
there, you have to assume a very advanced intelligence, and you have to
assume that it's highly motivated to talk to us, and neither of those
things may be true of a broadcaster. They might not be so rich, or
profligate with their energy, or, for that matter, very interested in
talking. They might use some other cheaper strategy---brief periodic
broadcasting, a sweeping lighthouse beam, or other methods.
you may know, there's another thrust in SETI, which has become the
focus of a lot of people's interest over the past ten years and that's
optical SETI, where you look at starlight and see if you find any
sudden, brief, flashes of light that are much stronger than what the
star normally puts out. The idea is that you might find
extraterrestrials communicating by shining a giant laser at us, and it's
an idea that's become quite popular. But as with most SETI projects,
they're simply scanning the sky, looking at each spot for roughly a
minute. And at the end of a couple of years they can tell you they've
looked at every spot in the sky and they didn't see any flashes, but of
course there you have the same problem as you do with radio surveys. You
look in every direction, but you only do it for a couple of minutes,
and so if anyone were broadcasting with the lighthouse method, you'd be
unlikely to find them.
Did the "Wow!" signal come from a particular star or group of stars?
Gray: That's a good question, and the short answer is that there's no way to tell.
Even though the Ohio State radio telescope is really big, it looks at a rather large spot in the sky---a spot shaped like an ellipse that's taller than the moon and about a quarter as wide. In a spot of that size, you have literally millions of stars. I've looked at the photographs for that area of the sky, and there are tons of stars there---no particularly intriguing star that stands out as being a likely source of the signal. Now, several years later I looked for the signal with the Very Large Array in New Mexico. Unlike some of the older telescopes it can give you a pretty good radio image of the sky, because its various telescopes make up one giant antenna that's twenty miles across. And it gives you pretty good resolution, so if you'd seen the "Wow!" with the VLA you really could tell which star a radio signal would have come from.
The Mount Pleasant Radio Observatory in Tasmania
What was it like working with the Very Large Array in New Mexico? Did you get a thrill out of that?
Gray: I did. The Very Large Array was, until the end of the twentieth century, the largest radio telescope ever built. It's the same array of antennas featured in the film Contact. It's an unbelievable machine. It can take pictures of the radio sky with the same resolution as an optical telescope, allowing you to see literally millions of objects across the sky. Most of them are distant galaxies with wild things going on at their core, most likely having to do with black holes.
Getting to use the Very Large Array to look for the 'Wow!" was very unexpected. As far as I can tell, no amateur astronomer had ever done it. Nobody had ever used the full array to look for an extraterrestrial signal at all. It's funny when you show up, they give you a rundown of all the technical stuff, but they also give you a brochure on how to survive rattlesnake bites, because if you go wandering into the desert out there you might get bitten.
But it's a credit to Big Science that they let me use the Very Large Array to look for the "Wow!" signal. I wouldn't have expected it, and it suggests that Big Science, as an enterprise, isn't quite as ivory tower or exclusive as you might think.
You're coming at this as from the field of data analysis, rather than as a professional astronomer, do you think you brought a
special skill set to this problem? Were there any insights you had that might
not have been as intuitive to an astronomer?
astronomers generally look at things like stars, things that aren't quite
eternal, but that last for a really long time. As a result some astronomers may
bring a certain expectation to a radio signal, an expectation that it's going
to be there all the time. The people who do SETI, who are often but not always
astronomers, have a mindset that it's sensible to look for the really strong
signal that is going to be there all of the time.
Because my education is not in astronomy or engineering, it may be that I bring a kind of practicality to this, especially as it concerns the
practicality and economics of what it takes to broadcast a signal like that. Broadcasters, just like those of us who are listening, might not be able to command enormous
resources, they might not be in charge of whatever political systems are
responsible for distributing resources to science in their little corner of the universe. And so as a result they might be forced to use signals that are not present all of the time and therefore those signals may be difficult to find.
The other thing is: Over the years I've talked to a lot of
astronomers and a lot of people involved with SETI, and whenever the topic of the
"Wow!" comes up, they seem to believe that everybody has looked for
it, that it's been checked out. But I've never been able to find anyone else who looked for it. In fact, nobody other than Ohio
State seemed all that interested in trying to confirm it at all. Now
fortunately that created a situation where I was able to convince several
scientists to help me look for it, using various kinds of radio telescopes,
including the Very Large Array, the Mount Pleasant Radio Observatory in Tasmania, and the small one that I built myself. So it's
possible that what I bring to this is simply the willingness to go out and
In a hundred years from now it's likely that we won't be
limited to these giant dish things that stare at the sky and only see one
little spot. It's possible that there will be some sort of technology that can
look at the whole sky at the same time, with the same sensitivity as you get
with a big dish, and perhaps, when we look, at some interval we'll see a flash, a
signal, and maybe that's the way we'll find broadcasters, if any are out
there. But in the meantime, you know, you have to keep a line in
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