On a balmy August evening in 1977, an enormous radio telescope in a field in the middle of Ohio sat silently listening to the radio universe. Shortly after 10:00 PM, the Earth’s rotation slewed the telescope through a powerful radio signal whose passage was noted only by the slight change in tone in the song sung every twelve seconds by the line printer recording that evening’s data.
When the data was analyzed later, an astronomer’s marginal exclamation of the extraordinarily powerful by vanishingly brief blip would give the signal its forever name: the Wow! Signal. How we came to hear this signal, what it could possibly mean, and where it might have come from are all interesting details of an event that left a mystery in its wake, one that citizen scientists are now looking into with a fresh perspective. If it was sent from a region of space with habitable planets, it’s at least worth a listen.
The Big Ear
Understanding the Wow! Signal requires a look at the instrument that produced it. Affectionately known as “The Big Ear”, the Ohio State University Radio Observatory was the vision of John D. Kraus, a physicist at Ohio State. Dr. Kraus was no stranger to big science — during WWII he developed methods for degaussing naval ships to protect them from magnetically detonated mines, and he worked on a massive cyclotron for the University of Michigan.
Dr. Kraus first described his idea for a telescope capable of detecting extraterrestrial radio signals in an article for Scientific American in 1955. The design of the telescope would be extremely simple, especially compared to the more typical fully steerable dish antenna. It consisted of a large, flat reflector section of steel mesh standing across an open space from a wide, stationary paraboloid reflector. Between these two elements lay a large, flat ground plane area of aluminum-covered pavement. At the focal point of the paraboloid reflector was a small shack containing the feed horns, which could move across the width of the telescope on railroad tracks. Although in general the telescope was static and pointed wherever the Earth’s rotation took it, the feedhorn tracking coupled with adjustments to the tilt of the flat reflector gave some control to which part of the sky was being surveyed.
The Big Ear was big: the flat reflector alone was 33 meters tall and 100 meters wide, and the ground plane stretched 150 meters between the two reflectors. But Dr. Kraus had actually designed a much, much bigger antenna.
His original design called for 600-meter-wide reflectors, but when the National Science Foundation grant came through in 1955 at a paltry $48,000, the design was reduced to what was possible. And even then, a great deal of “sweat equity” went into the construction of the Big Ear, with graduate students learning to weld specifically to build the telescope, and with critical equipment such as the parametric amplifiers needed for the receiver being built at cost by an OSU alumnus.
It took the better part of a decade to build the Big Ear, and once it was switched on, its main goal was the completion of the Ohio Sky Survey for extragalactic radio sources. The observatory was also used to study the Andromeda galaxy. But by the early 1970s, an interest in searching for potentially intelligent extraterrestrial civilizations by listening for specific radio signatures began to take hold of the radio astronomy community. The Ohio observatory, by virtue of its unique construction and its ability to do sky surveys, was identified early on by SETI scientists as the perfect tool for the job. And so, in 1973, the Big Ear began its near-constant search for ET.
Despite capturing the popular imagination through the star-power of backers like Carl Sagan, SETI was not particularly well-funded. Most SETI programs were filler projects, designed to take advantage of downtime on radio telescopes between observations for more traditional studies. Running on shoestring budgets, most of the early SETI programs devoted the bulk of their funding to telescope time, and little to data analysis. As a result, a lot of manual effort went into searching through the data for interesting signals.
And so it was that in August of 1977, astronomer Jerry Ehman was poring through data from Big Ear’s survey of the sky, recorded on page after page of fanfold printer paper. On the left of each sheet were 50 vertical columns of characters, one column for each channel monitored by the Big Ear’s receivers. The program for recording the data, written by Ehman and his colleague Bob Dixon, basically recorded the signal-to-noise ratio on each channel over a 10-second observation period as a single character. An SNR of zero was recorded as a blank space, followed by numerals for SNRs from one to nine. From 10 onward, increasing SNRs were represented by a single letter in alphabetical order.
The paper before Ehman was a sea of blanks and ones, with the occasional spike to six or seven scattered about. But as he scanned the data captured around 10:15 PM Eastern time on August 15, 1977, he noticed a sequence of numbers at the extreme left-hand side of the data that astounded him. On channel 2, the sequence “6EQUJ5” appeared, an outburst lasting 72 seconds before fading back to the noise floor (each observation was ten seconds long, plus an additional two seconds for computer processing.) The signal had been so strong at its peak that it went through all the numbers and 80% of the alphabet — between 30- and 31-fold higher than the noise floor. Ehman was so floored when he saw the signal that he grabbed his red pen, circled the vertical column of characters, and wrote “Wow!” in the left margin. His ebullience at the result stuck, and the signal has been known as “The Wow! Signal” ever since.
The Where of the Wow!
The Wow! Signal has been subject to a lot of scrutiny since 1977, and rightly so. Initial thoughts were that it was of terrestrial origin, perhaps a stray signal from an unknown military satellite, or a signal that had bounced off the Moon. But no other radio telescope operating that night had heard the same thing, which would be the case for a locally generated signal, and the Moon wasn’t in the right place at the time to act as a reflector. Other hypotheses such as a star going supernova, a computer glitch, radio-emitting comets, or “interstellar scintillation” have been blamed, but nothing has stuck. Then again, despite nearly 50 years of looking, nothing even remotely close to the Wow! Signal has been recorded. So it’s hard to draw any firm conclusions about the observation.
It should go without saying that the possibility of the Wow! Signal being from an extraterrestrial intelligence is among the lowest probability explanations for the event. Extraordinary claims require extraordinary evidence, and every mundane possibility needs to be eliminated before one starts blaming ET for the signal. But there are a couple of tantalizing facts about the Wow! Signal.
First, its frequency. The Wow! Signal occurred at 1,420 MHz — the famous “hydrogen line” frequency that occurs during changes in the energy state of neutral hydrogen atoms. These 21-cm wavelength microwave signals would be known to any civilization that had mastered enough physics to have radio communications. Whether this frequency could be considered a universal “hailing frequency” between technically literate civilizations is open for debate, but it is interesting that the signal was heard on a frequency long-speculated to be just that.
Second, the profile of the signal is exactly what would be expected of an extraterrestrial signal — in the sense of having an origin outside of the local Earth environment, as opposed to from a non-human intelligence. A telescope located at the latitude of Ohio using the rotation of the Earth to scan the sky would be expected to have a 72-second window on any point in the sky, so a signal coming from a long way off would increase for 36 seconds before decreasing and tailing off again. The “6EQUJ5” signal strength forms a curve that’s exactly the right shape.
The other intriguing aspect of the Wow! Signal is its apparent origin. The Big Ear had two feed horns at its focus, long axes parallel to each other but each pointed in a slightly different direction. It’s impossible to say exactly which feed horn heard the signal because of the way the data was processed, but it is known that only one of the feed horns heard it. That means that, barring some Earthly origin or natural phenomenon, there are two relatively small areas, both within the constellation Sagittarius, where the signal could have come from. And that gives astronomers somewhere to start looking.
One astronomer on the hunt for a potential source for the Wow! Signal is Alberto Caballero. Alberto has been spearheading the Habitable Exoplanet Hunting Project for a while now, which aims to entice amateurs to use their telescopes to detect the subtle changes in apparent brightness of a star when one of its planets transits across its face. It seems like the kind of measurement that would take millions of dollars of equipment to accomplish, but as we discussed with Alberto in a Hack Chat last January, the gear needed is surprisingly accessible and affordable.
As an offshoot to that effort, Alberto has taken an in-depth look at the stars located within the two slivers of sky that the Wow! Signal might have come from. In a preprint of the paper documenting the work, Alberto studied 66 G- and K-type stars in the target area, among many thousands of others. (Our own Sun is a G-type star, while K-type stars are smaller and cooler; both are attractive SETI targets.)
Out of that group, fifteen stars appear to be both Sun-like. And only one of those stars, 2MASS 19281982-2640123, has the luminosity data needed to start a serious exoplanet hunt.
Of course, all this is just speculative. The Wow! Signal could just as easily have been an artifact, a natural phenomenon, or even have come from outside the galaxy. But if one of the stars within the area the Big Ear was listening to on August 15, 1977 proves to have a potentially habitable planet orbiting it, it’ll be yet one more layer on the mystery of the Wow! Signal.
[Featured images: North American Astrophysical Observatory]