SETI, the search for extraterrestrial intelligence, has seen astronomers scouring the sky for decades in hopes of receiving artificially generated radio signals sent by alien civilizations. But then, there's a good chance we've already found just such a signal. And 1977 saw the most tantalizing glimpse ever.
Nicknamed the "Wow!" signal, this was a brief burst of radio waves detected by astronomer Jerry Ehman who was working on a SETI project at the Big Ear radio telescope, Ohio. The signal was, in fact, so remarkable that Ehman circled it on the computer printout, writing "Wow!" in the margin - and unintentionally giving the received radio signal the name under which it would become famous.
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Despite a lot of effort, no identification has been found for the signal's source, and no repeat signal has ever been found. It's a complete mystery. The only conclusion that can be drawn is if the signal truly did originate in deep space, then it was either an astrophysical phenomenon of which we've never seen before, or it truly was an intercepted alien signal.
To explain scientific observations, the normal method is to construct hypotheses and then test them. If your hypothesis is incorrect, it will fail to explain the observation. You can then continue this way, using different hypotheses, until you find something which can accurately describe what you've observed (if you ever watch Mythbusters, you may be familiar with how this works).
But with the Wow! signal, researchers ran into difficulty. After trying and failing to find any repeat of the signal, Ehman was skeptical of its origin, stating that "something suggests it was an Earth-sourced signal that simply got reflected off a piece of space debris." But when he tried to investigate that explanation, he only found more problems.
Investigations found that it was scarcely possible the signal could have originated on Earth, and reflection off a piece of space junk was equally unlikely. The received signal was very specific, and these explanations required too many assumptions. A pattern of logical thinking known as Occam's razor was pointing towards this signal having an astrophysical origin. But this provided no explanation for what it might be.
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The Hydrogen Line The curious Wow! signal is more or less a perfect match for what would be expected from a received extraterrestrial transmission. It's been closely analyzed as a result, but to date no one has come up with a satisfactory explanation for where the signal came from.
For a start, the signal's intensity was observed to rise and fall over a period of 72 seconds, consistent with the rotation of Earth, and a single source tracking across the sky, through the Big Ear telescope's view. This gave the signal a characteristic signature, caused by objects seen in the sky. It would be nearly impossible for any Earth-bound object to match it.
It also stood out dramatically over the background noise found in deep space, being about 30 times louder than anything else around it. But by far the most interesting thing about this signal was its frequency.
This signal was very sharp, transmitting at only a single frequency. Natural radio sources don't work like that. They spread across a range of frequencies, meaning that the same signal covers a broad band of transmission. The Wow! signal is not like this at all, showing only one very specific frequency at approximately 1420 MHz.
1420 MHz, also known as the hydrogen line, is a frequency internationally banned from use by terrestrial radio signals because of its use in radio astronomy. Astronomically, it's usually emitted by neutral hydrogen atoms in interstellar space. It's observed roughly evenly in all directions, and has been used before to help map out the galaxy. But in the SETI program, it has another use.
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Hydrogen is the most simple and abundant element in the universe, and any intelligent civilization would know of this frequency's presence in space and probably be using it to make astronomical observations. As a result, SETI researchers consider it a logical frequency to check for any alien transmissions intended to be heard. It's just as logical that any astronomers elsewhere in the galaxy might think the same way.
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The final question in the mystery is where exactly the signal came from. Because of the way in which the Big Ear was designed, the signal's source can be narrowed down to one of two small regions in the sky. But that's as precise as it gets.
This puts the source of the signal somewhere in the constellation of Sagittarius. There are a handful of nearby stars, but it's impossible to tell precisely where the signal originated. At least, not unless we ever hear a repeat signal. And given that no repeat signal has yet been found in any of the searches, it's probably best not to hold your breath.
To date, most SETI searches have operated by sweeping the sky, observing any spot for only a few minutes at a time. While this allows a lot of coverage, it also means that the likelihood of eavesdropping on any signals that happen to be pointing in our direction is minimal. The other approach would be more like the way the Kepler mission worked, by staring at one particular patch of sky and waiting. Of course, while we now believe that exoplanets are common across the galaxy, we have much less idea when it comes to alien transmissions - for all we know, we may waste years looking in the wrong direction.
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Unfortunately, there's no way for us to know what exactly caused the Wow! signal. As much as some of us would love to use it as proof of extraterrestrial life, that would be a leap of faith, and unscientific at best. Astronomer Robert Gray describes it as "...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."
Logically, the conclusion that must be drawn is that the Wow! signal very likely originated in deep space, but if it did then it was either a completely unknown astronomical phenomenon, or an intercepted alien broadcast - but with nothing else to go on, there's no way to prove or disprove either of these ideas.
For now, the Wow! signal remains as nothing more than a vague but enthralling hint that there may be more things lurking out there in this galaxy of ours than we currently know of.
Image: Composite showing a scan of the original computer printout with Jerry Ehman's annotations, and a star chart showing the possible source of the Wow! signal. Credits: Ohio State University Radio Observatory/NAAPO/Benjamin Crowell/Wikimedia Commons