One ping only

Wow! Astronomers have an intriguing hypothesis for a decades-old extraterrestrial mystery

The Wow! signal is still an enigma, 45 years after its detection, but at least one possibility so far can’t be ruled out.

Milky way galaxy has stars and space dust in the universe.
Pakin Songmor/Moment/Getty Images

Forty-five years ago, radio astronomers at Ohio State University detected a strong, clear radio signal from somewhere in the direction of the Sagittarius constellation.

It had all the features SETI (Search for ExtraTerrestrial Intelligence) researchers expected to see in an actual alien radio signal. The Big Ear radio telescope’s first antenna listened to the signal for 72 seconds before moving on in its scheduled sweep of the sky. Lead astronomer Jerry Ehman hastily wrote “Wow!” next to the signal on a printout of Big Ear’s data, and it’s been known as the Wow! signal ever since.

Three minutes later, when Big Ear’s second antenna swept toward Sagittarius, it found only silence. No one ever heard the signal again, despite hundreds of hours of trying. And for the last 45 years, scientists have been searching for an explanation.

In a recent paper, Columbia University astronomer David Kipping and the late astronomer Robert Gray explore an often-overlooked possibility — but as Kipping tells Inverse, there really are no good explanations. The Wow! signal remains an enigma.

What’s New — The real mystery is this: If the Wow! signal was a beacon from a distant world or something else, why hasn’t anyone heard it since? So far, it’s eluded nearly 200 hours of radio astronomers’ observations, often with more advanced, more sensitive telescopes than the now-retired Big Ear.

And to pull that off, according to a 2020 study, there would have to be at least 40 hours of silence between repetitions of the signal (if, of course, the signal actually repeats).

If the signal repeats came any closer together than that, then statistically, somebody should have heard it again. But with such a long pause between signals, it’s astounding that Big Ear was lucky enough to catch one in progress in the first place. Statistically speaking, Ehman and his colleagues should have missed it, too.

And if the signal was really a one-time event, the interstellar equivalent of somebody accidentally setting off a car alarm and shutting it down after the first bleep?

“The probability of the Wow! patch of sky harboring just one civilization that sent out just one signal, over all of cosmic time, *and* that the Big Ear just happened to be listening to at the right time and at the right spot is extremely small. I think we can fully discard that as being absurdly contrived,” Kipping says.

“I’m not saying alien civilizations don’t emit one-off signals — they might well do so — but if they truly only ever sent one signal towards us in both all history and all future time, then the chances of Big Ear seeing it are extremely small, far smaller than the probability of repetition,” he adds.

A scan of a color copy of the original computer printout.

Big Ear Radio Observatory and North American AstroPhysical Observator

Kipping and Gray, in the paper, say there’s another possibility that hasn’t been considered, and it’s called a stochastic repeater.

Digging Into The Details — Stochastic means, essentially, random — if a process is stochastic, you can model general patterns, but you can’t make specific predictions about when something will happen. For example, in the early Solar System, the way grains of ice and dust stick together and eventually clump into planets is a stochastic process. Models can describe how and why the grains stick together, but they can’t predict which grains will bump into each other, or when.

And maybe somebody on a planet in the direction of Sagittarius decided to build a radio transmitter that would fire off a signal every X hours, with a random number generator to give the transmitter a new value of X every time.

That seems like an odd thing to do, but it makes sense if you’re an alien radio astronomer hoping to get noticed by intelligent aliens. Sending a signal at random intervals means you avoid the risk of always catching distant listeners during the hours when they’re not actually listening, or hitting a spot on the planet that doesn’t have any radio telescopes operating. The randomness of the signal could also be thanks to scintillation in the interstellar medium it passes through, especially if it’s traveling more than 100 parsecs (~326 light years) to reach Earth.

The downside, as with the Wow! signal, is that you also might just confuse your listeners if they can’t manage to catch a repetition.

“Stochastic beacons are more frustrating because you don’t know when the next pulse will come, even if you’ve detected a bunch already. So if you want to sent useful information in a predictable manner, this isn’t the way,” says Kipping. “But if you just want to maximize the chance of someone seeing your beacon at some point, it is advantageous.”

The Wow signal came from somewhere in the direction of the constellation Sagittarius; can you spot the constellation in this photo?

By Alpsdake - Own work, CC BY-SA 4.0,

In a recent paper, Kipping and Gray set up a statistical version of a stochastic repeater, which produced a set of signals at random intervals over 90 days. Then he overlaid that with the dates and times of Big Ear’s observations of the patch of sky the Wow! signal had come from. Every time the signal happened at a moment when Big Ear was listening, Kipping and Gray counted a detection. They repeated the process 1,000 times.

The goal was to answer one question: if the Wow! signal repeated at random intervals, what were the chances that Big Ear would detect it once, and only once? The answer turned out to be about 32.3 percent, which sounds encouraging — a nearly one in three chance! But then Kipping and Gray factored in all the many hours of listening with other telescopes in other places in the decades since “Wow!” and the chances dropped to 1.78 percent.

In other words, there’s less than a one in fifty chance that the Wow! signal could be a randomly repeated signal that astronomers have somehow only heard once in 45 years. From a statistical standpoint, the random repeater idea still makes more sense than the other explanations — but it’s not great. There’s just not enough data yet to say that it’s not the answer.

“Every hypothesis looks improbable right now,” says Kipping. “And that’s kind of the problem with black swan events like this: they aren’t the product of typical behaviors, and thus we’re forced to compare a set of very short straws.”

What’s Next — To rule out a randomly repeating signal as an explanation for “Wow!” (at least to the satisfaction of statisticians, who have very specific requirements for deciding whether an explanation fits the data), Kipping and Gray calculate that astronomers need to spend another 62 days listening for a repetition of the Wow! signal. And while crossing something off the list won’t solve the mystery, it’s at least a step closer.

Meanwhile, Kipping and Gray say the statistical work laid out in their recent paper could help astronomers interpret signals on other SETI projects, like Breakthrough Listen, by predicting how often they should repeat, among other properties.

Today almost no one disputes that Big Ear detected something. And given the signal’s frequency and other characteristics, it was very unlikely to be a naturally-occurring something. Kipping shared his own best guess with Inverse.

“My personal hunch is that it was an unrecorded satellite on a high Earth orbit that, for some bizarre reason, was using the hydrogen line for comms or testing. That satellite no longer is functional or performing such tests, hence why we can’t see it repeat,” says Kipping. “It’s not a great explanation, but as I said, none of the options appear to work very well.”

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