One of the most compelling potential signs of extraterrestrial communication might have an astrophysical explanation.
Called the “Wow!” signal, the bright burst of radio waves has defied our understanding since its discovery in the 1970s. Now, scientists using archived data from the Arecibo Observatory in Puerto Rico suggest a new possible source for the signal: a cosmic hydrogen cloud that emitted light like a laser.
“I think we have probably the best explanation so far,” says astrobiologist Abel Méndez of the University of Puerto Rico at Arecibo. Méndez, together with astrophysicist Kevin Ortiz Ceballos of the Harvard and Smithsonian Center for Astrophysics in Cambridge, Mass., and Jorge Zuluaga of the University of Antioquia, Colombia, submitted this idea to arXiv.org on August 16.
The original “Wow!” signal was detected decades ago by the Big Ear radio telescope at Ohio State University. As the telescope scanned the sky, a computer program converted incoming radio signals to a series of letters and numbers representing their intensities and printed it out overnight.
In the morning, astronomer Jerry Ehman and his colleagues would look over the printouts for anything interesting. When Ehman saw a signal from the night of August 15, 1977, he recognized it as something exceptionally bright.
Even more intriguingly, it was in a narrow wavelength range associated with neutral hydrogen atoms. Other astronomers interested in the search for extraterrestrial intelligence, or SETI, had suggested this wavelength could be a natural calling frequency for alien civilizations. Ehman circled the signal and wrote “Wow!” in the margin in red pen.
The signal has never been seen again. Astronomers have suggested several nonalien explanations for the original, including comets in our solar system and interference from Earth-orbiting satellites or space debris. But none of them fully hold up.
In search of similar signals, Méndez and colleagues sifted through some of the last data taken by the Arecibo radio telescope before it collapsed in 2020 (SN: 12/4/20). Between February and May 2020, Arecibo’s antenna tracked the sky similar to how the Big Ear had in the 1970s, letting the researchers compare the data directly.
Méndez wasn’t expecting to find much. “I knew about the ‘Wow!’ signal for a long time, like everybody. But I dismissed it, probably like many astronomers, as some fluke,” Méndez says. “Not an astronomical event. And definitely even less, aliens.”
But to his surprise, the Arecibo data showed several signals that looked a lot like “Wow!” — only dimmer. He realized that the signals corresponded to clouds of cold atomic hydrogen scattered around the galaxy.
“I said, ‘Wait, wait, wait!’ That was the moment,” Méndez says. “If it was brighter for a moment, that would be it. That would be the ‘Wow!’ signal.”
The next question was how to briefly brighten clouds of hydrogen. The details still need to be ironed out, but Méndez and colleagues have an idea: A bright radio source, from something like a magnetized dead star, a magnetar, could emit a flare and zap the cloud with energy. That energy could excite the hydrogen atoms in a particular way and trigger a laserlike effect, where all the atoms emit light in the same wavelength at the same time (SN: 4/23/10).
That would be an unusual phenomenon, Méndez admits. Such hydrogen masers have been built in labs on Earth, but few have been observed in space, and none at this frequency. The perfect alignment of a magnetar, a cold hydrogen cloud and the Big Ear would have been lucky, too — although that could help explain why the signal was seen only once.
If this explanation turns out to be correct, it could pose a problem for SETI searches (SN: 9/30/18). If astronomers ever detect another strong signal at this frequency, it would be unclear whether it was from aliens or glowing hydrogen clouds.
“The SETI project has been looking precisely for this kind of event,” Méndez says. “If we have a natural process that can produce that, that could be a false positive.”
Modern SETI techniques probably wouldn’t be fooled by a hydrogen maser, says astronomer Jason Wright of Penn State, who was not involved in the new work. But he is reserving judgement on the idea until the details of the maser effect are fleshed out more, which Méndez and colleagues plan to do in a follow-up paper.
“He’s suggesting a phenomenon that has never been observed,” says SETI astronomer Jason Wright of Penn State, who was not involved in the new work. “The set of physical conditions is extremely delicate and specific, and it’s not clear if that’s even possible.”
But even if the “Wow!” signal was naturally occurring, “that would be cool,” Wright says. “The false positives of SETI can lead to amazing science.” For example, when astronomers first spotted pulsars, they called the spinning stellar corpses “LGM” for “Little Green Men” (SN: 3/8/18). The seminal paper on their discovery dedicated a whole section to ruling out ET.
“It wasn’t aliens,” Wright says, “but it was still a Nobel prize.”