History ‘Wow!’ the signal may finally have a source. Sorry, they are not strangers

One of the most compelling possible signs of extraterrestrial communication may have an astrophysical explanation.

Called “Wow!” signal, the bright burst of radio waves has challenged 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 cloud cosmic hydrogen that emitted light like a laser.

“I think we have probably the best explanation so far,” says astrobiologist Abel Mendez of the University of Puerto Rico in Arecibo. Méndez, along 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 the idea to arXiv.org on August 16.

The original “Wow!” the 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 the incoming radio signals into a series of letters and numbers representing their intensity and printed them overnight.

In the morning, astronomer Jerry Ehman and his colleagues would look at the prints for anything interesting. When Ehman saw a signal from the night of August 15, 1977, he recognized it as something extremely bright.

Even more intriguing, 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 that this wavelength might be a natural calling frequency for alien civilizations. Ehman circled the signal and wrote “Wow!” in red pen border.

The signal was never seen again. Astronomers have suggested several non-alien explanations for the original, including comets in our solar system and intrusions from Earth-orbiting satellites or space debris. But none of them hold up completely.

In search of similar signals, Méndez and colleagues examined some of the last data received by the Arecibo radio telescope before it was decommissioned in 2020. (SN: 12/4/20). Between February and May 2020, Arecibo’s antenna tracked the sky much like the Big Ear did in the 1970s, allowing researchers to directly compare the data.

Mendez didn’t expect to find much. “I knew about the ‘Wow!’ signal for a long time, like everyone else. But I dismissed it, as many astronomers have,” says Mendez. “And definitely less, aliens.”

But to his surprise, the Arecibo data showed some signals that sounded a lot like “Wow!” – only fainter. He realized that the signals corresponded to clouds of cold atomic hydrogen scattered around the galaxy.

“I said, ‘Wait, wait, wait!’ This was the moment,” says Mendez. “If there was a moment brighter, this would be it. That would be ‘Wow!’ signal.”

The next question was how to briefly illuminate the hydrogen clouds. The details still need to be ironed out, but Méndez and colleagues have an idea: A bright radio source, from something like a dead magnetized star, a magnetar, could set off a flare and grab the cloud with energy. This energy can excite the hydrogen atoms in a special way and cause a laser effect, where all the atoms emit light at the same wavelength at the same time. (SN: 4/23/10).

This would be an unusual phenomenon, Méndez admits. Such hydrogen masers have been built in laboratories 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 also have been lucky – although this could help explain why the signal was only seen once.

If this explanation turns out to be correct, it could pose a problem for SETI research (SN: 30.9.18). 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 for exactly this type of event,” says Méndez. “If we have a natural process that can produce it, it 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 judgment on the idea until the details of the mass effect are more pronounced, which Mendez and colleagues plan to do in a follow-up paper.

“It’s suggesting a phenomenon that’s 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 subtle and specific, and it is not clear whether this is possible.”

But even if “Wow!” signal was occurring naturally, “it would be nice,” says Wright. “SETI false positives can lead to amazing science.” For example, when astronomers first discovered pulsars, they called the rotating stellar corpses “LGM” for “Little Green Men.” (SN: 3/8/18). The seminal paper on their discovery devoted an entire section to the exclusion of ET.

“It wasn’t aliens,” Wright says, “but it was still a Nobel Prize.”