Was that a dropped call from ET?

Nobody believes it was ET phoning, but radio astronomers admit they do not have an explanation yet for a beam of radio waves that apparently came from the direction of the star Proxima Centauri.

“It’s some sort of technological signal. The question is whether it’s Earth technology or technology from somewhere out yonder,” said Sofia Sheikh, a graduate student at Pennsylvania State University leading a team studying the signal and trying to decipher its origin. She is part of Breakthrough Listen, a $100 million effort funded by Yuri Milner, a Russian billionaire investor, to find alien radio waves. The project has now stumbled on its most intriguing pay dirt yet.

Proxima Centauri is an inviting prospect for “out yonder.”

It is the closest known star to the sun, only 4.24 light-years from Earth, part of a triple-star system known as Alpha Centauri. Proxima has at least two planets, one of which is a rocky world only slightly more massive than Earth that occupies the star’s so-called habitable zone, where temperatures should be conducive to water, the stuff of life, on its surface.

The radio signal itself, detected in spring 2019 and reported on earlier in The Guardian, is in many ways the stuff of dreams for alien hunters. It was a narrow-band signal with a frequency of 982.02 megahertz as recorded at the Parkes Observatory in Australia. Nature, whether an exploding star or a geomagnetic storm, tends to broadcast on a wide range of frequencies.

“The signal appears to only show up in our data when we’re looking in the direction of Proxima Centauri, which is exciting,” Sheikh said. “That’s a threshold that’s never been passed by any signal that we’ve seen previously, but there are a lot of caveats.”

Practitioners of the hopeful field of the search for extraterrestrial intelligence, also known as SETI, say they have seen it all before.

“We’ve seen these types of signal before, and it’s always turned out to be RFI — radio frequency interference,” Dan Werthimer, chief technologist at the Berkeley SETI Research Center, who is not part of the Proxima Centauri study, wrote in an email.

That thought was echoed by his Berkeley colleague Andrew Siemion, who is the principal investigator for Breakthrough Listen. “Our experiment exists in a sea of interfering signals,” he said.

“My instinct in the end is that it will be anthropogenic in origin,” he added. “But so far we can’t yet fully explain it.”

So there’s nothing to see here, folks. Until there is. Notwithstanding claims of biosignature gases on Venus and tales of UFO sightings collected by the Pentagon, the discovery of life — let alone intelligence — out there would be a psychological thunderclap of cosmic and historic proportions.

False alarms have been part of SETI since the very beginning, when Frank Drake, then at Cornell and now retired from the University of California, Santa Cruz, pointed a radio telescope in Green Bank, West Virginia, in 1960 at a pair of stars, hoping to hear aliens’ radio waves. He detected what seemed to be a signal. Could it be this easy to discover we are not alone?

It turned out to be a secret military experiment.

Sixty years later we are still officially alone, and SETI as an enterprise has been through the wars economically and politically, even as technology has enhanced humanity’s ability to comb the nearly infinite haystack of planets, stars and “magical frequencies” on which They might be broadcasting.

Breakthrough Listen was announced with much fanfare by Milner and Stephen Hawking in 2015, sparking what Siemion called a renaissance.

“This is the best time to be doing SETI,” he said.

The recent excitement began April 29, 2019, when Breakthrough Listen scientists turned the Parkes radio telescope on Proxima Centauri to monitor the star for violent flares. It is a small star known as a red dwarf. These stars are prone to such outbursts, which could strip the atmosphere from a planet and render it unliveable.

In all they recorded 26 hours of data. The Parkes radio telescope, however, was equipped with a new receiver capable of resolving narrow-band signals of the type SETI researchers seek. So in fall 2020, the team decided to search the data for such signals, a job that fell to Shane Smith, an undergraduate at Hillsdale College in Michigan and an intern with Breakthrough.

The signal that surprised the team appeared five times April 29 during a series of 30-minute windows in which the telescope was pointed in the direction of Proxima Centauri. It has not appeared since. It was a pure unmodulated tone, meaning it appeared to carry no message except the fact of its own existence.

The signal also showed a tendency to drift slightly in frequency during the 30-minute intervals, a sign that whatever the signal came from is not on the surface of Earth but often correlates with a rotating or orbiting object.

But the drift does not match the motions of any known planets in Proxima Centauri. And, in fact, the signal — if it is real — might be coming from someplace beyond the Alpha Centauri system. Who knows?

The subsequent nonappearance of the signal has prompted comparisons to a famous detection known as the “Wow! Signal” that appeared on a printout from the Big Ear radio telescope, operated by Ohio State University in 1977. Jerry Ehman, a now retired astronomer, wrote “Wow!” on the side of the printout when he saw it after that fact. The signal never appeared again, nor was it satisfactorily explained, and some people still wonder if it was a missed call from Out There.

Of the Proxima signal, Siemion said, “There have been some exclamations, but ‘wow’ hasn’t been one of them.”

Asked what they were, he laughed.

“Initially there were perplexed reactions from folks, but it settled down quickly,” he said.

Over a period of 24 to 48 hours at the end of this October, he said, the mood shifted from inquisitive and curious to “very serious scientific detective work.”

Sheikh, who expects to get her doctorate this coming summer, is leading the detective work. She got her bachelor’s degree at the University of California, Berkeley, intending to go into particle physics, but found herself drifting into astronomy instead. She first heard about the Breakthrough Listen project and SETI on Reddit while she was looking for a new undergraduate research project.

“I would say we were extremely sceptical at first, and I remain sceptical,” she said about the putative signal. But she added that it was “the most interesting signal to come through the Breakthrough Listen program.”

The team hopes to publish its results early in 2021.

The Parkes telescope — which once relayed communications to the Apollo astronauts — is notorious for false alarms, Werthimer said. In one recent example, he said, astronomers thought they had discovered a new astrophysical phenomenon.

“It was very exciting until somebody noticed the signals only appeared at the lunch hour,” he said. They were coming from a microwave oven.

Over the years SETI astronomers have prided themselves on their ability to chase down the source of suspicious signals and eliminate them before word leaked out to the public.

This time their work was reported by The Guardian. “The public wants to know; we get that,” Siemion said. But, as he and Sheikh emphasise, they are not nearly done yet.

“Frankly, there’s still a lot of analysis that we have to do to be confident that this thing is not interference,” Sheikh said.

Part of the problem, she explained, is that the original observations were not done according to the standard SETI protocol. Normally, a radio telescope would point at a star or other target for five minutes and then “nod” slightly away from it for five minutes to see if the signal persisted.

In the Proxima observations, however, the telescope pointed for 30 minutes and then moved far across the sky (30 degrees or so) for five minutes to a quasar the astronomers were using to calibrate the brightness of the star’s flares. Such a large swing might have taken the telescope away from whatever the source of the radio interference was.

If all else fails, Sheikh said, they will try to reproduce the results by replicating the exact movements of the Parkes telescope again on April 29, 2021.

“Because,” she said, “if it’s actually coming from Proxima, then maybe they would like send a hello once a year or something like that. But it’s more likely that there’s some sort of yearly event that happens at the visitor centre, or something like that, that causes an environmental effect that doesn’t happen the rest of the year.”

The Proxima signal could be destined to pass into legend like the Ohio State Wow! Signal, but in SETI, there is always another day, another star.

It’s been fun, Sheikh said, even if the Proxima signal ends up being interference. “This is extremely exciting, no matter what comes out of it.”

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