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Sunday, April 21, 2024

A Strange Radio Signal Was Just From Earth, Not Aliens

Last fall, a colleague of Sofia Sheikh’s posted a message in her group’s Slack channel, where members of the Breakthrough Listen Search for Extraterrestrial Intelligence (SETI) collaboration talk about the radio telescope signals they’re analyzing for possible signs of communications from space. Most of the ones they’d analyzed so far turned out to be clearly caused by radio interference on Earth, artifacts of the myriad human technologies and devices that emit signals in the frequency ranges the scientists were studying. But one seemed more promising.

The message was posted by a student studying radio telescope data that was originally taken to monitor stellar flares emitted by the star Proxima Centauri. He had picked up a single unusual signal, and Sheikh didn’t know what to make of it. “It had a lot of features that we would associate with a signal coming from space,” she says. The signal detected near 982 MHz, dubbed “blc1” for “Breakthrough Listen Candidate 1,” intrigued them from the start, since it came from a telescope trained on the nearest stellar system to our own, one that may host a habitable world. And it looked narrow on the electromagnetic spectrum, suggesting that it was generated by technology. But whose technology?

Collaborating with other astronomers, Sheikh and her team began a series of tests on the signal—radio waves measured at a range of frequencies that stand out above more ubiquitous noise, like the faint sound of a distant radio station, distinguishable from the static. They wanted to determine whether the signal was moving the way something in the sky would, and they compared it to radio interference they’ve encountered at other frequencies. And in a pair of new studies published this week in the journal Nature Astronomy, they published their bad news: It was a false alarm. The tantalizing signal did not come from space after all, but originated from Earthling technology, like the others. 

“This was the most promising signal that we’ve ever found with the Breakthrough Listen project,” says Sheikh, an astronomer at UC Berkeley and lead author of one of the papers. But, she says, their yearlong quest to study the mysterious signal and understand its origin “was the most exciting investigation in my career so far,” and has helped the scientists develop their tools as they prepare to analyze future signals.

Breakthrough Listen, a research program that began in 2015, makes use of data from radio telescopes in Australia, West Virginia, and California to listen for potential alien signals from nearby stars as part of the ongoing search for extraterrestrial civilizations. Because it can be competitive getting time on a radio telescope, that sometimes includes “piggybacking” off others’ observations, so that they and other astronomers benefit from the same data.

Proxima Centauri seems like a good candidate for the search for life outside our solar system. The star is “only” a little more than four light-years, or about 25 quadrillion miles, away from Earth. That’s nearby, from a cosmic perspective, and it’s within transmission distance for a message from intelligent life. In 2016, astronomers confirmed the existence of a planet orbiting the star, fueling hopes that it might be hospitable to alien life. If and when anyone sends a space mission to another star, that will probably be its destination. In fact, Breakthrough Starshot aims to develop a system to fire a powerful laser beam to propel a tiny spacecraft at high speed to one of the star’s neighbors, Alpha Centauri, to take images and send them back home. (Both Breakthrough Listen and Starshot are funded by billionaire philanthropist Yuri Milner’s Breakthrough Initiatives.)

One of the planets orbiting Proxima Centauri is about the size of Earth and orbits within the “habitable zone” of the star—not too close and not too far away—meaning it might have liquid water, one of the requirements for life as we know it. Nevertheless, the worlds around Proxima Centauri might not be the most friendly to life; the star is a red dwarf, and those frequently throw out stellar flares and harmful radiation that could burn off a planet’s atmosphere and fry any alien microbes on the surface.

Sheikh and her colleagues’ work began as they pointed the Parkes Murriyang radio telescope in the New South Wales region of Australia at Proxima Centauri. (Murriyang is an Indigenous name given last year, representing the Wiradjuri Skyworld where a creator spirit lives.) They first observed whether Proxima Centauri produces flares like similar stars do. (It does.) But that doesn’t rule out life: Hardy aliens might have evolved to withstand space radiation. Or they could live below ground. Or the planet might have a thick atmosphere and magnetic field for protection. Or perhaps the planet might not be aliens’ home world at all, and instead might serve as an outpost emitting beacons to anyone who’s out there to listen. They might even be listening to us.

Then the team tested whether the signal would vanish as they pointed their telescope towards or away from the target. It did. That boded well. If the signal showed up everywhere, then it could have come from a cell tower or a wifi router near the telescope. “Even things like cars’ starter plugs can cause radio frequency interference,” Sheikh says.

But then they examined something called the drift rate of the signal, tracking to what extent it moves. It seemed to drift slowly, inconsistent with a source in the sky. That was a bad sign. And the nail in the coffin was when they detected about 60 lookalike signals, mirroring blc1 at other frequencies, and those lookalikes were easily confirmed as being caused by interference. The team couldn’t determine which of the lookalikes was the original frequency of the signal, and they couldn’t pinpoint its precise source—but now they’re sure it didn’t come from space.

Sheikh and her colleagues speculate that the many signals could have been generated by clock oscillators used in digital electronics. If the electronics were malfunctioning in some way—say, if one was sitting in a sunlit window, and the signal generator heated up—that could shift the frequency to make a signal that would mimic a moving transmitter.

As they completed their analysis, they also developed a thorough framework for vetting future signals. It’s a flowchart with a series of up to nine tests, which begins by making sure the telescope has worked properly, then compares the signal to known sources of interference, and includes confirming the signal with additional observations. The blc1 signal passed only some of the tests; no one has yet spotted a signal that would pass all of them.

When an intriguing signal like blc1 appears, people love to ask questions like: “Do we message back?” says Danny Price, an astronomer at the International Centre for Radio Astronomy Research in Perth, Australia, and an author of the other new paper. But it takes a lot of work following the initial detection before astronomers can make the call whether the signal’s the real deal or just interference from Earth. “I don’t think when we find something it’s necessarily going to be a really clear signal, like in Contact. It’s going to be a low signal-to-noise, difficult-to-interpret signal that needs a lot of verification,” he says.

Contact, Carl Sagan’s novel, was made into a movie in 1997. Sagan based the protagonist, played in the film by Jodie Foster, on Jill Tarter, a leader in the SETI field and founder of the SETI Institute in Mountain View, California. While Tarter and other early SETI researchers sometimes struggled for funding and support, that’s not really the case anymore, says Jason Wright, an astronomer at Penn State University. He argues that Breakthrough Listen, based near the SETI Institute at UC Berkeley’s SETI Research Center, played a role in that improved status.

“I think Breakthrough Listen has breathed new life into the field. It has given the field much more visibility and scientific respectability. I think it’s entirely appropriate that they’re finally getting the telescope time worthy of the question they’re trying to answer,” Wright says. (He previously served as Sheikh’s thesis adviser but was not involved in this project.)

When SETI research began, astronomers didn’t have to contend with as much radio interference. But it has gotten worse, thanks to the proliferation of cell towers, satellites and satellite constellations, and today there are few places remote enough to avoid it. “The only place in the whole solar system that is almost free of radio interference is the far side of the moon. I say ‘almost’ because there are lunar orbiters, so it’s begun,” Wright says. (NASA has given early funding to two projects that are coming up with designs for a lunar radio telescope.)

New and updated Earthbound telescope arrays will soon help expand the search for radio signals from aliens, by enabling sensitive observations of many stars at once, in the hopes of spotting a real alien signal from any one of them. “The SETI Institute was a pioneer with array technology, with the development of the Allen Telescope Array,” says Andrew Siemion, an astronomer at the SETI Institute and a co-author of the new studies. The array is currently being refurbished and upgraded with new technologies, he says. It consists of 42 antennas, and it’s based at Hat Creek Observatory, about 300 miles north of San Francisco. The Dixie Fire in September burned within a few miles of the array, but the telescopes were spared.

Sheikh, Price, and their colleagues plan to continue monitoring Proxima Centauri and other targets with another radio telescope array, in the Northern Cape of South Africa, called MeerKAT. It currently has 64 satellite dishes, each 13.5 meters in diameter.

And in the meantime, Sheikh doesn’t feel dismayed that this signal didn’t turn out to be a long-distance call from ET. She’s ready to continue the search. “I think a lot of people will see this result and will be like, ‘Aw, man, you didn’t find aliens again,’” she says. “But we were able to conclusively prove that this was Earth-based interference, and to do that we had to develop new algorithms, new tools, and a new framework that will be hugely important in future surveys.”

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