12.09.2020

In 2015, Sofia Sheikh was at loose ends. Her adviser at the University of California (UC), Berkeley, with whom she studied hot, giant exoplanets, had left for a new job. Browsing reddit, she saw a post about a lavishly funded new search for extraterrestrial intelligence (SETI) and noticed that its leader was also at UC Berkeley: astrophysicist Andrew Siemion. She asked her former adviser for an introduction and met with Siemion when he was still unpacking boxes in a new office. “Everything’s kind of history from there,” says Sheikh, who became the team’s first undergraduate student.

Sheikh is now a Ph.D. student at Pennsylvania State University (Penn State), University Park, where she led a radio survey of 20 nearby star systems aligned with Earth’s orbital plane. If an intelligent civilization inhabited one of these systems and pointed a powerful telescope our way, they would see Earth passing in front of the Sun, and they might detect signs of life in our atmosphere. They might even decide to send us a message. The results, published in February in The Astrophysical Journal, were unsurprising. “Spoiler alert: no aliens,” Sheikh jokes.

SETI researchers are used to negative results, but they are trying harder than ever to turn that record around. Breakthrough Listen, the $100 million, 10-year, privately funded SETI effort Siemion leads, is lifting a field that has for decades relied on sporadic philanthropic handouts. Prior to Breakthrough Listen, SETI was “creeping along” with a few dozen hours of telescope time a year, Siemion says; now it gets thousands. It’s like “sitting in a Formula 1 racing car,” he says. The new funds have also been “a huge catalyst” for training scientists in SETI, says Jason Wright, director of the Penn State Extraterrestrial Intelligence Center, which opened this year. “They really are nurturing a community.”

Breakthrough Listen is bolstering radio surveys, which are the mainstay of SETI. But the money is also spurring other searches, in case aliens opt for other kinds of messages—laser flashes, for example—or none at all, revealing themselves only through passive “technosignatures.” And because the data gathered by Breakthrough Listen are posted in a public archive, astronomers are combing through it for nonliving phenomena: mysterious deep-space pulses called fast radio bursts and proposed dark matter particles called axions. “There are untapped possibilities here,” says axion searcher Matthew Lawson of Stockholm University.

Perhaps the most important consequence of Breakthrough Listen is that it has nudged SETI, once considered fringe science, toward the mainstream. “Journals are relaxing and letting good technosignature papers be published,” says astrobiologist Jacob Haqq-Misra of the Blue Marble Space Institute of Science. “The giggle factor is reducing.” After nearly 3 decades of eschewing SETI, NASA organized a technosignature workshop in 2018. In June, it awarded a grant to model the detectability of possible technosignatures in the atmospheres of exoplanets, its first ever SETI-related grant not involving radio searches.

But some astronomers worry the funding boon is distorting science. Fernando Camilo, chief scientist of the South African Radio Astronomy Observatory, says Breakthrough Listen’s voracious appetite for time on large telescopes leaves him uncomfortable. “It leaves less time to do astronomy.” Others say SETI’s high-risk, rush-for-the-prize approach could distract funders from a more rational, stepwise search for extraterrestrial life. “We do have a really thoughtful process on what gets funded and what doesn’t,” says Harvard University astronomer David Charbonneau. “That doesn’t happen with rich individuals.”

But SETI proponents don’t see themselves as separatists. They are increasingly working hand in hand with those searching for exoplanets and studying astrobiology. “Looking for intelligence is the logical conclusion of this search for life,” says astronomer David Kipping of Columbia University.

SETI STARTED SMALL. In 1960, astronomer Frank Drake pointed a 26-meter radio telescope in Green Bank, West Virginia, at two nearby Sun-like stars. He scanned frequencies around 1.42 gigahertz, which correspond to wavelengths of about 21 centimeters—the part of the spectrum where clouds of interstellar hydrogen emit photons. This 21-centimeter glow is ubiquitous, and Drake supposed it might be a universal channel on the cosmic dashboard, a natural place for a clarion “We are here!” But his targets, Tau Ceti and Epsilon Eridani, were expressionless. The survey, called Project Ozma, saw no sign of artifice, such as an intense spike squeezed into a narrow frequency band.

With funding from NASA and the National Science Foundation (NSF), however, searches continued, with bigger telescopes to listen for fainter signals and hardware that could scan thousands and eventually millions of narrow frequency channels at once. Drake devised his now famous, eponymous equation that estimates how many communicative extraterrestrial civilizations may exist in the Milky Way. It depends on seven variables, from the rate of star formation to the average lifetime of a civilization. Even though only one of the seven factors—star-formation rate—was known with any certainty, alien hunters were on the prowl.

In 1992, NASA decided to look harder, only to quickly reverse course. It embarked on the Microwave Observing Project, a 10-year, $100 million SETI search using several large telescopes. But the following year, the project was ridiculed and cut by lawmakers focused on reducing the federal budget deficit. Ever since, NASA has mostly shied away from SETI.

Even as federal funding shriveled, the 1990s gave SETI an unexpected gift. Until then no one had detected an exoplanet, much less a potentially hospitable one, but that decade brought a host of discoveries. Since then, missions such as NASA’s Kepler telescope have suggested that planetless stars are rare, and that about one in five Sun-like stars has potentially habitable Earth-size planets—two more factors in the Drake equation that have fueled optimism among SETI advocates. The turn-of-the-century tech boom offered another boost: newly minted billionaires with a taste for space. A high point came in 2007 with the inauguration of the Allen Telescope Array, a SETI observatory in California kick-started with $11.5 million from Microsoft cofounder Paul Allen.

Then the field took another plunge. The 2008 financial crisis struck and within a few years, with federal and state funding tight, UC Berkeley withdrew from the project. The array was put into hibernation for 8 months. A planned expansion from 42 to 350 dishes never materialized. “SETI was entirely decimated,” Siemion says. “I was one of maybe two or three in the whole world working on SETI.”

That was when Yuri Milner called.

BORN AND EDUCATED in Moscow, Milner worked as a particle physicist at the Lebedev Physical Institute. In 1990, as the Soviet Union collapsed, he left to study business at the University of Pennsylvania, and in 1999 he founded an internet investment fund. The fund was an early backer of Facebook and Twitter, and later Spotify and Airbnb. Forbes magazine puts Milner’s net worth at $3.8 billion. “I made some lucky investments,” he tells Science.

Milner says he’s always felt a connection with space and SETI. He was born in 1961, days after Drake convened the first SETI conference. He is named after Yuri Gagarin, the first cosmonaut. Once he had built up a fortune, “I discovered that now I can give back to science,” he says. He knew of SETI’s dire financial straits, and he believed his money and knowledge of the tech industry could help speed up the search. Siemion’s UC Berkeley center, across the San Francisco Bay from Milner’s home in Silicon Valley, became the beneficiary.

Breakthrough Listen set out ambitious goals. It would survey 1 million of the closest stars to Earth and 100 nearby galaxies using two of the world’s most sensitive steerable telescopes, the 100-meter Green Bank Telescope in West Virginia and the 64-meter Parkes radio telescope in Australia. Buying up about 20% and 25% of the time on those telescopes, Breakthrough Listen promised to cover 10 times more sky than previous surveys and five times more of the radio spectrum, and gather data 100 times faster.

LEFT TO RIGHT: © LAURIE HATCH; AUSCAPE/UNIVERSAL IMAGES GROUP VIA GETTY IMAGES; SOUTH

Achieving these goals required new hardware. The key electronic component is a digital backend, which chops telescope data into ultrathin frequency slices and records it. Siemion says Breakthrough Listen’s backends are “orders of magnitude more powerful than anything else on site.” The instruments are available for 100 hours every year to other astronomers interested in such fine frequency resolution. That allocation is often oversubscribed at Green Bank, Siemion says, ever since the backend helped characterize the first repeating fast radio burst.

The project is adding a major new telescope to its mix of collaborations: MeerKAT, a South African array of 64 dishes each 13.5 meters across. Instead of buying time on the array, Breakthrough Listen is tapping into the data stream while the telescope observes its regular targets—a procedure known as commensal observing. “You take what you can get,” Camilo says. “When it works, it’s fantastic.” Commensal observing will also be added to the Karl G. Jansky Very Large Array in New Mexico, the workhorse of U.S. radio astronomy, in a project led by the privately funded SETI Institute.

Gathering data sets is one thing; scouring heaps of them for alien messages is another. SETI researchers have long looked for energy packed into narrow frequency signals—something that is hard for nature to replicate, although astronomers need to exclude humanmade signals. One test is to see whether the signal’s frequency drifts over time: An alien transmitter would be on a moving planet, causing a Doppler shift. If the frequency is rock steady, it’s likely to be earthly interference. Similarly, if the signal persists when the telescope moves from its target, it’s noise from Earth.

But aliens might send something more complex than a single loud note. How do you scan SETI data for something that just seems anomalous or weird? Researchers have been trying to enlist artificial intelligence (AI), but it hasn’t been easy. One species of AI, natural language algorithms, can recognize key words in the flow of human speech—think of Amazon’s Alexa, or eavesdroppers at the National Security Agency—after being trained on vast speech data sets. But the huge number of narrow frequency channels in SETI data overwhelms these algorithms.

Converting the data stream into 2D diagrams that resemble images works better, at least in tests, in which machine vision algorithms picked out strange pictures from a torrent of similar ones. “We have to guess what an anomaly might look like and train the algorithm to look for this, or look for things that look similar,” says Steve Croft of UC Berkeley’s SETI Research Center.

THE FOCUS OF SETI searches tends to reflect the technology of the times. Radio was in its heyday when Drake started out. But as lasers have grown in power and sophistication, so have efforts to spot alien laser signals with so-called optical SETI.

Astronomers have carried out optical searches with modest telescopes since the 1990s. Breakthrough Listen is doing its own, with time on the 2.4-meter Automated Planet Finder (APF) telescope at the Lick Observatory in California. APF has been scanning a sample of stars to distances up to 160 light-years but will now work through a new list: stars with potentially habitable planets identified by NASA’s Transiting Exoplanet Survey Satellite.

Others are developing telescopes that wouldn’t need to target individual stars. The LaserSETI project, funded by the SETI Institute, is a collection of $30,000 miniobservatories, made up of an off-the-shelf fisheye lens, two cameras, and electronics that would gather light from the entire sky. The first was installed last year on an observatory roof north of San Francisco. Eventually, the institute wants to install 60 instruments around the world for 24/7 coverage.

LaserSETI’s small telescopes would only pick up an especially bright flash from a nearby source. Shelley Wright of UC San Diego hopes to see much farther with the Pulsed All-sky Near-infrared Optical SETI (PANOSETI), an all-sky telescope able to detect ultrashort laser pulses across all optical wavelengths.

PANOSETI’s design includes lightning-fast photon counters sensitive to pulses less than one-billionth of 1 second long. “It’s hard for nature to make that,” Shelley Wright says. It relies on a Fresnel lens, a type used in lighthouses to focus light into a narrow beam. Flipped over, a Fresnel can gather light from a 10°-wide patch of sky onto the photon counters. The team is building two observatories, each an array of 80 telescopes with lenses 50 centimeters across, bunched together in a fly’s eye arrangement. The plan is to site the pair 1 kilometer apart—to help root out false positives—at the Palomar Observatory in California. Funded by Qualcomm cofounder Franklin Antonio, the project has built five telescopes but has been stalled by the COVID-19 pandemic.

THEN AGAIN, even intelligent aliens might be too busy or too shy to send messages to the stars. So SETI researchers also hope to detect passive signs of technology. People’s ideas about what to look for often reflect their time: Consider the 19th century “discovery” of canals on Mars when canals were still a common form of transport on Earth. In 1960, amid rapid economic growth and concerns about energy shortages, physicist Freeman Dyson imagined an advanced society might build a megastructure surrounding a star to capture its energy. Such “Dyson spheres” continue to fascinate and were suggested as an explanation for the strange dimmings of the star KIC 8462852, known as Tabby’s Star. In 2015, Jason Wright led a search for the glow of Dyson spheres in 100,000 nearby galaxies, using data from NASA’s Wide-field Infrared Survey Explorer satellite.

Technosignatures could be more subtle. In the not-too-distant future, ultrasensitive radio telescopes might be able to pick up the beams of a radar, like the ones used for air traffic control, from a distant exoplanet. Future optical telescopes might reveal the glow of a city’s lights or its infrared warmth. Heavy industry or geoengineering might leave fingerprints in a planet’s atmosphere.

Just knowing we are not alone … is something that can bring us together here on Earth.

These efforts chime with searches for biosignatures, detectable marks that organic life might leave on an exoplanet. “The line between technosignatures and biosignatures is blurring,” Sheikh says. “It makes sense to observe both.” In deciding to fund the 2018 workshop on technosignatures, NASA felt that they could be discussed “on a firmer scientific foundation than before,” says Michael New, the agency’s deputy associate administrator for research. After the workshop, the wording in NASA funding calls that had for some years excluded SETI-related proposals quietly disappeared.

In June, Jason Wright and his colleagues benefited from the new openness when they were awarded a grant to model exoplanet atmospheres and put together a “library” of potential technosignatures, which astronomers can refer to when observing exoplanets. The team will first model chlorofluorocarbons—a pollutant that isn’t produced naturally—and vast solar power arrays, because they would leave an obvious cutoff in the ultraviolet part of the spectrum. “What we should look for is things that can’t be avoided, civilization’s manifestations in the biosphere,” says Adam Frank, lead investigator on the grant at the University of Rochester.

BUT EVEN AFTER the fanfare of Breakthrough Listen, SETI remains far from a central concern for most astronomers. In 2018, panels of researchers convened by the National Academies of Sciences, Engineering, and Medicine (NASEM) drew up strategies for NASA on astrobiology and exoplanets. They made scant mention of technosignatures and didn’t advise NASA to spend any money on the topic, or, more generally, SETI.

SETI enthusiasts say they are trying to avoid being shut out of an even bigger NASEM effort: its decadal survey of astrophysics, a once-a-decade priority setting exercise that is influential with funding agencies and legislators. The survey is due to report early next year. “We’ve made a big push to get the decadal survey … to explicitly say that NASA and the NSF need to nurture this field,” Jason Wright says. He and colleagues made nine submissions, known as white papers, to the survey, compared with a single white paper in the previous survey. Sheikh says: “There are signs the winds are starting to shift.”

But many astronomers think the more important hunt is for alien life of a more basic kind, not the higher risk search for technological societies. “We have to invest in general questions,” says Charbonneau, who co-chaired the NASEM panel that developed the NASA exoplanet hunting strategy. “If we just go for the prize and don’t find anything, what have we learned from that?”

Mainstream astrobiologists hope the decadal survey will give a thumbs up to the Large UV/Optical/IR Surveyor, or LUVOIR, a proposed NASA space telescope as much as six times wider than the Hubble Space Telescope. It would scrutinize habitable planets for biosignatures and estimate the fraction of them that support life—another term in the Drake equation. “The progress we’ve made as scientists follows the terms of the Drake equation in order,” says astrobiologist Shawn Domagal-Goldman of NASA Goddard Space Flight Center. “That progress could lead to a search for technosignatures. I could see LUVOIR being used to do that, even though it wasn’t designed for such a search.”

Jason Wright, however, thinks the potential payoff of SETI is just too tempting to put off the search. In July, he and his colleagues reported the “discovery space”—all the possible locations, frequencies, sensitivities, bandwidths, timings, polarizations, and modulations—that SETI radio surveys have so far explored. The result: If the entire discovery space is represented by the world’s oceans, SETI has so far searched the volume of a hot tub.

Milner seems ready to support at least a few more SETI hot tubs. He says he wants Breakthrough Listen to continue past 2025, when his initial funding runs out. “It’s one of the most existential questions in our universe,” he says. “Just knowing we are not alone … is something that can bring us together here on Earth.”

Quelle: AAAS