Over lunch one day in 1950, the Nobel Prize winning nuclear physicist Enrico Fermi posed a question that would reverberate through parts of astronomy for decades.

“Where is everybody?” he mused.

He and a few other physicists had been discussing technological extraterrestrials, and Fermi appeared to be making the innocuous argument that since no aliens had landed on Earth, interstellar travel must be a tall order.

Seventy years later, his question has morphed into something else — a case against the very existence of extraterrestrial civilizations and an implication that astronomers are wasting their time looking — the infamous “Fermi Paradox.” Its continued occupation of the popular consciousness tends to make the researchers doing this work salty.

“There is no Fermi Paradox,” says Sofia Sheikh, a postdoctoral researcher at the Berkeley SETI Research Center . “You can’t say something about why there isn’t something there if you haven’t searched for it.” (Others point out that the academic argument does not actually belong to Fermi, and is not a paradox.)

Despite decades of talk, the Search for Extraterrestrial Intelligence (SETI) has been a field of comparatively little action — largely starved of federal funding due in part to Fermi Paradox-influenced thinking. SETI pioneer Frank Drake spent four months in 1960 scanning two stars for radio signals. More than a half century later, SETI researchers have made only modest progress. The privately funded Breakthrough Listen project is currently conducting one of the most exhaustive searches to date, listening to nearby stars for roughly fifteen minutes each. Preliminary results suggest that of our closest couple hundred neighbors, no star system harbors a civilization that broadcasts a powerful radio signal in our direction all the time. That leaves a couple hundred billion stars in the galaxy yet to be searched.

In 2012, Jill Tartar, a foundational SETI astronomer and inspiration for Carl Sagan’s novel Contact, likened the search for signals that could arrive from any direction at any time to hunting for marine creatures in a volume as vast as the Earth’s oceans. She estimated that SETI efforts had, collectively, sampled roughly one glass of water. An academic calculation suggested that, as of last year, SETI astronomers were up to perhaps “a large hot tub” of water.

To speed the cosmic trawl, researchers are increasingly leaning on a concept first articulated in — of all places — economics. Due to the current limits of technology, the modern SETI enterprise is mainly a search for potential civilizations that want to be found. Extraterrestrial intelligences would, by definition, be rational agents, and might intentionally beam out signals indicating their presence, shouting “We are here!” into the void. If so, astronomers could turn our common intelligence to their advantage, working out how to cooperate even without communicating. All they need to do is think like aliens.

“This is clearly the way we should think about how to design things,” says James Davenport, an astronomer at the University of Washington. “It’s a natural framework to think about how you might communicate with an unknown actor.”

A galactic game of seek and don’t hide

Stripped of its science-fiction trappings, the challenge of making contact looks like a special kind of game in economic theory: two players share one goal, but they can’t communicate as they attempt to achieve it.

While such an exercise might initially seem futile, Thomas Schelling, an iconoclastic and Nobel prize winning economist who popularized the Cold War concept of Mutual Assured Destruction, realized that games where players can’t communicate are still games. They may lack sure-fire paths to victory, but some strategies beat others. In Schelling’s 1960 book The Strategy of Conflict, he described how to identify such “focal” or “Schelling” points; focus on what you suppose your counterpart might know and what your counterpart supposes you might know.

A classic example is two strangers tasked with finding each other in Manhattan. An organized, but rather hopeless plan might be to walk the streets in a grid from Battery Park to Inwood, from the Hudson to the East River. Rather, Schelling reasoned, canny players might consider unique places and times that jump out to both parties as special, such as Grand Central Terminal at noon. Schelling’s theory has been born out in real demonstrations. In 2006, ABC staged just such a game, dropping off six pairs of people at random spots in the city. Within hours, the teams converged on two spots: Time Square and the observation deck of the Empire State Building. All six groups independently choose noon as their meeting time.

As technology improves and bigger astronomical data sets become available, SETI astronomers are rolling out a wide variety of novel searches based on the same principle. But it’s  easier said than done. What, if anything, can SETI researchers hope to have in common with alien civilizations? What do we know extraterrestrials know, and what do they know we know?

“We’ve got to pick some signal strategy or signal reception strategy that will match with what someone else comes up with,” Sheikh says. “Otherwise, the task is hopeless.”

Special frequencies

Human astronomy remains, for the time being, firmly attached to Earth. Presumably, other civilizations have a home base from which they broadcast too. In SETI, the question of “where to meet” often becomes a question of “what frequency to chat on.” After all, even just here on Earth humans reach each other on a dizzying array of radio channels, microwaves, and with beams of visible and infrared light.

In a foundational SETI publication appearing in Nature in 1959, physicists Giuseppe Cocconi and Philip Morrison proposed that 1420 MHz or thereabouts would be a good place to start the conversation. Hydrogen gas buzzes at precisely this radio frequency, and since hydrogen is the most common element in the galaxy (and the universe), this channel might be of particular interest. Schelling himself called out the frequency as an example of a Schelling point the next year in The Strategy of Conflict, writing in a footnote, “In the most favored radio region there lies a unique, objective standard of frequency, which must be known to every observer in the universe.”

The “hydrogen line” has since fallen out of favor in SETI, partially because all that hydrogen makes the channel rather noisy, and partially because astronomers no longer need to spend months turning the radio dial by hand as Drake did. “In the first SETI searches you’d look at a channel at a time,” Sheikh says. “Now we're doing billions of times that with a single observation with a single instrument.”

Yet researchers continue to think about special frequencies. When analyzing a billion signals at once, there are a billion chances for the algorithm to mistakenly flag one channel as artificial. Identifying the most promising candidates ahead of time could lend confidence to future detections.

Jason Wright, a Penn State astrophysicist, described a novel set of Schelling point frequencies last year in the International Journal of Astrobiology. Using base ten numbers and the units of Hertz to measure radio frequencies are merely conventions of human culture, so Wright sought culture-independent frequencies in fundamental physics. He found inspiration in research from the pioneer quantum physicist, Max Planck, who in 1900 wrote about physical quantities that “remain meaningful for all times, and also for extraterrestrial and non-human cultures, and therefore can be understood as ‘natural units.’”

These fundamental constants of nature describe the speed of light, the strength of gravity, and the relationship between a photon’s energy and its frequency. Any civilization capable of building a radio beacon would likely be able to measure these numbers as humans have, and by mixing them together could find a particular frequency — a universal frequency specified by fundamental physics. “If you know those three things, you say huh, something funny happens at that frequency,” Wright says.

Electromagnetic radiation with the fundamental frequency would be impossible to detect, so Wright added the fundamental charge of atomic particles to the mix and used the four constants to construct a base — like we use the number 10 as a base — to formulate a list of more reasonable frequencies in both radio waves and visible light, a frequency “comb,” that SETI researchers could use to sift through the haystack of radio channels.

Special times

As some SETI researchers ponder how aliens might reach out, others wonder when. Transmitting beacons take energy, and civilizations may not transmit all the time. (We certainly don’t. Our highest profile message lasted for three minutes in 1974, a powerful blast from the recently ruined Arecibo radio dish in Puerto Rico.)

To get picked up during Breakthrough Listen’s 15-minute scans, for instance, our nearest neighbors would need to be beaming out an all-directional signal using around a trillion watts of power, according to Wright, or about five percent of humanity’s total energy consumption. That’s an expensive porch light to leave on all the time. To hear from cultures on a budget, researchers may need to work out the cosmic equivalent of noon — a universal hailing time.

Sheikh reasons that any civilizations engaged in sending interstellar messages are likely to be at least as good at astronomy as we are. In recent decades, astronomers have spotted thousands of exoplanets, often by looking for stars that regularly dim as planets pass in front of them. Any aliens doing the same could already know Earth is here.

That knowledge could focus their efforts to make contact by specifying a unique time to say hello — the moment when Earth eclipses the sun, dimming our star and revealing our presence. Sheikh is moving to the SETI Institute in California in January, and for her first project there she plans to use the institute’s Allen Telescope Array to sweep the night sky directly overhead when the sun is positioned “behind” the Earth from the perspective of any inhabitants of star systems in view.

She estimates that the survey will be sensitive to radio broadcasts from an extraterrestrial dish analogous to Arecibo transmitting within about 220 light years. Weaker signals, or signals coming from deeper in the galaxy — which spans 100,000 light years — would go unnoticed.

Sheikh admits that her plan might seem like a long shot, but argues that it beats scanning the sky from one side to the other — the celestial equivalent of wandering Manhattan south to north. “You have to start somewhere,” she says. “Why not start somewhere you think is more likely?”

Another Schelling-inspired lesson is to test out as many Schelling points as possible. If your counterpart doesn’t show up at Grand Central at noon, try Times Square at midnight. In that spirit, Sheikh is collaborating with Davenport, at the  University of Washington to study another temporal landmark.

Nearly 35 years ago, astronomers witnessed a star just outside the Milky Way explode like a bomb. “SN 1987a” quickly became the subject of more academic papers than any other supernova.  “It’s the only supernova that’s gone off in our neighborhood in the last 100 years,” Davenport says. “It’s kind of a big event. It’s rare, visible, and outshone the entire galaxy.

If any civilizations were poised, waiting for a special galactic moment to announce their presence, SN 1987a would have been a great opportunity, suggested Argentinian astrophysicist Guillermo Lemarchand in 1994. Now Sheikh and Davenport, together with undergraduate physics and astronomy student Bárbara Cabrales at Smith College, are working out the math to look for signals that would just be reaching Earth now, had they been sent in response to SN 1987a.

As light from the supernova reaches new stars, and potential signals from those stars ripple out through space, the zone of the sky to search changes. Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), which watches for stars being eclipsed by exoplanets, the group is developing an algorithm to look for stars that dim or flash at precisely the right moment. They don’t expect to find a smoking gun with TESS, which keeps tabs on hundreds of thousands of stars. Their main goal is to get the software tools ready for the upcoming Vera Rubin Telescope, which will monitor 10 to 20 billion stars on a weekly basis.

Further down the line, other SETI researchers could tweak the software to search for radio signals, rather than looking only for civilizations with the technology to make their star flicker. “We’re looking at signals now in [visible light] because that’s where the data is,” Davenport says. “We’re scavengers. We’re taking what is already available to us. Astronomers have to be crafty.”

Shy Civilizations

The central conceit of seeking Schelling points in modern SETI is the assumption that both sides are playing the same game. But with more powerful instruments, researchers might be able to start looking for more bashful  civilizations. In that case too, crafty astronomers are already thinking about what common behaviors might give an inhabited planet away, even if it’s not actively broadcasting.

Humanity, for instance, has benefited from putting television, radio, and weather satellites in high enough orbits that they continuously face the same part of the Earth. These spacecraft in “geosynchronous” orbits form an artificial ring around our planet roughly 22,000 miles above the ground. If another planet sported a thick enough artificial ring, it might block the light from its star in a peculiar way that astronomers could spot.

Our ring is sparse today but gets slightly denser every year. In 200 years, the satellite belt would become notable enough to be seen by extraterrestrial astronomers at a distance of ten light years using current telescopes, astrophysicist Hector Socas-Navarro calculated in 2018.

Another proposal assumes that civilizations have a vested interest in their long-term survival and may develop the technology to watch out for catastrophic asteroid collisions. As Carl Sagan reportedly quipped, “If the dinosaurs had had a space program, they would not be extinct."

Some of the loudest radio pulses humanity has sent out into space have been for exactly this purpose, with much of the responsibility of monitoring the trajectories of asteroids falling to Arecibo’s 2.5-million-watt radar beam, before its destruction. Most of that radio signal bounced back to Earth carrying vital information about its target, but some would spill past the asteroid into the galaxy. Similar signals from another world would come sporadically. But since the orbits of asteroids and planets form a flat disk, any radar signals originating from a planet and directed toward an asteroid would always spill outward from the plane of the disk. Future eavesdropping attempts could use this fact to prioritize systems that are oriented “edge on” to Earth, rather than “top down.”

Some researchers recoil from efforts to get into the heads of extraterrestrial beings, considering them too outlandish, and Sheikh understands the instinct to avoid assumptions. Nevertheless, since coming to view SETI through a Schelling-tinted lens, she has realized that every project assumes some shared attributes between humanity and whomever else could be out there. Even the fact that many SETI searches target stars betrays a presumption that other lifeforms, like us, are more likely to live on planets than in deep space.

“Everything is a Schelling point, Sheikh says. “You can't get away from it.”

Seeking universality

The more Schelling-inclined researchers can intuit commonalities between terrestrials and hypothetical extraterrestrials, the better their odds of success. And the only commonalities likely to span light years are going to be potential universalities, like knowing how fast sunbeams travel, or not wanting to be taken out by an asteroid. In the interest of ferreting out such universalities, some researchers point out that Earth might hold more than just one example to learn from.

Human cultures have risen and fallen for millennia,and have a long history of misinterpreting each others’ legacies. Spanish conquistadors, for instance, mistook the Great Pyramid of Cholula (the world’s largest pyramid by volume) for a hill and built a church on it. “Looking at the full sweep of human behaviors, anthropology-style, is useful for shaking us out of our own cultural biases,”  Kathryn Denning, an anthropologist at York University in Toronto.

She goes even farther, suggesting that observing how dolphins, whales, birds, and other social animals interact while sharing or dividing up territory would be a “good way to broaden our thinking beyond the human.”

Penn State’s Wright conceives of Schelling points in a similar way. The goal isn’t to get into an alien’s head per se, but rather to decipher the essential behaviors of all intelligent beings, starting with animals here on Earth. “They have to use energy. They have to move around. At some point they have to interact with each other. They have to eat. And so, we hope that there are similar fundamental things the alien species out there might be doing,” he says.

Perhaps in another 70 years, equipped with more sensitive instruments and smarter searching algorithms, SETI astronomers will have checked enough galactic Schelling points to start to answer Fermi’s question. If we show up at enough interesting landmarks at enough unique times and fail to connect, we’ll have to conclude that no one else is trying to meet up. Or, if they are, they’re going about it in a way that’s truly alien.