8.04.2020

In 1985, when I was a baby journalist writing my first college newspaper story, I covered a symposium at Harvard inaugurating the Megachannel Extraterrestrial Assay (META), a computer system designed by physicist Paul Horowitz to sift through millions of narrow radio channels for signals from other civilizations.

Carl Sagan was on hand that weekend to represent the Planetary Society, which had helped fund the project. So was Steven Spielberg, who’d written a $100,000 check. Having grown up on Sagan’s Cosmos and Spielberg’s Close Encounters of the Third Kind and E.T.: The Extraterrestrial, I was star-struck. But I was also thrilled to witness what felt like the launch of a voyage that would finally turn science fiction into science reality.

No one at the symposium was rash enough to predict whether or when Horowitz’s project would succeed. But if you’d told the assembled scientists that 35 years would go by without META or any of its successors detecting even a hint of a signal, they’d have reacted with disappointment and disbelief. The aliens ought to be out there; they ought to be broadcasting; we ought to be able to hear them. But a 2020 Astronomical Journal paper detailing a search of 1,327 nearby stars at the highest sensitivity to date found zero candidate signals. So how is it that the Great Silence—to use the title phrase from astronomer Milan Ćirković’s 2018 book— continues?

Well, having just written my own book about the history of that question (Extraterrestrials, MIT Press, April 2020), I’ve come to suspect that there’s something missing in our approach to the search for off-world intelligence. This search is built around the hope that if technological societies are out there, they’re communicating (1) using the parts of the electromagnetic spectrum we can most easily scan from Earth’s surface, namely radio and optical frequencies, and (2) using encoding schemes such as pulse modulation that we can easily recognize. Those assumptions made sense in the early days of SETI in the 1960s, when the field was still a quirky offshoot of radio astronomy.

But today they seem fatally Earth-centric and human-centric. As Nathalie Cabrol of the SETI Institute wrote in a paradigm-busting 2016 Astrobiologypaper, “[S]o far, in our quest to find ET, we have only been searching for other versions of ourselves.”

What we didn’t know in the 1960s is that there are planets around most stars—and that while many are in the “habitable zones” of their systems, where surface water would neither boil nor freeze, few of them precisely resemble Earth. We also didn’t understand how hardy and adaptable life can be: We’ve found it in places with crushing pressures and scalding temperatures, in Antarctic lakes cut off from the sun for thousands of years, and even inside nuclear reactors, where it feeds on radiation. And we didn’t appreciate the dazzling variety of communications styles among the sentient beings we doknow—the other animals who share Earth.

Cabrol is right: it’s time to move beyond the idea that extraterrestrials would think like us or use technologies like ours. Sure, let’s keep listening for technosignatures such as radio signals. As SETI Institute founder Jill Tarter has pointed out, our search so far amounts to sampling a single glass of water from the ocean. But let’s also look for biosignatures, such as signs of industrial activity in the atmospheres of exoplanets—data that we’ll soon be able to gather using NASA’s James Webb Space Telescope. Let’s expand the search beyond familiar sunlike stars and red dwarfs. Let’s look at planets where exotic biochemistry might reign. Let’s use our computers to model how the universe might look to beings who evolved in different environments and might have very different sense organs and neural systems. And then let’s build new observing and filtering systems to look for the kinds of messages they might be sending.

Maybe we’ll get lucky and detect a radio signal tomorrow that says “hello” in simple mathematical code, the way Sagan predicted in his 1985 novel Contact. But more likely, if we want to find what Cabrol calls “life as we do not know it,” we’ll have to get outside our own heads and think more like aliens.

Quelle: SCIENTIFIC AMERICAN