martin_rees's picture
Former President, The Royal Society; Emeritus Professor of Cosmology & Astrophysics, University of Cambridge; Fellow, Trinity College; Author, From Here to Infinity
Breakthrough Listen

Searching for extraterrestrial intelligence (SETI) has for decades been a “fringe” endeavor. But it’s moving towards the mainstream. In 2015, it gained a big boost from the launch of Breakthrough Listen—a ten-year commitment by the Russian investor Yuri Milner to scan the sky in a far more comprehensive and sustained fashion than ever before.

It’s a gamble: even optimists rate the probability at only a few percent. And of course radio transmission is only one channel whereby aliens might reveal themselves. But the stakes are high. A manifestly artificial signal—even if we couldn’t decode it—would convey the momentous message that “intelligence” had emerged elsewhere in the cosmos.

These searches are more strongly motivated than they were in earlier decades. The Kepler Spacecraft, surely one of the most cost-effective and inspirational projects in NASA’s history, has revealed that most stars in the Galaxy are orbited by retinues of planets. There are literally billions of them in our Milky Way galaxy with the size and temperature of our Earth. 

But would these planets have developed biospheres? Or is our Earth unique, while all others are sterile and lifeless? Despite all we know about life’s evolution, its actual origin—the transition from complex molecules to the first replicating and metabolizing systems that we’d deem to be “alive”—has remained a mystery, and relegated to the “too difficult box.” But it is now being addressed by top-ranking scientists. We may soon know whether life’s emergence was a “fluke,” or whether it’s near-inevitable in the kind of “chemical soup” expected on any planet resembling the young Earth—and also whether the DNA/RNA basis of terrestrial life is highly special, or just one of several possibilities.

In seeking other biospheres, clues will surely come from high-resolution spectra, using the James Webb Space Telescope, and the next generation of 30+metre ground-based telescopes that will come on line in the 2020s.

Conjectures about advanced alien life are of course far more shaky than those about simple life. We know, at least in outline, the evolutionary steps whereby nearly 4 billion years of Darwinian evolution led to the biosphere of which we humans are a part. But billions of years lie ahead. I would argue that our remote, posthuman descendents will not be “organic” or biological; and they will not remain on the planet where their biological precursors lived. And this offers clues to the planning SETI searches.

Why is this? It’s because post-human evolution will be spearheaded by super-intelligent (and super-capable) machines. There are chemical and metabolic limits to the size and processing power of “wet” organic brains. But no such limits constrain electronic computers (still less, perhaps, quantum computers); for these, the potential for further development could be as dramatic as the evolution from monocellular organisms to humans. So, by any definition of “thinking,” the amount and intensity that’s done by organic human-type brains will be utterly swamped by the cerebrations of AI. Moreover, the Earth’s biosphere is not essential—indeed, it’s far from an optimal environment—for inorganic AI. Interplanetary space will be the preferred arena where robotic fabricators will have the grandest scope for construction, and where non-biological “brains” may develop insights as far beyond our imaginings as string theory is for a mouse.

This scenario implies that, even if life had originated only on Earth, it need not remain a trivial feature of the cosmos: humans may be closer to the beginning than to the end of a process whereby ever more complex intelligence spreads through the Galaxy. But in that case there would, of course, be no “ET” at the present time.

Suppose, however, that there are other biospheres where life began, and evolved along a similar track to what happened on Earth. Even then, it’s highly unlikely that the key stages would be synchronized. A planet where it lagged significantly behind what has happened on Earth would plainly reveal no evidence of ET. But on a planet around a star older than the Sun, life could have had a head start of a billion years—and already transitioned to the futuristic post-human scenario.

The history of human technological civilization is measured in centuries—and it may be only one or two more centuries before humans are overtaken or transcended by inorganic intelligence, which will then persist and continue to evolve for billions of years. This suggests that if we were to detect ET, it would be far more likely to be inorganic. We would be most unlikely to “catch” it in the brief sliver of time when it took organic form. A generic feature of these scenarios is that “organic” human-level intelligence is just a brief prelude before the machines take over.

It makes sense to focus searches first on Earth-like planets orbiting long-lived stars (the “look first under the lamp-post” strategy). But science fiction authors remind us that there are more exotic alternatives. In particular, the habit of referring to “alien civilizations” may be too anthropocentric—ET could be more like a single “mind.”

Breakthrough Listen will carry out the world’s deepest and broadest search for extraterrestrial technological life. The project involves using radio dishes at Green Bank and at Parkes—and hopefully others including the Arecibo Observatory—to search for non-natural radio transmissions using advanced signal processing equipment developed by a team based at UC Berkeley. Moreover, the advent of social media and citizen science will enable a global community of enthusiasts to download data and participate in this cosmic quest. 

Let’s hope that Yuri Milner’s private philanthropy will one day be supplemented by public funding, I’d guess that millions watching Star Wars would be happy if some of the tax revenues from that movie were hypothecated for SETI.

But in pursuing these searches we should remember two maxims, both oft quoted by Carl Sagan. First, “extraordinary claims require extraordinary evidence,” and second, “absence of evidence isn’t evidence of absence.”