by Bryan Walsh
Exactly what Fermi thought the answer might be isn’t clear—the physicist died a few years later, and his lunch companions all remembered the conversation differently. Teller and York thought Fermi was talking about whether interstellar travel was even possible—1950, remember, was before Sputnik and the first attempts to search for extraterrestrial signals. If it was indeed impossible to travel faster than light—as Einstein’s theory of special relativity holds—then perhaps it wouldn’t be so surprising that we hadn’t found any evidence of extraterrestrial intelligence. As Teller told the physicist Eric Jones years later, Fermi wondered if “we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center.”
To this day the brute fact of Einsteinian speed limits and interstellar distances helps explain why we’ve never had a confirmed alien visitation, even if intelligence did exist elsewhere in the galaxy. But York remembered Fermi probing deeper, following up his question with a quick calculation on the probability of Earth-like planets, the probability of life arising, the duration of a technological civilization—a Drake equation before Drake. “He concluded on the basis of such calculations that we ought to have been visited long ago and many times over,” York told Jones. “As I recall, he went on to conclude that the reason we hadn’t been visited might be that interstellar flight is impossible, or, if it is possible, always judged to be not worth the effort, or technological civilization doesn’t last long enough for it to happen” (emphasis added).
Fermi’s lunchtime question came to be known as the Fermi Paradox, although it’s not really a paradox, and it was later writers who eventually built it out, well after Fermi’s death.41 There are dozens upon dozens of possible solutions to the Fermi Paradox. One of the most likely is that the sheer amount of space in outer space means that there could be many independently evolved civilizations in the galaxy that would nonetheless never run into each other, a possibility Fermi himself noted. But the problem with that answer is that the galaxy is not just huge, but old. Even if it remains physically impossible to travel close to the speed of light—let alone beyond it like the starship Enterprise on Star Trek—an alien civilization could spread out at sub-light speeds and still colonize plenty of the galaxy, if it had enough time.42
According to the mathematicians Duncan Forgan and Arwen Nicholson of Edinburgh University, spacecraft traveling at one-tenth the speed of light—fast but not inconceivable—could cross the entire Milky Way galaxy in just 10 million years. That’s a long, long time by human standards—more than a thousand times longer than human civilization to date—but less than 0.1 percent of the age of the galaxy.43 It’s doable not just once, but many times over throughout the history of the Milky Way, just as empires on Earth have risen and fallen over and over again. And if those alien empires have been superseded by artificial intelligence, which isn’t hampered by the unfortunate organic trait called death, such expansion would presumably be even easier.
So we can judge that galactic expansion is possible, yet we see no evidence of it. There are no visible colonies here on Earth—the most habitable planet in the galaxy, as far as we know—or in our solar system or anywhere we point our telescopes. So again—where is everybody?
It’s at this point that attempts to solve the Fermi Paradox start to get creative. One hypothesis is that aliens exist but are deliberately hiding their presence from us. The MIT astronomer John Ball proposed that the first alien civilizations to arise over the history of the galaxy would accrue so much power—because they would inherit all the resources of space—that they would essentially run the cosmos.44 Perhaps just as we have chosen to create nature preserves and leave some remote tribes of indigenous people uncontacted—because we know from experience that an encounter would destroy their culture—these superpowerful aliens would leave us alone to develop.
Or perhaps aliens are trying to talk to us, but instead of using radio, they’re employing a technology that we haven’t yet mastered. Imagine a modern human trying to call up a caveman on a cell phone. At least they would share a biology and perhaps some points of common reference, but a truly advanced civilization attempting to communicate with us might be more like a modern human trying to talk to an ant. Would you be interested in putting forth the effort that would be needed to understand what little this tiny insect is capable of communicating? Probably not—you have important human things to do. And so it might be with any sufficiently advanced extraterrestrial intelligence, which is to say, almost any that we might encounter. Instead of the Great Silence, we would have the Great Indifference.
But even if aliens refused to make contact with us—whether out of enlightened policy or simple apathy—it doesn’t explain why, if they exist, we would see no evidence of their presence. An alien civilization that expanded to the stars would presumably use tremendous amounts of energy. Between 1775 and 2009, energy consumption in the United States alone increased by more than 450 times as it went from thirteen preindustrial colonies to the world’s preeminent economic and political power.45 Evidence of America’s unquenchable thirst for energy can be seen everywhere from the vast pits that have been dug up to extract coal to the measurable increases in global temperatures due to greenhouse gas emissions. Imagine how much more energy would be required to run an interstellar empire, and imagine then how much physical evidence it should leave. The physicist Freeman Dyson, whom we met in chapter 6, even pictured artificial power stations that could be built enclosing a star in order to harness raw stellar energy at the source—so-named “Dyson spheres.” While Dyson spheres are thought experiments, not anything ET might actually build, Dyson believed that meeting the energy needs of an advanced technological civilization would require something at this scale, and that we might look for such megastructures as we searched for intelligent life. Yet all our stargazing has found nothing of the sort—indeed, nothing clearly artificial at all.
Perhaps then the aliens are hiding—and not just hiding from us because they don’t want to interfere with our development, but because they don’t want to be found by anyone or anything. If we’re worried about potentially hostile aliens, the aliens might be worried too. Rather than call attention to themselves, extraterrestrial civilizations could learn to stay quiet and out of the way—perhaps because those races that stuck their head out met an early demise.
This is one solution to the Fermi Paradox that should send a shiver down our spines, because it asks us to wonder what might happen to us as our cosmic profile begins to rise. Humanity has been releasing radio and other signals into space for decades, and any aliens looking closely enough could notice the electric lights that burn on our planet’s nighttime side. Perhaps alien life-forms are observing us in the dark, and if we begin to demonstrate dangerous qualities—say a tendency toward relentlessly using up all the resources we can find—the cosmic cops will put a stop to us. If this sounds familiar, it’s the plot of the 1951 sci-fi classic The Day the Earth Stood Still—and the 2008 remake with Keanu Reeves.
It’s also possible that aliens are not hiding but hibernating. Anders Sandberg and Stuart Armstrong of the Future of Humanity Institute, and Milan Ćirković of the Astronomical Observatory of Belgrade,46 suggest that highly advanced alien civilizations might choose to go into a state of dormancy that could last for billions of years, all in the name of energy efficiency. As the universe expands and ages, it cools, and one side effect of cooler temperatures is that computing power becomes more efficient. (Technically the aliens wouldn’t be hibernating but aestivating, which is a dormant period that takes place during summer rather than winter.) Wait long enough to awaken—trillions of years—and the aliens might be able to get 1030 more computing per unit of power than they could wield now. So aliens exist in this scenario—we just happen to be awake while they are sleeping.
Solving the Fermi Paradox is like fan fiction for SETI devotees: weird, creative, and, because we have few ways of actually falsifying our guesses, potentially endless. But ther
e is one solution that should keep us awake at night, even if it means that we’ll never have to worry about an invasion by ET. It’s this: aliens existed once, but they’re all gone. The Great Silence means absolute silence—and that silence could be our future as well.
Absence of evidence is not evidence of absence. It’s a scientific maxim, and one relied on by SETI advocates to explain why we have yet to find proof of alien life. We’ve so far looked at thousands of star systems out of a couple hundred billion,47 which amounts to much less than 1 percent of what’s out there. “I didn’t see any gazelles in my front yard this morning when I backed out of my driveway,” Seth Shostak told me. “Maybe gazelles don’t exist, or maybe just none of them are in my driveway.”
But the longer we search and find nothing, the harder that argument is to make.
Let’s go back to the original Drake equation. The second variable is the fraction of stars that have planets. When Drake wrote his equation in 1961, scientists had no evidence of planets outside our solar system. But it turned out that such exoplanets existed—we just didn’t yet have the technology needed to see them.48 The next variable in the Drake equation is the number of planets per star system with an environment suitable for life. That’s less certain, but many of the exoplanets we’ve discovered are in an orbital range that suggests they could support life as we know it.
This puts to bed one solution to the Fermi Paradox: the Rare Earth hypothesis, the idea that planets like Earth capable of supporting life are scarce in the universe. That shouldn’t surprise us—since Copernicus displaced the Earth as the center of the solar system, advances in astronomy have shown that our beloved planet is more ordinary than we thought. That means there should be plenty of planetary space for life to arise, however, which makes the Great Silence all the more confounding.
Enter “the Great Filter.” First coined by the economist Robin Hanson, the Great Filter posits an explanation for the Fermi Paradox that is vitally important to our future on this planet. Hanson theorizes that there is, essentially, a great filter somewhere along the evolutionary path from the emergence of organic molecules on a life-supporting planet to the development of a civilization capable of leaving a mark on the stars. Whether that Great Filter falls before humanity’s current point of development or after it is, as the philosopher Phil Torres puts it, “the ultimate question for existential risk scholars.”49
Picture the Great Filter this way. Imagine that you’re a year away from your fortieth high school reunion. You look around and realize that no one from the class ahead of you—the class that would be having its own fortieth reunion this year—is still alive. What happened? If most of the deaths occurred when the alumni were 25 years old, or 35, or 45, that’s tragic—but you can take some comfort in the fact that you’ve already passed those thresholds. But if most of the deaths occurred a year ago—when those alumni would have been the same age you are now—you should be very worried, as it would mean that you’re about to hit the Great Filter of, in my case at least, Central Bucks High School East.
You’ll remember that the fourth variable in the Drake equation is the fraction of hospitable planets on which the earliest forms of life develop. This is the first possible spot for the Great Filter. It may be that abiogenesis—the process by which raw organic ingredients evolve from nonliving matter to become what we would recognize as basic microbial life—simply doesn’t happen very often. Or, following the chain of the Drake equation, it might be that basic life is common, but the billions of years of survival and evolution and sheer luck required for that life to become something we would term “intelligent” is a bar that isn’t often cleared. Remember how many times mass extinctions nearly wiped out life on Earth, and how close humanity came to extinction in the past, well before we could ever send signals to the stars. The story of Earth—from the first instances of life 3.5 billion years ago50 to space-faring Homo sapiens today—could be a serendipitous one, and one unlikely to be repeated even in a galaxy as vast and as old as the Milky Way.
If the Great Filter falls somewhere before humanity’s current level of climate changing, nuclear-powered technological development, it would be disappointing for those hoping to find other intelligences to share the universe with—but we could also breathe a sigh of relief. It would mean that there is nothing in the history of the cosmos to suggest that we face a Great Filter of our own in the future. We’ve already passed the test. We could still be annihilated by any number of existential threats outlined in this book, but our odds of survival would increase.
We don’t know yet where the Great Filter may fall, but evidence is beginning to trickle in that basic life, at least, can come into existence elsewhere. In 2018, NASA’s Curiosity rover identified a variety of organic molecules on Mars that could form the foundation of life, as well as evidence that contributions of methane—a gas that on Earth is mostly produced by living organisms—cycles seasonally in the Martian atmosphere.51 Mars isn’t the only possibility in the solar system—Saturn’s moon Titan has a chemically active and carbon-rich atmosphere, and may even host liquid water beneath its frozen icy surface.52 Where there is water, there may be life.
And that life could be hardy. A 2017 study by researchers from the Max Planck Institute for Astronomy and McMaster University argues that life sprang from meteorites and ponds only a few hundred million years after Earth had cooled enough to allow liquid water to form.53 Microbial life known as extremophiles can be found in punishing environments ranging from the minus-250-degree temperatures of submarine hydrothermal vents to the alkaline lakes of high-altitude Chile54 to the punishing undersea pressure of the Marianas Trench.55 NASA astrobiologists actually undertake field trips to the most extreme environments on Earth to study the kind of life that might be able to survive off planet. “Life may be sacred,” Shostak writes, “but it may also be commonplace.”56
If that turns out to be the case, it will be great news for astrobiologists, but a “bad omen for the future of the human race,” in the words of Nick Bostrom.57 The more proof we have that the development of life isn’t all that challenging, the more worried we should be. “If the Great Filter is ahead of us, then we’re in trouble,” Olle Häggström told me. “And when you discover that basic life is out there, it tells us that the filter is later rather than earlier.”
Worst of all would be the discovery of “necrosignatures”—evidence of extinct alien civilizations. Just as archaeologists sift through the dirt in search of vanished human civilizations, astronomers can hunt for evidence of extinct aliens. In 2015 the astronomers Adam Stevens, Duncan Forgan, and Jack O’Malley-James published a guide to searching for necrosignatures that could be observed from Earth. If a species destroyed itself with a biological weapon that killed everyone on the planet, microbes would feast on the corpses, excreting methanethiol and ethane in levels that might be detectable. Evidence of a nuclear holocaust could be discovered in the depletion of a planet’s ozone layer, or an increase in the air-glow brightness that would leave the atmosphere emitting a green radiance. If an inhabited planet had actually been destroyed—perhaps by some kind of “death star”—astronomers might be able to detect artificial compounds in the disk of debris.58
The astrophysicist Adam Frank of the University of Rochester has pondered the possibility of alien extinction through the lens of an existential threat we’re very familiar with here on Earth: climate change. Frank—who writes about his research in his excellent 2018 book, Light of the Stars—theorizes that if other intelligent species arose elsewhere in the galaxy, they may have employed energy sources that cause global warming as a side effect, just like us.59 Climate change in this view is something that universally accompanies technological development—which is bad news, because of the four potential outcomes of Frank’s models, three are catastrophic.60
In a die-off, population and temperature rise together rapidly, before the numbers peak and then crash, leaving a small remnant—too small, perhaps, to broadcast th
eir presence to the universe. In a collapse without resource change, the aliens keep using climate-change-causing resources—like the coal and oil we still burn on Earth—until their planet passes a tipping point, leading to eventual collapse and potential extinction. The same thing happens in the third outcome even if the population eventually switches to more sustainable resources, but too late to halt the momentum of catastrophic change. Only in the fourth outcome, sustainability, does the alien population switch to lower-impact resources in time, and achieve a survivable equilibrium with their planet. Climate change becomes an all-purpose solution to the Fermi Paradox—one that augurs poorly for our future here on Earth, where the heat is very much on. “What’s happening here may have happened millions of times before,” Frank told me. “If we don’t do anything about it, that’s our folly.”
We haven’t seen any astronomical evidence of alien civilizations that have been destroyed by climate change—but we also haven’t seen any evidence of civilizations that have managed to live in sustainable harmony with their environment. Climate change is as good an explanation as any, because it stands to reason that if there is a Great Filter that cuts short the life span of intelligent civilizations, the cause might be a common one, embedded in the way life uses energy and develops technologically, just as past civilizations here on Earth have tended to rise and collapse in common and even predictable ways.
It doesn’t have to be climate change, however—almost any of the existential threats we examine in this book could be the culprit. A nuclear chain reaction isn’t something that only works on Earth; a version powers our sun, and all stars. Joshua Cooper, a mathematician at the University of South Carolina, has suggested that civilizations may reach the space-faring age around the same time that they gain the ability to manipulate their own genetic code, and that they therefore may be wiped out again and again by sophisticated bioweapons of their own making.61 Just about the only man-made—or alien-made—existential risk that doesn’t fit in with the Great Filter is artificial intelligence. A superintelligent AI would presumably survive the species it had displaced and go on to “make a physical impact on the universe,” as Hanson told me—perhaps by turning the galaxy into paper clips.
