Alan Dunn’s cartoon of UFOs abducting New York City garbage cans, which appeared in The New Yorker in 1950.
One warm and bright summer day in 1950, four colleagues walked through the atomic weapons complex at the Los Alamos National Laboratory in the high desert of northern New Mexico. The Cold War with the Russians was in full swing, and there were new faces everywhere. Each of the men, however, was an old hand at the lab, and each played a key role in developing the bombs that helped win World War II.
First among them was Enrico Fermi, the Italian-born Nobel laureate whose brilliance had pierced the mystery of the atomic nucleus. Fermi was famous for his almost superhuman scientific abilities. C.P. Snow once wrote that if Fermi had been born just a bit earlier, he might have invented all of atomic science by himself. “If [that] sounds like hyperbole,” Snow wrote, “anything about Fermi is likely to sound like hyperbole.”1
Walking alongside Fermi was Edward Teller, the brooding Hungarian physicist whose work would become synonymous with the terrifying hydrogen bomb. While Fermi was not in favor of Teller’s push for the “super” bomb, the two men remained friends throughout their lives.2 Rounding out the group that day were American nuclear scientists Emil Jan Konopinski and Herbert York, both highly regarded researchers in their own right.
The four scientists made their way from the lab buildings to the Fuller Lodge where lunch was served (it was one of the few structures left over from the site’s earlier incarnation as a boys’ camp). As they walked, the conversation turned to unidentified flying objects.3 Since the end of the war, sightings of mysterious lights in the sky had been increasing. A recent incident had just made the local papers, reminding York of a whimsical New Yorker cartoon in which flying saucers were blamed for a rash of Manhattan garbage can disappearances. Given the physicists’ inclination for analysis, the UFO story led to a tussle of questions about faster-than-light travel and its limitations. Soon, however, the conversation wandered off to other topics as the four scientists continued along the path lined by pine trees and juniper. It was only later, in the middle of lunch, that Enrico Fermi blurted out, “But where are they?” 4
Alan Dunn’s cartoon of UFOs abducting New York City garbage cans, which appeared in The New Yorker in 1950.
Teller, York, and Konopinski all broke into laughter at Fermi’s outburst. They recognized their colleague’s sharp insight. Fermi had a habit of reducing complex problems to the barest essentials. Present at Trinity, the desert test of the first atomic bomb, Fermi had famously calculated the explosion’s power by simply dropping scraps of paper and noting how far sidewise they were swept by winds from the blast.5
But on that summer day over lunch, Fermi had identified a core question destined to haunt all subsequent discussions of intelligent life in the cosmos. Fermi’s observation was as straightforward as it was penetrating. If the evolution of extraterrestrial intelligent species was common, why didn’t we see them? Why hadn’t our telescopes found indications of their existence? Why hadn’t aliens already landed on the White House lawn?
Fermi’s question was not aimed at UFOs. That topic was, and continues to be, a morass of weak reasoning, poor observations, fakery, and conspiracy theories. Instead, his question would come to represent one of the first distinctly modern and scientifically manageable questions about extraterrestrial technological civilizations (we’ll use the term exo-civilizations).6
Over time, Fermi’s question would come to be known as Fermi’s Paradox. Its formal statement might go as follows: If technologically advanced exo-civilizations are common, then we should already have evidence of their existence either through direct or indirect means.
In the decades to come, other scientists would give Fermi’s question the precision it needed to have a scientific bite. In 1975, astrophysicist Michael Hart’s paper “An Explanation for the Absence of Extraterrestrials on Earth” addressed a number of objections to the reasoning behind Fermi’s paradox, including ones associated with physics, biology, and sociology.7 His conclusion was that none of the objections was strong enough to put off the paradox’s logic. Hart laid bare the essence of Fermi’s insight by demonstrating that just one species could “quickly” colonize the galaxy. Assuming an exo-civilization appeared that built ships capable of traveling at 10 percent of the speed of light, Hart showed that within just 650,000 years these creatures would cross the width of the galaxy. In this way, a single species could send ships in all directions, radiating outward from their home world, and quickly colonize every star system.
Of course, a few million years seems like a long time to most of us. Our species, Homo sapiens, has been around on the Earth for less than a million years. But what’s long for us is short for the life of the galaxy. The Milky Way, our home galaxy, is a vast and ancient metropolis of stars. It was born some ten billion years ago. So it would take about one ten-thousandth of the Milky Way’s age for Hart’s spacefaring civilization to cross the galaxy. Hart had demonstrated that, in a time scale that is small compared to the galaxy’s existence, just one randomly appearing interstellar civilization could reach all the planets orbiting all the stars in the sky—including our own.
For some researchers, Hart’s work filled the night with a disquieting emptiness. In their eyes, there was a straightforward logic to the Fermi Paradox that said we must be alone. The obvious absence of an alien civilization in our solar system, along with the lack of evidence for those civilizations’ existence among the stars, must mean no other form of life anywhere had reached our level of intelligence and technology. We were the sole species in the Milky Way that had made it up the ladder of evolution to build an advanced civilization. In response to the Fermi Paradox, physicist and science fiction writer David Brin spoke of the stars’ “Great Silence.” It was an apt term, capturing the cosmic loneliness that Fermi’s Paradox seemed to imply.8
Along with the Great Silence, an increasing interest in Fermi’s Paradox led to the idea of a “Great Filter.”9 The absence of evidence for advanced civilizations in the galaxy does not imply that Earth is the only life-bearing planet. The Fermi Paradox only speaks to the existence of technological civilizations like ours, or ones even more advanced. Microbes or shellfish or even dinosaurs might exist on every world in the cosmos. So if we don’t see exo-civilizations, some scientists argued, there must be a filter keeping evolution from spawning them. In other words, if we are alone in the cosmos, then some kind of evolutionary wall blocks other planets from reaching our level.
But a Great Filter might lie anywhere along that evolutionary path. Perhaps simple life is so difficult to form that it constitutes the Great Filter. In that case, Earth would be one of the few worlds with life. On the other hand, the emergence of even simple forms of intelligence might be the Great Filter. So, while lizards might appear on many worlds, dolphins and apes would not. If that were true, the difficulty in evolving intelligence filters out even those worlds where life has formed from moving further toward a technological civilization.
Ironically, at the exact historical moment that Fermi and his colleagues were sitting down to lunch, a new kind of evolutionary dead end for the Great Filter made its appearance. Fermi posed his question at a laboratory dedicated to developing weapons of unprecedented destructive energies. It was in the 1950s that humanity first gained the power to bring civilization to an abrupt and decisive end through full-scale nuclear war.
Atomic Armageddon made it possible to imagine that the Great Filter lay not in the distant evolutionary past (in which case we had been lucky to avoid it); instead, it might wait like a viper, hiding in the tall weeds of our future. Maybe the night sky was silent—and our planet unvisited—because no advanced civilization was smart enough to handle the pressure of its own existence.
If someone could have asked Fermi for his top choice for a Great Filter, he would likely have answered nuclear war. These days, however, we have a broader understanding of civilizations and their existential challenges. In the 1950s, when Fermi posed his question, there was only a small community of Earth scientists awakening to the possibility of human-driven climate change. The idea that humans could unintentionally change the behavior of the entire planet through nothing more than our collective daily activity was an idea so radical, it had barely been formulated in a scientific way. Now, however, we know better.
Earth’s passage into its human-dominated era, increasingly known as the Anthropocene, shows us a potentially more potent candidate for the Great Filter. Civilizations like our own are a complex web of interdependent systems. Where would you get your food if the electricity went out for a year? How would you heat your home if the pipelines delivering petrochemicals shut down? There are a million ways we all rely on the smooth operation of these systems. But a significant shift in Earth’s climate state would upend those systems in ways that would deeply challenge their operation.
Think about the Gulf Stream for a moment. It cycles warm water (and warm weather) up from Florida to Boston, and then out across the Atlantic. Hundreds of millions of people in some of Earth’s most technologically advanced cities rely on the mild climate delivered by the Gulf Stream. But the Gulf Stream is nothing more than a particular circulation pattern formed during a particular climate state the Earth settled into after the last ice age ended. It is not a permanent fixture of the planet. If the climate changes enough, the Gulf Stream, and the mild weather it delivers, could become a thing of the past.10
So what we call the Anthropocene may be a far more potent candidate for the Great Filter than nuclear war. An all-out nuclear exchange would, after all, be intentional. It would be someone’s decision. But it’s easy to imagine other civilizations less aggressive and warlike than ours. They might not even think to build nuclear weapons. Climate change, however, is likely to be universal. As we will see, it is likely to be a consequence of any project of advanced civilization building on any planet. Long-term dramatic climate change need not lead to a civilization-building species’ extinction. It only needs to make conditions difficult enough that their project of technological civilization is disrupted and unable to recover on the now-climate-changed planet.11
All these issues surrounding the Great Filter really illustrate the power of Fermi’s insight. Making progress in science often hinges on asking the right kind of question. Without a well-posed question, discussions become little more than people talking (or yelling) past each other. And without a well-posed question, there’s no clear path toward gathering data that will yield answers.
Finding a good question is like throwing open the shades in a dark room. It’s the first step in finding a new way to tell a story about the world because it lets us see the world in a new way. A good question reframes what we think is important. It tells us where we should be looking, where we should be going, and how to begin organizing our efforts to get there.
Fermi’s 1950 question helped play that role for the issue of exo-civilizations. As developed by Hart and others, Fermi’s Paradox asks us to consider if and why humanity might be alone in the universe.
But to truly understand the importance of Fermi’s question for our future, we need to travel back a few thousand years into our past.
THE PLURALITY OF WORLDS
The Greek philosopher Epicurus made the first expression of what we might call “exo-civilization optimism” almost 2,200 years ago: “There are infinite worlds both like and unlike our own. . . . Furthermore we must believe that in all worlds there are living creatures and plants and other things we see in this world.”12
Epicurus’s interests ranged from ethics to the nature of suffering, but first and foremost he was an atomist. The world for him was composed of an infinity of tiny components, arrayed in infinite combinations. That belief served as a foundation for the atomists’ belief that the universe must also be infinite, and thus must contain an infinite number of other inhabited planets.
Not all Greek philosophers, however, shared the atomists’ faith in a fecund cosmos. “There can not be several worlds,” wrote Aristotle in nearly the same era.13 Aristotle was an exo-civilization pessimist. For him, the Earth was the center of the entire universe. Since there can only be one center, the Earth must be unique. Aristotle was certain that no other worlds, and certainly no other worlds like Earth, existed.
The conflict between these convictions—of fecundity of the universe on one hand and the uniqueness of the Earth on the other—would echo down the next twenty centuries. From the Greeks, through the Middle Ages, to the Renaissance, and on into the early twentieth century, optimism concerning other inhabited planets waxed and waned.
From one century to the next, philosophers, physicists, theologians, and astronomers asked the same questions: Are we alone? Are we the first? Each generation posed the question using the prejudices, ideas, and tools of their time. The arguments were always fierce; sometimes they even turned deadly. In the medieval period, the Catholic Church considered discussion of other worlds to be heresy. That did not stop philosophers and theologians from struggling to understand why an infinitely powerful God would create only a single inhabited world. In the thirteenth century, Thomas Aquinas answered this dilemma by claiming God could have created other inhabited planets, but had chosen not to (a distinctly unsatisfying solution).14
By the sixteenth century, a new generation of thinkers was pushing back on the question of other worlds. Copernicus famously dethroned the Earth from the center of the universe in On the Revolution of Heavenly Spheres, first published in 1543. In his version of astronomy, radical for its time, our world was just one more planet orbiting the Sun.15 Copernicus never expressed opinions about other planets orbiting other stars. But his work removed Earth from its privileged cosmic position and opened the door for others to publicly explore what became known as “the plurality of worlds” question.
For a time, the Church tolerated some discussion of Copernican astronomy. But in the late 1500s the radical Dominican monk Giordano Bruno pushed the limits of that tolerance until it broke. Bruno not only publicly advocated for Copernican astronomy, he went further, arguing that the universe must contain infinite worlds with infinite varieties of inhabitants. These views helped earn him the attention of the Inquisition, and in 1600 the Church burnt Bruno at the stake for heresy.16
By the time the scientific revolution was in full swing, Isaac Newton had revealed powerful, unifying laws governing the motion of celestial and terrestrial objects. Astronomy was making swift progress, as new planets such as Uranus and Neptune were discovered and the orbits of comets were understood. The intellectual tumult shifted debate about life on other worlds for both scientists and an increasingly literate public. The influential French writer Bernard de Fontenelle, for example, scored the equivalent of an Enlightenment-era best seller with his 1686 book Conversations on the Plurality of Worlds.
The book was framed as a series of late-night discussions between a philosopher and a quick-minded young baroness. Expressing the optimism of his age, de Fontenelle imagined that many of the planets orbiting the Sun hosted peoples. He even thought the Moon had intelligent inhabitants. Turning his sights beyond our solar system, de Fontenelle wrote, “The fixed stars are so many Suns, every one of which gives Light to a World.” And on many of these worlds, de Fontenelle was certain that life thrived.17 One influential image from the book gives a graphic representation of de Fontenelle’s optimism. The frontispiece of an early edition shows our solar system nestled snuggly amidst a cosmos dense with other stars and other worlds.
It was an optimism that prevailed well into the nineteenth century. Darwin’s theory of evolution brought a new twist to the discussion of life and planets. Writers like Camille Flammarion, the French Carl Sagan of his day, thrilled audiences with visions of life evolving in entirely novel forms on a fertile Mars and Venus.18 Adding evolution theory to debates about the plurality of worlds gave writers like Flammarion the chance to imagine how nature shaped the inhabitants of other planets. Since evolution responded to the specific conditions on a given planet, the transformations a species undergoes must fit those conditions. In this way, Flammarion argued that life on Mars must be very similar to life on Earth, since both planets (he thought) presented similar environments.19
Illustration of our solar system surrounded by other stars and their planets from Bernard de Fontenelle’s 1686 book, Conversations on the Plurality of Worlds.
Mars would later become the focus of a very public version of optimism. At the turn of the twentieth century, American millionaire Percival Lowell founded an observatory in Flagstaff, Arizona (which had yet to achieve statehood and was still a territory), to study so-called “canals” on the Red Planet.20 Lowell was convinced that Mars was inhabited. Through books and lectures, he dedicated his final years to convincing others. His efforts were successful enough that many in the general public took it as a given that Mars was a living world.
During the latter half of the nineteenth century, however, a pessimistic pushback on exo-civilizations emerged, both from outside and within science. In 1853, William Whewell, an English scientist, philosopher, and Anglican priest, wrote a scathing critique of the optimists’ position in his book Of the Plurality of Worlds. Turning from mere hopes expressed by other writers to the astronomical facts of his day, Whewell wrote, “No planet, nor anything which can fairly be regarded as indicating the existence of a planet revolving about a star, has anywhere been discovered.”21 Whewell also argued strongly against using Earth’s history as a guide for life’s progress on other worlds. “The assumption that there is anything of the nature of a regular law or order of progress from [interstellar] material to conscious life . . . is in the highest degree precarious and unsupported.”22
Another dissenting voice came from Alfred Russel Wallace, who, along with Darwin, is considered one of the founders of evolution theory. In his 1904 book Man’s Place in the Universe, Wallace applied his own detailed understanding of biology to the question of life on other worlds. Using the availability of liquid water as a guide, Wallace concluded Earth was the only habitable solar system world. Going further, he claimed few planets in the galaxy would be earthlike enough to allow for intelligence.23
By the early twentieth century, a more determined pessimism about the existence of planets around other stars (now called “exoplanets”) took hold. It was a view that proved damning for scientific views of exo-civilizations as well. This new pessimism focused on the prevalence of planets and rested on a model for planet formation called collision theory. Theoretical studies by astronomers in the early 1900s argued that planets could only form when two stars passed in a close encounter. As the suns shot past each other in a near collision, gravity would pull some of their gas into space, leaving it to fall into orbit around one of the stars. Eventually, the extruded gas would cool and coalesce into a planet. James Jeans, the leading astronomer of his day, soon demonstrated that these kinds of stellar near misses would be exceedingly rare. Because of Jeans’s work, by the middle of the twentieth century, many astronomers believed planets were few and far between in the universe.24 That meant life would also be rare.
So, by the time Fermi and his companions sat down to lunch that day in 1950, the buoyant optimism of de Fontenelle and Flammarion had been stalled. Many scientists thought planets were rare. Even if they weren’t, biological arguments like those of Alfred Wallace could be marshaled to make life seem like an improbable event. Even worse for those who wanted to take life on other worlds seriously, Lowell’s observations of Martian canals had become a joke in the scientific community.25 In the early 1950s, the possibility of life and intelligence in the universe remained a question that few scientists were seriously considering.
But science does not exist in a vacuum. It is a human endeavor, and its story evolves with the stories laid out by the rest of a culture, even as it shapes that culture. The narratives we could tell ourselves about life in space were set to shift for the worst of reasons.
ROCKETS, BOMBS, AND SATELLITES
When Fermi blurted out his famous question in 1950, the US was still reeling from news that Russia had detonated its own atomic weapon. At that time, the total US inventory of atomic bombs numbered in the hundreds. By 1960, however, the global weapons stockpile had grown to more than twenty-two thousand.26 More importantly, the early bombs had been based on nuclear fission—the splitting of the nucleus of a heavy atom like uranium. The carnage at Hiroshima and Nagasaki had demonstrated that these “atomic” weapons could wipe out a large portion of a city in an instant. By 1960, both the US and Russia had deployed weapons based on thermonuclear fusion. These bombs were powered by slamming atoms of hydrogen, the simplest element, together to create something heavier, following the same basic process that powers stars like the Sun. The new hydrogen weapons were terrifyingly powerful.27 A medium-sized H-bomb could destroy an entire metropolitan area. The largest H-bomb could blow a small portion of the Earth’s atmosphere into space.
The race toward ever more powerful nuclear weapons defined much of the 1950s. But the bombs triggered another race during that decade, and this second technological sprint would have an even greater impact on reimagining the fate of far-flung exo-civilizations.
Building more powerful bombs meant little to nuclear weaponeers if they couldn’t be delivered to their targets more quickly than those of the enemy. In this way, the logic of the Cold War moved inexorably from the technologies of jet bombers to those of rocket-powered missiles.
In the final years of World War II, Nazi V-2 guided missiles had terrorized Britain and proven the power of long-range rockets. After the war, both the Russians and the US snapped up captured German V-2 scientists, and each nation vigorously pursued the development of continent-crossing rockets called intercontinental ballistic missiles (ICBMs). The Russians proved faster and more nimble in their development. On August 21, 1957, a Soviet R-7 missile blasted across 3,700 miles, reaching an altitude of ten miles.28
The true power of these rockets became apparent two months later, when the world woke up to find we’d acquired a second moon. On October 4, 1957, another Russian R-7 rocket punched the 184-pound Sputnik above the Earth’s atmosphere and into orbit, where it became the Earth’s first artificial satellite. Wheeling a few hundred miles overhead, Sputnik broadcast perfectly timed radio “beeps” for anyone with the right equipment to hear.29 And the world was listening. While Russian politicians gloated and Americans panicked, it was clear that an ancient threshold had been breached. Humanity’s space age had begun.
There was, however, only one way to talk to a hypersonic rocket in the atmosphere or a satellite orbiting high above the planet. Communications at these ranges required the use of sophisticated radio technology. And it was exactly in those technologies that the political and military urgencies of the 1950s dovetailed with the first scientific effort to detect alien intelligence.
Until the 1950s, astronomy was carried out with telescopes fashioned with glass lenses and mirrors. That meant astronomy was done only with visible light—the kind our eyes had evolved to perceive. But visible light is nothing more than waves of electromagnetic energy with wavelengths that fall within a certain range. (Wavelength is the distance between the peaks in a wave.)
In the mid-1800s, physicists discovered there was an entire spectrum of electromagnetic waves. These waves stretch from very short, atomic-scale X-rays and gamma rays all the way to radio waves the size of buildings. Astronomical objects tend to emit energy across a large fraction of this electromagnetic spectrum.
Evolution tuned our eyes to see electromagnetic waves only in the visible “band” of the spectrum. It’s no coincidence that this visible band happens to be where the atmosphere is most transparent to sunlight. But the Sun also produces X-ray “light,” ultraviolet “light,” and radio “light.”
Buoyed by the advances in radio engineering during World War II, astronomers in the 1950s began opening their first new “window” on the night sky by using light outside the spectrum’s visible band. With radio waves, researchers found they could map out the entire galaxy or capture the echo of long-dead stars in ways that were impossible using visible light.
Radio astronomy, as it was called, constituted one of the most exciting frontiers in science as the 1950s progressed. If you were young, gifted, and scientifically ambitious, radio astronomy was the place to be. That was how, at the end of the decade, a newly minted astronomer named Frank Drake found himself in the wilds of West Virginia, searching for signals of alien civilizations.
LISTENING TO THE SKY
Frank Drake had always been a gearhead with vision. The man who would help define much of the modern science around exo-civilizations was born in 1930 on the south side of Chicago, just as the Great Depression began. His father, a chemical engineer for the city, often brought home gadgets for his son that ended up in the boy’s basement “lab.” The young Drake spent hours in that basement, playing with motors, radios, and chemistry sets. But it was the frequent bike trips to the city’s Museum of Science and Industry that took Drake’s imagination beyond the details of his radios. There, he and a friend found full-scale models of atoms that made the invisible real. “Some of the exhibits were so dramatic, it would almost knock you to the floor,” Drake later wrote.30
When Drake was just eight years old, his father told him there were other worlds “just like Earth.” The idea gave him a vision of other life and other planets that never faded. The Oz stories were also a favorite of young Drake. As a child, he owned many of these books about another world. Author L. Frank Baum had written thirteen volumes beyond the first one, The Wonderful Wizard of Oz, many of them featuring Princess Ozma, the ruler of Oz.31
The boy grew into a tall, handsome young man with an affinity for science that landed him at Cornell University with a Reserve Officers’ Training Corps scholarship. While Drake didn’t begin his undergraduate work with a specific interest in astronomy, he soon found himself drawn to the subject. And throughout his introductory astrophysics courses, he never lost his fascination with the question his father introduced to him as a boy: Are there other inhabited worlds in the universe? But it was not a question he was willing to pose to his professors, for fear of sounding like a fool. That reticence would fade through a chance encounter with Otto Struve, one of the world’s most famous astrophysicists.
Struve was a large, intimidating man who was a leader in the study of stars. In 1951, he was invited to present a lecture to the Cornell community, and Drake was in attendance. The lecture focused on what was known about how stars formed from clouds of interstellar gas. As he neared the end of his talk, the imposing Russian-American pivoted to the topic of life in its cosmic context. He claimed there was mounting evidence that at least half of the stars in the galaxy had their own planetary systems. The old collision theory of planet formation was falling from favor, and Struve said there was no reason why life couldn’t exist on some of those planets.32 A light went on in Frank Drake’s head. Here was someone older and established, asking the same question he’d been fascinated by since he was a boy.
Struve’s inspiration was still alive in Drake in the spring of 1958 as he piloted an old white Ford, stuffed with all his belongings, through the backwoods of rural West Virginia. He was on his way to the newly minted National Radio Astronomical Observatory’s Green Bank facility. There, he was to become a member of the observatory’s fledgling scientific staff.
The research engines of the Cold War were churning, and funding had been opened for any project that could push American capabilities forward. In Drake’s words, Green Bank “had been given what amounted to unlimited funds to build the best radio observatory in the world.”33 Nestled in a remote, verdant valley valued for its radio (and actual) isolation, Green Band was the new home of American radio astronomy.
Soon after Drake’s arrival, the towering metalwork of an eighty-five-foot radio dish was completed. The astronomers at Green Bank planned to use the newly commissioned telescope to study everything from the structure of our galaxy’s pinwheel shape to its hidden center.34 Drake would be part of many of these efforts. But the inhabited worlds in Drake’s imagination wouldn’t leave him alone. It wasn’t long before he was thinking of ways to use this giant radio ear to find them.
“I calculated just how far our new 85-foot telescope could detect radio signals from another world if they were equal to the strongest signals [on earth],” Drake later wrote.35 The answer turned out to be about ten light-years, or sixty trillion miles. Since he believed stars like the Sun had the best chance of hosting a world like Earth, his next step was to check the star charts. Luckily, there were at least few sunlike stars within ten light-years.36 Drake saw he had the beginnings of a real research project.
After his initial calculation, Drake needed to get his colleagues at the observatory to buy into something as seemingly crazy as a search for alien civilizations. The scientists who lived at Green Bank often ate together at a roadside diner a few miles away. Over lunch there one winter day, Drake made his pitch to use the telescope to search for signs of intelligent life on other worlds.
Frank Drake and the early telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia, in 1964.
“At the time, the director of the National Radio Astronomy Observatory was Lloyd Berkner, [who was] something of a scientific gambler, and he was all for it. So as the last greasy french fry was washed down by the last drop of Coke, Project Ozma was born.”
“Project Ozma.” True to the exuberance of his childhood dreams, Drake named his search after the princess of the Emerald City. With the blessings of the observatory administration, the team began building the equipment needed to carry out Project Ozma. By the spring of 1960, the amplifiers, filters, and other radio engineering gear were ready.37
For six hours each day that year, from April to July, Drake aimed the telescope at one of two target stars. The first was Tau Ceti in the constellation Cetus (the Whale). The second was Epsilon Eridani in the constellation Eridanus (the River).38
He later wrote of remembering “the battle against the cold each morning as I would climb to the focus of the dish. . . . And then of that moment on the first day of the search when a strong, pulsed signal came booming into the telescope just as soon as we had turned it towards Epsilon Eridani.”39
The heart-pounding excitement of the “booming signal” turned out to be a false alarm. That source turned out to be man-made. It was, in fact, just about the only time Drake thought they’d detected another civilization. Project Ozma never captured any alien signals, but it did capture something else of great importance: the world’s imagination.40 Just ten years after Fermi had asked his question among a small group of friends, at least some in the scientific community were ready to take the question of exo-civilizations seriously.
As Drake was working out the details of his search at Green Bank, two physicists named Giuseppe Cocconi and Philip Morrison published a groundbreaking study titled “Searching for Interstellar Signals.” The paper appeared in a 1959 issue of Nature, one of the most prestigious journals in science. The two physicists argued that the best way to look for signals from advanced exo-civilizations was by using radio astronomy. Dust blocks visible light, making the Milky Way seem blotchy to our eyes. But radio “light” has long wavelengths that pass unobstructed through dusty regions of the galaxy. So, with radio waves, the galaxy becomes transparent, allowing astronomers to “see” from one end to the other. This meant a civilization emitting radio waves could be seen at far greater distances than one emitting visible light signals.41
Drake had already reached the same conclusion. But the publication of Cocconi and Morrison’s paper meant others were thinking exactly along his lines. It was a development that worried the new director of Green Bank, who was none other than Drake’s inspiration, Otto Struve. Until then, Drake had been keeping a tight lid on his search. Struve, however, feared getting scooped. Within a few weeks, Struve used an invited lecture at MIT as an opportunity to reveal Project Ozma’s existence to the world.42
Soon, Drake was hosting a steady stream of visitors. Award-winning journalists, theologians, and leading businessmen made the trek to Green Bank. Project Ozma, along with the publication of Cocconi and Morrison’s paper, marked a turning point in the way science engaged with the issue of alien civilizations. By 1960, humanity was dogged by questions of its own imminent destruction on one hand, while it watched the space age dawn, offering fresh possibility, on the other. These two technological developments were reshaping politics and culture, and they served as a kind of imaginative ether, launching the first true scientific search for other civilizations.
With Project Ozma, a specific scientific question about exo-civilizations had finally been posed in a way that could be explored using a specific set of appropriate scientific tools. As this crucial threshold was crossed, exo-civilizations rose for the first time from the purely speculative realm of science fiction. One year later, the young Frank Drake would see the consequences of this work become manifest in a fateful call from Washington, D.C.
J. Peter Pearman was a staff officer of British origin at the National Academy of Sciences. In the summer of 1961, he called Drake with a remarkable request. Pearman was part of the Academy’s Space Science Board, and he wanted Drake to host a meeting exploring the research possibilities for “extraterrestrial communications.” Drake had spent the year after Project Ozma nervously wondering which of his colleagues might be snickering behind his back. He agreed immediately to run the meeting.43
The discussion then turned to invitations. Drake was happy to discover from Pearman that not only were other scientists taking up the question of extraterrestrial life, but there were two government-sponsored committees already exploring the problem. Together, they drew up a list of ten scientists for the meeting.
First, there would be Cocconi and Morrison, the authors of the Nature paper. Drake then suggested Dana Atchley, a radio engineer who’d donated a key piece of equipment for Project Ozma. Barney Oliver, a Hewlett-Packard “research magnate” who’d visited Drake during Ozma, was also included. As a leading astronomer and head of Green Bank, Otto Struve was asked to serve as the meeting’s chairperson. Struve then asked that his former student Su-Shu Huang join the group. For expertise in the chemistry of life, the pair chose Melvin Calvin, a Berkeley scientist who discovered the chemical pathways of photosynthesis that allow plants to turn sunlight into food. Rumors were flying that the next Nobel Prize in chemistry would have Calvin’s name on it.
Running over their list, Drake joked, “We’ve got astrophysicists, astronomers, electronics inventors, and exobiology experts. All we need now is someone who’s actually spoken to an extraterrestrial.” 44 Without missing a beat, Pearman, in his perfect Oxford accent, told Drake he had exactly that. John C. Lilly was a biologist who had become famous for his work with dolphins. Lilly claimed his research demonstrated that dolphins were as intelligent as people. Lilly also believed they possessed a sophisticated form of language that he could decipher. Drake agreed that Lilly should be on the list.
There was one more scientist Pearlman and Drake wanted to invite. He was younger than all the other invitees, but his name, like Drake’s, would shape the future of astrobiology. In the summer of 1961, Carl Sagan was newly minted PhD with a fellowship at Berkeley. There, he’d been working with Calvin, developing laboratory experiments on the formation of life. Though only twenty-seven, Sagan had already made a name for himself as both brilliant and brash.45
The meeting was scheduled for October 31, 1961. Invitations were sent out, and Drake and Pearman were soon delighted to find that almost all were accepted. Only Cocconi declined (he would never engage in astrobiological research again). But as the meeting approached, a conflict appeared. The group had gotten word that Calvin was going to get his Nobel Prize in chemistry, and the announcement would come during the three days of the Green Bank meeting. Calvin was more than willing to take the call from Sweden at Green Bank, but Pearman and Drake knew some champagne would be needed for a celebration. Procuring bubbly, however, posed its own kind of challenge.
“[Getting champagne was] no mean feat in the semidry state of West Virginia,” Drake later recalled. “West Virginia apportioned one state-operated liquor store to each county. The one closest to the observatory stood in a little lumber town called Cass, about ten miles away. The observatory’s staff now included a driver—a West Virginian with the fairly common (for those parts) first name of French, and the improbable surname of Beverage. For a moment I considered sending him to buy the champagne, but it would have been too silly. Instead, I drove over to Cass myself that weekend.” 46
Drake purchased a case of champagne and made his way back to Green Bank.
With the invitations complete and the champagne hidden away, the only thing left for Frank Drake to do was to set an agenda. “I sat down and thought, ‘What do we need to know about to discover life in space?’ ” 47
Drake simply wanted a way to organize the discussion, but the path he chose had consequences far beyond the Green Bank conference. Though Drake could not have known it at the time, his idea would establish an organizing principle for the entire future of astrobiological science.
Since the purpose of the meeting was to explore possibilities for communication with exo-civilizations, Drake understood that the first and most important question would be how many exo-civilizations there were to communicate with. That translated into a single, specific question the meeting needed to answer: What is the number of technologically advanced civilizations in the galaxy that can emit radio signals detectable on Earth?
The galaxy contains about four hundred billion stars.48 If the number of technological civilizations (call the number N) turned out to be small, then the search for exo-civilization would be unlikely to succeed. There would be just too many stars to search and too few inhabited systems to find. But if N were large (in the billions, perhaps), then astronomers wouldn’t have to search many stars before an exo-civilization popped up.
So, what Drake needed was a way to estimate the value of N. To accomplish this, he broke the problem up into seven pieces. Each piece represented a distinct subproblem the scientists at the meeting could discuss in detail. Most importantly, each piece could be expressed as a factor in an equation for the number of galactic exo-civilizations—the all-important quantity N.
Let’s run down the seven pieces of Drake’s equation and his exo-civilization question.
1. The Birth Rate of Stars
Based on our own experience here on Earth, life will form on planets. Of course, it is perfectly reasonable to ask whether life can bypass planets by forming in something like an interstellar cloud (astronomer Fred Hoyle assumed this in his famous sci-fi story The Black Cloud).49 Given what we do understand about the mechanisms of life, however, it’s far more likely that a solid planetary surface with lots of liquid water and other chemicals is a requirement to get biology going. Assuming a focus on planets brings us straight to a focus on stars. If we want to know how many planets host exo-civilizations in the galaxy, we first have to know how many planets exist, and that means we first have to know how many stars exist.50 So Drake’s equation begins with the number of stars created in the galaxy each year. Astronomers represent this by the symbol N* (read as “N sub star”).
2. The Fraction of Stars with Planets
Once we know the number of stars forming per year, we can then ask how often planets get created around these stars. Is planet formation a very rare occurrence, or a common one? As we saw in our brief tour of history, this is an ancient question. And by the middle of the twentieth century, planet formation had once again become the subject of intense astronomical debate.
Drake expressed this question in terms of fractions. What, he asked, is the fraction of stars that host a planet? He wrote this term as the symbol fp (read as “f sub p”).
3. The Number of Planets in “The Goldilocks Zone”
It is not enough to just ask if a star hosts a planet. The planet’s orbit around the star is also a key factor in thinking about life, intelligence, and civilizations. If a planet is very close to its star, then the temperature on its surface will be so high that life gets fried down to its atoms. If, on the other hand, a planet’s orbit is very large, its surface will be perpetually frozen and in near darkness.
At the time of the Green Bank meeting, Otto Struve’s former student Su-Shu Huang had just finished work that showed how each star is surrounded by a “habitable zone of orbits.” Huang defined this zone as the band of orbits where liquid water can exist on a planet’s surface.51 Liquid water is thought to be a key factor in allowing life to form and thrive. The inner edge of Huang’s habitable zone was the orbit where a planet’s temperature was just cool enough to keep surface water from boiling. The outer edge was the orbit where the temperature was just high enough to keep water on a planet’s surface from freezing.
Drake and his colleagues at the Green Bank meeting needed to know how many planets (for those stars that had planets) were in the habitable zone. In other words, how many planets were on orbits that left their surfaces neither too hot nor too cold. Thus, the third variable in Drake’s equation would be the average number of planets in a star’s habitable zone, which is also sometimes called the “Goldilocks zone.” This term is expressed as np (read as “n sub p”).
4. The Fraction of Planets Where Life Forms
While the first three terms in Drake’s equation dealt purely with issues of physics and astronomy, the fourth brings chemistry and biology into the discussion. Given a star with a planet in an orbit that leaves it with liquid water on its surface, what are the odds that the simplest forms of life will appear? Once again, Drake expressed this question in terms of a fraction, which he called fl (read as “f sub l”).
It’s worth noting that discussions about fl hinge on the chemical pathways taking nonliving matter into a self-replicating state. The formation of life from nonlife is called abiogenesis. Experiments done by Harold Miller at the University of Chicago in the early 1950s had already provided compelling evidence that abiogenesis might not be difficult to obtain on a habitable-zone planet.52
5. The Fraction of Planets Where Intelligence Evolves
The fifth term moves us from the biochemistry of life’s origin into the dynamics of its changing forms. Assuming life begins on a planet, how often would evolution carry that life forward to intelligence? Drake expressed the fraction of planets where intelligence evolves with a term called fi (read as “f sub i”).
6. The Fraction of Planets with a Technological Civilization
The sixth term moves us from evolutionary biology to sociology. Given that a planet hosts an intelligent species, how often does a technologically advanced civilization then arise? This question was represented by the term fc (read as “f sub c”), the fraction of planets where a technological civilization begins.
For practical purposes, Drake saw “technologically advanced” as meaning a civilization with the capacity to broadcast radio signals.53 So, while the Romans were certainly a civilization, from Drake’s point of view, they don’t count as a technologically advanced one.54
7. The Average Lifetime of a Technological Civilization
The final factor in Drake’s equation is the most haunting: How long does a civilization like our own last? Can we expect another few centuries before our global society flares out, or are there many millennia of development ahead? Assuming that technological civilizations have occurred often enough for an average to be well defined, what is their average lifetime?
With this last term (written as L), Drake was asking the others at the meeting to consider alien sociology on a deeper level. Some discussion was devoted to the overconsumption of resources, but given the heightened fears of nuclear war in 1961, aggression was the focus of Drake’s final variable.55 Are most civilizations as aggressive and warlike as our own? Do they become more peaceful as they evolve? How long, on average, can they last without destroying themselves?
ONE EQUATION TO BIND THEM
Each of the seven terms Drake chose to set the agenda for his Green Bank meeting was a problem that, in principle, had a quantifiable answer. Each contained its own compelling mysteries, and each was a step on a ladder to that overarching question: Are we alone?
To be specific, though—and the whole point of the meeting was to be specific—Drake’s overarching question was: How many radio signal producing technological civilizations other than our own reside in the Milky Way galaxy? In the language of Drake’s agenda, what is the value of N?
With all his subproblems mapped out, Drake was finally in a position to put them together into a single equation. Here it is, written out in mathematical form:
N = N*fpnpflfifcL
In words, Drake’s equation says the number of exo-civilizations from which we can get signals equals the number of stars forming each year (N*), times the fraction of those stars with planets (fp), times the number of planets where life can form (np), times the fraction of planets where life actually does form (fl), times the fraction of those planets that evolve intelligence (fi), times the fraction of those intelligences that go on to create technological civilizations (fc), times the average lifespan of those civilizations (L).
Here, you can see why scientists like equations so much. An idea that takes a mouthful of words to express gets captured pretty cleanly in just one short line of symbols.
On the morning of November 1, 1961, with the participants at Green Bank meeting gathered around the conference table, Drake stood and wrote his new equation on the blackboard. Scrawled in chalk like a haiku, it was never intended to be anything more than a guide, an overview, an organizing principle.
It turned out to be much more.
A search of “Drake equation” on the Google Scholar search engine returns thousands of papers. A similar search of Amazon brings back scholarly books, science fiction novels, T-shirts, and even a tungsten carbide ring imprinted with the formula. Since the Drake equation was introduced, it has appeared in a stunning number of scientific conferences, magazine articles, and documentaries.
“It amazes me to this day,” Drake wrote later, “to see [the equation] displayed prominently in most textbooks on astronomy, often in a big, important-looking box.” With humility, Drake added, “I’m always surprised to find it viewed as one of the great icons of science because it didn’t take any deep intellectual effort or insight on my part. But then as now it expressed a big idea in a form that . . . even a beginner could assimilate.”56
In considering the importance of the Drake equation, you have to begin with what it is not. It is not a law of physics. Einstein’s famous equation E = mc2 expresses a fundamental truth about the behavior of the world. It is a statement of our understanding about how nature works on its own. The Drake equation, on the other hand, is really a statement of our lack of understanding. It tells us what we would need to know to get a specific answer to a specific question: How many exo-civilizations are out there?
Before Drake, the scientific consideration of exo-civilizations was unfocused. What existed was a mix of unconnected musings in scientific journals, books, and popular articles. There was no structure for building a coherent program of study, either through theory or observations. By breaking the big question into seven smaller questions, Drake crafted a useful way to think about the problem that also left scientists something they could work on. It gave them something to do.
Each of the terms in the equation could be explored on its own, using whatever means were available. Astronomers could work on the first three terms; biologists could think about the two that followed; sociologists and anthropologists could explore the last two. Of course, most of the work would be speculative. But at least it would be speculation with a focus and a scientific foundation.
With time and patience, advances were made from all sorts of directions. Computer studies of chemical reactions provided insights into abiogenesis. Evolutionary studies of life on Earth showed how cognitive patterns leading to intelligence first appeared. And while some terms, like the average age of a civilization, might never be known, others, like the fraction of stars with planets, were thought to be within grasp at the time of the Green Bank meeting. In addition, while even the closest stars were fifty trillion miles away, the planets in our solar system were relatively close. If we could find even one example of life—in its simplest form—on Mars or anywhere else in our solar system, that would tell us something powerful about the first biology term.
What Drake’s equation gave astrobiology was a way to think about itself. In the process, it changed how we understood life, civilization, and ourselves.
Drake’s equation also ensured the success of the Green Bank meeting. Beginning with the rate of star formation and marching all the way through to the average lifetime of technological civilizations, the nine participants did their best to make informed estimates of the different terms. History shows they were a hopeful group. They assigned values relatively close to one for all the fractions. Most telling, though, they reserved their pessimism for Drake’s final factor: the average lifespan of technological civilizations.
The capacity for a civilization to short-circuit its own evolution through self-destruction vexed the meeting’s participants. It would go on to become a bottleneck in all thinking about searches for extraterrestrial intelligence (SETI). The Green Bank participants believed, as Drake later wrote, that “the lifetimes of civilizations would either be very short—less than a thousand years—or extremely long—in excess of perhaps hundreds of millions of years.”57
In the end, the group agreed that the final factor was what mattered most. The number of stars was so vast that the galaxy could absorb a lot of what Drake and his colleagues considered pessimism regarding the other terms of the equation. But the galaxy also needed to be populated now. There needed to be an overlap in time between our civilization and theirs so that there would be signals for us to receive. That meant the other civilizations needed to last at least millions of years, which seemed like a stretch for the Green Bank group.
Just before the meeting ended, Drake and his colleagues broke out their one remaining bottle of champagne (Calvin’s call from the Nobel Committee had come late on the first night of the meeting). As they raised their glasses, Otto Struve offered a toast. “To the value of L,” said Struve. “May it prove to be a very large number.”58
THE DAY CLIMATE CHANGED
In 1965, little more than three years after Struve’s toast, President Lyndon Johnson would raise the same issue of civilizations’ longevity in the far more specific context of our own fate. Speaking before a joint session of Congress, he said, “[T]his generation has altered the composition of the atmosphere on a global scale through . . . a steady increase in carbon dioxide from the burning of fossil fuels.”59
It’s remarkable to note that, more than fifty years ago, an American president was already aware of, and acknowledging, human-created climate change. Johnson had been briefed on the dangers of CO2 increases by the famous climate scientists Charles Keeling and Roger Revelle, among others. So, not only was President Johnson aware of the issue, but he was already concerned enough to raise it before Congress. That single sentence in his address gives the lie to the claims of so many climate-change deniers that global warming is some kind of recent hoax. Indeed, the scientific understanding of our effect on the Earth dates back more than a century. As President Johnson’s speech demonstrates, even fifty years ago, that understanding was firm enough to gain notice at the highest levels of policy and politics.
But there is a difference between a community of scientists, at the vanguard of their fields, glimpsing human-driven climate change and the culture as a whole metabolizing the story. A single speech by a president can’t create the kind of intimacy that is the hallmark of humanity’s most powerful narratives about itself and its place in the world. That takes time and the play of events. The industrial revolution, for example, didn’t arrive as soon as the first factory was built. It took people moving en masse from farms to cities where day-to-day life took on new rhythms and textures. Only then did we begin to see ourselves as “industrial.” Only then could we tell new stories about ourselves as a civilization that conquered the planet with steel, rubber, and oil.
Likewise, we are just beginning our entry into the Anthropocene. Fifty years on from President Johnson’s speech, we have just started becoming familiar with images of melting glaciers, massive heat waves, and flooded cities. We are just beginning to experience what life on a climate-changed world looks like. But when President Johnson stood before Congress in 1965, that story was still new.
Conservation was the intended theme of the president’s speech that day. Only a few years had passed since biologist Rachel Carson had raised alarms over the environmental effects of pesticides in Silent Spring. Even less time had elapsed since the treaty banning atmospheric testing of nuclear weapons had gone into effect. While the Cold War made instant annihilation a credible threat in the 1950s, by the mid-1960s some were beginning to realize that even the everyday activities of our project of civilization were not, in total, going unnoticed by the planet.
Sustainability on a global scale, however, is a very different kind of story for humanity to tell itself. It demands a vastly enlarged imaginative palette. At the time of President Johnson’s speech, the picture of a climate-threatened future was just starting to be painted by scientists as they gained a first foothold on understanding the Earth as a planet. These researchers were recognizing, for the first time, that Earth needed to be understood in its entirety as single, tightly coupled system—a kind of vast, planetary-scale machine.
Ironically, and as is so often the case, the need for this new vision found its first urgency in the needs of warfare. With the rise of long-range bombers and intercontinental missiles, cold warriors were busy imagining Earth from well above the atmosphere. But they were also deeply concerned with how weather could tip the scales of battle. It was partly at their urging that resources poured into the scientific study of climate. A nuclear-powered laboratory was built under the ice of Greenland to understand how weather patterns changed over the course of millennia. Instrument-laden ships crisscrossed the oceans, studying the forces driving deep ocean currents. Most importantly, the same ICBMs threatening nuclear war were starting to lift scientific satellites into orbit, where their eyes would point downward to study the Earth.
These were expensive and globally extensive efforts. They laid the groundwork for a new vision of our project of civilization’s planetary context and impact.
In 1960, a still-wet-behind-the-ears NASA launched its first successful weather satellite, TIROS (Television Infrared Observation Satellite). By 1962, TIROS was offering continuous coverage of the Earth’s weather.60 In the wake of TIROS, no longer would a hurricane unleash its violence on an unsuspecting population. And for the first time, people were treated to images of Earth as a globe suspended in space. Even the earliest grainy videos showed the elegant arc of the world’s horizon as seen from high above the atmosphere, a vision that would rewire our collective imaginations.
By the mid-1960s, a convergence had begun. Images from TIROS, President Johnson’s address on carbon dioxide, Fermi’s lunchtime insight, and Drake’s Green Bank conference were pieces in a cultural jigsaw puzzle that was beginning to assemble itself. Each represented a tentative first step toward seeing our project of civilization in a new light—the light of the stars. Fermi and Drake represented a new awareness among scientists that the story of our own project of civilization must be set onto a cosmic stage, with all its stars, planets, and possibilities. Meanwhile, studies of climate funded via Cold War urgencies shaped an awakening among other scientists that Earth’s story must be told in terms of a mighty planetary system driven by sunlight and shaped by life—including our own. Finally, President Johnson’s address signaled that our civilization’s impact on the planet was making its way into the domains of culture and politics.
A new human story, a new human mythology, was emerging. The outlines of this narrative, in which human beings and our project would be inescapably bound to the machinery of planetary evolution, were beginning to take shape. Few at the time could recognize the power, the peril, and the promise growing in this new story. It was still too new and too unformed. To take the next steps in forging this new vision, we would have to leave home. We would have to become wayfarers and journey, for the first time in our long history, out to the high frontier of space. That was where the sibling worlds of our solar system were waiting to tell us their secrets.
