by Adam Frank
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 pr
oject 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.”