This piece is adapted from Five Billion Years of Solitude: The Search for Life Among the Stars by Lee Billings.
Recent headlines about alien life in our planet’s atmosphere have been shot down to earth. But scientists are still looking for messages from and signs of other civilizations among faraway stars—carrying on the tradition that started more than 50 years ago, when some of the era’s most brilliant scientific minds got together to discuss the astronomical possibilities.
In 1961, J.P.T. Pearman of the National Academy of Sciences approached astronomer Frank Drake to help convene a small, informal SETI conference at the National Radio Astronomy Observatory’s Green Bank observatory. The core purpose of the meeting, Pearman explained, was to quantify whether SETI had any reasonable chance of successfully detecting civilizations around other stars.
Besides Drake and Pearman, three Nobel laureates attended. The other attendees were only slightly less celebrated. They included physicist Philip Morrison, who had co-authored a 1959 paper advocating a SETI program just like the one Drake undertook in 1960. A dark-haired and brilliant 27-year-old astronomy postdoc named Carl Sagan was, at the time, the youngest and arguably least distinguished name on the guest list.
A few days before the conference began, Drake tried to categorize the key pieces of information needed to estimate the number, N, of detectable advanced civilizations that might currently exist in our galaxy. Drake reasoned that the average rate of star formation in the Milky Way, R, placed a rough upper limit on the creation of new cradles for cosmic civilizations. Some fraction of those stars, fp, would actually form planets, and some number of those planets, ne, would be suitable for life. Some fraction of those habitable planets, fl, would actually blossom into living worlds, and some fraction of those living worlds, fi, would give birth to intelligent, conscious beings. The fraction of intelligent extraterrestrials that developed technologies that could communicate their existence across interstellar distances was fc, and the average longevity of a technological society was L.
When the conference opened on the morning of Nov. 1, 1961, after the guests were seated and sipping coffee, Drake wrote on a chalkboard: N = R fp ne fl fi fc L
That string of letters has come to be known as the “Drake equation.” Though Drake had intended it only to guide the next three days of the Green Bank meeting, the equation and its plausible values would come to dominate all subsequent SETI discussions and searches. At the meeting, it was the two final and most nebulous terms of Drake’s equation: fc, the fraction of intelligent creatures who would develop societies and technologies capable of interstellar communication, and L, the average longevity of an advanced technological civilization, that caused the most vigorous philosophical debate.
According to Morrison, history suggested that the emergence of technological societies might be a convergent phenomenon. The early civilizations of China, the Middle East, and the Americas all arose independently and generally followed similar lines of development. And yet their paths had ultimately diverged—the drivers of social change and technological progress were not at all clear. Despite China’s development of technologies such as gunpowder, compasses, paper, and the printing press hundreds of years before Europeans did, China experienced nothing equivalent to the colonization of the New World, the European Renaissance, and the successive scientific and industrial revolutions. Whether sending ships across oceans or messages among the stars, a society's ability to explore and expand appeared to be a matter not only of technological prowess, but also of choice. Whether any particular technological culture would ever attempt something as wild as interstellar communication seemed unpredictable.
The Green Bank attendees eventually guessed that between one-fifth and one-tenth of intelligent species would develop the capabilities and intentions to search for and signal other cosmic civilizations. That left only L, the typical lifetime of technological civilizations, for the group to consider.
Drake suspected that what really controlled the number of technological civilizations living in the cosmos was almost solely their longevity. The thought made Morrison shudder. He had worked on the Manhattan Project during World War II and had witnessed the detonation of the first atomic bomb. Humans had developed a global society, radio telescopes, and interplanetary rockets at roughly the same time as weapons of mass destruction. Perhaps all societies would proceed on similar trajectories, becoming visible to the wider cosmos at roughly the same moment they gained an ability to destroy themselves. In fact, Morrison estimated, if the average civilization endured only a decade before passing into oblivion, at any time there would most likely be only one communicative planetary system throughout the galaxy. One of the most compelling reasons to search for evidence of extraterrestrial civilizations, Morrison thought, would be to learn whether our own had a prayer of surviving its current technological adolescence.
Sagan attempted to counter the doomsaying, noting that we could not rule out some technological civilizations achieving global stability and prosperity either before or even after developing weapons of mass destruction. They might master their planetary environment and move on to exploit resources in the rest of their planetary system. Such a society could, in theory, persist for geological timescales of hundreds of millions or even billions of years, potentially lasting as long as its host star continued to shine. And if, somehow, that civilization managed to escape its dying sun and colonize other planetary systems ... well, perhaps then it would endure practically forever. Of all the attendees, Sagan was by far the most optimistic that technological civilizations could solve not only their many planetary problems, but also the manifold difficulties associated with interstellar travel. Somewhere out there, Sagan suggested, immortals passed their unending days amid the stars.
Drake’s best guess in 1961 walked the line between Morrison and Sagan: He speculated that L might be about 10,000 years, and that consequently perhaps 10,000 technological civilizations were scattered throughout the Milky Way along with our own. It was probably not coincidental that Drake’s personal estimate rendered the successful detection of alien civilizations still quite difficult but not entirely beyond our capabilities: By his reckoning, only 10 million stars would need to be monitored to obtain an eventual detection, though the search could take decades, even centuries.
At the conference’s end, as the guests drank Champagne, Otto Struve, the director of Green Bank observatory, offered up a toast: “To the value of L. May it prove to be a very large number.”
Adapted from Five Billion Years of Solitude: The Search for Life Among the Stars by Lee Billings with permission of Current, a member of Penguin Group (USA) LLC, A Penguin Random House Company. Copyright © Lee Billings, 2013. Five Billion Years of Solitude is available Oct. 3.
More from Slate’s series on the future of exploration: Is the ocean the real final frontier, or is manned sea exploration dead? Why are the best meteorites found in Antarctica? Can humans reproduce on interstellar journeys? Why are we still looking for Atlantis? Why do we celebrate the discovery of new species but keep destroying their homes? Who will win the race to claim the melting Arctic—conservationists or profiteers? Why don’t travelers ditch Yelp and Google in favor of wandering? What can exploring Google’s Ngram Viewer teach us about history?