Light of the Stars Read online

  “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.

  THE GREEN BANK CONFERENCE

  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 b
e 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 mouthf
ul 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?