ARE WE ALONE? the drake equation AND THE ODDS OF INTELLIGENT LIFE BEYOUND EARTH
I was a student at the University of Virginia when I first encountered what some call the Green Bank Equation — more formally known as the Drake Equation. The course, Life Beyond Earth, examined two questions: Are we alone in the universe? And can we build colonies in space? The second was inspired by the work of Dr. Gerard K. O'Neill at Princeton University, published as The High Frontier: Human Colonies in Space, and will be the subject of a future newsletter. This newsletter focuses on the first question: Are we alone?
Green Bank, 1961
On November 1, 1961, roughly a dozen of the most distinguished scientists in America gathered at the National Radio Astronomy Observatory in Green Bank, West Virginia, for a meeting that helped shape the scientific search for extraterrestrial intelligence. J. Peter Pearman of the National Academy of Sciences’ Space Science Board had asked astronomer Frank Drake to convene a serious discussion about whether SETI, the Search for Extraterrestrial Intelligence, had any reasonable scientific basis for optimism. To focus the conversation, Drake wrote a simple equation on the blackboard.
The attendees carried exceptional credentials. Biochemist Melvin Calvin received a phone call during the meeting informing him he had just won the Nobel Prize. Biologist Joshua Lederberg, himself a Nobel laureate, was in the room. So was physicist Philip Morrison, one of the original architects of the Manhattan Project. And sitting among them, just days short of his twenty-seventh birthday, was Carl Sagan, who would go on to become one of the most influential astronomers in the world, and a vocal advocate for the search for extraterrestrial intelligence.
The Equation
N = R* × fp × ne × fl × fi × fc × L
Drake's equation calculates a single number: N, the estimated number of civilizations in our galaxy currently producing signals we might detect. It gets there by multiplying seven factors:
R* The rate at which new stars suitable for life form in the galaxy
fp The fraction of those stars that have planetary systems
ne The average number of planets per system that could support life
fl The fraction of those planets where life actually develops
fi The fraction of life-bearing planets where intelligence emerges
fc The fraction of intelligent species that develop technology capable of communicating across space
L The length of time such a civilization continues to broadcast detectable signals
The first three terms, star formation, planetary systems, habitable planets, were highly speculative in 1961. Today, thanks to the Kepler space telescope and its successors, we have far fewer observational constraints. Planets are extraordinarily common. The Milky Way contains an estimated 100 to 400 billion stars, and planetary systems appear to be the rule rather than the exception. NASA estimates the number of planets in the habitable zone around stars like our sun is at least 300 million.
The last four terms are where the equation becomes speculative. How often does life begin on a suitable planet? How often does it reach intelligence? How often does intelligence develop communications technology? And how long does a technological civilization broadcast? Those questions remain unanswered, and the range of defensible estimates spans from 'the galaxy is teeming with civilizations' to 'we are probably alone.'
Sagan's Optimism — and the Paradox It Creates
At Green Bank in 1961, Carl Sagan was among the most optimistic voices in the room. He believed technological civilizations were not rare, that intelligence was a natural product of evolution, and that many civilizations would find ways to survive their own technological adolescence. Two decades later, he brought the Drake Equation to a mass audience through his landmark television series Cosmos. The question of whether we are alone in the universe became part of our cultural conversation.
Sagan's optimism creates a problem, however. If the equation's optimistic estimates are correct, if the galaxy hosts millions of communicating civilizations, then physicist Enrico Fermi's famous question demands an answer: where is everybody? We have been listening for only a short time, and we have found no definitive evidence of alien civilizations. Silence is not what you would expect from a galaxy teeming with civilizations; hence the Fermi Paradox.
Scientists have detected organic molecules, including amino acids, in comet.
Various Explanations
The silence we observe from Earth is not evidence that the galaxy is empty. Below are some of the most prominent explanations for the Fermi Paradox:
The Great Filter. Economist Robin Hanson hypothesized that either life rarely begins, rarely develops complex cells and intelligence, or that technological civilizations usually destroy themselves. If he’s correct, have we passed the filter, or does it lie in front of us?
Civilizations Destroy Themselves. Technology produces nuclear weapons, engineered pathogens, and climate disruption. The variable L in Drake's equation, how long a civilization broadcasts, may be very short for most species. This is the most pessimistic reading of the Fermi Paradox. Nevil Shute’s, On the Beach, a classic nuclear apocalypse novel, exemplifies this. It was turned into a 1959 movie starring Gregory Peck and Ava Gardner. Another favorite, Forbidden Planet, offers a non-human example. The 1956 sci-fi classic stared a serious Leslie Neilsen, Walter Pidgoen and Anne Francis. The poor Krell never knew what killed them. I pay homage to the movie in my Atlantis Trilogy by referencing “Krell metal.”
The Dark Forest. Popularized by Chinese science fiction author Liu Cixin in The Dark Forest (2008; English translation, 2015), the second novel of his Three-Body Problem trilogy, this hypothesis proposes that the universe is silent because broadcasting your location is lethal. Any civilization advanced enough to detect another has rational cause to either hide or destroy it before it becomes a threat. Independence Day offers another reason to fear attacking extraterrestrial attention: rapacious aliens intent on stealing other species’ resources and destroying them in the process.
Non-intervention. Perhaps advanced civilizations operate under something like Star Trek's Prime Directive, a deliberate policy of not contacting species that have not yet reached a certain threshold.
A corollary to the non-intervention concept was exemplified by the 1951 film The Day the Earth Stood Still (remade in 2008). In that film, the alien Klaatu warns that Earth's violence could threaten other worlds. Humanity’s growing capacity for destruction triggered alien intervention. Another vivid illustration comes from Carl Sagan. His novel Contact (1985), adapted into a film in 1997, depicts contact unfolding in stages: a radio signal is received, but only when humanity develops the ability to understand and act on the embedded instructions does a genuine meeting become possible.
The Zoo Hypothesis. A variant of non-intervention: we are being deliberately observed, kept in an undisturbed state like animals in a nature preserve. In Star Trek: The Next Generation, the episode Who Watches the Watchers follows Federation anthropologists observing a less advanced civilization from a concealed post. In the classic Twilight Zone episode People Are Alike All Over, Roddy McDowall plays an astronaut who is welcomed by the Martians and given an Earth-like home which turns out to be a cage in a zoo.
The Planetarium Hypothesis. A darker variant: we are not merely uncontacted but deceived. The universe we observe may be a simulation, carefully constructed to appear empty. What we take for cosmic silence may be a stage set. The closest example I can find is the Star Trek episode For the World is Hollow and I have Touched the Sky. A sick Dr. McCoy stays on Yonada, an asteroid ship whose inhabitants are unaware that their world is an enclosure. If you have a better example, let me know.
Rare Earth. Perhaps the conditions that produced complex life on Earth — a large stabilizing moon, a gas giant like Jupiter that helps shield the inner solar system, plate tectonics, our position in a quiet galactic neighborhood — are so improbably specific that intelligent life is extraordinarily rare, regardless of what the optimistic terms of Drake's equation suggest. Isaac Asimov's Foundation trilogy and Frank Herbert's Dune series offer a suggestive fictional parallel: both depict a galaxy populated exclusively by humanity and its descendants, with no independent alien civilizations to encounter.
Communication Issues. We have been listening for radio messages for years. The "water hole" is a relatively quiet band of radio frequencies between the natural signals of hydrogen and hydroxyl, the ingredients of water. Lying roughly between 1,420 and 1,660 megahertz, the water hole has been a favored frequency range on the theory that civilizations associated with water-based life might recognize it as a natural meeting point. But a civilization millions of years more advanced than us might communicate by means we have not yet mastered: neutrino beams, quantum entanglement, optical laser pulses, or methods entirely outside our current physics. The Polish science fiction writer, Stanislaw Lem wrote several novels, His Master’s Voice and Solaris, where we got a message or encountered aliens, but couldn’t understand them.
Robots and Artificial Intelligence (a corollary to civilizations destroying themselves). Given the current debates about artificial intelligence, I cannot leave this one out. For decades, scientists and science fiction writers have explored the possibility that robots and artificial intelligence could destroy or subjugate a civilization. They’ve even speculated that the first extrasolar intelligence we meet will be artificial. Hollywood loves the concept. Colossus: The Forbin Project (1970) depicts enforced machine rule; The Terminator (1984) and The Matrix (1999) depict catastrophic war and human survival under machine domination. There are many more examples.
If you have a favorite film or book that exemplifies one of these theories, let me know.
In Sum
Frank Drake wrote seven variables on a blackboard in West Virginia in 1961 and asked humanity to do the arithmetic. Those of us who believe that we are not alone are still waiting for definitive evidence of extrasolar intelligence. Those who believe we are alone point to silence as evidence of the uniqueness of intelligence on Earth.
Sources: Frank Drake, NRAO Green Bank conference, November 1–2, 1961. Interesting Engineering, "The Drake Equation and Carl Sagan's Unshakable Optimism." SETI Institute, Drake Equation overview. Slate, Green Bank conference retrospective. Astrobites, "Frank Drake's Equation and Legacy." Robin Hanson, 'The Great Filter — Are We Almost Past It?' (1998). Liu Cixin, The Three-Body Problem (2008, English trans. 2014) and The Dark Forest (2008, English trans. 2015). Peter D. Ward and Donald Brownlee, Rare Earth: Why Complex Life Is Uncommon in the Universe (2000).
Legacy of Atlantis by John Topping — available now from The Story Plant / Gramarye Media.
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