The Great Silence: a biologist's Fermi Paradox

The trash cans of New York

In May of 1950, trash cans were disappearing from the streets of New York City. Not a metaphor. Actual municipal garbage cans, gone overnight, one after another — a civic embarrassment that nobody could quite explain. The May 20th issue of The New Yorker ran a cartoon by Alan Dunn depicting the only reasonable culprits: a flying saucer parked on a Manhattan sidewalk, little green men loading stolen cans into the hold. It was a joke layered on two genuinely unsolved mysteries at the time — the cans, and a postwar wave of UFO sightings sweeping the United States.

A few weeks later, at Los Alamos National Laboratory in New Mexico, a group of physicists saw the cartoon and laughed. Then they went to lunch.

The laboratory was in the middle of a crash program to build a thermonuclear bomb, five years after the first atomic one had been tested a hundred miles to the south. Walking toward the Fuller Lodge dining hall were three men with considerable blood on their conscience and a fourth about to join them. Edward Teller, the future father of the hydrogen bomb. Emil Konopinski, a specialist in atomic-nucleus structure. Herbert York, who would go on to direct Lawrence Livermore. And, ambling up as they walked, a small, brilliant Italian named Enrico Fermi, who had built the world's first nuclear reactor under a squash court at the University of Chicago.

They talked, as they walked, about the cartoon. About flying saucers. About whether any of this was real. By the time they sat down at the table the conversation had drifted somewhere else. And then, mid-bite, out of nowhere, Fermi blurted out the question everyone around him would remember for the rest of their lives.

Don't you ever wonder where everybody is?

The table laughed. Then the table went quiet. Because everyone sitting at it understood, instantly, that Fermi was not talking about lunch. He was talking about the universe.

A galaxy one thousand times too old

To feel the question the way Fermi felt it, you have to sit for a moment with the numbers.

Our galaxy, the Milky Way, contains somewhere between one hundred and four hundred billion stars. The observable universe contains something like two trillion galaxies. The Milky Way is about a hundred thousand light-years across. The galaxy is roughly thirteen billion years old.

Now imagine a civilization — any civilization — that develops starships capable of one percent of the speed of light. Not warp drive. Not wormholes. Just a modest, boring, we-already-know-the-physics kind of speed. With those ships, a patient species could cross the Milky Way in about ten million years. It could island-hop, settle, send probes, build colonies; it could fill the galaxy the way a dandelion fills a lawn.

Ten million years is the timescale. The galaxy is a thousand times older than that.

Fermi did the math in his head over lunch. If even a small fraction of stars produced intelligent life, and if even a small fraction of those civilizations built starships, the galaxy should be colonized many times over. Our solar system should have been visited, catalogued, settled, or at the very least littered. Earth's sky should be loud with signals, our telescopes should see stars wrapped in engineered structures, our geology should hold evidence of ancient probes.

Instead, the sky is quiet. The solar system is empty. We have searched for seventy-six years — with radios, with optics, with telescopes that would have seemed like fever-dream fantasy to the men at that lunch table — and we have heard nothing.

This is the paradox. Not that aliens don't exist — they might, somewhere, right now. The paradox is that if the universe works the way most of what we know suggests it works, then we should already have met them, and we haven't.

A chalkboard at Green Bank

Eleven years after Fermi's question, in November of 1961, a young radio astronomer named Frank Drake stood in front of a chalkboard at the Green Bank Observatory in West Virginia and tried to turn Fermi's intuition into math.

Drake was thirty-one. The year before, he had quietly pointed the observatory's eighty-five-foot telescope at two nearby sun-like stars — Tau Ceti and Epsilon Eridani — and spent two hundred hours listening for deliberate signals near 1,420 megahertz, the natural broadcast frequency of neutral hydrogen. He called the project Ozma, after L. Frank Baum's princess of a world "both wonderful and far away." He heard nothing. But the National Academy of Sciences was impressed enough with the seriousness of the attempt that they asked Drake to organize a small conference to discuss whether the search for extraterrestrial intelligence was a scientific question worth asking.

He invited about a dozen people. Among them was Philip Morrison, who had published the foundational paper on interstellar radio search two years earlier. The biochemist Melvin Calvin, who would receive a phone call from Stockholm during the conference informing him he had just won the Nobel Prize. The Nobel-laureate biologist Joshua Lederberg. The neuroscientist John C. Lilly, who studied dolphins. And, youngest at the table by a decade, a twenty-seven-year-old astronomy postdoc named Carl Sagan.

Drake needed an agenda. So a few days before the meeting, he wrote down every factor he could think of that would determine whether we could ever detect another civilization. There were seven of them. He strung them together with multiplication signs. And on the morning of November 1, 1961, he turned to the chalkboard and wrote:

Each term asked a different question. How many stars are forming in the galaxy each year? (R_*). What fraction of them have planets? (f_p). How many of those planets are habitable? (n_e). On how many does life actually begin? (f_l). On how many of those does intelligence evolve? (f_i). How many intelligent species develop communicative technology? (f_c). And the ominous one at the end — how long does such a civilization last before falling silent? (L).

When they multiplied optimistic guesses, they got millions of civilizations. When they multiplied pessimistic ones, they got one. The group was sobered but charmed. As the conference ended, Otto Struve — director of Green Bank, their host — raised a glass and offered what may be the most poignant toast in the history of astronomy.

In 1961, only the first term was known with any confidence. Sixty-five years later, we have pinned down three. The fraction of stars with planets, f_p? Essentially one. Nearly every star in the Milky Way has planets. The number of habitable planets per system, n_e? Somewhere around 0.2 — one in five. As of September 2025, NASA's exoplanet archive crossed six thousand confirmed worlds, with another eight thousand candidates waiting in line behind them. Kepler, TESS, and ESA's Gaia have delivered a universe of planets more abundant than the Green Bank table dared hope.

And yet the silence has only deepened. Because the terms we still cannot estimate — f_l, f_i, f_c, L — turn out to be, almost entirely, questions of biology. And biology, it turns out, is a much stranger and lonelier subject than the physicists of 1961 quite realized.

The hardest thing life ever did

Frank Drake and his 1961 colleagues set f_l — the fraction of habitable planets where life actually begins — at essentially 1.0. Life, they thought, was a chemistry problem, and chemistry is universal. Give a wet rocky planet a few hundred million years and something would begin to self-replicate. Easy.

Sixty-five years later, that confidence looks increasingly romantic. We still do not know how the first self-replicating chemistry got started on Earth. The Miller-Urey experiment of 1952 showed that amino acids form easily from a simulated primordial atmosphere, which was wonderful news. But amino acids are not life, any more than a pile of bricks is a house. The gap between chemistry and biology — the moment one becomes the other — is called abiogenesis, and nobody yet has a confirmed account of how it happened. Estimates of its probability in the scientific literature span more than thirty orders of magnitude. We do not know whether life is chemically inevitable or stupidly lucky. Hold onto that fact; we will need it later.

But even if life is everywhere, complex life might not be. And here the biology lens sharpens the paradox into a knife.

The two-billion-year standstill

For roughly the first two billion years of Earth's biological history, life was bacterial. Single-celled, simple, unambitious. Prokaryotes — cells without a nucleus — covered the planet, thrived in vents and oceans and rocks, invented photosynthesis, re-plumbed the atmosphere, and then, for an astonishingly long time, did almost nothing else.

Then, about two billion years ago, something extraordinary happened, and it has happened exactly once in the four-billion-year history of life on Earth. One prokaryotic cell swallowed another, and instead of digesting it, kept it. The swallowed cell — a bacterium — became the mitochondrion, the tiny internal power plant that every cell in your body, and every cell in every plant and animal and fungus on the planet, still carries. This merger is called eukaryogenesis, and from it comes everything you have ever recognized as life: algae, trees, mushrooms, fish, birds, you, the dog asleep on your floor.

The evolutionary biochemist Nick Lane, who has spent his career at University College London trying to explain why this happened, has come to a striking conclusion. Without mitochondria, cells are energetically capped. They cannot grow large or genetically complex, because they cannot generate enough energy across their membranes to run a large genome. Lane and his collaborator William Martin showed, in a landmark 2010 Nature paper, that the acquisition of mitochondria enabled a two-hundred-thousand-fold expansion in the number of genes a cell could express. That jump is the difference between bacteria and everything else.

Here is the unsettling possibility. The jump happened once. In four billion years. On a planet that was otherwise ideal for it. That suggests eukaryogenesis is not an inevitability — it is a freak accident, a cosmic coin flip that came up heads exactly one time.

If Lane is right, the universe may be teeming with bacterial life and nonetheless almost entirely empty of anyone to talk to. We would be the first species, orbiting the first sun, to have crossed the membrane between being alive and being complex. Microbes everywhere; minds almost nowhere.

The opposite argument — intelligence is inevitable

Not everyone buys this. The Cambridge palaeontologist Simon Conway Morris, famous for his work on the Cambrian explosion, has spent decades arguing the exact opposite position. His evidence is a phenomenon called convergent evolution: the tendency of natural selection to independently reinvent the same solutions in unrelated lineages.

Eyes, Conway Morris points out, have evolved independently on Earth at least forty times. Camera eyes — the sophisticated kind with lens, iris, and retina — have evolved in vertebrates (like us) and, completely separately, in cephalopods like octopuses and squid. Flight has been invented four times: by insects, by pterosaurs, by birds, by bats. Echolocation in dolphins and bats. Sabre-teeth in three different mammal lineages. Agriculture, in ants and humans. If life's outcomes were random, this kind of parallel rediscovery shouldn't happen. The fact that it happens constantly suggests that evolution is not a drunkard's walk; it is a funnel, pushing life toward a limited menu of viable designs.

Conway Morris's conclusion: on any Earth-like planet, intelligence is "almost guaranteed." Limbs, brains, eyes, tool-making, perhaps even curiosity — the universe should be full of beings who, if we met them, would be unsettlingly familiar.

Which means that if Conway Morris is right, Fermi's question gets harder, not easier. Because in his universe, the silence has no biological excuse.

And then there is K2-18b

While the biologists argue, the telescopes listen. And in April of 2025, they thought they might have heard something.

A team at the University of Cambridge, led by the astronomer Nikku Madhusudhan, announced that the James Webb Space Telescope had detected traces of dimethyl sulfide — DMS — in the atmosphere of a planet called K2-18b, a sub-Neptune orbiting a red dwarf 124 light-years away. On Earth, DMS is produced almost exclusively by marine phytoplankton. A headline-grabbing announcement called it "the strongest evidence yet that life may exist on a planet outside our solar system." The finding was at three-sigma significance — not confirmation, but a genuine signal above the noise.

Within weeks, the case fell apart. Jake Taylor at Oxford ran a quick independent reanalysis and argued the data were consistent with a flat line — no DMS signal at all. A larger team led by Kevin Stevenson at Johns Hopkins APL found that 87.5% of retrievals with a better binning scheme failed to recover the signal. By August 2025, a paper in Astronomy & Astrophysics combining every available JWST observation concluded that the DMS detection was not robust. A PNAS analysis in late 2025 added that the biological flux required to produce the claimed concentrations would be twenty times Earth's.

As of this writing, the consensus is that K2-18b has water, methane, and carbon dioxide, and no convincing biosignature. The headlines were overexcited; the science, once it settled, was not.

But the episode is instructive in a way the headlines were not. Because it points at a deeper problem the biology lens forces us to confront. We do not actually know what a signal of life looks like. We are looking for chemistry that matches our chemistry — oxygen, methane, DMS — because it is the only kind of biology we have ever seen. If alien life is chemically different, we might already be staring directly at it without recognizing it. The hardest part of the search is not finding the signal. It is knowing what the signal is.

Five ways to explain the silence

Over seventy-six years, the question Fermi asked at lunch has attracted dozens of proposed resolutions. None are proven; most are untestable; several are unfalsifiable. But five of them have become the canonical menu, and nearly all of them are, at heart, questions about biology.

Rare Earth — we are a statistical fluke

In 2000, the University of Washington palaeontologist Peter Ward and the astronomer Donald Brownlee published a book called Rare Earth: Why Complex Life Is Uncommon in the Universe. The book, by their own account, grew out of a conversation about how ridiculous the cantina scene in Star Wars was. The real universe, they argued, was not going to be that crowded.

Their case: Earth is a spectacularly unlikely planet. We orbit a stable, single star (more than half of all star systems are binary). We sit in a narrow continuously-habitable zone. Jupiter, a gravitational bodyguard parked conveniently outward, catches most of the comets and asteroids that would otherwise sterilise the inner solar system. Our Moon — abnormally large for a planet our size — stabilises our axial tilt, which stabilises our climate. We have a molten iron core that generates a magnetic field, shielding the surface from solar wind. We have plate tectonics, without which carbon would never cycle out of the atmosphere. We are in a quiet, metal-rich neighborhood of a spiral galaxy, far from the radiation-drenched core. Hit any one of those switches the wrong way and complex life never arrives.

Ward and Brownlee concede that microbes are probably everywhere. It's just that us — complex, oxygen-breathing, brain-having us — might require a combination of cosmic luck that only pays out once every many galaxies. Critics, including the biologist Jack Cohen, counter that this is circular reasoning: describing the recipe that produced Earth and then calling that recipe necessary. The debate is unresolved.

The Great Filter — we are lucky, or we are doomed

In 1998, the George Mason economist Robin Hanson gave the problem its most haunting framing. Between inanimate matter and a galaxy-colonizing civilization, he argued, there must be at least one evolutionary step that is staggeringly improbable — a filter so restrictive that almost no lineage gets through. Because we observe a dead universe, we know the filter exists. The only question is where it sits.

If the filter is behind us — if it is abiogenesis, or eukaryogenesis, or the leap to multicellularity — then we are the rare, lucky, staggering winners. The galaxy may be ours for the taking. If the filter is ahead of us — if every civilization eventually destroys itself through war, climate collapse, misaligned AI, engineered pathogens, or something we cannot yet imagine — then we are on borrowed time, and the silence is the echo of everyone else's extinction.

Hanson's most counterintuitive corollary is this. Finding microbial life on Mars would be catastrophic news for humanity, because it would mean abiogenesis is easy, which would mean the filter is probably still ahead of us. The empty universe may be the best piece of news we could hope for.

The Dark Forest — everyone is hiding

The logic is brutal, and it is game-theoretic rather than biological. Assume every civilization wants to survive. Assume resources are finite across the galaxy. Assume you cannot verify a stranger's intentions from light-years away. Now ask yourself: if you detect another civilization, do you announce yourself, or do you hide? And if they detect you, will they hide, or will they strike first just in case?

Liu's answer is that every rational civilization, reasoning correctly, concludes silence is the only survival strategy, and that pre-emptive destruction of loud neighbors is prudent insurance. The universe, therefore, is a forest at night, full of hunters, all of them armed, all of them quiet, all of them terrified of each other. Similar ideas had been floated by David Brin in 1983 under the name "deadly probes," but Liu gave it a name that stuck because the image was unforgettable.

As a solution, the Dark Forest has a problem — it assumes every civilization adopts the same risk calculus, and biological diversity suggests that is unlikely. But as a story, it is the cleanest reason the silence could be chosen rather than forced.

The Zoo — they're watching, not talking

In 1973, the radio astronomer John Ball proposed in the journal Icarus that the galaxy is not empty but curated. Advanced civilizations know about us, have for a long time, and have agreed — or been compelled — to leave us alone until we reach some developmental threshold. Star Trek fans have a tidier name for this. It is the Prime Directive. It is the Galactic Zoo hypothesis, and it is unfalsifiable by design. We cannot distinguish being left alone from being alone. If we ever do meet them, the zookeepers will owe us a rather awkward explanation.

Kardashev and Dyson — where are the megastructures?

In 1960, the physicist Freeman Dyson published a short paper in Science — "Search for Artificial Stellar Sources of Infrared Radiation" — arguing that any sufficiently advanced civilization would eventually surround its star with a vast swarm of energy-collecting satellites in order to harvest all available stellar output. Dyson envisioned a loose orbiting collection, not the solid shell that later science fiction borrowed his name for; he would eventually regret the phrase "Dyson sphere" for that reason. Such a star, from the outside, would look strange: dim in visible light, unusually bright in waste-heat infrared. Four years later, the Soviet astronomer Nikolai Kardashev extended the idea at a small conference in Armenia and proposed a three-level classification for civilizations. Type I harnesses a planet's energy. Type II harnesses a star's. Type III harnesses a galaxy's. Earth, today, sits at about 0.73. We have a long way to go.

The point of the Kardashev scale, for Fermi's question, is this: a Type II civilization should be trivially detectable from across the galaxy. A Type III should be visible from other galaxies. A 2015 excitement around KIC 8462852 — "Tabby's Star," which flickered in odd ways — was briefly entertained as a Dyson-swarm candidate before being explained by circumstellar dust. Decade after decade, we have scanned infrared sky surveys for the waste heat of engineered stars. We have found none. Not one. Across a galaxy of four hundred billion stars, nobody appears to have built anything.

Seventy-six years of listening

The search has not been idle. Since Drake's Project Ozma pointed its telescope at Tau Ceti in April of 1960, we have been listening with increasing seriousness and dramatically improving instruments. Here is what, in those sixty-six years, we have actually heard.

Once.

On August 15, 1977, at roughly 10:16 in the evening, the Ohio State University radio telescope known as the Big Ear — a three-acre metal structure aimed at a fixed patch of the sky that the Earth's rotation was sweeping past it — registered, for seventy-two seconds, a narrowband radio signal so strong and so perfectly-shaped that, when Jerry Ehman, a volunteer astronomer, reviewed the computer printout a few days later and saw the intensity code "6EQUJ5," he circled it in red pen and wrote a single word in the margin: Wow!

The signal came from the direction of the constellation Sagittarius. It was centered on 1,420 megahertz — the same hydrogen-line frequency Drake had picked for Ozma, the frequency every serious SETI researcher had agreed was the natural interstellar calling channel. It matched the expected shape of a sky-fixed source rather than a terrestrial one. And then it was gone. No repeat. No follow-up. Every attempt to re-observe that patch of sky in the forty-nine years since has heard nothing.

For a generation, the Wow! signal was SETI's great unsolved mystery. In 2022, the Breakthrough Listen program conducted the first targeted search of the best candidate sun-like star in that sky region and found nothing. In 2024 and 2025, a team led by Abel Méndez at the University of Puerto Rico went back into decades of archived Ohio SETI data and managed to refine the signal's properties rather than solve it — narrower source region, peak flux above 250 Janskys, frequency pinned to 1420.726 megahertz. But the same team also identified weaker analogues in archival Arecibo data: cold, compact clouds of neutral hydrogen can, under rare alignments with a bright transient source like a magnetar flare, brighten into narrowband signals that look exactly like the Wow! profile.

The case is not closed. But the evidential weight has shifted, in 2026, away from "message from aliens" and toward "rare astrophysics."

Beyond the Wow! signal, the sheet stays blank. A 2020 candidate from the direction of Proxima Centauri, called BLC1, turned out to be terrestrial interference. The six thousand confirmed exoplanets have produced no unambiguous biosignatures. The Breakthrough Listen initiative, a hundred-million-dollar private SETI program funded by Yuri Milner with support from Mark Zuckerberg and the late Stephen Hawking, has surveyed a million nearby stars and a hundred nearby galaxies since 2016 and reports, as it concludes this year: nothing.

The galaxy, as far as our ears can hear, is a place where nobody is broadcasting.

Dissolving the paradox

In 2018, three researchers at the Future of Humanity Institute at Oxford — the philosopher Anders Sandberg, the nanotechnology pioneer Eric Drexler, and the moral philosopher Toby Ord — published a paper with a title that was either a shrug or a bombshell, depending on whom you asked. It was called Dissolving the Fermi Paradox, and what it did, if its argument held, was make most of the resolutions above unnecessary.

Their argument is quietly devastating, and it is a lesson in statistical humility. When you multiply seven numbers together to get N — the expected number of civilizations in the galaxy — each of those numbers carries its own uncertainty. If you multiply best guesses for each term, as the 1961 Green Bank group did, you get a single point estimate — usually a large one. But that is the wrong math. The right math is to treat each term as a probability distribution representing our actual state of knowledge, and then to multiply the distributions.

Sandberg, Drexler, and Ord did this properly. They surveyed the published literature for each term in Drake's equation and measured how wide the real uncertainties are. For some terms — f_p, n_e — the uncertainty is a factor of a few. For f_l, the probability of life beginning at all, the literature spans more than thirty orders of magnitude. When they propagated these real uncertainties through the equation, the resulting distribution for N had an enormous left tail, meaning that a significant chunk of probability mass sits at N less than one.

Their number: something between 38% and 85% probability that we are alone in the observable universe. Not alone in the galaxy — alone in everything we can see.

If this analysis is right — and several respected astronomers, including Jason Wright at Penn State's SETI Center, regard it as essentially correct — then there is no paradox at all. The universe being empty is not surprising; it was always one of the most plausible outcomes. The real error was the expectation, not the observation. The Dark Forest, the Zoo, the Great Filter — we may need none of them. We may simply be early, or alone, and statistics is enough to explain everything we have not heard.

Seventy-six years of asking

Contributors

What if we are everybody?

Return, for a moment, to the Fuller Lodge dining hall in the summer of 1950. Four physicists at a table. A cartoon on the wall. Missing trash cans, flying saucers, laughter trailing off into something like unease. Don't you ever wonder where everybody is?

Seventy-six years later, with six thousand catalogued exoplanets, a hundred-million-dollar listening program, a space telescope that can smell the chemistry of atmospheres around distant stars, and a radio signal from Sagittarius that nobody has quite explained, we still do not have an answer. What we have instead is a question that has matured into something stranger than the men at that table could have imagined.

The lunchtime joke was where. The modern question is whether. And if the biology is right — if eukaryogenesis happens once every many galaxies, if the leap from chemistry to self-replication is as improbable as the uncertainty distributions suggest, if Sandberg and his colleagues have done the math correctly — then the silence has a simpler explanation than any of the hypotheses. It is not a dark forest. It is not a zoo. It is not a conspiracy of quiet civilizations waiting for us to prove ourselves. It is an empty room.

We might be first. That thought ought to land with more weight than it usually does. In a previous post on these pages, we followed the idea of panspermia — the possibility that life did not begin on Earth but hitchhiked here on the back of a comet or a meteorite, seeded from somewhere else. Whether life arose here or drifted in from the dark, the implication for the Fermi question rhymes. If we are the first complex minds in our observable patch of the universe — whatever the path that got us here — then the cosmos is not ignoring us. It is waiting on us. Every story that has ever been told about the universe realizing it exists has had someone in it who did the realizing. That someone, as far as the evidence currently goes, may be us.

Which would make us, in a very literal sense, the universe's first draft of a mind. The first species to look up from the dirt and ask where everybody else is. The trash cans of New York, Alan Dunn's cartoon, Fermi's mid-bite question, the Voyager Golden Record that Frank Drake co-designed hoping someone, someday, would decode it — they become something more than a sequence of hopes. They become an address label mailed into the dark, sent on the assumption that there is somebody to receive it.

Maybe there is. Maybe we have not listened hard enough, or for long enough, or in the right language. Maybe the Dark Forest is real and our broadcasts are deeply stupid. Maybe the filter is behind us, and we are about to inherit the galaxy. Maybe it is ahead, and we will become a cautionary silence for some future species to wonder about.

Or maybe — this is the answer the data currently favors — there is nobody else yet. And the question Fermi asked at lunch, which has been pointing outward for seventy-six years, turns around and points back. Where is everybody? stops being a question about them and becomes a question about us. If we are the first, the silence is not a mystery. It is an invitation. It is the sound of a universe with nothing to say yet, because the ones who will eventually say something are still learning how to speak.

Four physicists walked to lunch in the summer of 1950. They laughed at a cartoon. They asked the question that named the silence. And the silence answered, eventually, as clearly as silence ever does.

Not yet.