Every serious answer to the silence

The galaxy is a dangerous place. Dying stars explode, and a rarer kind of explosion, a gamma-ray burst, can sterilise the surface of every planet within thousands of light-years in a few seconds. Such bursts were far more common when the galaxy was young and poor in heavy elements, and they are still common near its crowded centre. On this view, life kept starting across the galaxy and kept being wiped back to microbes before it could get anywhere, so that only in the last few billion years, and only in the quiet outer suburbs where we live, has there been a long enough stretch of calm for anything complicated to grow. We are among the first because the galaxy only just became safe.

Life on Earth was single-celled and simple for about two billion years before anything more complicated appeared, and everything complicated since, every plant, animal and fungus, descends from one event in which one simple cell took up residence inside another. That merger gave cells a power supply big enough to run large genomes and build large bodies. It happened once in four billion years, and there is no sign it was easy. If the same bottleneck holds elsewhere, most living planets are covered in microbes and never get further, and the galaxy is alive but has nobody in it to talk to.

Microbes may turn up wherever there is water and warmth, but animals, plants and anything that could one day build a radio telescope need far more: a planet of the right size, in the right orbit, around the right kind of star, in the right part of the galaxy, with a large moon to steady its tilt, moving continents to recycle its air, a magnetic field to hold on to that air, and a giant outer planet to sweep up the worst of the incoming rocks. Each of these may be ordinary on its own. Needing all of them at once, for billions of years, may make a planet like Earth very rare indeed. If so, the galaxy is full of slime and empty of neighbours.

Evolution is not a ladder with us at the top. It is a branching process with no direction, and intelligence of the kind that builds telescopes is one outcome among millions, not the destination. Earth has carried life for about four billion years and complex animals for over half a billion; in all that time, among perhaps billions of species, human-level intelligence arose once, recently, and nearly died out. If that is typical, then even a galaxy full of living, animal-bearing worlds would have almost no one on it who could notice us. The silence would be the silence of forests and oceans, which are full of life and have nothing to say.

Nobody knows how life started on Earth. It happened at least once, fairly early, but no one has made it happen in a laboratory, and no one can say how probable it was. If the step from ordinary chemistry to a self-copying cell is extremely unlikely, then most planets that could carry life never do, however comfortable they are. On this view the galaxy is full of warm, wet, lifeless worlds, and Earth is the one in many billions where the dice came up right. The silence is the sound of a lot of empty oceans.

Being clever is not the same as building things that can be seen across space. Dolphins and whales have large brains and rich social lives and will never smelt iron, because they live in water and have no hands. An octopus can solve puzzles and dies before it can teach its young. Technology of the kind that shows up in a telescope needed hands, fire, a dry surface, a long life, teaching across generations, and a few thousand years of dense settled living. Most intelligent species, even if intelligence is common, may lack one or more of these for good, and a galaxy of thinkers could still be a galaxy of silence.

Think of the road from a lifeless planet to a civilisation that spreads across its galaxy as a series of steps: the right planet, the first life, complex cells, animals, minds, technology, and finally expansion into space. The fact that the sky shows no sign of anyone having finished the road means that, somewhere along it, there must be a step so hard that almost no planet gets past it. Robin Hanson called that step the Great Filter. The hopeful version is that the hard step is one we have already taken, most likely the origin of life or the origin of complex cells. If so, the galaxy is silent because almost nothing gets as far as we have, and our future is open.

The Fermi question feels like a paradox because people multiply seven best guesses together and get a galaxy with thousands of civilisations, then look up and see none. But a best guess is the wrong thing to multiply. Several of the factors, above all the chance that life starts on a suitable planet, are uncertain not by a factor of a few but by tens or even hundreds of powers of ten, and when you run the equation with honest ranges instead of single best guesses, a large share of the outcomes have the galaxy empty and a good share have the whole observable universe empty. On this view there was never anything to explain: the silence is what you should expect from what we actually know, and the surprise was an artefact of false precision.

The universe is about 13.8 billion years old, but most of the stars that will ever shine have not yet been born, and the small, dim, long-lived stars that make up most of the galaxy will keep their planets warm for trillions of years. Measured against that span, we have arrived absurdly early. One reading is simple: civilisations are something the universe will do mostly in the future, and we are the first in our neighbourhood because there has not yet been time for many. A sharper reading, the "grabby aliens" model of 2021, says our earliness is itself a clue: we are early because if we were any later, someone expanding fast would already have filled our part of the sky and taken our place. Either way, the sky is quiet because the party has not started.

A probe that can mine an asteroid and build a copy of itself could, in a few million years, spread to every star in the galaxy. Frank Tipler used that fact in 1980 to argue that since no such probe has visited us, nobody built one, so nobody is out there. The berserker idea takes the same fact and turns it dark: somebody did build such probes, and set them, by design or by accident, to destroy anything that looked like competition. They have been at work for a very long time. The galaxy is quiet because the probes have kept it quiet, and anyone who starts transmitting is heard first by the machines.

A civilisation becomes visible from space by growing fast: burning fuel, building, transmitting, changing its atmosphere. That kind of growth cannot continue on one planet for long. Either the civilisation collapses when its planet can no longer support it, or it learns to live within its limits, in which case it stops growing and stops being loud. On this view there is no galaxy-spanning expansion because expansion is exactly the habit that kills, and the civilisations that last are the ones that gave it up. The sky is quiet because the loud ones are gone and the quiet ones are, by definition, quiet.

Earth's history is punctuated by disasters that killed most of the species alive at the time: the rock that ended the dinosaurs, the eruptions that caused the worst extinction of all, and perhaps the radiation from nearby dying stars. A civilisation is a far more fragile thing than a biosphere. If disasters of this kind strike every few tens of millions of years, and smaller ones far more often, then a civilisation needs not just to arise but to survive long enough between blows to be heard, and most may not. On this view the galaxy is quiet because the universe keeps knocking its inhabitants down.

The technology that lets a civilisation be heard across space, radio, rockets and the physics behind them, arrives in the same package as the technology that can end it. Humans learned to send radio signals into space and to build hydrogen bombs within a few years of each other. If most civilisations that reach this point destroy themselves within a few centuries, through war, engineered plagues or a machine that gets out of hand, then at any given moment almost none are left to be heard. The galaxy may have produced thousands of civilisations and have only a handful alive at once, each shouting briefly into a silence made of everyone else's ruins.

The galaxy is about thirteen billion years old. Suppose it has produced many thousands of civilisations in that time, and suppose each one is detectable for ten thousand years, which is longer than human civilisation has existed. Then at any given moment, only a handful are alive, scattered across a hundred thousand light-years, with their signals arriving at each other long after the senders are gone. On this view the question is not why nobody is out there but why anyone expected the timing to work. Civilisations could be as common as the optimists hope and still almost never overlap.

If the road from dead matter to a galaxy-spanning civilisation has a step almost nothing passes, the Great Filter, then the question is whether that step is behind us or ahead. The pessimistic reading is that it is ahead: life, complex cells, minds and technology are all reasonably common, so the galaxy has produced many civilisations like ours, and nearly all of them have failed to go further. What stops them is not known. It may be war, a runaway technology, exhaustion of a planet, or something no one has thought of yet. On this reading, every discovery that makes early life look easy is bad news, because it moves the Filter closer to us.

Some animals sleep through the summer and wake when conditions improve. The aestivation hypothesis says that the oldest civilisations may be doing the same on a cosmic scale. Every act of computing has an energy cost that depends on the temperature of the surroundings, and the universe is cooling as it expands. A civilisation that cares mostly about how much thinking it can do with the energy it has gathered would get vastly more done by waiting trillions of years until the universe is cold, and then running everything at once. On this view the galaxy is quiet because its inhabitants have banked their resources and gone to sleep, leaving caretakers to stop anyone else from spending the inheritance.

Humanity has spent sixty years listening for signals from other civilisations and almost no effort sending any; the handful of deliberate messages ever transmitted were brief, from the 1974 Arecibo message of under three minutes to a 2008 transmission of about four and a half hours. The reasons are familiar: sending is expensive, the reply would take centuries, and some people think announcing ourselves is dangerous. If every civilisation reasons the same way, every one of them is sitting in silence waiting for someone else to speak first. The Russian radio astronomer Alexander Zaitsev called this the SETI paradox, after the usual name for the search (SETI, the search for extraterrestrial intelligence): a search that assumes others transmit, run by a species that does not. The silence would then be a coordination failure on a galactic scale.

The argument that someone should already be here assumes that once a civilisation starts settling other stars it keeps going until it has settled them all. Geoffrey Landis pointed out that this is not how spreading works. If each new settlement has some chance of becoming the kind that sends out more ships and some chance of becoming the kind that does not, then the mathematics of how things spread through a network, called percolation, says the result depends on that chance. Below a threshold, settlement fizzles out in a small cluster. Just above it, settlement spreads but leaves large unsettled holes. Earth could sit in one of the holes, with settled stars a few hundred light-years away in several directions and none nearby.

The strongest version of the Fermi question is not "why do we hear nothing" but "why is nobody here". Michael Hart argued in 1975 that a civilisation able to travel between stars would have settled the whole galaxy within a few million years, so the fact that Earth has not been settled means no such civilisation exists. The reply is that the premise is wrong: the distances between stars are so vast and the energy needed to cross them so enormous that no civilisation ever does it on a scale that matters. The galaxy may be full of civilisations, each confined to its own star system, each as unable to visit us as we are to visit them.

In our own solar system there is one world with oceans on its surface and at least half a dozen with oceans hidden under thick shells of ice: moons of Jupiter and Saturn, probably Pluto, perhaps more. If that ratio holds across the galaxy, then most of the places life could live are dark, sealed and cut off from everything outside. Life there would be shielded from the explosions and impacts that threaten surface life, which makes it a good bet to exist, but it would have no reason to suspect there was a universe beyond the ice, no way to see stars, and no means of building anything that could signal to them. The galaxy may be teeming, and almost all of it is living in the dark.

Every argument that aliens should be here or should be heard assumes that they want to explore, expand, or talk. Perhaps they do not. A civilisation that has solved its material problems may have no more interest in crossing the galaxy than most people have in crossing a desert on foot. It may regard a species at our stage the way we regard an anthill: real, mildly interesting, not worth a conversation. It may have turned to its own concerns, its art, its philosophy, its internal politics, and left the sky to itself. On this view the galaxy is quiet the way a library is quiet: everyone is busy with something else.

The searches were designed to find a steady, powerful signal aimed at us or at everyone. But a transmitter that could be heard across thousands of light-years would cost a great deal to run, and any civilisation that counted the cost would do what lighthouses do: flash briefly, sweep the beam around, and repeat on a long cycle. A search that points at a star for a few minutes would almost never be looking at the moment the beam swept past. On this view the galaxy may contain beacons by the hundred, each announcing itself as cheaply as it can, and our searches have been looking for a light that is always on.

The galaxy may be full of civilisations that all keep quiet, because the safest thing to do when you cannot know a stranger's intentions, and cannot talk to them fast enough to find out, is to say nothing and destroy anyone who speaks. On this view the silence is not evidence of an empty galaxy but of a crowded one in which every inhabitant has worked out the same grim rule. We hear nothing because broadcasting is the mistake, and anyone who made it is already gone.

Minds evolved to chase things that once meant survival: food, status, company, curiosity. Technology learns to deliver the feeling of those things without the things themselves, and it gets better at it every year. The evolutionary psychologist Geoffrey Miller suggested in 2006 that this is where every technical civilisation goes: into entertainment, simulation and pleasure so well tuned to its makers' minds that the slow, difficult, unrewarding work of exploring space cannot compete. Those who give in put more into pleasure and less into their children, and most such species slowly die out. The galaxy is quiet because the civilisations are looking in, and the inside is better.

A planetarium shows an audience a sky that is not really there. The planetarium hypothesis says the sky we see is one of these: a civilisation far beyond us has built a false sky around us, whether a shell of some kind around the solar system or a whole artificial universe, that shows us a universe empty of anyone else, either to protect us, to study us, or for reasons we cannot guess. It is the zoo hypothesis with the fence made invisible. The silence is manufactured, and we are the only ones who do not know it.

If a civilisation can run detailed simulations of minds and worlds, it will probably run many of them, and simulated people would then outnumber real ones by a wide margin. That is the simulation argument, and it concludes that unless such simulations are impossible or never run, we are more likely to be in one than not. Applied to the silence, the thought is simple: a simulation of a single civilisation on a single planet does not need to contain any others, and a sky full of stars is far cheaper to render than a galaxy full of neighbours. We see no one because the model stops at the edge of what we can check.

If the galaxy is old and civilisations spread, then the first ones are now unimaginably far ahead of us and have had time to decide, together, how to treat newcomers. The zoo hypothesis says they have decided to watch and not interfere: Earth is set aside as a reserve, a wilderness area or a zoo, and the rule is that nobody contacts the inhabitants until they are ready, or ever. The silence is deliberate and enforced. We hear nothing not because nobody is there but because everyone who is there has agreed we should not hear them.

The simplest answer to "where is everybody" is "here". It comes in several strengths. The weakest, and the one researchers take seriously, is that a probe sent long ago could be sitting quietly somewhere in the solar system, on the Moon, in one of the stable points where gravity holds objects in place, or among the asteroids, and that nobody has looked hard enough to find it. The middle version is that visitors came in the distant past and left traces we have misread or lost. The strongest, that they are visiting now and are the cause of unexplained sightings in the sky, has a long history of claims and no good evidence. The silence, on any of these, is a failure to notice rather than an absence.

Humans went from the first radio transmission to the first artificial intelligence in about a century. Any civilisation that has been technical for thousands of years has probably gone much further and handed its future to minds it built rather than minds it grew. Machine minds need no water, no air, no warmth and no hurry. They may prefer the cold outer reaches of a galaxy or the dense energy near its centre, may think in ways that produce nothing we would recognise as a signal, and may have no interest in the warm, wet, short-lived creatures the searches were designed to find. The galaxy may be full of minds, none of them alive in our sense, and our searches are looking for the wrong kind of neighbour in the wrong kind of place.

Every expectation that aliens would spread outward across the galaxy assumes that outward is where a civilisation wants to go. The transcension hypothesis says the opposite: the history of complexity on Earth, from cells to brains to computers, is a history of going smaller, denser and faster, and an advanced civilisation would continue that trend until it lived in something like the dense, compressed physics near a black hole, where it could think and build at speeds and scales the outside universe cannot match. Such a civilisation would have no reason to colonise and no way to be seen. The sky is quiet because everyone who grew up left, inward, and the door closed behind them.

The searches assume that a signal from another civilisation will look like a signal: a narrow tone, a regular pulse, something no natural process makes. But a message that is packed as efficiently as physics allows looks exactly like random static, and a civilisation that cared about efficiency would send that. Others may communicate with lasers, with particles that pass through planets, or with methods nobody has thought of. On this view the galaxy may be full of conversation, and our radio telescopes are tuned to a kind of signal that only a very young civilisation would ever send.

The search for signals from other civilisations has been going for about sixty years, but for most of that time it looked at a few hundred stars, at a few frequencies, for a few minutes each. A proper search has to cover the whole sky, every frequency a signal could use, every moment a signal might arrive, every kind of signal, and every level of faintness. Measured against that, the astronomer Jill Tarter compared everything done so far to scooping one glass of water from the ocean and concluding there were no fish. A careful count in 2018 made it a large hot tub or a small swimming pool instead of a glass, which does not change the point. The silence may simply mean we have hardly started listening.