The Seventy-Two Seconds

A reading of the Wow! Signal — the seventy-two-second radio transmission received in Ohio on 15 August 1977 and never received since. Whatever happened in those seventy-two seconds was not the act of a civilisation that had thought it through. It was rogue, or accidental, or panicking — one of the three things that happen when broadcasting outruns deliberation. The uncomfortable parallel is what this essay is about. We have been transmitting for ninety years, without thinking, and we have built every one of the failure modes that not-thinking can produce.

A reading of the Wow! Signal — what we heard, what we think, and what must have happened at the other end.

I. What we heard

The room in Ohio was empty when it happened. Nobody was watching. The telescope, called Big Ear, was an unmanned instrument, a flat metal field the size of three football pitches lying open to the sky in a clearing behind the Perkins Observatory in Delaware County. It did not need a watcher. Every twelve seconds it took a sample of the sky overhead, measured the strength of radio waves arriving in fifty narrow frequency channels, and printed the result on a long roll of fanfold paper that accumulated, hour after hour, in a tray beside the printer. The paper ran continuously, day and night, building up a record that someone would look at later, when they got around to it.

At ten-sixteen in the evening on the fifteenth of August, 1977, eastern daylight time, a patch of sky in the constellation Sagittarius drifted into the telescope’s field of view. The patch was empty by all available cataloguing. There was no star there bright enough to matter, no known radio source, no reason to expect anything. Big Ear watched it, the way it watched every other patch of sky, without preference or expectation, and for the first twelve-second sample the readings were what they had always been — the low background hum of the universe, the cosmic microwave background, the faint thermal noise of warm equipment and a warm Earth. The printer rendered these as single digits. One. Two. One. Two. One. The kind of numbers that had filled the printout for years.

Then, in one frequency channel near the hydrogen line, the next sample arrived and the number was 6. The one after that was an E, which in the encoding the telescope used meant a reading higher than any single digit could express. Then Q. Then U — a letter that had, as far as anyone could later determine, never been printed before by Big Ear in its years of operation. Then J. Then 5. And then the readings fell back to the background, and the channel went quiet, and the sky kept turning, and the telescope kept watching, and nothing else happened that night, or the next night, or any of the nights since.

The whole event lasted seventy-two seconds. Six samples, six characters, one perfectly shaped curve rising and falling exactly as a fixed point in the sky would rise and fall as it drifted through the telescope’s stationary beam. The signal was narrow in frequency — confined to a single channel, the way a deliberate broadcast would be, not spread out the way natural radio sources usually are. It was strong, roughly thirty times the background. It was on the hydrogen line, the one frequency a radio astronomer anywhere in the galaxy would already be paying attention to. And it was coming from the direction of the densest part of the visible sky, the bulge of stars at the center of the Milky Way.

The paper accumulated in its tray. The samples after the event were ones and twos again, the same as before. The printer kept running. The night went on. In the morning, the telescope’s operators came in and did what they always did, which was nothing in particular — they would not see the page with the strange sequence on it for several more days. The paper sat in the tray, growing under more paper, holding the only physical record of the event that any human being would ever have. The seventy-two seconds had passed. The signal was already, by the time anyone noticed it, four days into its silence.

When Jerry Ehman finally pulled the page out and ran his eyes down the columns, he was at his kitchen table. He had taken the printouts home to review them at his own pace, the way he did. He saw the sequence — 6EQUJ5 — running down one column where everything around it was background noise, and he understood within a few seconds what he was looking at, and he picked up a red ballpoint pen and circled the column and wrote Wow! in the margin, and put an exclamation point after it, because there was nothing else to write. He did not know yet that he had just given the signal the name it would carry for the rest of his life and beyond it. He only knew that he was holding, on a piece of fanfold paper at his own kitchen table, something that the instrument had been built to find, and that he had no idea what to do next.

What he did next was check. He checked the satellite catalogues — nothing was in that part of the sky that night. He checked for aircraft, for military transmissions, for asteroid radar, for known terrestrial interference. Nothing. He went back to the telescope and pointed it at the same patch of sky as soon as he could, on the chance that whatever it was might still be there. It was not. He pointed it again. And again. The patch of sky in Sagittarius held its position and held its silence, and Ehman, who would live another forty-eight years, would never hear it again, and neither would anyone else.

That is what we heard. A perfect, narrow, strong, single, unrepeated transmission from the direction of the galactic center, on the frequency a galaxy would broadcast on if it wanted to be found, lasting just long enough for a fixed point in the sky to drift through a stationary beam, and then nothing — not for an hour, not for a day, not for a year, not for the rest of a human lifetime. One signal. One time. And forty-eight years of the same patch of sky giving back nothing but the cosmic background, as if it had never spoken at all, as if we had imagined it, except for the piece of paper with the red pen circle and the single word in the margin.

II. Why the frequency matters

To understand why the Wow! Signal is the kind of evidence that has resisted explanation for almost half a century, you have to understand what hydrogen is doing out there in the dark, and why one particular frequency is unlike all the others in the radio spectrum.

Hydrogen is the simplest thing in the universe. One proton, one electron. It is also, by an enormous margin, the most common substance in existence — roughly three quarters of all the ordinary matter in the cosmos is hydrogen, and most of it is not in stars or planets but drifting in the vast cold spaces between them, in clouds so diffuse that a cubic centimeter of one of these clouds might contain only a single atom. There is more empty space between the atoms than there is anything else. But the clouds are unimaginably large, and there are unimaginably many of them, and taken together they constitute the largest single feature of the universe by mass.

These hydrogen atoms, drifting in the cold, occasionally do a very small thing. The electron in a hydrogen atom can be in one of two states relative to the proton — it can be spinning, loosely speaking, in the same direction as the proton, or in the opposite direction. The same-direction configuration has slightly more energy than the opposite-direction configuration. Once in a very long time — on average, once every eleven million years for any given atom — the electron will spontaneously flip from the higher-energy state to the lower-energy state, and in doing so it will release a tiny amount of energy in the form of a single photon. That photon has a very specific wavelength. It is twenty-one centimeters long. Its frequency is 1,420 megahertz.

Eleven million years per atom sounds like never. But there are so many hydrogen atoms in the galaxy that at any given moment, an enormous number of them are flipping. The universe is glowing at 1,420 megahertz, faintly and everywhere, because the hydrogen between the stars is performing this same small quantum gesture in numbers vast enough that the total emission is detectable from anywhere with a radio telescope. Every galaxy gives off this glow. Every cloud of cold gas between the stars gives off this glow. The hydrogen line is the universal background hum of ordinary matter doing the simplest thing it can do.

This is the first reason the frequency matters. It is the most natural frequency in the universe. Any civilization that develops radio astronomy at all will discover the hydrogen line almost immediately, because they cannot avoid discovering it. The moment they point a radio receiver at the sky, the hydrogen line is what they will hear. It is how they will map their own galaxy — we mapped ours that way in the 1950s, by tracing the cold hydrogen in the spiral arms. Any radio astronomer anywhere will have spent their formative years staring at this frequency. It is, in a meaningful sense, the frequency every radio astronomer in the universe has in common.

This is what the Cornell physicists Philip Morrison and Giuseppe Cocconi realized in 1959, in the paper that effectively founded SETI. They were trying to answer a practical question. If you wanted to broadcast a signal across interstellar distances, and you wanted to maximize the probability that someone on the other end would actually be listening on the right frequency, which frequency would you choose? The radio spectrum is enormous. There are billions of channels to choose from. If you picked at random, the probability that any given listener happened to be tuned to your particular channel would be essentially zero. You needed what they called a natural meeting place — a frequency that any technological civilization would already be paying attention to, for reasons that had nothing to do with you, simply because the physics made it interesting.

The hydrogen line is the obvious candidate. It is conspicuous to anyone with the relevant technology. It is the same frequency everywhere in the universe, because the underlying quantum mechanics is the same everywhere. It is unmistakable — you cannot confuse it with anything else, because the physics that produces it is so simple and so specific that it makes a sharp, narrow feature in the spectrum that nothing else mimics. If you are a young civilization with a transmitter, and you want to be heard by an older civilization that has been doing radio astronomy for a long time, you broadcast at 1,420 megahertz. You will know they are listening, because they have to be.

There is a second reason, which makes the choice even more compelling. The region of the radio spectrum around the hydrogen line is unusually quiet. The galaxy produces less background noise here than at most other frequencies. The Earth’s atmosphere is relatively transparent at this wavelength — radio waves at 21 centimeters pass through atmospheres easily, which means any planet with an atmosphere similar to ours would receive them cleanly. And just slightly higher in frequency, around 1,660 megahertz, is the corresponding emission line for the hydroxyl molecule, OH. Hydrogen and hydroxyl, taken together, are the chemical components of water — H and OH combine to make H₂O. The band of frequencies between the hydrogen line and the hydroxyl line is sometimes called the water hole, after the watering holes on the African savanna where animals of different species would gather, by long convention, in a kind of truce. It is the place where everyone in the galaxy might be expected to meet.

So when SETI was built — when the first radio telescopes were pointed at the sky in deliberate search of artificial signals — they were pointed, overwhelmingly, at the water hole. They were looking at the hydrogen line and the frequencies immediately around it, because that is where any civilization trying to be found would broadcast, and because that is where any civilization trying to find others would listen. Big Ear, the telescope in Ohio, was specifically configured to monitor a narrow band of frequencies centered on the hydrogen line. It was, in a real sense, a hydrogen-line detector with SETI as a secondary purpose, listening for natural hydrogen emission and watching for anything that stood out from it.

And this is the staggering thing about the Wow! Signal. It came in at 1,420.4556 megahertz. Almost exactly on the hydrogen line. Not in some random part of the radio spectrum. Not at the frequency of a satellite, or a piece of military hardware, or a known astronomical source. At the specific frequency that physicists had identified, eighteen years before the signal arrived, as the frequency a deliberate extraterrestrial broadcast would most plausibly be transmitted on. The signal arrived in the one channel where it would be most conspicuous, most unmistakable, most certain to be noticed by anyone listening.

If it was natural, then nature produced a narrowband signal at exactly the frequency that the founders of SETI had predicted, eighteen years earlier, that a transmission would arrive on. That is a coincidence of a particular kind. It is the kind of coincidence that is not impossible — coincidences happen, the universe is large, strange alignments occur — but it is the kind that requires you to do some work to explain. Nature emits hydrogen radiation continuously and weakly, in broad smooth profiles, from clouds of gas across the sky. Nature does not, as far as we have ever observed, emit narrowband radiation at the hydrogen line, in the profile of a fixed point, for seventy-two seconds, once. The frequency is the frequency of nature; the shape of the signal is not.

If it was a transmission, then someone, somewhere, had made the same calculation that Morrison and Cocconi had made in 1959. They had reasoned about which frequency to broadcast on. They had thought about who might be listening, and on what channel, and what physics those listeners would share with them. They had arrived at the same answer that two physicists in upstate New York had arrived at almost two decades earlier, working from first principles in a Cornell office with a chalkboard. And they had pointed their transmitter at the galactic center and broadcast on the frequency of the simplest, most common, most universal substance in the cosmos, because they understood — because anyone would understand, because the physics is the same everywhere — that this was the frequency on which the universe could be expected to be listening.

The signal showed up on the channel SETI was built to monitor, at the frequency physics had identified as the natural meeting place, in the direction of the most listeners — and it did this once, for seventy-two seconds, and never again. Whatever it was, it knew where to be heard. Or, if it did not know, then nature, on this one occasion, produced something that was indistinguishable from a thing that knew.

III. What we think

We do not know what it was. After almost half a century of investigation, every confident answer has failed.

It was not a satellite. The orbital catalogues for August 1977 are good enough to be sure, and no satellite was in the right position; moreover, a satellite would have moved against the stars, and this thing did not. It was not aircraft, not military radar, not the reflection of any known terrestrial transmitter off any known piece of space debris. The frequency band where it appeared is internationally protected for radio astronomy precisely because the hydrogen line is too important to allow human transmissions in it; nothing on Earth is supposed to be there, and forty-eight years of careful searching have found nothing on Earth that was.

It was not, as a 2016 paper proposed and a flurry of subsequent papers refuted, a hydrogen cloud excited by a passing comet. The comets in question were checked when their orbits brought them back into view, and they emitted nothing of the kind. The frequency was wrong by enough to matter. The width of the signal was wrong by enough to matter. The comet hypothesis is the most prominent natural explanation ever advanced for the Wow! Signal, and it has not survived contact with the data.

It was not, as far as any astronomer has been able to determine, a known kind of astrophysical event. Pulsars do not do this. Fast radio bursts do not do this — they are broadband; this was narrowband. Masers do not do this at this frequency in this part of the sky. We know of no natural process that produces a narrowband signal, on the hydrogen line, with a profile matching a fixed point in the sky, for seventy-two seconds, exactly once.

This leaves us with two possibilities that are both uncomfortable in different ways. The first is that it was a natural phenomenon of a kind we do not yet understand — some process that produces this exact signature, that operates on a timescale long enough that we have only seen it once in almost fifty years of searching, that happens to mimic everything we would expect from an intentional transmission. This is possible. The universe has surprised us before, and what looked like little green men in 1967 turned out, two years later, to be the first known pulsar. The history of radio astronomy is in part a history of mysteries that became, over decades, ordinary. The Wow! Signal could be one of these, still waiting for its explanation. We just have not found it yet.

The second possibility is that it was a transmission. Not in the loose sense in which any radio source is a transmission but in the specific sense of being produced, deliberately or otherwise, by something with intent. By something, in the most freighted use of the word, that meant to be heard.

There is no neutral way to weigh these two possibilities, because the prior probabilities are themselves the thing under dispute. If you think technological civilizations are common in the galaxy, the second possibility looks reasonable; if you think they are vanishingly rare, the first does, however strained it must become to fit the data. The Wow! Signal does not settle the argument. It only sharpens it. It is the cleanest piece of evidence we have, and it points in a direction that the evidence alone cannot resolve. We are obliged either to extend the natural explanations into territory where they have so far failed, or to accept that we may have heard, for seventy-two seconds in 1977, something that no other moment in human history has heard before or since.

IV. What must have happened at the other end

If it was a transmission, then somewhere — twenty-six thousand light-years away, give or take, in the direction of the galactic center — something happened in the year roughly 24,000 BC that produced seventy-two seconds of high-power, narrowband, hydrogen-line broadcast aimed in the direction of our small, unremarkable solar system, and then stopped, and never started again. The light from that event has been traveling toward us since before the last ice age peaked, since before humans had crossed into the Americas, since before the cave paintings at Chauvet were made. It arrived in Ohio on the fifteenth of August, 1977, and was caught by an unmanned instrument and printed on a piece of fanfold paper, and we are still, half a century later, trying to read its shape.

What kind of event has that shape? What does a transmission look like that lasts seventy-two seconds and is never repeated?

It does not look like a planned broadcast. A planned broadcast, by any civilization that understood interstellar distances and the realities of interstellar listening, would be repeated. It would be repeated for years, for centuries, because the broadcasters would understand that no single seventy-two-second pulse, traveling through twenty-six thousand light-years of interstellar medium, has any meaningful probability of being heard. The galaxy is vast and patient, and the listeners — if there are any — are not always pointed in the right direction at the right time. To be received, a deliberate signal essentially has to be sustained. Any civilization sophisticated enough to build the transmitter would understand this, the way the engineers of the Arecibo message in 1974 understood this when they designed a signal meant to be sent and re-sent across an extended period. A one-shot pulse is a near-guarantee of being missed by everyone it was intended for. The probability that we heard it — that one young species, with one strange telescope, pointed at the right patch of sky on the right night — is already vanishingly small. For a planned broadcast designed to reach the galactic center, being heard by Earth would be the equivalent of throwing a single message in a bottle into the Pacific and having it wash up in one specific tide pool on the first try.

So the event has the wrong dimensions to be a considered act. It is too short, too singular, too unrepeated. It does not have the shape of strategy. It has the shape of an accident.

And once you accept that, the candidate explanations start to narrow in a particular direction. There are really only a few ways a civilization produces seventy-two seconds of perfectly-tuned, high-power, hydrogen-line transmission aimed at the densest part of the galaxy, and then nothing.

The dark forest

Before we look at those few ways, we have to look at the larger argument that frames all of them, because it changes the weight of each of the candidate explanations and rules out one that might otherwise seem obvious.

The argument has come to be called, after the Chinese novelist Liu Cixin who gave it its most famous fictional form, the dark forest hypothesis. The underlying logic is older than the novel — Stephen Hawking made versions of it publicly, and a long and bitter argument has gone on inside SETI for decades about whether broadcasting our own existence into space is wise — but Liu’s image is the one that has stuck, and it is worth taking on its own terms.

The image is this. The universe is a forest at night. The forest is full of hunters. The hunters cannot see each other; they can only listen. Any hunter who speaks reveals their position. Any hunter who reveals their position can be found. The hunters cannot tell, from a single voice in the dark, whether the voice belongs to a friend, an enemy, or something for which neither word applies. They cannot tell whether the voice is young or old, armed or unarmed, alone or accompanied. They can only tell that it is there, and that the speaker now exists as a target on a map that did not contain them a moment ago. The rational strategy in such a forest, for any hunter who has been there long enough to think clearly, is silence. Listen. Never speak. If you hear another voice, do not answer it. If you can tell where the voice came from, you have a choice — to ignore it, or to remove it before it can grow into something that might one day remove you. The forest is quiet because every hunter who has lasted has learned to be quiet, and every hunter who did not learn is no longer there to break the silence.

The dark forest argument does not require that the hunters be malicious. It only requires that they be uncertain about one another, and that the cost of being wrong about a neighbor’s intentions be high enough that caution dominates. In a universe with even a small number of civilizations that would attack on discovery — and you cannot tell, from a signal alone, whether you are dealing with such a civilization — the safe move for everyone else is to assume the worst and stay quiet. The strategy propagates. Anyone who broadcasts is selected against. Over enough time, the only civilizations left broadcasting are the ones too young to have learned the lesson, and they tend not to remain broadcasting for long.

If the dark forest description of the universe is even partially right, it has a sharp implication for the Wow! Signal. It rules out the explanation that would otherwise be the most natural one — that the signal was a deliberate, considered broadcast by a mature civilization trying to make contact. No mature civilization, in a dark forest universe, broadcasts. Especially not toward the galactic center. Especially not on the hydrogen line. Those are the choices a civilization makes when it does not yet understand what it is doing. They are the choices of a species that has not yet thought through what being heard would actually mean. A civilization sophisticated enough to build the transmitter, and old enough to have used it deliberately, would also be old enough to have realized that the loudest possible shout in the most populated possible direction on the most conspicuous possible frequency is exactly the gesture a hunter who wants to live does not make.

So the dark forest does not, by itself, explain the Wow! Signal. What it does is narrow the field of possible explanations on the transmitting end. If the universe is the kind of place the dark forest argument describes, then whatever produced those seventy-two seconds was not a deliberate act by anyone who understood what they were doing. It was something else. It was an accident, or an act of disobedience, or an act of desperation. It was, in other words, one of the three things that follow — and the dark forest is the reason it had to be one of them.

The leak

One possibility is that it was never meant to leave. A test. Someone in a laboratory or an engineering facility was calibrating a transmitter, or running a diagnostic, or testing a beam-forming array, and a switch was in the wrong position, or a shutter failed to close, or a containment that was supposed to keep the test signal local failed for a little over a minute and was then restored. The transmitter was built for some entirely internal purpose — planetary communication, perhaps, or interplanetary, or something we do not have a category for — and for seventy-two seconds it accidentally leaked into the sky. The reason it never repeated is not that the broadcasters stopped. It is that, on their end, nothing was ever broadcasting in the first place. They never knew.

This explanation has a quiet horror to it, if you sit with it. It would mean that the most significant event in the history of human radio astronomy, the one piece of evidence we have that may point to other minds in the universe, was, on the other end, not even noticed. A glitch. A flickered light on a console that someone reset without thinking. We caught the radio equivalent of a sneeze in a sealed room, and the sneezer does not remember it.

The rogue

A second possibility is the rogue actor. One person, or one small group, with access to a transmitter powerful enough to broadcast interstellar distances, doing something they were not supposed to do. Maybe they believed in making contact and their society did not. Maybe they were a researcher acting against the protocols of their own equivalent of SETI, who had been told repeatedly that broadcasting was forbidden and decided, for reasons of their own, that they would broadcast anyway. Maybe they were a young person with access they should not have had, doing the cosmic equivalent of what humans do when they get unsupervised access to powerful tools, which is to try them out and see what happens. The signal lasted seventy-two seconds because that is how long it took for someone else to notice and shut it down. The reason it has never been repeated is that the person who did it was stopped, and stopped permanently, in whatever way that civilization stops people who do things like that.

This version has a particular kind of plausibility because it explains something the deliberate-broadcast theory cannot explain, which is the carelessness of the choice. Why aim at the galactic center? Why use the hydrogen frequency? Both of those choices are the choices that maximize the probability of being heard by anyone — including the wrong anyone. A cautious civilization, broadcasting deliberately, would never choose them. A rogue actor, acting against the cautious instincts of their own society, would choose exactly them, because they would be optimizing for the goal of contact rather than the goal of safety. They would be making the loudest possible shout in the most populated possible direction, because for them the point was to be heard, and they were not paying the price of being heard. The price would be paid by everyone else.

This is exactly the kind of act the dark forest argument predicts will happen, and predicts will end badly. A civilization that has collectively learned to be quiet is still a civilization made of individuals, and not every individual will agree with the collective decision. Some will find the silence intolerable. Some will believe that the long policy of caution is itself the mistake, that the universe is not as dangerous as the cautious have made it out to be, that contact is worth the risk. They will be, in the terms of their own society, dissidents — and they will have access to equipment that was built for legitimate purposes but that can, with the right repositioning, be turned into a transmitter. They will make their broadcast. And then the security of their own society will reach them, because the same civilization that learned to be quiet will have learned to enforce that quiet, and will treat any rogue broadcast as a threat to the survival of everyone. The seventy-two seconds is the length of time it took to reach them.

You can almost construct the scene. A facility somewhere, on some world, with a transmitter that was built for purposes their society considered legitimate — astronomy, perhaps, the way our largest dishes are built for astronomy and could in principle be used as transmitters. A person who has decided that the silence of their civilization, the long policy of listening but never speaking, is wrong. They have argued the point and lost. They have access. They wait until the rotation of their world brings the galactic center into view, and they point the transmitter, and they push whatever the equivalent of a button is. They get seventy-two seconds before someone reaches them, or before the alarm propagates to whoever has the authority to cut the power. And then the door opens, and the broadcast ends, and on a different world twenty-six thousand light-years away, a piece of paper comes out of a printer in Ohio with the sequence 6EQUJ5 in one column.

The dying world

A third possibility, and this is the one I find I keep returning to, is panic. Something happened on that world during those seventy-two seconds that caused someone to do the broadcast, and to do it in haste, and then to either die or be silenced or lose the ability to continue. A distress call. An impact event, a war, an extinction-class disaster, a civilization in the last moments of itself, deciding that the long policy of silence no longer mattered because there was no longer anything to protect, and reaching for the transmitter and aiming it at the largest possible audience and sending whatever they could in the time they had. Seventy-two seconds is the length of a final message. It is the length of please remember that we were here. It is the length of the time between realizing what is happening and being unable to continue.

This explanation, more than the others, accounts for the strange particulars. It accounts for the direction — toward the galactic center because that is where the most witnesses live, and if you are sending a last message you want the largest possible audience. It accounts for the frequency — the hydrogen line because it is the frequency anyone listening would be listening on, and you do not have time to be clever about channels. It accounts for the brevity, because seventy-two seconds is what they had. And it accounts for the silence afterward, because in this version, there is no one left to send a second message. The broadcaster is gone. The thing we received in 1977 was not the first transmission of a long conversation, and it was not the only transmission of a careful policy. It was the last transmission of someone who no longer exists.

V. The common shape

All three of these possibilities — the accident, the rogue, the dying world — share a common structure, and I think it is the structure that makes the Wow! Signal sit in the mind the way it does. They all explain the silence the same way. The silence is not a choice. The silence is a condition. The broadcasters did not decide to stop. They were stopped. Either because they never knew they had transmitted, or because they were prevented from transmitting again, or because they no longer exist to transmit. The signal is unrepeated not because someone learned a lesson but because someone lost the ability to continue.

The shape of the event, in other words, is the shape of an interruption. Something began, and then something ended it. We do not know which side of that interruption we should be paying attention to — the brief, hot, deliberate-looking burst that started it, or the long quiet that has followed for almost half a century — but the two are connected, and the connection is the thing the Wow! Signal is really about. Something tried to speak, and could not finish.

We have been treating it for forty-eight years as a question about whether anyone is out there. But the more carefully you look at the shape of the event — the duration, the direction, the frequency, the singularity — the more it begins to look like the answer to a different question. Not is anyone there, but what happens to them. And the answer, if it is an answer, is that whatever was there in 1977 was there for seventy-two seconds and then was not. Something about the act of transmitting, or the circumstances of it, ended the possibility of transmitting again.

And here is the part that has to be said plainly, because the careful prose above can disguise it. Whatever happened in those seventy-two seconds was not the act of a civilisation that had thought it through. Each of the three explanations is, in its own way, the same thing: a civilisation that had not understood, until it was too late, what broadcasting actually was. The leak: nobody on their end had thought about what their test rig was leaking. The rogue: an individual broadcast against the policy of a society that had thought about it, but the individual had not. The dying world: they had no time to think; the broadcast was reflex. In every case, the transmission was an act of unconsideration. It was not the act of a civilisation that knew what it was doing. It was the act of a civilisation that was doing what it was doing before it could finish working out whether it should.

There is a thought here that I find harder to shake than I would like. If the silence is a condition rather than a choice, and if the conditions that produce that silence are conditions that any civilization might encounter — accidents in laboratories, individuals who broadcast against the will of their societies, disasters that overtake worlds — then the Wow! Signal is not a message from elsewhere. It is a description of a recurring outcome. It is the shape that emerges when a young species reaches a certain stage and tries, in one of the available ways, to speak across the dark, and is met by one of the available endings. The fact that we observe it once and not many times might mean that we have been listening for a short while; it might also mean that the events that produce such signals are rare enough that we are unlikely to catch many of them in any human lifetime. Either way, what we caught was not a beginning. It was a thing that had already been interrupted.

We are, right now, the young civilization with the loud transmitters. We have been broadcasting in every direction at once since the 1930s, with our television signals and our radar pulses and our deliberate messages. We have not yet had our own moment of accident, or of rogue actor, or of disaster. We are still, in the timeline that contains the Wow! Signal, in the early phase — before the seventy-two seconds. Whatever the broadcaster was doing in the moment we caught them, we are doing now, more loudly, with less consideration, for longer. Theirs was an act of unconsideration that lasted seventy-two seconds. Ours has lasted ninety years and is accelerating.

The dark forest argument, if you take it seriously, says that the silence around us is not evidence of an empty universe but of a universe full of careful listeners. If it is right, then we are doing the thing the careful listeners have learned not to do, and we are doing it from the same kind of position the Wow! broadcaster did it from — a young world, with new equipment, on a planet that has only recently figured out how to make noise across the dark. We have not yet been interrupted. The seventy-two seconds we caught in 1977 may be the shape of how interruption arrives, and the only thing we can say about our own version of it is that it has not happened yet.

VI. What we are doing

It is worth being concrete about what we are doing, because the abstraction of broadcasting makes it sound like a thing that happens occasionally, in laboratories, under controlled conditions. It is not that. It is continuous. It is happening as you read this sentence. It has been happening, without serious interruption, for almost a hundred years.

The first sustained leakage began in the 1930s with high-powered television broadcasting and the development of early radar. Those signals went up into the ionosphere, partially penetrated it, and continued outward at the speed of light. They have been traveling ever since. The sphere of detectable human radio emission is now roughly ninety light-years in radius. Every star within that sphere — and there are several thousand of them — has, in principle, been bathed in evidence of our existence for some portion of the last century. The signal weakens with distance, and a great deal of what we transmit is too weak to be detectable from any meaningful range. But the loudest things we do — military radars, planetary radars used to map asteroids, intentional transmissions like the Arecibo message of 1974 or the various smaller broadcasts beamed at nearby stars since — those are, by the standards of what a serious listener could detect, loud enough to matter.

We are louder now than we have ever been. There are something like ten thousand active satellites in orbit, many of them emitting continuously. There is a growing constellation of communication satellites — Starlink and its competitors — that has multiplied the radio output of low Earth orbit by orders of magnitude in the last few years. Aviation radar, weather radar, military early-warning radar — all of these are running, twenty-four hours a day, every day, painting the sky in radio frequencies that any civilization with our level of technology would notice immediately if they happened to be pointed at us. The planetary radar at Goldstone, used to characterize near-Earth asteroids by bouncing high-powered radio pulses off them, transmits at strengths that are detectable across interstellar distances by any receiver comparable to our own. Every time it pings an asteroid, the bulk of the signal misses and continues outward into space, carrying with it the unambiguous signature of a deliberate technological source.

And we have, in addition, been doing the deliberate version. The Arecibo message in 1974 was the first, beamed at the globular cluster M13, twenty-five thousand light-years away, designed by Frank Drake and Carl Sagan as a coded greeting. There was the Cosmic Call in 1999 and 2003, aimed at several nearby Sun-like stars. There was the Teen Age Message in 2001, broadcast to six Sun-like stars by Russian scientists. There was the Across the Universe transmission in 2008, when NASA beamed a Beatles song toward Polaris. There has been the continuous, low-level enthusiasm of various METI projects — Messaging Extraterrestrial Intelligence — arguing that we should not only be willing to broadcast but should be actively, deliberately, repeatedly broadcasting, in coordinated programs designed to maximize the probability of being heard.

The decisions about these deliberate transmissions were not made by humanity collectively. They were made by small groups of scientists, sometimes by single individuals, sometimes by private organizations or corporations using rented time on existing instruments. There is no international body that approves interstellar transmissions. There is no treaty governing them. There is no equivalent of the protocols that exist for nuclear testing or biological research. A person with access to a large radio dish and an opinion about whether contact would be a good thing can, in principle, broadcast whatever they like in whatever direction they like, and there is no mechanism that will reliably stop them.

This is the part of the comparison with the Wow! Signal that I find I cannot put down. If one of the candidate explanations for the signal we caught in 1977 is that it was a rogue act — an individual or small group acting against the cautious instincts of their own society — then we should notice that we have built, on our own world, a situation in which exactly that kind of act is possible at any time. We have the transmitters. We have the individuals with strong opinions about contact. We have the absence of any meaningful enforcement of caution. We are one well-funded enthusiast away, at any given moment, from broadcasting deliberately and repeatedly at any star we choose. Some of those enthusiasts already have, in smaller ways, and have published proudly about it.

We are also, in the more passive sense, broadcasting constantly without anyone having chosen to. Every aircraft transponder, every cell tower, every satellite uplink, every radar installation is contributing to a continuous leakage that no individual is responsible for and that no decision-making body has approved. We do not collectively decide to broadcast. We broadcast as a side effect of being the kind of civilization we are. The leakage is not a policy choice; it is a metabolic condition. Stopping it would require dismantling most of the infrastructure that modern life depends on, and no one is going to do that, and so the broadcasting will continue, and it will get louder, because every year we build more transmitters and more satellites and more radars, and every year the sphere of detectable human emission expands by another light-year.

Notice what we have built. All three of the conditions that could plausibly explain those seventy-two seconds on the other end — the accident, the rogue, the disaster — we have already assembled here. We have the accident: thousands of transmitters running continuously, with no oversight of what they are leaking. We have the rogue: an individual with access to a large dish and a strong opinion can broadcast tomorrow, and there is no mechanism that will stop them. We have the disaster, or the conditions for one: nuclear weapons, engineered pathogens, climate collapse, all of which could, at any time, produce the kind of last-broadcast moment the dying-world explanation describes. The Wow! signal lasted seventy-two seconds because the broadcaster could not or did not think it through. We have spent ninety years not thinking it through, and we have built every one of the failure modes that not-thinking can produce.

The implication of this, if the dark forest argument is anywhere close to right, is uncomfortable in a specific way. It is not that we are choosing to take a risk. It is that we have already taken the risk, and we took it before we understood what we were taking it for. The first generation of high-powered radar operators in the 1940s were not thinking about extraterrestrial listeners. They were thinking about German aircraft. The television engineers of the 1950s were not thinking about civilizations twenty-six thousand light-years away. They were thinking about reception in Cleveland. The decision to make our planet visible was not really a decision. It was the accumulated by-product of a thousand smaller decisions, none of which were about the sky.

And the by-product is now traveling outward at the speed of light in a sphere that we cannot recall. Anyone within ninety light-years who happens to be listening has, in principle, already been listening. They have had time to notice us. They have had time, in some cases, to point their own instruments at our sun and look more carefully. They have had time, if they wanted to, to begin doing whatever they would do in response to having found us. We will not know that they have done so until whatever they do reaches us, which will be at the speed of light, which means we will have no warning beyond the time it takes the warning itself to arrive.

This is the part of the Wow! Signal that I think is the actual lesson, if there is a lesson. Not that aliens exist; we do not know that. Not even that the dark forest is true; we do not know that either. The lesson is about what kind of civilization we are. We are the kind that broadcasts before it thinks. We are the kind that builds the transmitter and turns it on before anyone has worked out whether turning it on is wise. We are, in the specific sense the Wow! Signal makes possible, the kind of civilization that produces seventy-two-second events. We have not had ours yet. But we have built the conditions under which ours becomes possible, and we are adding to those conditions every day, and the only question is which of the available endings — the accident, the rogue, the disaster — will be the one that finally interrupts us.

Or perhaps none of them will. Perhaps the universe is empty, or perhaps the listeners are not the kind of listeners the dark forest argument imagines, or perhaps the distances are great enough and the times are long enough that no one within our broadcasting sphere has been able to do anything about us yet. Perhaps we will be lucky. Perhaps the seventy-two seconds we caught in 1977 was, after all, a comet or a maser or some natural process we have not yet characterized, and the silence afterward is just the silence of an empty universe, and the whole framework I have been constructing is a misreading of a single piece of paper in an archive in Ohio.

But the framework is at least worth taking seriously. Because the framework, if it is right, says something we are not in the habit of saying about ourselves — that we have made a decision, collectively and without deliberation, to do the thing that older and more careful civilizations have learned not to do. That we are, right now, transmitting. That we have been transmitting for almost a century. That every day we transmit more, and that every day the consequences of having transmitted propagate further into the dark. That we are, in the precise terms of the Wow! Signal, doing what the broadcaster on the other end did in the seventy-two seconds before whatever happened to them happened. And that we have no idea, and no way of finding out, what the equivalent of that whatever might be for us.

The piece of paper with the red pen circle is still in the archives of the Ohio History Connection. The patch of sky in Sagittarius continues, as far as anyone can tell, to be empty. The hydrogen line continues to glow faintly across the universe, the way it has glowed for thirteen billion years, the simplest substance in the cosmos doing the simplest thing it can do, and the channel that physicists in 1959 identified as the natural meeting place for civilizations to find each other continues to carry nothing in particular from the direction we have most reason to be watching. The seventy-two seconds happened. The forty-eight years have happened. We are inside the silence now, on whichever side of it we are on, and we do not know yet what the silence is.

We are also, as we sit inside the silence, still transmitting. Somewhere on Earth, right now, a radar dish is pinging the sky. A satellite is uplinking. A television tower is leaking the day’s broadcast outward at the speed of light. The signals are going. They have been going for ninety years. They will keep going whether we decide they should or not, because we have built a civilization that cannot stop emitting them without ceasing to be the civilization it is. The Wow! broadcaster had seventy-two seconds. We have had nine decades. The question is not whether we are doing what they did. The question is only how long we have before we find out what doing it costs.