by Carl Sagan
• • •
The two-lane blacktop highway, Ellie noticed with a pleasant little tremor, was lined with rabbits. She had seen them before, especially when her drives had taken her as far as West Texas. They were on all fours by the shoulders of the road; but as each would be momentarily illuminated by the Thunderbird’s new quartz headlights, it would stand on its hind legs, its forelimbs hanging limply, transfixed. For miles there was an honor guard of desert coneys saluting her, so it seemed, as she roared through the night. They would look up, a thousand pink noses twitching, two thousand bright eyes shining in the dark, as this apparition hurled toward them.
Maybe it’s a kind of religious experience, she thought. They seemed to be mostly young rabbits. Maybe they had never seen automobile headlights. To think of it, it was pretty amazing, the two intense beams of light speeding along at 130 kilometers an hour. Despite the thousands of rabbits lining the road, there never seemed to be even one in the middle, near the lane marker, never a forlorn dead body, the ears stretched out along the pavement. Why were they aligned along the pavement at all? Maybe it had to do with the temperature of the asphalt, she thought. Or maybe they were only foraging in the scrub vegetation nearby and curious about the oncoming bright lights. But was it reasonable that none of them ever took a few short hops to visit his cousins across the road? What did they imagine the highway was? An alien presence in their midst, its function unfathomable, built by creatures that most of them had never seen? She doubted that any of them wondered about it all.
The whine of her tires on the highway was a kind of white noise, and she found that involuntarily she was—here, too—listening for a pattern. She had taken to listening closely to many sources of white noise: the motor of the refrigerator starting up in the middle of the night; the water running for her bath; the washing machine when she would do her clothes in the little laundry room off her kitchen; the roar of the ocean during a brief scuba-diving trip to the island of Cozumel off Yucatan, which she had cut short because of her impatience to get back to work. She would listen to these everyday sources of random noise and try to determine whether there were fewer apparent patterns in them than in the interstellar static.
She had been to New York City the previous August for a meeting of URSI (the French abbreviation for the International Scientific Radio Union). The subways were dangerous, she had been told, but the white noise was irresistible. In the clacka-clacka of this underground railway she had thought she heard a clue, and resolutely skipped half a day of meetings—traveling from 34th Street to Coney Island, back to midtown Manhattan, and then on a different line, out to remotest Queens. She changed trains at a station in Jamaica, and then returned a little flushed and breathless—it was, after all, a hot day in August, she told herself-to the convention hotel. Sometimes, when the subway train was banking around a steep curve, the interior bulbs would go out and she could see a regular succession of lights, glowing in electric blue, speeding by as if she were in some impossible hyper-relativistic interstellar spacecraft, hurtling through a cluster of young blue supergiant stars. Then, as the train entered a straight-away, the interior lights would come on again and she would become aware once again of the acrid smell, the jostling of nearby straphangers, the miniature television surveillance cameras (locked in protective cages and subsequently spray-painted blind), the stylized multicolored map showing the complete underground transportation system of the City of New York, and the high-frequency screech of the brakes as they pulled into the stations.
This was a little eccentric, she knew. But she had always had an active fantasy life. All right, so she was a little compulsive about listening to noise. It did no harm that she could see. Nobody seemed to notice much. Anyway, it was job-related. If she had been so minded, she could probably have deducted the expense of her trip to Cozumel from her income tax because of the sound of the breakers. Well, maybe she was becoming obsessive.
She realized with a start that she had arrived at the Rockefeller Center station. As she quickly stepped out through an accumulation of daily newspapers abandoned on the floor of the subway car, a headline of the News-Post had caught her eye: GUERRILLAS CAPTURE JOBURG RADIO. If we like them, they’re freedom fighters, she thought. If we don’t like them, they’re terrorists. In the unlikely case we can’t make up our minks, they’re temporarily only guerrillas. On an adjacent scrap of newspaper was a large photo of a florid, confident man with the headline: HOW THE WORLD WILL END. EXCERPTS FROM THE REV. BILLY JO RANKIN’S NEW BOOK. EXCLUSIVELY THIS WEEK IN THE NEWS-POST. She had taken the headlines in at a glance and tried promptly to forget them. Moving through the bustling crowds to the meeting hotel, she hoped she was in time to hear Fujita’s paper on homomorphic radio telescope design.
• • •
Superposed on the whine of the tires was a periodic thump at the joins of swathes of pavement, which had been resurfaced by different New Mexico road crews in different epochs. What if an interstellar message were being received by Project Argus, but very slowly—one bit of information every hour, say, or every week, or every decade? What if there were very old, very patient murmurs of some transmitting civilization, which had no way of knowing that we get tired of pattern recognition after seconds or minutes? Suppose they lived for tens of thousands of years. And t a a a a a l k e d v e r r r r y s l o o o o o w w w w l y. Argus would never know. Could such long-lived creatures exist? Would there have been enough time in the history of the universe for creatures who reproduced very slowly to evolve to high intelligence? Wouldn’t the statistical breakdown of chemical bonds, the deterioration of their bodies according to the Second Law of Thermodynamics, force them to reproduce about as often as human beings do? And to have lifespans like ours? Or might they reside on some old and frigid world, where even molecular collisions occur in extreme slow motion, maybe only a frame a day. She idly imagined a radio transmitter of recognizable and familiar design sitting on a cliff of methane ice, feebly illuminated by a distant red dwarf sun, while far below waves of an ammonia ocean beat relentlessly against the shore—incidentally generating a white noise indistinguishable from that of the surf at Cozumel.
The opposite was possible as well: the fast talkers, manic little creatures perhaps, moving with quick and jerky motions, who transmitted a complete radio message—the equivalent of hundreds of pages of English test—in a nanosecond. Of course, if you had a very narrow bandpass to your receiver, so you were listening only to a tiny range of frequencies, you were forced to accept the long time-constant. You would never be able to detect a rapid modulation. It was a simple consequence of the Fourier Integral Theorem, and closely related to the Heisenberg Uncertainty Principle. So, for example, if you had a bandpass of a kilohertz, you couldn’t make out a signal that was modulated at fasted than a millisecond. It would be kind of a sonic blur. The Argus bandpasses were narrower than a hertz, so to be detected the transmitters must be modulating very slowly, slower than one bit of information a second. Still slower modulations—longer than hours, say—could be detected easily, provided you were willing to point a telescope at the source for that length of time, provided you were exceptionally patient. There were so many pieces of the sky to look at, so many hundreds of billions of stars to search out. You couldn’t spend all your time on only a few of them. She was troubled that in their haste to do a full sky survey in less than a human lifetime, to listen to all of the sky at a billion frequencies, they had abandoned both the frantic talkers and the laconic plodders.
But surely, she thought, they would know better than we what modulation frequencies were acceptable. They would have had previous experience with interstellar communication and newly emerging civilizations. If there was a broad range of likely pulse rates that the receiving civilization would adopt, the transmitting civilization would utilize such a range. Modulate at microseconds, modulate at hours. What would it cost them? They would, almost all of them, have superior engineering and enormous power resources by Earth standards. If
they wanted to communicate with us, they would make it easy for us. They would send signals at many different frequencies. They would use many different modulation timescales. They would know how backward we are, and would have pity.
So why had we received no signal? Could Dave possibly be right? No extraterrestrial civilizations anywhere? All those billions of worlds going to waste, lifeless, barren? Intelligent beings growing up only in this obscure corner of an incomprehensibly vast universe? No matter how valiantly she tried, Ellie couldn’t make herself take such a possibility seriously. It dovetailed perfectly with human fears and pretentions, with unproved doctrines about life-after-death, with such pseudosciences as astrology. It was the modern incarnation of the geocentric solipsism, the conceit that had captured our ancestors, the notion that we were the center of the universe. Drumlin’s argument was suspect on these grounds alone. We wanted to believe it too badly.
Wait a minute, she thought. We haven’t even examined the northern skies once with the Argus system. In another seven or eight years, if we’ve still heard nothing, that’ll be the time to start worrying. This is the first moment in human history when it’s possible to search for the inhabitants of other worlds. If we fail, we’ve calibrated something of the rarity and preciousness of life on our planet—a fact, if it is one, very much worth knowing. And if we succeed, we’ll have changed the history of our species, broken the shackles of provincialism. With the stakes this high, you have to be willing to take some small professional risks, she told herself. She pulled off the side of the road and did a shallow racing turn, changed gears twice, and accelerated back toward the Argus facility. The rabbits, still lining the roadside, but now pinked by dawn, craned their necks to follow her departure.
CHAPTER 4
Prime Numbers
Are there no Moravians in the Moon, that not a missionary has yet visited this poor pagan planet of ours to civilize civilization and Christianize Christendom?
—HERMAN MELVILLE
White Jacket (1850)
Silence alone is great; all else is weakness.
—ALFRED DEVIGNY
La Mort du Loup (1864)
The cold black vacuum had been left behind. The pulses were now approaching an ordinary yellow dwarf star and had already begun spilling over the retinue of worlds in this obscure system. They had fluttered by planets of hydrogen gas, penetrated into moons of ice, breached the organic clouds of a frigid world on which the precursors of life were stirring, and swept across a planet a billion years past its prime. Now the pulses were washing against a warm world, blue and whit, spinning against the backdrop of the stars.
There was life on this world, extravagant in its numbers and variety. There were jumping spiders at the chilly tops of the highest mountains and sulfur-eating worms in hot vents gushing up through ridges on the ocean floors. There were beings that could live only in concentrated sulfuric acid, and beings that were destroyed by concentrated sulfuric acid; organisms that were poisoned by oxygen, and organisms that could survive only in oxygen, that actually breathed the stuff.
A particular lifeform, with a modicum of intelligence, had recently spread across the planet. They had outposts on the ocean floors and in low-altitude orbit. They had swarmed to every nook and cranny of their small world. The boundary that marked the transition of night into day was sweeping westward, and following its motion millions of these beings ritually performed their morning ablutions. They donned great-coats and dhotis; drank brews of coffee, tea, or dandelion; drove bicycles, automobiles, or oxen; and briefly contemplated school assignments, prospects for spring planting, and the fate of the world.
The first pulses in the train of radio waves insinuated themselves through the atmosphere and clouds, struck the landscape and were partially reflected back to space. As the Earth turned beneath them, successive pulses arrived, engulfing not just this one planet but the entire system. Very little of the energy was intercepted by any of the worlds. Most of it passed effortlessly onward—as the yellow star and its attendant worlds plunged, in an altogether different direction, into the inky dark.
WEARING A Dacron jacket displaying the word “Marauders” above a stylized felt volleyball, the duty officer, beginning the night shift, approached the control building. A klatch of radio astronomers was just leaving for dinner.
“How long have you guys been looking for little green men? It’s more than five years, isn’t it now, Willie?”
They chided him good-naturedly, but he could detect an edge to their banter.
“Give us a break, Willie,” another of them said. “The quasar luminosity program is going great guns. But it’s gonna take forever if we only have two percent of the telescope time.”
“Sure, Jack, sure.”
“Willie, we’re looking back toward the origin of the universe. There’s a big stake in our program, too—and we know there’s a universe out there; you don’t know there’s a single little green man.”
“Take it up with Dr. Arroway. I’m sure she’ll be glad to hear your opinion,” he replied a little sourly.
The duty officer entered the control area. He made a quick survey of dozens of television screens monitoring the progress of the radio search. They had just finished examining the constellation Hercules. They had peered into the heart of a great swarm of galaxies far beyond the Milky Way, the Hercules Cluster—a hundred million light-years away; they had tuned in on M-13, a swarm of 300,000 stars, give or take a few, gravitationally bound together, moving in orbit around the Milky Way Galaxy 26,000 light-years away; they had examined Ras Algethi, a double system, and Zeta and Lambda Herculis—some stars different from the Sun, some similar to it, all nearby. Most of the stars you can see with the naked eye are less that a few hundred light-years away. They had carefully monitored hundreds of little sectors of the sky within the constellation Hercules at a billion separate frequencies, and they had heard nothing. In previous years they had searched the constellations immediately west of Hercules—Serpens, Corona Borealis, Boötes, Canes Venatici…and there also they had heard nothing.
A few of the telescopes, the duty officer could see, were devoted to picking up some missed data in Hercules. The remainder were aiming, boresighted, at an adjacent patch of sky, the next constellation east of Hercules. To people in the eastern Mediterranean a few thousand years ago, it had resembled a stringed musical instrument and was associated with the Greek culture hero Orpheus. It was a constellation named Lyra, the Lyre.
The computers turned the telescopes to follow the stars in Lyra from starrise to starset, accumulated the radio photons, monitored the health of the telescopes, and processed the data in a format convenient for their human operators. Even one duty officer candies, a coffee machine, a sentence in elvish runes out of Tolkien by the Artificial Intelligence Laboratory at Stanford, and a bumper sticker reading BLACK HOLES ARE OUT OF SIGHT, Willie approached the command console. He nodded pleasantly to the afternoon duty officer, now collecting his notes and preparing to leave for dinner. Because the day’s data were conveniently summarized in amber on the master display, there was no need for Willie to inquire about the progress of the preceding hours.
“As you can see, nothing much. There was a pointing glitch—at least that’s what it looked like—in forty-nine,” he said, waving vaguely toward the window. “The quasar bunch freed up the one-tens and one-twenties about an hour ago. They seem to be getting very good data.”
“Yeah, I heard. They don’t understand…”
His voice trailed off as an alarm light flashed decorously on the console in front of them. On a display marked “Intensity vs. Frequency” a sharp vertical spike was rising.
“Hey, look, it’s a monochromatic signal.”
Another display, labeled “Intensity vs. Time,” showed a set of pulses moving left to right and then off the screen.
“Those are numbers,” Willie said faintly. “Somebody’s broadcasting numbers.”
“It’s probably some Air Force inter
ference. I saw an AWACS, probably from Kirtland, about sixteen hundred hours. Maybe they’re spoofing us for fun.”
There had been solemn agreements to safeguard at least some radio frequencies for astronomy. But precisely because these frequencies represented a clear channel, the military found them occasionally irresistible. If global war ever came, perhaps the radio astronomers would be the first to know, their windows to the cosmos overflowing with orders to battle-management and damage-assessment satellites in geosynchronous orbit, and with the transmission of coded launch commands to distant strategic outposts. Even with no military traffic, in listening to a billion frequencies at once the astronomers had to expect some disruption. Lightning, automobile ignitions, direct broadcast satellites were all sources of radio interference. But the computers had their number, knew their characteristics and systematically ignored them. To signals that were more ambiguous the computer would listen with greater care and make sure they matched no inventory of data it was programmed to understand. Every now and then an electronic intelligence aircraft on a training mission—sometimes with a radar dish coyly disguised as a flying saucer camped on its haunches—would fly by, and Argus would suddenly detect unmistakable signatures of intelligent life. But it would always turn out to be life of a peculiar and melancholy sort, intelligent to a degree, extraterrestrial just barely. A few moths before, an F-29E with state-of-the-art electronic countermeasures passed overhead at 80,000 feet and sounded the alarms on all 131 telescopes. To the unmilitary eyes of the astronomers, the radio signature had been complex enough to be a plausible first message from an extraterrestrial civilization. But they found the westernmost radio telescope had received the signal a full minute before the easternmost, and it soon become clear that it was an object streaking through the thin envelop of air surrounding the Earth rather than a broadcast from some unimaginably different civilization in the depths of space. Almost certainly this one was the same thing.