The Clockwork Rocket

by Greg Egan

  Yalda emitted a bitter, truncated buzz. “The worst? The Hurtlers will keep coming, ever larger and in ever-greater numbers, until the odds that we’re struck approach a certainty. If we survive that, we’ll probably collide with an orthogonal clump of gas—turning the world itself into something like a giant Hurtler. Somewhere along the way, there will be gravitational disruption, maybe ripping us free from the sun completely—or maybe tossing us into it. And if none of these things sound sufficiently fearsome, the encounter might scramble our arrow of time completely, leaving us with no past and no future. The world will end as a lifeless mass of thermal fluctuations in a state of maximum entropy.”

  Eusebio heard her out without flinching, without disputing anything. Then he said, “So how can we survive that?”

  “We can’t,” Yalda said bluntly. She pointed to his chest. “If it’s more than a glancing blow—if you’re going to deny me my choice of impact parameter—then we’re all dead.”

  “Are you telling me that it’s physically impossible to protect ourselves?”

  “Physically impossible?” Yalda had never heard an engineer use that phrase before. “No, of course not. It’s not physically impossible that we could shield ourselves from all of these collisions, or side-step them, or simply flee from the whole encounter. It would violate no fundamental law of physics if we built some kind of magnificent engine that carried the whole world off to a safer place. But we don’t know how to do that. And we don’t have the time to learn.”

  “How long would it take?” Eusebio asked calmly. “To learn what we need to know to make ourselves safe.”

  Yalda had to admire his persistence. “I can’t honestly say. An era? An age? We still don’t know the simplest things about matter! What are its basic constituents? How do they rearrange themselves in chemical reactions? What holds them together and keeps them apart? How does matter create light, or absorb it? And you want us to build a shield against collisions at infinite velocity, or an engine that can move an entire world.”

  Eusebio looked around at a group of students chatting happily near the food hall, as if they might have overheard this catalog of unsolved problems and decided to rise to the challenge.

  “Suppose we’d need an age, then,” he said. “A dozen gross years. How long do we actually have before the danger becomes acute?”

  “I can only guess.”

  “Then guess,” Eusebio insisted.

  “A few dozen years,” Yalda said. “The truth is, we’re blind to whatever’s coming; a whole world, a whole blazing orthogonal star might already be disposed to strike us tomorrow. But from the progression in the size of the Hurtlers that we’ve seen so far, unless we’re especially unlucky…” She trailed off. What difference did it make? Six years, a dozen, a gross? All she could do was go on living day by day, averting her gaze from the unknowable future.

  Eusebio said, “We need an age, and we don’t have it.”

  “Exactly.”

  “That’s what I thought,” he said, “but I had to hear it from you to be sure.”

  His tone was solemn but far from despairing. Yalda stopped walking and turned to face him. “I’m sorry I had no good news for you,” she said. “Perhaps I’m wrong about all of this. Perhaps our luck will be far better than—”

  Eusebio raised a hand, cutting her off. “We need an age, and we don’t have it,” he repeated. “So we find the time elsewhere.”

  He wiped the colliding clusters from his chest. Then he drew two lines, one straight, one meandering, and added a few simple annotations.

  “We make a rocket,” he said, “powerful enough to leave the world behind. We send it into the void and accelerate it until it matches the velocity of the Hurtlers. Once it’s done that, there’ll be very little chance that anything from the orthogonal cluster will strike it—but we might need to offset its position at the start, to keep it from colliding with gas and dust in our own cluster.

  “The complete journey is as I’ve drawn it. The time that passes for the world will be the time it takes to rotate the rocket’s history by a full turn: one quarter-turn to accelerate, one half-turn to reverse, one final quarter-turn to decelerate. If the rocket accelerates at one gravity—giving the passengers no more than their ordinary weight—the time back home for each quarter-turn will be about a year, making four years in all.

  “The time that passes on the rocket for those stages of the journey won’t be much greater: each curved segment is only longer than its height by a factor of pi on two. But when the rocket’s history is orthogonal to the history of the world, no time at all passes back home. So for the travelers, the journey can last as long as it needs to. If they require more time to complete their task, they can prolong the flight for another era, another age; it won’t delay their return by one flicker.”

  Yalda was speechless. Their roles really had been reversed: the physics Eusebio was presenting was so gloriously simple that she was ashamed she hadn’t thought of it herself—if only in the same whimsical spirit as that in which she’d first thought of the Hurtlers as past-directed fragments of the primal world.

  But when it came to practicalities, where did she begin?

  “What kind of rocket can you hope to build,” she said, “in which generations can survive for an age—let alone flourish to the point where they have any chance of fulfilling their purpose? The largest rocket I’ve seen with my own eyes was the size of my arm; the largest I’ve heard of was smaller than my body. If you can send my optics workshop into the void that would be the talk of Zeugma for a generation, but I don’t know where we’d put the wheat fields.”

  Eusebio hesitated, considering his reply, but he did not appear the least bit discouraged by her response.

  “I believe you’ve been on Mount Peerless,” he said.

  “Of course. The university has an observatory there.”

  “Then you’ll know that it’s far from any permanent habitation.”

  “Certainly.” Yalda thought she knew where he was heading: an isolated, high-altitude site would be the perfect place to test new kinds of rockets.

  Eusebio said, “The geologists tell me that the core of Mount Peerless is pure sunstone. I plan to tunnel into it and set it alight, and blast the whole mountain into the sky.”

  Yalda collected the children from school and took them back to the optics workshop. Amelia and Amelio were happy playing on the floor with a box of flawed lenses, lining them up to form impromptu telescopes and buzzing hysterically at the sight of each other’s distorted images. Valeria and Valerio were going through a stage of drawing pictures of imaginary animals that they insisted had to be preserved; Yalda gave them some old student assignments that were blank on one side of the paper, and some pots of dye that Lidia had brought home from the factory.

  Then she stood at her desk watching over them while she tried to decide what to make of Eusebio’s plan.

  Giorgio brought a group of students into the workshop to use the heliostat for an experiment in polarization. The children ran to greet him, and he accepted their embraces without a trace of annoyance or embarrassment before gently shooing them back to their activities. Yalda produced a stack of mechanics assignments and proceeded to mark them, marveling that she could feel guilty for taking a couple of chimes away from her conventional duties to ponder the correct means of averting the planet’s annihilation. She’d shared her ideas about the Hurtlers with Giorgio—and he’d offered his usual perceptive comments and objections—but in the end he’d still treated the whole thing as if it were an exercise in metaphysics.

  Yalda arrived home with the children just as Lidia returned from her shift.

  “Did you bring some more dye?” Valeria nagged her. She and Valerio had used up the last of their supplies on a series of images of giant worms with six gaping mouths arrayed along the length of their bodies.

  Lidia spread her arms and jokingly opened six empty pockets. “Not today. Every batch was perfect.”

 
; Valeria went into a sulk, which meant wrapping six arms around her co and trying to pull his head off. Yalda warned her three times, increasingly sharply, then stepped in and physically disentangled them.

  “You always take his side!” Valeria screamed.

  Yalda struggled to hold her still. “What side? What did Valerio do?”

  At a loss for an answer, Valeria changed tactics. “We were just playing, but you had to spoil it.”

  “Are you going to be sensible now?”

  “I’m always sensible, you fat freak.”

  Lidia made a reproving hum. “Don’t talk to your Aunty Yalda that way.”

  “You’re a bigger freak!” Valeria declared, turning on her. “At least it’s not Yalda’s fault she doesn’t have a co. She might have swallowed hers by accident before she was born, but everyone knows you killed your co with a rock.”

  Yalda tried to draw on her reserves of patience by reminding herself how well Valeria had behaved in the workshop.

  Daria had actually predicted that the third year of school would be the time when Tullia’s children started lashing out at their adoptive parents, punishing them for the derision they received from their increasingly unsympathetic classmates. So before the school year began, Yalda had encouraged the four of them to talk about the ignorance and hostility they were facing, and tried to suggest strategies for dealing with it. At the time, the recipients of her advice had promised that nothing in the wicked world could stop them feeling the same love and loyalty for their Aunties as ever.

  Amelio had been looking on impassively, but now he decided that this was the right time to announce in a tone of weary resignation, “Women are for making children, not raising them. You can’t expect them to do a good job.”

  Yalda thought: Bring on the Hurtlers.

  Lidia said, “Why don’t we try making our own dye? We can go to the markets and look for the ingredients.”

  Valerio’s face lit up with excitement. “Yes!”

  “I want to come too,” Amelia pleaded.

  Valeria held out for a few pauses, then decided that the whole thing had been her idea. “I know where we should go first. I already made a list in my head.”

  Yalda exchanged a glance with Lidia, thanking her for halting the descent. “Why don’t we all go,” Lidia said. “Before it gets dark.”

  Yalda woke on the first bell after midnight. Daria still hadn’t come home; some nights she went straight from the university to the Solo Club, then slept in her own apartment. That left Yalda or Lidia to get the children to school in the morning, depending on Lidia’s shift, but it was hard to complain that Daria wasn’t pulling her weight when she paid more than half the rent.

  There was a patch of light on the floor beside the window, a hint of color in its diffuse edges. Yalda rose and walked quietly over to it. Through the window, four pairs of trails were visible, spreading slowly but still bright enough to bury most of the stars. In the early days every Hurtler reported had been fast and close, presumably because the more distant ones had been too dim to see with the naked eye. If anything in the current crop ever came that close, it would be spectacular. Maybe that would be enough to jolt her out of her stupor—assuming it didn’t achieve something far worse.

  If a part of her had trouble believing that the world could end in a barrage of rocks from the distant past, the part that did believe struggled even harder to imagine that such grand cosmic mayhem could be avoided. Maybe everyone was simply born predisposed to expect life to go on as normal—and whatever the benefits of her education, hoping to overturn that innate conviction with an argument that started with the right triangles she’d found hidden in her plots up on Mount Peerless was asking too much of her animal brain.

  Yalda glanced over at the sleeping children. However much she resented them at times, she certainly wasn’t indifferent to their fate—but she could summon no deep, visceral sense that their lives, and the lives of their own children, now lay in the hands of her earnest, enthusiastic, possibly deranged ex-student.

  What would Tullia have done? Joked, reasoned, mocked, argued, probed all the competing theories for weaknesses, shone some light into other people’s blind spots, then followed her own imperfect instincts like everyone else. Yalda had never stopped missing her, but she was confused enough already without resorting to begging ghosts for advice.

  The light on the floor brightened; Yalda turned back to the window. A fresh, dazzling streak of violet had appeared; it spread out slowly, parallel to the older trails.

  She needed to make some kind of decision, if only to let herself sleep. So… she would take an interest in Eusebio’s plans and offer him whatever guidance she could, trying to help him spot the pitfalls in his strange endeavor. She could do that much without agreeing to be a passenger on his mad flying mountain—and without abandoning hope that the cosmic pyrotechnics that obsessed them both might yet turn out to be as harmless as a swarm of mites.

  Eusebio led Yalda on foot for the last few saunters across the dusty brown plain. “Sorry to make you walk so far,” he said. “But it’s not a good idea to bring the trucks too close.”

  Yalda took a moment to understand what he meant. The liberator for truck fuel wasn’t identical to that used with sunstone, but it could still cross-react. A pinch of gray powder spilt from a tank and carried the wrong way on the wind could heat things up very quickly.

  “So how did you get the sunstone out here in the first place?” she asked.

  “The way it’s always transported: well wrapped, in small pieces.”

  They reached the test rocket: a cone of hardstone about Yalda’s height. Apertures near the top revealed an intricate assembly of cogs and springs secured within. “Most of this is for attitude control,” Eusebio explained. “If the sunstone burns unevenly, that puts a torque on the rocket and the whole thing will start to swerve. The mechanism needs to detect that and respond quickly, adjusting the flow of liberator between the combustion points.”

  “Detect it how?”

  Eusebio took a crank from a box of tools that had been left beside the device, and began winding the main spring. “Gyroscopes. There are three wheels set spinning rapidly on gimbals; if their axes shift relative to the housing, that means the rocket is veering off a straight course.”

  When he’d finished winding the spring he squatted down and gestured with the crank at the bottom of the cone, which was held half a stride above the ground on six stubby legs. “There are four dozen tapered holes drilled into the sunstone, lined with calmstone most of the way. The conical plug of sunstone that was cut out to make each hole is also lined and put back in, but with grooves carved into it that leave a gap between the pieces. The liberator flows down through the gaps and ignites the unlined part at the bottom. As the sunstone burns, the lining’s corroded, progressively exposing the fuel.”

  Yalda joined him and peered up at the orderly array of not-quite-plugged holes. Sunstone lamps used the tiniest sprinkling of liberator, diluted with some kind of inert grit, to keep them blazing for a night’s performance in the Variety Hall—and they still killed a few careless operators every year. Deep inside this rocket, waiting to trickle down into the fuel, was a whole tank full of the stuff in its pure form.

  “I remember when you were afraid to visit the chemistry department,” Yalda said.

  “I was never afraid!” Eusebio protested. “You told me not to waste my time with chemists, because they couldn’t get their energy tables straight.”

  “Yes, of course, that’s exactly what I would have said.” Yalda straightened and stepped back from the rocket.

  Eusebio tossed the crank into the toolbox, then looked back across the plain toward Amando, one of the three assistants who’d been working at the site when Yalda arrived. Eusebio waved broadly with two hands, and received the same gesture in reply. Then he reached into the cone and released a lever.

  “What does that do?” Yalda asked.

  “One chime to ignition.
”

  “Thanks for the warning.”

  “One chime!” Eusebio scoffed, picking up the toolbox. “We’ll be behind the barrier long before then.”

  Yalda was already ten strides ahead. “Did your father really let you design his trains?” she called back to him.

  “Yes, but I just tweaked someone else’s plans,” Eusebio admitted. “Trains are complicated. I wouldn’t want to have to invent them from scratch.”

  When Yalda reached the truck closest to the rocket she saw the other two assistants, Silvio and Frido, lying chest-down on the tray. A sturdy timber barrier some four strides high had been attached to the side of the vehicle; the two men were propped up on their elbows behind narrow slits in which theodolites had been fitted. The theory was that the rocket would ascend vertically, run out of fuel, rise a little farther from momentum alone, then plummet back to the ground to make a crater at more or less the point from which it had risen. Knowing the height it reached would allow the team to quantify the amount of energy produced that actually ended up doing something useful. It was one thing to measure the effects of a quarter of a scrag of sunstone incinerated in a workshop experiment, where the gases produced in a sealed chamber drove a piston against a load. Expecting the same yield when gas was spilling off the sides of the rocket and fragments of heat-cracked fuel were free to sprinkle down onto the desert might be a bit optimistic.

  Eusebio caught up and joined Yalda and Amando behind the shield. Amando said, “One lapse to go”; he’d synchronized his own small clock with the one Eusebio had set ticking inside the rocket. Yalda stared anxiously at the clock’s face. If the sunstone burned hotter and faster than expected, it wasn’t inconceivable that the flame could rise up and eviscerate the tank full of liberator, bringing everything together in one mighty flash when the rocket was too high for the barrier to protect them.