The Clockwork Rocket

by Greg Egan

  Eusebio said, “Have you heard of the Eternal Flame?”

  Yalda was glad she’d already committed to a policy of benign inexpressiveness.

  Paolo buzzed amiably—not mocking his guest, but treating this invocation as if it could only have been meant as deliberate, flippant hyperbole.

  Eusebio’s tone remained scrupulously polite, but Yalda could see him struggling to keep his frustration in check. “The old stories of the Eternal Flame were misguided in their details,” he said, “but modern ideas suggest that such a process might actually be possible.” He turned to Nereo. “Am I wrong about that?”

  Nereo said cautiously, “Rotational physics doesn’t rule it out immediately, the way our earlier understanding of energy did.”

  Paolo was surprised. “Honestly?” He put down the loaf he’d been chewing. “So all those wild-eyed alchemists might simply have given up too soon? Ha!” He shot Nereo a reproving glance, as if his science adviser might have thought to mention such an interesting fact a few years earlier.

  Eusebio said, “Sir, may I tell you one message we received very clearly from the people of Red Towers last night?”

  “Certainly,” Paolo replied.

  “My venture, as it stands, is too modest,” Eusebio confessed. “Nobody believes that a few dozen people in a vehicle perhaps the size of this compound could survive long enough to reap the benefits of their situation. The basic physics of the trajectory puts no limit on the length of their journey—no limit on the advantage they could have over us in years. But the practicalities of their situation will be the determining factor. A robust society requires a certain scale, in both people and resources. An isolated camp in the desert with well-chosen supplies might endure for a generation or two, but it takes a whole city to flourish for an age.”

  Paolo said, “I understand.” He was silent for a while. “But how large a rocket would be large enough? No one knows. It’s a very big risk to take, based on nothing but guesswork.”

  “If it stops the Hurtlers from destroying us,” Eusebio replied, “how could it not be worth it?”

  “But that judgment depends not only on the travelers succeeding,” Paolo reasoned, “but also on an absence of other solutions. The same resources spent here on the ground might solve the problem more efficiently. I can’t speak for others, but I do like having my own money doing its work somewhere nearby, where I can watch over it.”

  “Yes, sir.” Eusebio lowered his gaze. The rejection could not have been clearer.

  Paolo turned to Nereo. “So, perhaps the Eternal Flame could be made real?”

  “Perhaps,” Nereo conceded reluctantly. “But there are a number of subtleties to be considered, some of which we barely understand.”

  “What if I hired a gross of chemists to go out into the desert and start testing every possible combination of ingredients? Somewhere far from any people they could harm?” Nereo didn’t reply immediately, but Paolo was already warming to his own vision. “We could require that each experiment take place at a location on the map that encoded the particular choice of reagents. That way, it would be apparent from the positions of the craters which reactions should never be attempted again.”

  “Ingenious, sir,” Nereo declared. He was being sarcastic, but Paolo chose to take him at his word.

  “The credit,” Paolo replied, “should go to our guest, who delivered this inspiration to me.” He bowed his head toward Eusebio.

  Throughout their second, final show in Red Towers, Eusebio kept up an admirably professional veneer of optimism, but as soon as they were back in Nereo’s guest room he slumped against the wall.

  “It’s too much,” he said numbly. “I can’t do this anymore.”

  “We can find someone else to take over the recruitment drive,” Yalda suggested.

  “I’m not talking about the recruitment drive! The whole project is impossible. I should give up this idiocy and go back to the railways; let someone else worry about the Hurtlers. I’ll probably be dead before the worst of it, anyway. Why should I care?”

  Yalda walked over to him and touched his shoulder reassuringly. “So Paolo won’t invest in the rocket. He’s not the only wealthy person on the planet.”

  “But Red Towers is as good as it gets,” Eusebio said. “Journalists here understand the whole message. People listen to our plan and offer intelligent, constructive criticism. But no one here will volunteer to be a passenger, and the man who owns half the city would rather try to resurrect alchemy as his weapon against the Hurtlers than watch his money disappear into the void.”

  “It’s a setback,” Yalda admitted. “But don’t make any decisions right now. Things might look different after a few days.”

  Eusebio was unconvinced, but he tried to receive her advice graciously. “Don’t worry,” he said. “I couldn’t throw this all away in an instant if I wanted to. It’s not as if someone’s going to walk up to me tomorrow and offer to buy the mining licenses.”

  Yalda woke in the night, unsure for a moment where she was. She raised herself up on her elbows to look around the room. Soft-edged shadows tinged with spectral colors stretched across the floor from beneath the window, framing Eusebio’s sleeping form.

  He was beautiful, she realized: tall and strong, perfectly shaped even in slumber. How had she never seen that before?

  But it was the thought of him that had woken her. If they brought their bodies together now, she was sure she could extract the promise from him. Her flesh wouldn’t die; she’d let her mind and its anxieties fade away, leaving her children with a devoted protector. He was the closest thing to a co she could hope for, and she did not believe he would refuse her. Not here, alone with her; not if she insisted.

  She rose to her feet and stood watching him, imagining his skin pressed against her own, rehearsing the words that would convince him. If Paolo could have six sons, why should Eusebio limit himself to two? She would not say a word against his co; she wasn’t asking him to betray his lifelong partner, only to enlarge his prospective family.

  Eusebio opened his eyes. She saw him register her presence, her gaze. She expected to be questioned, but he remained silent, as if everything between them was clear to him already. If she lay beside him now, it would not take much persuasion. They both wanted the comfort of this, and the promise of life for their children.

  But as she took a step toward him, Yalda felt a chilling clarity spread across her mind. She wanted comfort—not oblivion. And whatever Eusebio wanted, it was not the encumbrance of her children. Nothing in this beguiling vision had any connection to their real plans, their real needs, their real wishes. The oldest part of her believed it could survive this way—as her mother had survived in her—but even that blind hope was misplaced. Eons of persistence would count for nothing when the sky lit up with orthogonal stars.

  She said, “The Hurtlers woke me. I’m sorry I disturbed you.”

  Eusebio said, “That’s all right, Yalda. Just try to sleep.” He closed his eyes.

  Yalda returned to her bed, but she was still awake at dawn.

  After breakfast, Amando came in his truck to take Eusebio to another test launch. Eusebio didn’t demur; the sheer momentum of the project still counted for something. At least until the money ran out.

  Nereo walked Yalda to the station. “I’m sorry we didn’t have a chance to talk optics,” he said. “I’ve been tinkering with your light equation recently, trying to find the right way of adding a source.”

  “Really?” Yalda was intrigued. The equation she’d come up with five years before described the passage of light through empty space, but said nothing about its creation. “How far did you get?”

  “I took some inspiration from gravity,” Nereo said. “Think about the potential energy of a massive body, such as a planet. Outside the body, the potential obeys a three-dimensional equation very similar to the one for light: the sum of the second rates of change along the three directions of space is zero. Inside the body, instead o
f zero that sum is proportional to the density of matter.”

  “So you think I should add a similar term to the light equation, to represent the light source?” Yalda thought about this. “But a light wave involves a vector with four components; the source would have to be the same kind of thing.”

  Nereo said, “What about a vector that’s aligned with the history of the light source—pointing into its future—with the length of the vector proportional to the density of the source?”

  “That’s the right kind of vector,” Yalda conceded, “but what would this ‘density’ actually be describing?” He didn’t mean mass; this was something else entirely.

  “Whatever property matter needs in order to produce light,” Nereo replied. “We don’t have a word for it yet; maybe ‘source strength’? But assuming we can put a number on it, we can talk about how tightly it’s packed: the ‘source density’.”

  “Hmm.” Yalda worked through the implications as they entered the station. “So if we’re looking at the simplest case, where everything is motionless, only the time component of the equation would be non-zero, and it would have solutions a bit like gravitational potentials.”

  “But not quite the same,” Nereo stressed. “These solutions oscillate as you move across space.”

  Yalda’s train was boarding; they didn’t have time to take the discussion further. Nereo said, “I’ll send you a copy of my paper when I’ve written it.”

  On the journey back to Zeugma, Yalda managed to find one solution to Nereo’s equation herself: the equivalent of the potential energy around a motionless, point-like mass.

  For gravity, it had been known since the work of Vittorio an era and a half ago that the potential was inversely proportional to the distance from the mass. For light, the overall strength of the wave diminished with distance in exactly the same fashion, but it also underwent an oscillation with the smallest possible wavelength—that of a wave traveling at infinite speed. All of this was just an idealization—the need to wrap around the cosmos smoothly would impose additional constraints and complications—but it was a start.

  Yalda pondered the curve. Perhaps there was something like this present in everything from sunstone to a flower’s petals. Light at the minimum wavelength would be invisible, so when the source was motionless the petals would be dark. But a system of oscillating sources, suitably arranged, might well be able to twist the original pattern into a set of tilted wavefronts, corresponding to light slow enough to lie in the visible realm.

  Exact solutions when the source was in motion wouldn’t be easy to find, but the general idea made sense; to generate a wave that moved energy around, something had to feed in the energy in the first place, whether that meant jiggling the end of a string or vibrating your tympanum in air. That the creation of light would draw true energy out of an oscillating source—increasing its kinetic energy, rather than draining it—was a peculiar twist, but that was rotational physics for you.

  There was, Yalda realized, another strange twist, arising from Nereo’s choice of a vector aligned with the particle’s history as the term to add to the equation. If you accelerated a light source long enough to curve its history around a half-circle and send it backward in time… then once it was settled on its new, straight trajectory it would be surrounded by a wave that was the opposite of the original. Compared with its earlier self, the pattern of light the source generated would now appear upside-down.

  But how could a featureless speck of matter tell which way it was traveling through time? The arrow of time was meant to come solely from entropy’s rise; a lone particle couldn’t grow disordered. A wave changing sign wasn’t as dramatic as the difference between a stone being smashed and the shards spontaneously reassembling, but it did imply that there might be two distinguishable kinds of source, at least when they weren’t moving too rapidly: one “positive”, one “negative”.

  It was evening when the train pulled into Zeugma. Yalda didn’t have the energy to face the four squabbling brats at bedtime, so she wandered around the city center for a while, killing time until she was sure they’d be asleep.

  When she finally braved the apartment, she was surprised to find Lidia sitting in the front room, in the dark.

  “I thought you’d be at work,” Yalda said. “Did they change your shift—or was there some problem with the helper?” The whole point of asking Eusebio to pay her had been to hire someone to watch the children in her place.

  “The helper was fine,” Lidia said, “but we didn’t need her tonight.”

  “So you’re working mornings again?”

  “No, I’ve lost my job.”

  “Oh.” Yalda sat beside her. “What happened?”

  “Did I tell you about my new supervisor?”

  “The idiot who kept asking you to become his deserted brother’s co-stead?” Yalda didn’t believe the rumor that Lidia had murdered her own co, but there were times when it might have been helpful if more people had taken it seriously.

  Lidia said, “Two days ago he came up with a new offer: either I hand the children over to his brother, or he tells the factory owner that I’ve been stealing dye.”

  “But you only ever took from the spoiled batches. I thought everyone did that!”

  “They do,” Lidia replied. “But he told the owner I was taking good jars as well. He had some kind of paperwork prepared, inventory records that made it look plausible. So the owner believed him.”

  “What a piece of shit.” Yalda put an arm across her shoulders. “Don’t worry, you’ll find a better job.”

  “I’m just tired,” Lidia said, shivering. “I used to think everything would be different by now—by the time I was this old. But what’s changed? There aren’t even women on the Council.”

  “No.”

  “Why doesn’t your friend Eusebio do something about that?” Lidia demanded.

  “Don’t blame him,” Yalda pleaded. “He’s already fighting the Council on a dozen different fronts.”

  Lidia was unimpressed. “The whole point of being a new member of the Council is to fight the old guard and get something useful done. But sharing power never seems to be a priority with anyone.”

  “Shocking, isn’t it?”

  “Will he have free holin on his rocket?” Lidia wondered. “I might join up for that alone.”

  Yalda said, “Of course there’ll be free holin. In fact, it looks as if the entire crew will consist of solos and runaways: one of each, in a rocket the size of this room.”

  “That’s beginning to sound less tempting.”

  Yalda rose. “I’m sorry about your job. I’ll ask around, see if anyone knows of a position…”

  “Yeah, thanks.” Lidia put her head in her arms.

  As Yalda crossed the room, a slab of sharp-edged brightness on the floor beside the window caught her eye. The diffuse light from the Hurtlers didn’t look like this; it was as if one of their neighbors had stolen a spotlight from the Variety Hall and mounted it on their balcony.

  She walked over to the window and looked out. The neighbors weren’t to blame; the light was coming from high above the adjacent tower. A single bluish point was fixed in the sky, displaying no discernible color trail.

  Lidia had noticed the light too; she joined Yalda by the window.

  “What is it?”

  Yalda suddenly realized that she’d seen the same object high in the east when she’d walked out of the railway station—but it had been a great deal paler then, so she hadn’t given it a second thought. “Gemma,” she said. “Or Gemmo.” The naked eye couldn’t separate them, so there was no point guessing which of the two had suffered the change.

  Lidia hummed with exasperation; she was in no mood to be mocked. “I might not be an astronomer,” she said, “but I’m not a fool. I know what the planets look like, and none of them are that bright.”

  “This one is, now.” A dark, lifeless world that had once shone by nothing but reflected sunlight was turning into a sta
r before their eyes.

  Lidia steadied herself against the window frame; she’d grasped Yalda’s meaning. “A Hurtler struck it? And this is the result?”

  “It looks that way.” Yalda was surprised at how calm she felt. Tullia had always believed that a large enough Hurtler could set the world on fire. Dark world, living world, star; they were all made of the same kinds of rock, the distinction was just a matter of luck and history.

  Lidia said, “Now tell me the good news.”

  Good news? Gemma and Gemmo were far away, and much smaller than the sun, so at least the world wouldn’t suffer intolerable heating from the new star.

  In fact, the two planets were so far from the sun that the density of the solar wind in their vicinity was believed to be a tiny fraction of its value around the nearer worlds—and no Hurtlers had ever been seen at such a distance, in accord with the idea that it was friction with that gas that made the pebbles burn up. But the lack of the usual pyrotechnics hadn’t prevented this unheralded impact.

  The Hurtlers were everywhere, visible or not—and Ludovico’s absurd explanation for them that blamed the solar wind alone was now completely untenable. Yalda didn’t expect him to recant, but the people who’d voted with him against Eusebio’s offer didn’t have the same degree of pride invested in the matter.

  Perhaps the new star was no more exotic than the Hurtlers themselves, but its meaning was far easier to read: the only thing that had spared their own planet so far was luck. Any day, any night, the world could go the same way.

  “The good news,” Yalda said, “is that we might just get our flying mountain after all.”

  12

  “Stay close to me!” Yalda called out to the group as they approached a bend in the tunnel. “Anyone suffering from nausea? Weakness? Dizziness?”