The Clockwork Rocket

“The stem of each stand represents a short stretch of the history of the Peerless,” Yalda explained, “before it was launched. Time is measured vertically, straight up from the floor; space is horizontal. Back then, the stars were only moving slowly in relation to us, so we can think of them as being spread out evenly across the floor, with their histories rising up almost vertically.” She glanced across at Fatima’s neat, stylized rendering.

“And the pyramids are light?” Ausilio asked.

“Exactly,” Yalda confirmed. “Incoming light, emitted long ago by the surrounding stars and finally reaching us at the apex of the pyramid. The two pyramids represent violet light and red light, as seen by us. The steepest one is…?”

“Red,” Prospera volunteered. “The edges cross less space in a given time—a slower velocity.”

Yalda said, “Right. A cone would provide a more detailed model, showing all the rays of a given color, but the eight edges of each of these pyramids are enough to give us a good idea of how the light behaves—and the fact that they mark off equal angles around the Peerless is going to be helpful to us.”

Everyone had finished the first view. “Could you look down from above now, please,” Yalda instructed them, “and draw what you see.”

She waited until most of the students had new sketches on their chests before continuing. “Think about the light rays that reach us,” she said, “between the edges of each of these triangles. When the Peerless was motionless compared to the stars, each of these equal segments in our view of the sky took in light from an equal slice of our surroundings. The stars were arranged uniformly around us in space, more or less—so we saw them scattered uniformly across the sky, with no one direction appearing very different from another.”

Yalda looked around and chose one of the quieter students: Ausilia, whose co did most of the talking for the pair. “Could you tip the stems over for me, please? Try to make them both as close as you can to a one-eighth turn down from the vertical. Halfway to orthogonal. The speed of blue light.”

The stems were connected to the base with a swiveling joint; Ausilia approached the task diligently, stepping back several times to check the angles.

“Could everyone draw the new configuration, please,” Yalda said. “From the side first.”

Severa approached her and whispered, jokingly, “You know you’re robbing them of the big payoff when we learn to do all of this algebraically.”

“Ha! How far away is that?”

“A couple of years, I expect.”

“And how many of this class will stick with it that long?”

Severa thought for a while. “More than half.”

Yalda was encouraged; for the first generation that would be a good result. But right now, she was going to ensure that every one of these people could make sense of the view around them using nothing but their eyes and their intuition.

She addressed the class again. “This drawing tells us something straight away, about the view we can expect from the Peerless. Any suggestions?”

Prospera said, “The violet light coming in from behind us has been tilted so far that it’s… gone past horizontal.” Her tone made it clear that she knew the change had to be significant, but she couldn’t quite see what it implied.

“So if you follow the light in toward us,” Yalda suggested, “what happens to its height?”

“It gets less, as you move in,” Prospera replied.

“Its height gets less. And what does height stand for, in this picture?”

“Time.” Prospera pondered this for a moment. “So the light would have to come from the future?”

“Exactly. It would have to be traveling back in time. Not for us—it’s still coming from our past—but for the star that emitted it. So what you’ve found tells us that no ordinary star that lies directly behind us—in the rear one-eighth of our view, or a bit beyond that—can appear to us in violet, because that would require the star to have emitted light into its own past.”

“But it would be different for an orthogonal star, wouldn’t it?” Fatima asked eagerly.

Yalda said, “Well, their time is horizontal in this picture, and their future is aligned with the direction in which we’re traveling, but—”

Fatima ran forward to the edge of the cave and peered down the slope of the mountain.

“—but unfortunately, the rock below us hides that part of the view.” Between the mountain and the haze from the engines, there was no chance at all of observing the orthogonal stars yet.

Yalda asked the students to draw the tilted pyramids from above. A few people became confused, or drew some preconception rather than the actual view, but after noticing the emerging consensus of their peers they looked again and refined their own versions.

She waited until everyone had the essential features correct.

“Each of the eight segments still represents an equal portion of our view,” she reminded them. “But their relationship with the surroundings has changed. Let’s start with the violet, the broader pyramid. Can someone tell me what’s going on?”

Ausilia spoke up. “At the front,” she said, pointing out the triangle on her chest, “the angle between the edges is much bigger than one-eighth now, seen from above.”

“Which means…?” Yalda pressed her.

Ausilia hesitated, but then followed through. “Our one-eighth of the view is taking in light from more than one-eighth of the stars?”

“Exactly!” Yalda approached her and had her turn so the whole class could see her sketch. “In the direction in which the Peerless is traveling, this slice of the view has a wider reach, so light from more stars gets crammed into it. We still see it as an equal eighth, but as far as our surroundings are concerned it’s much more.” She stepped away from Ausilia and gestured toward the zenith. “Focus on the violet ends of the trails. They started out scattered uniformly around us, before the launch; now they’re crowded together around the direction in which we’re traveling. And the reason is simple: when you take two lines that are a fixed angle apart—like the edges of that front triangle—the more you tilt them, the greater the angle between them will seem to be.”

She waited for the simple logic of it to mesh with the evidence before their eyes, then added, “In the opposite direction there’s an opposite effect. The mountain makes that harder to see—and we’ve already shown that there’s a region behind us where we won’t be getting violet light from the ordinary stars anyway—but in general, looking back the view is sparser.”

Fatima was standing closer to her than Ausilia now, so Yalda moved beside her and pointed to her drawing of the red pyramid.

“What about red light? If you compare the rear triangles in the two pyramids, it’s clear that the angle for the red light is even smaller than it is for violet—so we should see the red images behind us spread out across the sky more than the violet, pushed forward compared to the violet. And that difference persists as you move away from the rear. For any given star, the red light ends up further from the nadir. Does that sound familiar?” Yalda pointed to the vertical trails behind her, the red ends all higher than the violet.

“But what happens with the red light,” she continued, “when we look in the direction in which we’re traveling? There are only five triangles from the pyramid visible here. What’s going on with the three triangles that point to the front for us?”

Fatima helpfully added three lines that made the hidden triangles visible:

“They’ve ended up pointing backward,” Ausilia said.

“Yes!” Yalda raised her eyes to the zenith. “See those strange red ends of trails poking out the wrong way? They’re stars that are actually behind us! The pyramid shows us that in red light, we can’t see anything that’s in front of us—in the sense that an onlooker fixed to the stars would judge something to be ‘in front’. But our view isn’t empty of red in that direction; instead of seeing what’s in front of us, we’re seeing some of what’s behind us.”

“And all of it twice,” Fatima said, running a fingertip across the diagram toward the apex of the pyramid. “Every star we see behind us in red… we also see in front of us in red.”

“That’s right,” Yalda said. “But though it’s light from the same star—and it looks the same color to us—it’s not the same light.”

Fatima thought for a moment. “The red light we see, looking back, left the star at a greater angle than the angle it makes with us. So it left the star as faster-than-red light… but because we’re fleeing from it, it’s not gaining on us as quickly as it would if we were still. Our motion has changed the color from violet or ultraviolet to red.”

“Yes.” Yalda pushed her, “And the other light? The red light we see looking forward that came from the very same star?”

Fatima gazed down at the diagram, struggling. “From the angles, I think it must have left the star moving quite slowly. But if it’s moving so slowly, how could it ever catch up with us?”

Yalda said, “If you’re getting confused, just draw… whatever it is you need to draw.”

Fatima made a new sketch, paused, then added some annotations.

“The red light we see as coming from ahead,” she said, “must have left the star behind us long ago… but now we’ve caught up with it, we’re overtaking it. That’s why it strikes us from the front. The star is behind us, but the light was ahead of us.”

Fatima looked up at the zenith, then a fresh revelation struck her. “That’s why those upside-down star trails sputter out at green! However long ago the light left the star, the angle it made with our history could never end up greater than the angle for blue light. But blue would be the absolute limit—light from infinitely long ago. In real life we can’t expect to see that far back.”

She modified her diagram to show what she meant.

Yalda said, “That’s all true—though the reason we don’t see any blue in those trails is also a matter of how much power the star emits in different parts of the spectrum. The light we’d see as blue would have to leave the star as far-infrared, traveling incredibly slowly. So it can’t be carrying energy out of the star at a very high rate… which means the star itself simply won’t be shining very brightly in that color.”

Ausilia had been following the discussion closely, though Yalda wasn’t sure how much she’d understood. But then she pointed to Fatima’s chest and said, “If that star happened to be in front of us instead, its slowest light would still end up looking blue, wouldn’t it? It would just approach it from the other end of the spectrum. So its trail would start out violet, but never quite get as far as blue.”

She hesitated, then produced a diagram, echoing Fatima’s, to illustrate her point.

Yalda chirped with delight. She gestured to Ausilia to turn so that everyone else could see the picture. “That’s the last puzzle solved: why some trails above us are just violet and indigo. And that’s it: between you, you’ve unwoven the whole sky.”

In fact, not everyone had caught up with Fatima and Ausilia, but Yalda stood back and let the students help each other past their lingering puzzlement. As they looked to the stars and back, connecting the details of the color trails to the figures before them, the thrill of understanding spread.

This alien sky belonged to them, now. Its transformation would become even more extreme as the Peerless moved faster, but the new ways of seeing it that they had acquired would handle those changes with ease.

Yalda knew that only a few of these people would end up as researchers, only a few as teachers. But even if they did no more than pass their understanding on to the children of their friends, it would all be a part of strengthening the culture, ensuring that their descendants were at ease in this strange new state.

And the most beautiful thing of all, she realized—struck by it anew, because she’d almost begun taking it for granted—was that every one of these solos and runaways, every one of these partnered women and their cos, would have the chance to live out their lives without coercion, making use of their talents, untrammeled by the customs of the old world.

Forget the Hurtlers, forget the orthogonal stars. For that alone, the struggle had been worth it.

16

Strapped to her bench in the navigators’ post, Yalda counted down the pauses. It had always been Frido or Babila doing the honors before, but she’d taken the role for herself this time, knowing it would be her last chance.

“Three. Two. One.”

The anticlimax that followed was welcome; any sudden, perceptible change would have meant that something had gone horribly wrong. The clock advanced another two lapses before Yalda noticed anything at all—and even then she had her doubts; the hint of dizziness, of balance gone awry, could as easily have been nothing but anticipation. The machinists were tapering off the flow of liberator in an excruciatingly protracted manner; it would take a full chime for the engines to shut down completely.

“Can you hear that?” Frido asked.

“Hear what?” Babila raised her head to listen.

Yalda said, “The rock.” Over the hammering of the engines, she could make out a low creaking sound coming through the ceiling. The mountain had lost only a fraction of its weight, but it was already beginning to rearrange itself, stretching out beneath the diminishing load. That was not a bad sign; better that it start adjusting now than save up the changes for a sharp transition later that released all its energy at once.

Four lapses into the shutdown, Yalda could have sworn that the skin on her back was growing numb—and knowing the true reason that she was starting to register less pressure did nothing to make the illusion less compelling. At seven lapses her dwindling weight began triggering flashes of panic, in which she was convinced—for a moment or two—that the legs of the bench had given way beneath her. The engines were producing a strange, soft patter now; the rock above had fallen silent. For the first time since the launch, she could hear the ticking of the clock from across the room.

Babila turned and vomited up her last meal, thoughtfully depositing it out of sight of her companions—though the floor itself might not hold the mess down much longer. With no hope of reconciling the room’s apparent steadiness with the alarming sense that everything was slipping, Yalda closed her eyes. She found herself visualizing the Peerless from a distance, a dark cone against the color trails. But in this fanciful vision the middle third of the mountain had turned as soft as resin, and she watched in horrified fascination as it stretched out into a narrow tube, then snapped—

She braced for the impact that must have followed the same plummeting sensation for every one of her ancestors who’d had the misfortune to experience it. That the crash never came was no surprise, but nor was it a relief; the threat of impending damage refused to be dispelled.

Yalda lay on her bench humming softly, waiting for something to change. Finally, she grew sufficiently inured to the sense of dread to open her eyes and look around. Frido had removed most of his straps and was sitting up; Yalda did the same, and felt no worse for it. In fact, the actions were reassuring, proving that she still had control over her body.

Half a dozen ropes had been strung across the room at shoulder height. Frido finished unstrapping himself and reached up to take hold of the nearest of them. At first he tried to walk across the floor, using the rope as an aid, but his feet kept slipping on the stone. Then he changed his approach, curling his body up and gripping the rope with his feet as well, forming them into a second pair of hands. After a few unsteady moments, he mastered the technique and scurried along the rope, hand over hand, as far as the wall. Then he swung onto a second rope that was fastened to the stone beside him, and set off in a new direction.

Babila watched him, stupefied. “I’m not doing that for the rest of my life,” she moaned. “You can send me home right now.”

Yalda removed the strap from around her waist and took hold of the nearest rope. Following Frido’s example, she re-formed her feet and tried to raise them, but then she found herself tumbling slowly in mid-air, still clinging to the rope with a single hand but unable to seize it with any other appendage.

“Hunch up, you fool!” Babila suggested irritably; in her state of nausea even Yalda’s clumsiness was a personal affront. But it was good advice; Yalda had no control over her body’s orientation, but she could still bring all four hands together at the point where she held the rope. From there, she slid them along it, spacing them out more comfortably. She looked across the room to study Frido’s technique—he was never taking more than one hand off the rope at a time—then she began tentatively pulling herself along.

She was fine at first, until her sense of the vertical flipped and the cozy illusion that she was hanging down from a horizontal rope was replaced by the equally false conviction that she was perched above it, precariously balanced, certain to tumble over at any moment. She closed her eyes and pictured herself ascending instead: climbing up a vertical rope. When she opened her eyes and started moving again, her chosen illusion persisted; the small forces on her body as she dragged it along the rope were oriented in the right direction to reinforce the idea.

After practicing for a while she became reasonably proficient, but it was disconcerting to be so dependent on the ropes. If one of them snapped, installing a replacement would not be easy; it was clear now that they’d underestimated the number of handles needed on the walls to ensure that a chamber like this remained navigable, come what may. And if threading a new rope into place was a major task, any kind of construction work would be impossible.

Frido left the navigators’ post, dragging himself through the doorway to see how the neighboring machinists were faring. Babila was still sitting on her bench looking miserable. Yalda approached her.

“Try the ropes,” she said. “I’ll stay close to you.”

“I can’t do it,” Babila declared.

“You can’t hurt yourself. You can’t fall.”

“What if I get stranded?” Babila retorted. “Drifting in mid-air?”

It wasn’t an entirely ridiculous objection; the chamber was high enough that someone really could end up out of reach of anything solid—let alone anything they could actually grasp.

“Even if you let go of the rope accidentally,” Yalda pointed out, “you won’t drift away from it very quickly. You’ll always have time to grab hold of it again. And I’ll stay in front of you, I’ll make sure you’re all right.”

Babila wasn’t happy, but she reached up with one hand and grabbed the rope beside her, released the strap around her waist, then refashioned her obsolete feet and curled her body up so she could grip the rope in four places.

“We’re all animals now,” she declared forlornly. “I feel like an arborine.”

“Is that so bad?” Yalda wondered. “We’re going to have to re-learn everything we do, but if we’ve done something similar before, in the forests, that can only help.”

“And which zero-gravity forests were they?” Babila began pulling herself along the rope with surprising speed.

Yalda backed away from her hastily. “None in the past,” she said, “though it might be interesting to see how they deal with it now. We might learn something from all of the animals.”

“They won’t know what hit them,” Babila predicted gloomily. “They’ll cope much worse than we do.”

“Maybe.”

For all her reticence, Babila proved to be quite agile. Yalda suspected that most of her pessimism was just the nausea talking, and that both would wear off soon enough.

“A part of me keeps thinking that this is temporary,” Yalda admitted, clinging to the rope near the center of the room; the chamber now seemed to her like a disk-shaped space standing up on one edge. “As if it’s a trick that’s all down to some clever new way of using the engines, and if we get bored with it we can always just stop.”

“I know what you mean,” Babila said. “How can a condition that came without effort back home require a whole burning mountain to sustain it… while one that was impossible for more than a pause or two becomes the natural state?” She shivered. “Think of all the people who’ll live and die like this: feeling as if they’re endlessly falling.”

Yalda listened to the silence of the dead engines. She’d always expected that she’d welcome it ecstatically when it finally came, but it was going to take a while to grow accustomed to the absence.

“They won’t feel as if they’re falling,” she said. “They’ll feel the way they always feel. Only the old books will tell them that there was once a thing called ‘falling’ that felt the same.”

A day after the shutdown, Frido, Babila and a group of the machinists set off up the mountain. New jobs were waiting for them, close to the summit. Yalda lingered in the navigators’ post, promising to follow them later; nobody pressed her to explain why.

When she opened the door to the cell a thick haze of dust spilled out, red in the moss-light. The soil on the floor here was covered with the same netting as they’d used in the gardens, but without plant roots to help bind it, it was scarcely contained.

Nino was at the back of the cell, clinging to the netting; bundles of paper tied with string drifted around him, along with several clumps of faeces and half a dozen dead worms.

“Come out of there.” Yalda heard the anger in her voice, as if it were Nino’s fault that he’d been living in this squalor. She should have checked on him much sooner.

“Is there anyone around?” he asked.

“No.”

Nino used the netting to crawl across the floor. He hesitated at the doorway, confused for a moment, then Yalda backed away and made room for him on the rope that was anchored to the wall beside the entrance. He took hold of the rope and drew himself toward it, then reached back and swung the door closed, stopping any more of the dust escaping.

He looked over at the navigators’ bed, fully enclosed beneath a tarpaulin. “I was thinking you must have done something like that. Is it easy to use, without everything spilling out?”

“Not really,” Yalda confessed. “I think we’re going to have to start adding some kind of resin to the sand.”

Nino said, “My only problem is that it’s been hard to read, through the dust. If you could spare a couple of those tarpaulins—”

“Forget about that mess.” Yalda gestured dismissively toward his cell. “I’ll make sure you have a proper bed, upstairs.”

Nino hesitated; she recognized the way he held the muscles around his tympanum when he was struggling to find the most tactful way to phrase something. “That’s kind of you,” he said, “but it would be better if I could fix what I have already.”

“Nobody’s staying down here,” she said. “Now that we’re orthogonal, barring an emergency the engines won’t be fired again in our lifetimes.”

“I understand,” Nino replied. “There’ll be no full-time navigators, and you’ll have work to do elsewhere. But it’s better if I stay.”

“Are you worried about the trip?” Yalda asked him. She hadn’t handled the last move as well as she might have. “I’ll get some of the machinists to play guard on the way up. No one will be able to accuse you of running wild, if you have a whole escort.”

“No one will accept me being up there at all,” he said. “Let alone accept the sight of you coming to my cell—”

Yalda cut him off irritably. “If you think that’s a problem, I’ll put your cell inside my apartment. Then no one need know how often I visit you.”

Nino buzzed with bleak amusement. “Do that, and we’d both be dead in a stint.”

“I don’t believe that.”

“No? Then you don’t know what people are capable of.”

Yalda was angry now. “Don’t patronize me. I was in prison myself, remember.”

Nino said, “You suffered for a while at the hands of a spoilt brat who was more interested in hurting someone else. That’s not the same as trying to live in a world where everyone is your enemy.”

“And the stunt you pulled for the same spoilt brat,” Yalda retorted, “is not the be-all and end-all of life on the Peerless. People have more important things to think about.” She took her hands off the rope and drifted free for a moment. “Do you know how to make a loaf, like this? How to fix a lamp? How to sow a crop?”

“So everyone will be preoccupied with weightlessness for a while,” Nino conceded. “That’s no reason for us to push our luck. Leave me here, let people forget about me. Or if they think of me at all, let them be satisfied that I’ve been banished as far from them as possible. Banished and abandoned.”

Yalda could not accept this. “Abandoned to starve? Abandoned to go crazy?”

Nino said, “The moss is edible; have you really never tried it? But if you want to help me… choose someone you trust—someone whose movements will attract no attention—and send them down here with a few loaves and books every couple of stints. If I can read something new every now and then, I won’t lose my mind. And I still have another draft of the sagas I can work on.”

“If I leave you here alone,” Yalda said, “what’s to stop someone from coming down and killing you? You’re afraid that if I take you to the summit and make it clear that you have my protection, people will be so outraged that they’ll turn against me… but how long do you think you’ll last with no protection at all?”

Nino thought about this, seriously. “If you put enough locked doors in the way, that might help. You can justify it as a way of keeping me down here, even if I manage to break free from my cell. Some people will be happy enough with the thought of me buried in an impenetrable dungeon—and I’ll be a little safer from the others, who won’t be happy until I’m dead.”

Yalda said, “If I call a meeting and explain to everyone why you did what you did, they should accept that your imprisonment is punishment enough. And they should respect me more, not less, for refusing to bow to tradition. The Peerless exists to bring change. Every last runaway here should be ready to shout: octofurcate the old ways! If they really wanted to live by those rules, they should have stayed in a world where they still held sway.”

Nino took his time replying, striving for tact again. “That’s a brave speech, Yalda,” he said, “and I can’t fault it, myself. But before you try it on the whole crew… can you name one person who started out opposing your decision, who you’ve managed to bring around with the same fine words?”

“Yalda! Are you busy? Please, you have to see this!”

Isidora was calling from outside Yalda’s office, too excited to waste time dragging herself into the room. Yalda was in the middle of a long calculation on the energetics of oscillating luxagens, but after a moment she slipped her notes into a hold and latched it. Isidora’s bursts of enthusiasm were annoying at times, but it was thanks to her efforts that the optics workshop was functioning again so soon. If she wanted to share her excitement at having rendered one more piece of apparatus usable without gravity, it would be churlish to refuse her.

Yalda dragged herself across the room and through the doorway, four hands shuttling her along the two parallel ropes. She retained the extra pair of hands she’d been using on her papers, in anticipation of having to twiddle a focusing wheel or adjust the angle of a prism.

Before Yalda had come within half a stretch of her, Isidora was already backing away down the corridor toward the workshop.

“What’s the great achievement?” Yalda called after her.

“You have to see this for yourself!” Isidora replied.

The walls of the optics workshop were kept free of the ubiquitous luminous moss, so the room’s deep shadows and controlled lamplight made it eerily reminiscent of its counterpart in Zeugma University—with the surreal placement of people and equipment only heightening Yalda’s sense of wandering into a nostalgic hallucination. Isidora was waiting in a corner where Sabino, a young researcher, was operating one of the microscopes, while clinging to two wooden bars that ran between the erstwhile floor and ceiling.

The microscopes had been back in action for days. Yalda approached, intrigued.

“What’s the new development?” she asked. Two closely spaced clearstone slides were positioned at the focus of the instrument; whatever they enclosed was—unsurprisingly—too small to discern with the naked eye, but they were attached to an elaborate mechanism of levers and wheels that Yalda hadn’t seen before, with a slender rod reaching into the space between them. In front of the small sunstone lamp that was illuminating the specimen was a thin slab of material that she recognized as a polarizing filter.

Sabino said, “Please, take a look for yourself.” He was shy with Yalda, but she could see that he was at least as excited as Isidora.

He moved aside and let Yalda take the bars in front of the microscope. Even the solid wood trembled a little from the shifting forces as they changed places; Yalda waited for the vibrations to die down, then peered into the eyepiece.

The field was full of translucent gray specks, most of them roughly spherical, albeit with jagged outlines. Shape aside, they possessed no visible features, no apparent parts or fine structure. Not all of the specks were in focus; the slides hadn’t been pressed together tightly enough to touch the material, to hold it in place. But the focal plane of the microscope had been adjusted to take in one particular speck; this one was fixed, gripped by a tiny pair of callipers that appeared solid black in their opacity. The other specks, though unconstrained, were barely quivering, demonstrating that the air between the slides was almost motionless.

“What am I looking at?” Yalda asked.

“Powdered calmstone,” Sabino replied.

“Under polarized light?”

“Yes.”

A pinch of fine sand—ground from calmstone or anything else—would not normally look like this. The grains would generally appear variegated in polarized light, made up of half a dozen regions of very different shades of gray. These were uniform, homogeneous.

“So you sorted them?” Yalda asked Sabino. “You picked out the purest grains you could find?”

“Yes. Maybe one in ten gross were like this.”

“One in ten gross? You’ve been busy.”

Yalda hadn’t had time to learn about Sabino’s project since coming up from the navigators’ post, but she could guess the rationale for his painstaking efforts. If solids such as calmstone were composed of regular arrays of indivisible particles—such as Nereo’s putative luxagens—then the best way to study their properties would be to obtain pieces of the material in question in which the array was as close to geometrically flawless as possible. An array of particles that maintained a regular pattern should have the same optical properties throughout; the usual mottled appearance of sand under polarized light ruled that out, but by chance there could always be exceptions. Sabino had found those exceptions, and discarded everything else.

“Try moving the wheel,” he suggested. “The top one on your right.”

Without looking away from the eyepiece, Yalda reached up with the right hand of the pair that sprouted from her chest, and found the wheel. She drew her fingertip along the rim, nudging it very slightly. In response, the callipers between the slides shifted, dragging their tiny cargo some fraction of a scant.

“What am I missing?” she asked. She didn’t think anyone expected her to be impressed by the fact that they could move single grains of sand around.

“Don’t just look at the callipers,” Isidora urged her. “Watch what’s happening around them.”

Yalda turned the wheel gently again; something caught her eye, but as soon as she stopped to try to scrutinize it, it ceased attracting her attention.

She moved the wheel a little more, then when the unexpected thing she hadn’t quite seen began to happen again, she started jiggling the wheel back and forth: jiggling the callipers, jiggling the tiny piece of calmstone it held.

As she did so, a second piece beside it moved in lockstep. Light was visible between the two; they were not touching. But whatever she did to the captive grain, its mimic followed as if they were two parts of a single, rigid body.

“Nereo’s force,” she said softly. “This is it? We can actually see it?”

Isidora chirped with glee, treating the question as rhetorical. Sabino was more cautious. “I hope that’s what it is,” he said. “I can’t think of any better explanation.”

According to Nereo’s equation, every luxagen should be surrounded by furrows of lower potential energy, within which any other luxagen nearby would prefer to reside. For a single luxagen, the furrows would simply be a series of concentric spherical shells, but the same effect acting on a multitude of particles could bind them together in a regular array—and in that case, the pattern of indentations in the energy landscape would extend beyond the array itself, offering the chance for another fragment of a similarly composed material to become ensnared in it. In effect, a sufficiently pure speck of rock could “stick” to another such speck, without the two actually touching.

“You tried this before, when the engines were running?” Yalda asked Sabino.

“Stint after stint,” he replied. “But gravity and friction must have overwhelmed the effect, because I never saw anything like this.”

Which meant that nobody back home could have seen it, either; it was only the condition of weightlessness that had made the experiment viable.

Yalda had been watching Sabino with her rear gaze; now she leaned back from the microscope and turned to face him. “This is excellent work!” she declared. “I’ll want you to give a talk on it to all the researchers, sometime in the next few days. Have you done anything on the theoretical side?”

Sabino produced a sheet of paper from a hold beside the microscope. “So far, only this,” he said.

“These are the energy troughs around a hexagonal array of luxagens,” he explained. “I drew it when I was first thinking about this project, back on the ground. It took about four stints to calculate.”

“I can believe that,” Yalda replied. It was a nice example of the kind of pattern that could persist beyond the edge of a solid—and she could easily picture a second array getting caught in those energy pits, like a truck sinking into another’s wheel ruts. She said, “We’re going to need to find ways to estimate the forces arising from much larger arrays, and to take account of the whole three-dimensional geometry. But don’t worry about that for now; you should concentrate on refining this setup.”

“All right.” Sabino was still a bit dazed, and though Yalda was trying to keep him as grounded as possible, he could not have failed to realize the importance of his discovery. If this experiment could be repeated and elaborated upon, it promised to make the nature of matter the subject of systematic inquiry—ending the days when the differences between a stone and a puff of smoke had no better explanation than the empty incantation that “solid objects occupy space”. Nereo had paved the way, but until now all his beautiful mathematics had remained untested guesswork. It was possible that Sabino and Nereo would be spoken of alongside Vittorio, who had made sense of the orbits of the planets—but Yalda thought it best not to overwhelm the young researcher with florid praise and promises of immortality. What he needed to do now was pursue the work itself.

The three of them talked through some possibilities for the next step; simply measuring the force that had to be applied to pull one grain of calmstone free of another was one obvious goal, but the torques required to twist them out of their preferred alignment might also yield information about the underlying geometry.

They took the discussion to the food hall, where it turned to the question of other minerals: were they all made of the same kind of luxagens, differently arranged? Could geometry alone account for all the differences between hardstone and clearstone, calmstone and firestone? The experiments they’d envisaged so far would only be the start; Yalda could see the chase that Sabino had begun lasting a generation.

But as she finally dragged herself off to her apartment to sleep, she thought: That’s the beauty of it—there is no rush. Time back home had come to a standstill, and any Hurtler that struck the Peerless now would barely leave a mark. The mountain’s resources would not last forever—and they certainly didn’t have enough sunstone to get themselves home by burning it the old way—but at last a tiny crack had opened up in their ignorance as to what a slab of sunstone might actually be.

Yalda climbed into her bed, shrugging at the resin-sticky sand until it covered her body beneath the tarpaulin, more hopeful now than ever that they were following the right course.

Fatima appeared outside Yalda’s office, back from her latest errand. Yalda ushered her in, then asked quietly, “How was Nino?”

“He didn’t look too bad,” Fatima replied. “He said to thank you for the books.”

Yalda was embarrassed. “You’re the one who should be thanked.”

“I don’t mind taking things to him,” Fatima said. “Climbing all those stairs would have been hard work, but now it’s not much different than going anywhere else.”

Yalda did not believe that she was endangering her with these trips—Fatima wouldn’t be blamed merely for following instructions—but she was worried about the effect on the girl of being Nino’s only visitor.

“It doesn’t upset you, having to see him like that?”

“I’d rather he was free,” Fatima said candidly. “He’s been punished enough. But I know you can’t let him out yet. He was kind to me, back when we were both still recruits, so I’m happy to go there and try to cheer him up.”

“All right.” This was the arrangement Nino had wanted, and for now Yalda had no better ideas. “Just promise that you’ll tell me if you start finding it difficult.”

“I will.” Fatima swung back on the ropes as if to depart, but then she stopped herself. “Oh, I checked in on the forest too, on my way back.”

Yalda had almost forgotten that she’d asked her to do that. No one had been officially assigned to monitor the Peerless’s tiny patch of wilderness, and she’d been loath to divert any of the farmers to the task while they were still coming to terms with the onset of weightlessness. “How’s it looking?”

“It’s less dusty there than in the fields and the gardens,” Fatima said. “There were a lot of twigs and petals and dead worms in the air, but nothing larger—the trees haven’t become uprooted, and I didn’t see any arborines flailing around on the ceiling.”

“That’s a relief.”

“I don’t think the wheat’s doing too well there, though,” Fatima added.

“Wheat?”

“There’s a plot of wheat in one of the clearings,” Fatima explained. “It looks as if the stalks were moved there whole—dug up from a field and replanted, not grown there in the plot. But none of their flowers were open when I was there.”

“I see.” Yalda was perplexed; whoever was conducting the experiment hadn’t mentioned it to her.

She sent Fatima to rejoin her physics class, and went looking for Lavinio, the chief agronomist. A note at the entrance to his office said he’d be down in the fields for another two stints. Yalda tried counseling herself to be patient; she didn’t expect to be kept informed about every last scientific activity on the Peerless, and it might attract Lavinio’s resentment if she showed up far from her usual haunts for no other reason than to question him about some trivial experiment.

But how trivial was it? The farmers were far too busy addressing the logistics of weightless harvesting to go and plant wheat in the forest just to test an idle conjecture about the effects of companion species on growth rates. No one would have done this unless it was important.

She couldn’t wait two stints.

Weightlessness had transformed the stairwells from sites of interminable drudgery to the mountain’s smoothest thoroughfares. With a pair of ropes all to herself and no one else in sight, Yalda switched to her high-speed gait: propelling herself forward with all four limbs at once, then releasing the ropes and moving ballistically for as long as possible before brushing them again with whichever hands were necessary to correct any sideways drift and replenish her speed. The moss-lit walls flew by, while the threatening edges of the helical groove that wrapped around her, its jagged steps proclaiming a vertiginous descent guaranteed to end with her head split open, only added to her triumphant sense of control. Once you could survive throwing yourself down a staircase as tall as a mountain, anything seemed possible.

Yalda reached the level of the forest in what felt like less than a bell. When she moved from the stairwell to the access tunnels, her mind insisted on treating all the arborine-proof doors along the way as hatches, and she emerged into the chamber with a strong sense of ascending through a floor. The trees stretched out “above” her did their best to persuade her to realign her sense of the vertical, but all the loose detritus suspended around them rather undermined their case.

The refitting of this chamber had been perfunctory, with just a few unpaired guide ropes suspended between hooks on the wall, so Yalda had to push off from the rock and drift freely through the air to enter the forest itself. Once she was among the trees, though, the branches offered plenty of hand-holds. Tiny dark mites darted past her with exuberant energy, coming and going in a flicker. A green-flecked lizard scampered out of her way, its claws still finding easy purchase in the bark. However ancient and unvarying their instincts, these animals had not been defeated by the change.

She found the clearing Fatima had described—and Lavinio with it. He’d crisscrossed the small treeless space with ropes, the better to access the dying wheat plants. Only now did Yalda feel that the netted soil was below her: she was an aerial spy, sneaking through the canopy like the arborine in her grandfather’s story. She descended with as much creaking of branches as possible, to dispel any appearance of furtiveness.

Lavinio watched her in silence as she approached. He looked grimly unsurprised by her presence, as if he’d already faced such a run of bad luck that an unwelcome visitor right in the middle of it was just what he’d expected.

“Can you tell me what this is for?” she asked him, clambering down a trunk then taking hold of one of his ropes.

“I was hoping the wheat might learn from the trees,” he said.

“Learn what?”

“Up.”

Yalda dragged herself nearer. Disconcertingly, the floor of the forest had become vertical to her again, a cave wall from which the trunks around them sprouted like giant, bristling outgrowths. The wheat stalks were aligned with the trees—but presumably they’d been planted that way, so what was there to learn?

“I don’t understand,” she said. “Is something going wrong in the fields?” She gestured at the limp gray wheat-flowers.

“Not like that,” Lavinio replied. “Here, the flowers don’t know when to open; something in the light confuses them. But up in the fields the mature plants are still healthy.”

“That’s good to hear. And the seeds?”

Lavinio reached down into the soil between the stalks and scrabbled around for a while, then pulled out a seed. It must have been put there by hand, in a separate experiment; none of the sickly plants around it could have produced it, let alone possessed the means to embed it in the ground.

Yalda took the seed from Lavinio and examined it carefully. It was covered with dozens of fine white rootlets that had broken through the skin in all directions, favoring no particular side. There was no shoot, though, no beginning of a stalk. The seed did not know which way to grow.

“I thought light and air were the cues for stalk formation,” she said.

“That’s what I was taught. That was the dogma; I never questioned it.” Lavinio took the seed back and turned it between his fingers. “But however shallow the placement… they still don’t seem to find up. Even if half of the seed is uncovered—exposed directly to the light and the air—they don’t get the message.”

Yalda said, “So when the test seeds you sowed in the fields wouldn’t grow, you came down here to see if the forest had a stronger message?”

“That was the idea,” Lavinio said. “With all of this plant material oriented the same way, I was hoping some kind of influence could pass from the trees to the wheat. But the mature wheat just dies here, and the seeds do exactly what they do in the fields.”

Yalda forced herself to remain calm. The mature plants in the fields were still healthy, so the coming harvest wouldn’t be affected; they weren’t facing imminent starvation. But they did not have long to solve this, or there would be no harvest after that.

“What’s happening in the medicinal gardens?” she asked.

“All those shrubs grow from runners, not seeds,” Lavinio replied. “Some of them are sprouting at odd angles, but once the gardeners correct them by hand they’re fine.”

“That’s something.”

Lavinio made a sound of begrudging assent; the disaster was not all-encompassing. But they couldn’t live on holin and analgesics.

Yalda said, “I wish you’d brought this to me sooner.” She could understand him wanting to prove his expertise by dealing with the problem himself, but there was too much at stake for that.

“Frido thought it would be best to find the solution first,” Lavinio explained. “Instead of spreading panic when there was no need.”

Yalda pondered this revelation. Frido knew about the wheat, and he’d kept it from her? Lavinio might have felt that the responsibility for the crops was his alone, but what was Frido’s excuse?

“I’m not interested in spreading panic,” she said. “But we’re going to need as many people thinking about this as possible.”

“I’ve already set up every experiment you could wish for,” Lavinio insisted. “I’m looking at every combination of factors: light, soil, air, neighboring plants… what is there left to test?”

“And nothing appears to be working?”

“Not so far,” Lavinio admitted.

“Then we both know what’s needed,” Yalda said. “The wheat’s been fine until now—and only one thing has changed.”

Lavinio buzzed humorlessly. “So what are we going to do? Fire the engines again, until the next crop is established? And the next one, and the next?”

“Hardly. We’d run out of sunstone in a generation, and then just starve to death a few years later.”

“Then what?” Lavinio demanded. “If only gravity will make the wheat grow—?”

Yalda held up a hand and twirled a finger around. “Spinning creates gravity too. We could put the seeds in a rotating machine—a centrifuge—until they germinate.”

Lavinio considered this. “It’s an idea,” he said. “But what if germination’s not enough? What if it takes half a season under gravity to establish the plant’s growth axis?”

Yalda was reluctant to answer that. The crew was still struggling to adapt to the last change: refitting every apartment, every workshop, every corridor; relearning every daily routine. How much discontent would it foster, to announce that all their efforts had been misdirected, and that everything they’d achieved was about to become obsolete?

Without wheat, though, they couldn’t survive. And it was no use wishing that the cure would be painless; they needed to be prepared for the worst.

She said, “If germination isn’t enough, we’ll have to set the whole mountain spinning.”

The meeting hall continued to fill slowly long after the scheduled time had passed, but Yalda had no intention of starting until everyone had arrived. People were coming from every corner of the mountain, many of them making a journey they had never attempted before under weightlessness.

Yalda stayed close to the entrance, greeting people and marking off their names on a list. Frido had offered to do the job for her, but she’d insisted on making the most of this chance to come face-to-face with every member of the crew again, however briefly.

Now Frido waited in the front tier, clinging to the ropes beside Babila and half a dozen of the old feed chamber machinists. Yalda hadn’t been able to bring herself to confront him, to accuse him of acting in bad faith. She suspected that he’d been keeping the problem with the wheat to himself as a way to strengthen his position, hoping to make himself a hero to the crew by announcing a simple, biological remedy that would save them all from starvation—courtesy of Lavinio, but still created under his patronage and Yalda’s neglect. No doubt he’d also been prepared to claim the rotational cure as his own, if it had come to that. In fact, Yalda remembered Frido as being part of a group who’d discussed the possibility of spinning the Peerless, when the first real plans were being made for the mountain. The consensus they’d reached was that it would have made navigation and course corrections far too complex, for the sake of some very uneven gains in comfort. It had never crossed their minds that gravity could be a matter of life and death.

Half a bell later, the list of non-arrivals was down to one unavoidable entry. Yalda gave a few quick words of thanks then introduced Lavinio, who explained what he’d seen, and the experiments he’d tried.

“There must be something within a wheat seed that’s sensitive to gravity,” he concluded reluctantly. “Three days in a centrifuge will make the seed sprout, but then it stops growing when the signal is taken away. The established crop didn’t die in the fields when the engines were switched off, so we’re going to keep trying longer periods in the centrifuge in the hope that we’ll find a point where the seedlings can be taken out and planted. But there is no guarantee that such a point exists, short of maturity.”

He moved aside, and Yalda dragged herself back on stage. She clung with four hands to the ropes behind her, surveying the anxious crowd, wondering what would happen if someone took this opportunity to lambast her over her leniency toward the saboteur. But these people had just learned that they risked starvation; Nino was a long-vanquished enemy, rotting away out of sight.

“Sometime in the next dozen stints,” she began, “we might discover that a few more centrifuges and a bigger workforce in the farms are all we’re going to need to fix this problem. But if that turns out to be a false hope, the only alternative will be to spin the Peerless itself, which is not going to be quick and easy. So we need to begin work to prepare for that immediately, doing all we can to make it possible in time for the next harvest—even as we hope that we won’t need to do it at all.

“It might seem tempting to try to spin the Peerless around a horizontal axis, in the hope of making the gravity in the fields as close as possible to the old direction—but I’m afraid the mountain’s center of mass is so low that it wouldn’t work out that way. There’s also a question of stability: if you try to spin a cone around anything other than its axis of symmetry, the slightest disturbance can set it wobbling. So we really have no choice: the mountain needs to spin around a vertical axis, running from the summit to the base.”

She glanced down at Frido. Should she have had him stand beside her, backing her up, confirming these technical claims? Everyone understood centrifugal force, but half the crew would still have to take the finer points on faith.

Frido gazed back at her with a neutral expression. They both knew that he’d been preparing to move against her. It was too late to try to bring him on side.

“We’ll need to install two dozen small engines,” Yalda continued, “spread out down the slope of the mountain, along two lines on opposite sides of the axis. These will be very gentle devices compared to the ones we’ve used to accelerate, but we’ll still need to put them in deep pits so their thrust doesn’t merely tear them loose—or peel away parts of the surface of the mountain.

“That means digging into the rock out there, with no gravity to hold us down. It also means working in an air-filled cooling bag, to avoid hyperthermia. No one has ever done anything like this before. And however optimistic we are about it, it’s more work than the usual construction crews could hope to complete in time for us to sow the crops. Everyone who isn’t working in the farms will have to help. Once the construction crews have worked out the protocols, they’ll start training other people to join in. I’ll be among the first of their students, myself—because nothing could be more important than this.”

“Stints of dangerous work in the void, possibly for nothing?” Delfina interjected. She was in the front tier, a few strides left of Frido. “That’s your solution to an agricultural problem?”

“What do you suggest instead?” Yalda asked her.

“Find another food source that isn’t so dependent on gravity,” Delfina replied. “What are the arborines living on, in their forest?”

“Lizards, mostly. Which are living on mites—which in turn feed on bark and petals.”

“We could get used to lizard meat,” Delfina declared. “If it’s good enough for our cousins, why not eat it ourselves?”

“I’m sure we could,” Yalda conceded, “but the whole forest only supports about six arborines.”

“We can’t farm the lizards more intensively?”

“That’s… worth considering,” Yalda said. “But it would be another gamble, and even if we could make it work the payoff would come too late. The only thing we know for sure is that we can raise a wheat crop under gravity. Once we spin the Peerless, all we’ll have to do is prepare new fields and plant the seeds.”

“Where, exactly?” Delfina pressed her. “Which chambers were built with their floors pointing away from the mountain’s axis?”

“We’ll have to improvise for the first crop,” Yalda admitted. “We’ll have to lay down fields on surfaces that used to be walls—we won’t have time to carve out new chambers with the ideal geometry.”

“And what happens if we need to fire the engines? To avoid some unanticipated obstacle?” Delfina was enjoying this; someone had prepared her well.

Yalda did her best not to grow flustered. “As things stand, we’d need to get rid of the spin first. But there’s no reason in principle why we couldn’t redesign the attitude controls and the engine feeds to work while the Peerless was rotating.”

Delfina hesitated, as if she’d finally reached the end of a list of objections that she’d committed to memory. But her contribution wasn’t over.

“I’m sorry,” she said, “but I’m not convinced. On balance, I don’t think your plan is worth the risk. I won’t be joining any work team for this purpose.”

Yalda said, “There is no coercion here. You’re free to make your own decision on this.”

“And free to persuade my friends to make the same decision, I hope,” Delfina added cheerfully.

“Of course.” Yalda was angry now, but she was not going to change her stance and start making threats. Help spin the mountain, or you can go without food next harvest.

Far better, she decided, to call the spoilers’ bluff.

“But we’ll need to start drawing up the rosters,” she said, “so I’d like to get the numbers clear right now. How many people are prepared to work to make this happen—either in the farms, or out on the slopes? Please raise a hand if you’re willing to do that.”

About a third of the crew responded immediately. For a long, painful moment it looked to Yalda as if that burst of enthusiastic support was all she would get, but then the numbers began to grow.

In the end, only about two dozen people chose to side with Delfina. Most were from the feed chambers, sending her a message about Nino. No doubt there were many more who wanted the saboteur dead, but they weren’t going to risk the crops—or even risk being seen as risking the crops—just to express their anger over something else entirely.

Frido was not among the dissenters. At some point he had counted the numbers around him and decided to raise his hand.

17

As they waited to use the airlock, Yalda helped Fatima into her helmet and cooling bag. No one’s flesh was flexible enough to conform to the shape of the fabric perfectly—and the whole point was to ensure that there was air moving freely over your skin—but if you let the bag hang too loosely anywhere it just blew out into a rigid tent, leaving you fighting it with every move. The trick was to come close to filling the bag but to wrinkle your skin as much as possible, creating a series of small air channels between skin and fabric.

Yalda finished checking the fit. “I think you’re right now,” she said.

“Thanks.” Fatima reached into the hold beside them and took out two canisters of compressed air, passing one to Yalda. Yalda attached it to the inlet at the side of her own bag.

“Someone should find a better way to keep cool,” Fatima suggested.

“In time for the next shift?” Yalda joked.

Ausilio had finished pumping down the airlock pressure; he slid the external door open, took hold of the guide rail just outside the exit, then pulled himself through. As soon as he’d reached back to slam the door closed, Fatima opened the equaliser and air hissed slowly back into the lock.

Yalda was growing tired of these laborious preparations, shift after shift, but she kept her frustration to herself. Three more stints, and she’d never have to go through this rigamarole again.

Fatima entered the airlock and began working the pump energetically, bracing herself with three hands against the clearstone walls.

By the time Yalda was through onto the slope, Fatima and the rest of the team were already out of sight. Yalda swung herself between the guide rails and set off down the mountain, moving briskly but always keeping at least two hands on the rails. In the absence of gravity she ought to have been oblivious to the gradient of the slope, but the rim of the inverted bowl of garish color trails above her matched the old horizon perfectly, making it impossible to think of the ground as level.

The new horizon was a dazzling, multicolored circle where the fastest ultraviolet light from the old stars was shifted to visible frequencies before giving way abruptly to blackness. Straight ahead of her—“downhill”—the more modest trails of the orthogonal cluster shone sedately. Away from the guide rails, silhouetted in the starlight, dead trees sprawled at odd angles. Notwithstanding the high altitudes to which they’d been accustomed, their roots had not been enough to keep them cool in the complete absence of air. Patches of red moss had colonized the deadwood, but its faint light suggested that it was struggling.

A few saunters from the airlock, Yalda reached the pit. Lamplight from deep within the tunnel shimmered off the dust emerging from its mouth. At first glance it was easy for a planet-trained eye to see these motes as being borne on some kind of breeze, but then the thumb-sized fragments of rock scattered among the specks—moving more slowly, but just as freely—put an end to that illusion. Nothing was propelling the dust; it was flowing out of the tunnel for no other reason than its own chance collisions, inexorably driving it to occupy more space.

The guide rails, dating from before the launch, ran right past the tunnel’s entrance but couldn’t take her in. Yalda shifted her grip to a pair of ropes anchored to a series of wooden posts that veered off into the light. The floor of the tunnel sloped gently down into the rock; it was another half saunter before the roof was above her.

The haze of dust and grit thickened. When Yalda gripped the rope close to the posts, she could feel the vibration of the jackhammers. When she raised her hand, backlit motes of rock swirled away from it, driven by the air slowly escaping through the fabric. Fatima was right to be dissatisfied; it was a crude business when the only way they could cool themselves was to throw warm air away.

Gradually the rock face came into view, ringed by blazing sunstone lamps. Seven members of the team were working it with jackhammers, braced against the rock within their cages. Three taut guy ropes ran from the top of each cage to the tunnel wall, holding worker and cage in place against the tool’s relentless kick. Yalda had done that bone-shaking job for two stints, and then finally conceded that she was past it.

Four other workers were moving between the cages, clinging to the guy ropes and dragging the open mouths of their rubble sacks over the fragments of broken rock that were bouncing away from the hammers. It was impossible to scoop up all the debris, but their efforts kept the workspace more or less navigable.

Fatima spotted Yalda and waved to her, then turned her attention back to the rubble she was chasing. With the cooling bags covering everyone’s skin, communication was reduced to glances and hand gestures. If you brushed against someone you could exchange a few muffled words, but mostly the shifts were spent in a kind of tacit camaraderie, where the rhythms of the work itself—shifting the hammer cages, re-pinning the guy ropes—had to take the place of friendly banter.

There were already two full sacks waiting to be removed, the drawstrings at the top pulled closed and used to tie them to hooks on a pulley line that ran the full height of the tunnel. Yalda dragged the line around to bring the sacks within reach, slipped their drawstrings over her shoulders, then set off back to the mouth of the tunnel.

The catapult sat on the other side of the guide rails. Yalda put the rubble sacks on holding hooks at the side of the machine, grasped a nearby support post with her two left hands, then started turning the crank that ratcheted the catapult’s launching plate back along its rails, stretching a set of springs below. As the crank began stiffening its resistance, she could feel the support post working itself loose from the ground. Cursing, she shifted her lower hands to the catapult, dug a mallet out of the tool hold, and bashed the support post half a dozen times.

Yalda checked the post; it felt secure now. But as she bent to put the mallet back in the hold, she could feel a tiny rocking motion in the catapult itself: she’d managed to loosen some of the tapered wooden pegs that held its base against the ground.

Never mind; she’d deal with that later. She swung the first sack onto the launching plate, checked that it was properly closed and sitting squarely on the plate, then reached down and released the catch. The plate shot up a full stride before the springs stopped it, leaving the whole machine reverberating. The sack continued on, gliding away smoothly into the void. Yalda had had her qualms about disposing of the rock this way; who knew what demands their descendants might have for even the most mundane materials? But the effort that would have been needed to secure the rubble on the slopes—let alone cycle it all through the airlocks and stash it somewhere inside the mountain—was more than they could spare.

She launched the second sack into oblivion, then headed back down the tunnel.

The haze was growing thicker. Two of the hammers had hit a lode of powderstone, which left no solid pieces to collect and just wafted out like smoke, coating everyone’s faceplates with gray dust.

Four more sacks were waiting on the pulley line. Yalda brought down two of them, then paused to wipe her helmet clean and squint up at the rock face. The crumbling powderstone was a nuisance, but it would speed progress. Once the main excavation was completed, half a dozen small feed chambers would be constructed behind the rock face, accessible through a separate tunnel leading straight up to the surface. Apart from Benedetta’s probes, this would be their first real test of an engine that wasn’t gravity-fed, with the liberator pushed through the fuel by compressed air. Yalda was already feeling anxious about that, but in some ways the test would be forgiving. The geometry of the engine placement would be the most important thing; small variations in the thrust wouldn’t be critical.

She slogged her way back out to the catapult. As she cranked it, the support post she was holding came loose again. She fumbled for the mallet—the simple task of retrieving it made harder by the streaks of gray powder still stubbornly clinging to her faceplate—then she realized that one of the sacks was actually blocking the front of the tool hold, so she shifted it onto the launching plate. Then she gripped the base of the catapult with her lower pair of hands to brace herself, and started bashing the support post.

Yalda was upside-down and two strides above the ground before she felt the tightness around her lower wrist; she dropped the mallet and reached down frantically toward the catapult, but it was already too late to grab any part of it. With her rear gaze she stared up at the sack’s drawstring, twisted around her hand. She must have left the drawstring protruding from the side of the launching plate, and then slipped her arm through its loop.

Her first, idiotic, impulse was simply to disentangle herself from the sack—as if it alone were the cause of all her problems, and if only she were free of it she’d drift gently back to the ground. Her next thought was to pull it closer to her body, which she did. Then she freed her wrist from the string and clutched the coarse fabric of the sack against her chest, but she managed to stop herself from completing the plan: tossing the sack upward in order to propel herself back toward the mountain. Her instinctive sense that this tactic ought to work was almost overpowering; had she been stranded in the middle of a chamber inside the Peerless, it would certainly have done the trick. But even if she’d pushed the sack away with all the strength in her four arms—even if she’d burst the seams of her cooling bag and extruded two more—it would not have been enough. She knew how long she’d labored to turn the crank on the catapult, how much energy she’d put into the springs. A single burst of effort couldn’t match that. And a partial victory that merely slowed her ascent would be completely useless, if in the process she lost the means to do anything more.

Yalda glanced back at the receding light from the tunnel mouth. If she panicked and acted without thinking, she was dead. Her rapidly increasing distance from the ground was terrifying, but it was not her real enemy. It didn’t matter how long it took her to reverse direction; once she was headed back to safety, the length of her trip would be irrelevant. Or very nearly so: the sole criterion was that she needed to return before her air canister ran out—and it held enough for an entire six-bell shift.

Could the canister itself help? She ran a hand over its cool surface, imagining a swift burst of air that would send her hurtling down to safety. But without any tools, she doubted she could break open the valve that limited the outflow—and even if she could, the momentum of the entire contents might not be sufficient for the task. What’s more, a marginal success would make a lie of her indifference to distance; if she ended up drifting slowly back toward the ground, without cooling she could easily die of hyperthermia on the way. There were a dozen replacement canisters in the catapult’s tool hold, but did she really want to smash this one open and gamble on reaching the others in time?

No air rocket, then. All she had for exhaust was the rubble in the sack, and all she had to propel it was her own strength. But the catapult had put her in this plight using nothing but stored muscle power; if she parceled out the rubble in a manner that allowed her to expend more energy on this task than she had on turning the crank, she should be able to reverse the consequences.

Her body’s slow spin had her facing back down toward the light from the tunnel again. Her fellow workers would certainly miss her as the rubble sacks began filling up the pulley line, but they would not be in a rush to come looking for her; she could easily have spent all this time on minor repairs to the catapult. The rock face was where serious accidents happened; what kind of fool managed to shoot herself into the void? But whenever they did start to worry about her absence, she could forget about anyone throwing her a rope; she was already too far away for that.

No matter; if she stayed calm, she could fix this. She identified the point on the star-trail horizon that, as near as she could tell, marked the direction in which she was traveling: precisely opposite the shrinking patch of light on the ground. She loosened the drawstring and opened the sack a little, fearful of spilling the contents with her jostling, then reached in and took out a handful of rubble. She waited for her rotation to bring the target in front of her again; she wasn’t going to try to reconfigure her flesh to give equal strength to a backward throw. Then she brought back her arm and flung the handful of rock away with all her strength.

The effort felt puny and ineffectual—and she suddenly realized that in her haste and agitation she’d been laboring under yet another delusion. If she threw a heavy object, like the whole sack of rubble, the energy she put into it would be limited by the maximum force her muscles could apply. If she divided the sack in two and threw each half separately, the same force would let her throw a half-sack faster than a whole one, transferring as much energy to each half as she would have used on the whole.

Two throws, twice as much energy—hooray! But two throws would still not be enough, so why not four throws, a dozen, a gross, taking her time but increasing the total energy as much as she needed? That was what she’d been thinking: matching the energy she’d put into the catapult would simply be a matter of eking out the rubble with sufficient care.

But the pattern of throwing ever smaller weights ever faster only held true up to a point—for a half-sack, yes, but not for a handful of pebbles. By that point, the limiting factor would be the speed at which her muscles could contract, not the force they could apply. And when the speed was fixed, the energy she could put into a given quantity of rubble became proportional to its mass—which meant that it would add up to the same total, regardless of how many separate throws she made.

It didn’t matter how much strength she still had in her body; it didn’t matter that she could have cranked that catapult ten dozen more times without tiring. Her fate was completely determined by the total mass of rock in the sack, and the speed at which she could throw, not the greatest load, but the smallest.

Yalda looked back at the mountain. She could see three other worksites now, the bright mouths of tunnels further down the slope. But her trajectory was carrying her off to the side, and an expanse of dark rock was now spread out below her. There was an entire second line of worksites a half-turn around the mountain; the full set of engines would consist of a dozen diametrically opposite pairs. But if any of those sites came into view she’d know that something was wrong—that she’d been misdirecting her throws and inadvertently bending her trajectory.

She took another handful of rubble from the sack, waited for the target, then threw it. Her spin provided a rhythm for the process, giving her a chance to rest her arm without delaying the next throw too long. After a dozen cycles, she switched arms. She couldn’t extrude any new limbs without damaging her cooling bag, but although she felt some jarring at the end of each throw it didn’t build up to enough pain or damage to slow her down.

What she could have done with, though, was a good slingshot.

Yalda could see ten of the worksites now, with the remaining two from her side of the mountain probably just hidden behind small outcrops. All these engines would be completed, with or without her. The Peerless would get its spin, the crops would thrive once more. The real purpose of their journey would soon come to the fore again. Sabino had opened up a path that the brightest young students—Fatima, Ausilia and Prospera—would follow. Her death would not mean the end of anything.

And Nino? She cut off the morbid train of thought. The rubble sack was still more than half-full, her situation had not yet been proved unsalvageable.

As she threw another handful of rubble, she saw a flash of light in her rear gaze. She tried to place it exactly, to backtrack from the afterimage, but her spin confused her. Had she glimpsed one of the other worksites, the lights from its tunnel peeking briefly over the edge of the mountain? It had been too bright for that, hadn’t it? The tunnel mouths all faced the same way around the mountain—so those at the other worksites would be pointing away from her. The most she could have seen was the spill of light from the ground near the pit, and the scatter in the dust haze. How could that have outshone the sites where she was facing straight into the tunnels?

A few turns later, she was facing the mountain when she saw a second flash: far from any of the worksites, surrounded by blackness. She wondered if someone might have lit a sunstone lamp inside one of the observation chambers—but why would they do such a thing, let alone light it only for an instant?

The third flash was at a different location, still nowhere near any worksite—and too brief and too bright, Yalda concluded, to be an artificial source at all. Something must have collided with the Peerless—something small that nevertheless carried enough energy to turn the rock white-hot.

The telescopes had shown a corridor devoid of matter, but there’d been a limit to the sensitivity of those observations. Any speck of dust here, drifting along at a leisurely rate relative to the ordinary stars, would now be like a Hurtler to the Peerless. That was the price of taming the Hurtlers by matching their pace: ordinary dust could now do as much damage to the mountain as a Hurtler could do to an ordinary world.

So much for the city of carefree scholars, working in safety and tranquility until the secrets of the cosmos were laid bare to them. Just like the people they’d left behind, they would be living with the constant threat of conflagration. And not for four years: for generations.

Worst of all, Yalda realized, she was probably the sole witness to these events. The dust could have been striking the mountain for days, but most of the surface was invisible from the worksites and observation chambers. She had to get back and organize a fire watch for the Peerless; they had to prepare themselves to reach and douse a wildfire anywhere on the slopes, or risk going the way of Gemma.

Yalda cast another few stones—imagining them heavier in the hope of tricking her body into dispensing a little more force. The sack was a quarter full. She believed she was still heading away from the mountain, but judging tiny changes in the view at this distance was almost impossible.

How could they keep a lookout for fires? From a cage tethered on a rope, high above the surface, stabilized… somehow. Once the mountain was spinning, though, the problem wouldn’t be stability, but the strength of the rope.

And once the mountain was spinning, it would be far harder to move around on the surface. Weightlessness had made it difficult enough, but every part of the slope would be transformed into a ceiling. How did you douse a raging fire on a ceiling?

The sack was empty. Yalda clutched it to her chest, unwilling to presume that she’d have no further use for it. Was she moving toward the mountain, or away from it? For some time now, she hadn’t discerned any change in the angle it occupied in the sky, but she’d been too distracted to give the task much thought. She needed to pick a few distinctive stars close to the edge of the mountain, then wait to see if they crept away from it, or whether its silhouette slowly grew and hid them.

Yet another flash of light came from the mountain, this one very close to one of the worksites. Perhaps someone there, outside the tunnel on catapult duty, would have seen it? Yalda counted the pit-lights down from the summit, and realized that the site was her own.

The light winked again, from exactly the same direction. Not an impact, then. By now, she realized, her team would be out scouring the area for her, their sunstone lamps occasionally turning up into the sky. Yalda pictured them inspecting the catapult, feeling how loose it was, wondering if anyone could possibly have been careless enough—

The same light appeared, brighter than before, crossing her line of sight so slowly that it dazzled her. When she completed a half-turn it struck her rear gaze and stayed—wandering a little, but never fading out completely.

The lamp wasn’t on the surface of the mountain; it was moving straight toward her through the void. And it couldn’t be aiming itself, searching her out itself.

Yalda spread the empty sack out in front of her, hoping to make a larger, more reflective target. The approaching light began wavering oddly, as if seen through a heat haze. Through a burst of air, spreading out through the void. Some beautiful idiot had come after her—launched along the same trajectory by the catapult—and now they were using compressed air to brake. Not from a tiny canister like her own, but from one of the giant cylinders that powered the jackhammers.

The dazzling light overshot her, passing to one side. It rebounded, then overshot in the other direction. It was excruciating, but Yalda could do nothing to meet her rescuer halfway. By trial and error, by eye and airburst alone, the distance and difference in speed that separated them was whittled down to the point where the lamp became superfluous and its owner shut it down. No longer blinded by its glare, by starlight alone Yalda could see the figure before her, clutching an air tank and a coil of rope, wrapped in a familiar cooling bag.

Fatima took hold of a portion of the coiled rope and tossed it toward Yalda. This sent her gliding backward, but she didn’t bother trying to compensate, she just let the rope uncoil. Yalda reached out and grabbed the end, then brought it around her waist twice and held on tight.

There was a jolt as the rope went taut, then they were bound together, moving in a broad circle around a common point. Yalda dragged herself along the rope a short way, then gestured to Fatima to use an airburst to get rid of some of their angular momentum. By the time they were within arms’ reach of each other, their spin was almost gone.

Fatima took hold of Yalda’s helmet and pressed it against her own. “Help me get down. Please.”

She sounded terrified, and for a moment Yalda couldn’t reply. How could she have come after her at all, if she was so afraid?

“Let me take the canister from you,” Yalda suggested gently. “Don’t release it until I’m holding it.”

Fatima had two arms wrapped around the cylinder. Yalda embraced it herself the same way, then eased it out of Fatima’s grip.

With her other hands, she rearranged the rope, forming two coils and bringing them around their bodies, then securing the connection with a series of knots. Fatima was shivering; she’d already done more than Yalda could have asked of anyone. It was her own job now to get them safely down.

“I keep thinking about Benedetta,” Fatima said. “Landing is the hardest thing.”

“This won’t be like that,” Yalda promised. “No fire, no heat, no danger—” She noticed the sunstone lamp still strapped to Fatima’s shoulder. “We won’t need that anymore.” She pulled it loose and swatted it gently away into the void; with all the jarring it had suffered already, it was a miracle it hadn’t exploded.

Yalda found her target on the horizon and opened the valve on the air cylinder a notch; the effortless kick against her arms was the most beautiful sensation she’d ever felt. She’d never know if she’d already been heading back toward the ground before Fatima reached her; she didn’t want to know.

A pinprick of light appeared on the dark rock below them. “Did you see that?” Yalda asked Fatima. She’d been hoping that she might have been delirious before—or that Fatima’s ascent might have included enough unlikely swerves for her search lamp to account for everything.

“Yes. What was it?”

“I have no idea,” Yalda lied. “Don’t worry; we’ll work it out later.”

As the mountain loomed closer, the line of worksites spread out beneath them, the most distant fading to black. Yalda made a sideways correction, steering them toward the mouth of their own tunnel. When that patch of bright rock began to grow alarmingly, she squirted air down, slowing their descent. For a pause or two she thought she might have overdone it and launched them away from the mountain again, but they were close enough now that the cues did not remain ambiguous for long. She used another quick burst to slow their horizontal motion, lest they scrape all their skin off on the rock.

As the guide rails running past the tunnel mouth rushed into view, Yalda discerned a new feature: the team had tied dozens of lengths of rope to the rails, spread out along a couple of stretches, pointing away from the rock with their free ends high above the ground. If she could steer into this soft, forgiving fence—

“Try and grab the ropes!” she urged Fatima, as they swooped toward them. “The more arms to share the jolt, the better.”

A flicker before the glorious fool-catcher came within reach, Yalda used a tiny kick from the cylinder to give them a slight upward velocity. Then she dropped the cylinder and flailed around, managing to seize one of the ropes. Fatima had gripped another one, in two places. Yalda brought all her own hands onto her rope before it went taut; the shock to her joints made her cry out in pain, but she didn’t lose hold of it.

They were a few strides above the guide rails. Yalda had been expecting to have to drag herself hand-over-hand down to the ground, but the ropes’ elastic tug had delivered a little more force than was needed to stop them, and they were actually drifting slowly toward the surface.

Fatima began humming from the shock. Yalda almost joined her, but she was afraid that if she started she’d never stop.

She said, “We’re safe. You did it, my friend, and now we’re both safe.”

18

Lavinio said, “Without gravity, I think this is the best we can do.”

Yalda bent down from the ropes that crossed the test field and examined the plants. The wheat stalks were barely two spans high.

“They’re… mature? They’re making seeds?” The tiny structures protruding from the stalks certainly resembled seed cases, but they were so small that it was hard to be sure.

“Yes, they’re mature,” Lavinio confirmed.

“But they’re a twelfth the size of normal wheat!”

“However long we keep the seedlings in the centrifuge,” Lavinio explained, “they always stop growing when we take them out—but if we raise them to this height first, they don’t die when we replant them in the fields. They don’t get any bigger, but they do form seeds of their own.”

“Wonderful.”

It was not the outcome they’d been hoping for, but Lavinio couldn’t hide his fascination. “It’s as if the maturation process is triggered directly by the cessation of growth, so long as the plants are larger than some critical size. If we really understood the mechanism, perhaps we could intervene further. But for now—”

“For now, we have the option of growing six crops a year—each with extremely low yields.” Yalda prodded one of the seed cases with a fingertip. “And these actually germinate?”

Lavinio said, “Yes—if we put them in a centrifuge, like their parents. The seedlings start out extremely stunted, but they catch up in size by about the fourth stint.”

Yalda had been expecting a clear-cut verdict, one way or the other, to force her hand. The utter failure of the centrifuged seedlings would have left her with no choice but to spin the Peerless, while a perfect fix that let them grow the old-style crops would have allowed her to declare that building the engines had been a worthwhile precaution, but actually firing them had mercifully proved to be unnecessary. “So where does this leave us?” she asked.

“It would be much more labor-intensive than ordinary farming,” Lavinio said. “And we’d need at least ten dozen centrifuges to yield the same total volume of grain as we were harvesting in a year, when we had gravity.”

Ten dozen centrifuges, running around the clock. Burning fuel, demanding maintenance. Spinning the whole mountain would eat into their sunstone reserves—but they would only need to do it once.

“It would be survivable,” Lavinio added. “Not ideal, but not completely impractical.”

Yalda thanked him, and promised a decision within the next few days.

She headed back to the summit, skimming along the stairwell’s ropes. With ordinary wheat in ordinary fields, it would be a simple matter to increase the size of the crop to feed a larger population. Having to build and run a dozen more centrifuges just to increase the yield by one tenth would change everything.

But if they went ahead and spun the Peerless, and then some wayward pebble set the slopes on fire, how much harder would it be to douse the flames when the mountain was flinging everything off into the void?

Yalda left the stairwell in the academic precinct and dragged herself down the corridor toward her office, trying not to betray her anxiety as she returned the warm greetings of passersby. Now that the tunnels were finished, the completion of the spin engines was in the hands of skilled machinists—but everyone here had been out on the slopes in the dust and danger, everyone had earned the right to think of the project as their own.

Some people flashed her looks of excitement and anticipation; some called out “Three stints to go!” If she turned around and announced that all of their work had been for nothing—and that they would now have to live on meager supplies of machine-raised, stunted wheat—she was going to need a spectacularly compelling argument to back up her decision.

Marzia was waiting outside her office. “The test rig’s ready,” she said. “Just give the word, and we’ll launch it.”

“Are you sure this is safe?”

“It will be five strolls from us when it ignites, and still moving away,” Marzia reminded her. “I don’t see how we could make it any safer without giving up the chance to observe it at all.”

Yalda accepted this, but it was hard to be relaxed about the experiment. The engines of the Peerless had failed to set the world on fire, but that had never been their purpose. Marzia’s rig was designed to ignite a mineral that had never been seen burning, except perhaps on the surface of a star.

“What if a spark comes back and hits the mountain?”

“Any debris that would be hot enough to harm us will be hot enough to burn up long before it reaches us.”

“Unless you ignite the Eternal Flame,” Yalda joked weakly.

Marzia gave an exasperated buzz. “If you’re going to start invoking those kinds of fantasies, why not throw in another twist and let us survive anyway? Then we can all head home to see our families.”

Yalda said, “Go ahead and launch the rig. Just make sure that the fire lookouts know what to expect.”

Three bells later, Yalda met Marzia in the precinct’s observation chamber. Marzia had set up two small telescopes and trained them on the rig, which from their point of view appeared almost fixed now as it drifted away from the mountain. By starlight the device was just a slender silhouette, but after Yalda had taken a peek to confirm that the instrument was aimed correctly, Marzia handed her a filter to slip into the optics. The image was about to brighten considerably.

As Yalda checked the wall clock with her rear gaze, a globe of light erupted at one end of the rig’s calmstone beam, spraying luminous shards into the void. The beam had been slotted straight through the middle of a spherical charge of pure sunstone, encased in a solid hardstone shell; on the timer’s cue, the fuel had been saturated with liberator and the heat and pressure had risen until the casing was blown apart. A slight equatorial thinning of the shell had directed the explosion outward from the beam, sparing the other equipment attached to it and leaving almost no net force or torque; the beam had acquired a barely perceptible rotation, and had remained squarely in view.

And it was burning. The sunstone had scattered, and the calmstone itself was ablaze.

Marzia let out a chirp of triumph at this unprecedented feat. Yalda would have been far happier to learn that calmstone was impossible to ignite—and that the stars, and Gemma, must have simply lacked the mineral that covered most of the surface of the world. Calmstone sand could douse burning fuel. Calmstone had contained the Great Fire of Zeugma. Calmstone had borne the launch of the Peerless without succumbing to the flames. But now—

“Air does make a difference,” Marzia muttered happily. Similar experiments had been attempted back home, but with air always present to carry away some of the heat, the calmstone had never reached its flashpoint.

They’d soon know if the same effect was enough to put out the flame once it was already burning. A few strides along the beam from the ignition trigger, four tanks of compressed air were fitted with clockwork ready to discharge their contents onto the flame. There was no missing this when it happened: as the air rushed down the beam the whole rig accelerated sideways, and Yalda had to start turning the scope to keep the apparatus in view. Once she managed to track it closely enough to steady the image she could see the artificial wind distorting the incandescent halo around the beam—but the calmstone itself grew no dimmer. The fire remained self-sustaining: the creation of light by each tiny patch of the disintegrating mineral was accompanied by enough heat to guarantee the same fate for its neighbors, with enough to spare to make up for whatever the surrounding gases were carrying away.

Yalda was dismayed, but there was one more stage to the rig, one more trick to test. A pause or two after the first four tanks emptied, a second set opened up—but now the air, though much gentler, was being routed through pipes half-filled with powdered hardstone. This was the ultimate bucketful of sand: a dose of the most inert mineral of all to draw the heat into itself and try to disrupt the cascade of energy.

The hardstone sand was poured radially, with four symmetrical flows directed straight down onto the beam to cancel out any rocket effects and allow the material to accumulate as much as possible in the absence of gravity. It was a model for the best-case scenario: the equivalent of dousing the mountain’s slopes in the absence of any confounding spin.

The timing of the release had been guesswork, chosen on the basis that earlier was better, and the portion of the beam subject to this treatment had not yet caught alight. Some sand was drifting away, but there was more than enough being added to make up for that; Yalda could see the mound growing by the light of the encroaching flames.

As the fire hit the mound, the view faded to black; with the filter in place even the stars were invisible. Yalda restrained herself; anything could still be happening beneath the sand. But if this worked, she thought, one more experiment would be enough. If they tried the same thing with a spinning rig and found that the centrifugal force ruined the dousing effect, then stunted wheat would be a small price to pay to retain the ability to protect themselves.

A light flickered and brightened, illuminating the remains of the rig. The fire had continued to consume the beam; it had merely been hidden. There was no “dousing effect” to be saved.

Yalda turned to Marzia. “What now?” she asked numbly.

“We could vary some parameters,” Marzia suggested. “Tweak the flow rate, or the quantity of the hardstone powder.”

“I thought this was already the best setup you could think of.”

“It was,” Marzia said. “But my guesses aren’t infallible; some small change might still improve it.”

“Enough to make a difference?” Yalda pressed her.

“It’s not impossible.”

Yalda said, “Then it’s worth trying.”

There had to be a solution. She could not accept a life for the travelers as hazardous as the life they’d left behind. The flashes of light on the surface had been harmless enough so far—but they wouldn’t get a second Gemma moment out here. The proof that the worst could happen would only come when the Peerless itself burst into flames.

Marzia said, “You know I studied chemistry in Zeugma?”

“Of course. I think we met once, after I visited Cornelio.”

“We always worked with a knife beside us,” Marzia said. “We protected our bodies as much as we could… but when something went wrong, you couldn’t really hope to find an effective extinguisher in time.”

Yalda was horrified. “And you think that’s the best we can do? Prepare ourselves for an amputation?”

“I had to cut off my own hand twice,” Marzia replied. “It was either that, or lose everything.”

“I admire your resolve,” Yalda said, “but hands can be reformed. Flesh can be replenished. Any rock we discard is gone forever.”

Marzia thought for a while. “Our ‘empty corridor’ has turned out not to be as empty of ordinary matter as we hoped,” she said. “Could we have missed some orthogonal matter as well?”

“That’s possible.” The orthogonal star cluster was more than a dozen blue light-years away, but the dust and pebbles of the Hurtlers themselves were all around them, and there could be larger non-luminous bodies as well.

“Whittling the mountain down to nothing over the generations is an alarming prospect,” Marzia said, “but if tossing the occasional fire-afflicted portion off into the void is the only way to protect ourselves… maybe we can take comfort in the possibility that what we’re losing isn’t really irreplaceable.”

Yalda said, “Comfort isn’t quite the word I’d use.”

Marzia persisted. “The idea of crossing the void to try to mine another body of rock might seem daunting to us now, but who knows what our descendants will be capable of?”

“How much more are we going to load onto them?” Yalda asked wearily. “It’s bad enough that we expect them to invent their way home with whatever fuel they have left. Now they’re meant to find mines in the void in time to patch up the mountain before fire damage shrinks it to an uninhabitable core.”

“What choice do we have?” Marzia replied. She gestured toward the dying embers of the rig. “I’m happy to try more experiments, but I can’t see our luck changing there. Whatever the solution is, we have to trust the people who come after us to play a part in finding it. If we’d had all the answers ourselves, we would never have needed to make this journey at all.”

Three times a day, the fire lookouts climbed down their rope ladders for the change of shift. The number of impact flashes they reported rose and fell, but no more than Yalda would have expected for random collisions.

If the dust had comprised some kind of well-defined obstacle with known borders, they could have planned a route around it, or at least done the calculations and decided whether it was worth the cost in fuel. But they had not seen any hint of this in advance, before their velocity blinded them to all the ordinary matter ahead, and now any maneuver that sought to escape the problem would amount to no more than trying out random detours one by one and then seeing if they’d made things better or worse. They did not have that much sunstone to burn.

Marzia’s follow-up experiments came to nothing. If burning calmstone could be extinguished at all, they were as far as ever from discovering how to do it.

Yalda sought out Palladia, the most experienced of the construction engineers, and asked her to consider the possibilities for discarding parts of the mountain. After a couple of days pondering the matter, she returned to Yalda’s office to sketch out her preliminary ideas.

“The two simplest options,” Palladia said, “would be to install a kind of sacrificial cladding—expendable tiles covering the surface that could be detached easily if they caught alight—or leaving the exterior as it is, and being prepared to blast an outer wall away, if necessary.”

“Blast an outer wall away?” Yalda was no longer prepared to rule out anything. “So we lose pressure, then spend a couple of years with everyone in cooling bags trying to make repairs?”

“Hardly,” Palladia replied, amused. “We’d divide the outer precincts into individual sections. We’d put pressure doors in all the access corridors, and pre-install a set of charges in each section. Once the lookouts identified the precise location of the fire, there’d be a procedure to follow: start the timers on the charges, evacuate everyone, seal the section… then the wall is blown into the void, taking the fire with it.”

“Tell me about the first option.” Yalda resisted adding: the sane one. “The tiles, the cladding.”

“There are two issues there,” Palladia said. “Can we mine enough material from the interior to put an effective layer of cladding on the surface, without causing structural problems? We need to be able to guarantee the integrity of every chamber under the loads arising from centrifugal force, not to mention the eventual re-use of the main engines. But even if we have enough raw material, the next question is whether we’d have time to clad the whole exterior before our luck runs out and the surface catches fire. That would be a massive task under any conditions—but with the mountain spinning it would be the hardest thing we’ve ever attempted.”

“We could delay the spin-up, if it was worth it,” Yalda suggested reluctantly. They could live off stunted wheat while they completed this shield, if it was actually going to be capable of protecting them.

Palladia said, “Let’s try to get some solid numbers.”

They worked together for ten days. Thanks to Marzia’s experiments they knew the rate at which calmstone burned, and though no one had yet been able to find one of the tiny impact sites on the surface, Yalda could estimate the depth to which dust particles of various masses would penetrate the cladding when they struck with infinite velocity. Palladia had surveyed the whole mountain during the construction phase, compiling the first detailed records of its composition, and she’d witnessed firsthand how various chambers had stood up to the stresses of the launch.

The numbers were not in their favor. To cover the mountain with a worthwhile protective layer would leave it gutted and weakened inside, to the point where its spin alone could start breaking it apart. But giving up on spin wouldn’t save them; the next time they fired the engines, to decelerate, the Peerless would turn to rubble.

“I want you to draw up plans for… our other option,” Yalda said.

Palladia regarded her with something close to panic.

“I’m not asking you to rush anything,” Yalda assured her. “You should take as long as you need to get this right. But you should make all your choices on the basis of structural considerations alone. We’ll address the other practicalities separately—if we have to move some pieces of equipment to safer locations, or duplicate some facilities, so be it.”

Palladia was still not happy. “When are you going to speak to Frido about this?”

Yalda said, “I’m speaking to you, because I know you can do the job. You can have as many assistants as you need—just pick whoever you want. You might have to wait until the spin engines are finished for some people to become available, but once that’s done this will be our highest priority.”

Palladia replied carefully, “I’m honored to be given this responsibility—but with respect, I think Frido and Babila should be involved. Assistants can follow instructions and check my calculations, but they won’t have the confidence to argue with me if I head down a wrong path. This is too important to be left to one person.”

Yalda could see the logic in that. “Why Frido and Babila?”

“They’re the most experienced engineers we have,” Palladia said. “Who else should I consult?”

She was afraid, Yalda realized. If something went wrong with the scheme and the Peerless ended up crippled and airless, the architects of the plan would be held accountable. Though Yalda would take most of the blame, anyone who had been too close to her on this would share the opprobrium. But if the most powerful members of the only other viable faction were equally enmeshed in the project, Palladia would have some protection in the aftermath.

Was that so unreasonable? And regardless of the politics, Yalda didn’t doubt that Frido and Babila would scrutinize the plans diligently. Whatever their disagreements, they were not going to jeopardize the Peerless itself just to undermine her.

“All right,” she said. “Let’s talk to Frido.”

They found Frido in his office. He listened patiently to their summary of the problem and the results of their calculations.

“Of course I’m happy to help,” he said. “But before we go any further, I think we should put this to a crew meeting—just as we did with the spin engines.”

Yalda said, “Why? The construction crews can handle this; we won’t be taking anyone away from their ordinary jobs.”

“No,” Frido agreed, “but it will still affect everyone. Having explosives set up all around the mountain is not the kind of change we should be making lightly.”

Yalda glanced at Palladia, but she remained silent. “It should be clear that we haven’t come to this lightly,” Yalda said. “Are you in favor of the plan, or not?”

“Of course I’m in favor of it,” Frido replied calmly. “And I want to do everything I can to see it carried out, safely and successfully. The question is, how can we bring the crew along? Can we convince them that, in protecting the Peerless from external threats, this won’t increase the risk to their lives from the enemies within—from saboteurs?”

Yalda traveled down from the summit to check the preparations before the spin engines were fired. In the fields, the last of the crops to grow up from the old cavern floors were being harvested. In the gardens, workers shifted plants and netted soil onto walls that would soon be horizontal. A haze of dust and organic detritus filled these chambers, leaking out into the corridors and stairwells to dim the moss-light and coat every surface with black grime.

After consulting with Lavinio and the other agronomists, Yalda had decided to leave the forest untouched. It was near enough to the axis of the mountain to remain unaffected by centrifugal force, and the effort required to shift the whole tangled maze of full-grown trees—as well as capturing and moving the arborines—seemed disproportionate to any benefit, when all the plants and animals it contained were doing well enough without gravity.

Boards were being fitted over the helical grooves in the outer stairwells, to bridge the gaps in the floors of the tunnels they’d become. The ring corridors could be left as they were, their walls already traversable, but crews were busy fitting rope ladders to their radial offshoots.

Every factory, every workshop, every office needed to be rethought, if not literally reconstructed. But as Yalda traversed the length of the Peerless from field to mill to kitchen, from plantation to carpenter’s workshop, from the medicinal gardens to the holin store, everyone she spoke with accepted the upheaval without complaint.

This was not the time to tear people away from their work to confront them with the news of Palladia’s plan, and she doubted that Frido would be foolish enough to do that himself. While they were as busy as this, united by the common cause of rescuing the crops, no one would be interested in hearing about anything else.

When the work was done, though? Frido could undermine her whisper by whisper, spreading his own message about the new project, leaving people wondering why she hadn’t explained it to them herself. However she handled this, she would not be able to put off the confrontation for long.

Yalda waited in the observatory for the fireworks to begin. She’d invited her old work team to join her, but not everyone had accepted; there were observation chambers lower down offering much better views of the pyrotechnics. But she had something different in mind: she’d locked the big telescope on a point just above the horizon, so her companions could take a look and commit what they’d seen to memory. The flames pouring from the tunnels they’d helped carve into the slopes would be spectacular enough, but the actual proof of the engines’ efficacy would first appear as a tiny shift in the view through the telescope.

Fatima let go of the ropes she’d been holding and curled up in midair. “This is where you discovered rotational physics, isn’t it?”

“It must be,” Yalda replied, “but I don’t really recognize anything. The ground, the buildings… everything’s changed.” Even the telescope itself had been rebuilt, with the original lens inserted into a new frame.

“Someone should put a sign here,” Fatima suggested. “To commemorate it.”

“I’m sure that can wait until I’m dead.”

Yalda glanced at the clock beside the telescope; there were still three lapses left to ignition. Ausilia and her co were clinging to the lowest of the cleaners’ handles at the edge of the dome, peering down the mountain expectantly. Prospera and her friends were over near the entrance, daring each other to attempt ever more intricate ricochets off the clearstone panes. It would be hard to end up stranded in midair, and Yalda had no fear that they’d break the dome, but if anyone collided with the telescope she’d be annoyed.

“I saw Nino yesterday,” Fatima said.

“How was he?” Yalda asked, wishing she didn’t have to hear the answer.

“Not so good.”

“Did you take him some books?”

“He’s not reading anymore,” Fatima said. “He told me he’s lost the power to concentrate; the words just make him dizzy.”

Yalda said, “I’m sorry. But I’m sure you cheered him up.”

Fatima’s expression hardened. “If he knew when he was getting out, it might be easier for him. If you could set a date—”

“Set a date? Do you think it’s that easy?”

“You’re the leader, aren’t you?” Fatima replied bluntly. “And everyone respects you even more, since you decided to build the spin engines. You’re going to save the crops, save us all from starving! Do you really think people will throw you out, after that?”

“It depends on what else I do,” Yalda said.

Fatima was drifting disconcertingly far from the support ropes; she reached down in time to pull herself back.

“If it’s getting too hard for you, maybe someone else could join you in the visits,” Yalda suggested.

Fatima turned to face her squarely. “I’ll tell you exactly what it’s like,” she said. “I go and see him every two stints. I bring him some loaves, tell him some gossip, try to make a few jokes. But that’s it, that’s all I can do. When I turn around and leave, nothing’s changed for him. He’s my friend, I’ll never abandon him… but it’s like holding someone’s hand while they’re being tortured.”

Yalda’s skin crawled. “I’m sorry.”

“Stop apologizing,” Fatima said angrily. “Just do something for him.”

Ausilio let out a chirp of delight, and Prospera’s group quickly scrambled to the edge of the dome to catch sight of the flames from the engines. Yalda motioned to Fatima. “Let’s take a look; it will be a while before anything shows through the telescope.”

They pulled themselves along the ropes to the nearest pane. Looking down the slope, they could see three pale cylinders of blue-white fire emerging sideways from the starlit rock. Yalda waited anxiously for something to go wrong; she’d had visions of one of the engines tearing itself out of the ground and cartwheeling off into the void, spraying the mountain with fire as it went. But the pale flames remained motionless and steady, and she could barely feel the vibration of the engines.

She should have been ecstatic. Their ignorance about the wheat might have killed them, but now they were close to guaranteeing the success of the next crop. She remembered when Nino had told her of Acilio’s sneering prediction of their fate: Eating the soil. Begging for death. The fact that it had almost happened only made it infinitely sweeter to imagine Acilio’s face when the Peerless next lit up the sky over Zeugma.

But what could she do for Nino? Stand up in front of the crew and declare that he deserved to roam freely now—right after informing them that she wanted to fit explosive charges in every wall that separated them from the void? Or simply wait for Frido to explain to them that Palladia’s plan required a new leader who would send the right message to all the would-be saboteurs lurking among them, by finally disposing of the last one who’d been caught in the act?

Yalda dragged herself back to the telescope, and called the team to gather around. The red end of one star trail that she’d centered in the view had now shifted, just detectably, out of the cross-hairs.

“How do we know the telescope didn’t get bumped by the engines’ vibrations?” Prospera asked, only half-joking. “How do we know that the mountain’s really turned at all?”

All that hard, dangerous work, all that beautiful fire pouring out across the slopes, for an incremental change that could as easily be an illusion.

Yalda said, “How do we know? Be patient, wait a while, then look again.”

Two days into the spin-up, one of the lookout posts—wisely left unoccupied for the duration of the process—snapped free of its ropes and was lost to the void. Isidora, whom Yalda had put in charge of the lookouts, had the other three reeled back in to be strengthened and tested before anyone tried to use them again.

By the time the engines were shut down there’d been no other reports of serious damage. In the academic precinct there was a series of small annoyances to deal with—most of them involving the realization that the centrifugal force here, though too strong to be ignored, was also too weak to produce enough friction to hold things in place in the conventional way. Equipment and furniture that would have stayed put under old-style gravity now had to be re-secured just as firmly as when it had been weightless, in order to resist the pushes and tugs of ordinary use.

Yalda quite liked the slight weight she’d acquired in her own office and apartment; she could still use the old system of ropes to get around, but she no longer found herself flailing in panic if she ended up out of reach of all the walls, ropes and handles around her. Slow as she was to fall toward the walls that had turned into floors, her body now accepted that she couldn’t end up stranded.

After helping to get the optics workshop functioning again—with Sabino moved to a perfectly weightless room of his own, dead on the axis—Yalda headed for the fields. As she soared down the central staircase it was as if nothing had changed, but when she took hold of the rope ladder at the mouth of the radial exit, she dutifully reformed her lower hands and descended feet-first.

The tunnel led into the top of the nearest chamber; the flat disk of the interior was now standing on edge. The rope ladder continued down one of the rock faces, and as Yalda moved between the sheer walls, even in the moss-light she found it hard to think of the place as an underground cavern anymore. It was more like descending by night into a secret valley.

The gravity was still weak here, but it had cleared all the dust out of the air. The floor of the valley was deserted, but when Yalda stepped carefully between the furrows she could see that the newly planted seeds had already sent up shoots. The sight sent a shudder of relief through her body.

A flimsy guardrail surrounded the mouth of the radial tunnel leading down into the next chamber; nothing about this exit now looked remotely sensible. “Ah, Eusebio,” Yalda whispered. “Everything’s turned sideways in your beautiful design.” She slipped between the rails and reached across to the rope ladder, which followed what had once been the corridor’s floor. As she gripped the ladder’s side and the structure swayed toward her, her old, dormant sense that a fall could injure her was abruptly reawakened.

The second field had been sown later than the first; no shoots were visible, but Yalda found a buried seed and checked that it was sprouting. Lavinio would have told her if there’d been any problems—but to touch the promise of the next harvest with her own hands reassured her, made her feel strong.

In the third field, the closest to the mountain’s surface, farmers were still at work. Half a dozen firestone lamps had been strung on a pulley line that stretched from the entrance at the top of the chamber to a corner of the field. As Yalda descended, she could see the giant shadows she cast sideways across the rock face.

When she reached the ground, one of the farmers, Erminia, approached and greeted her.

“Thank you for your work here,” Yalda said. “How long until you finish sowing?”

“One more day, but then there’s another field…” Erminia gestured in the direction of the summit, unsure how to refer to it now that “up” had two different meanings. “Two days there, then the whole crop is planted.”

“As soon as there’s a chance, we’ll join the two chambers,” Yalda promised.

“Really?” Erminia didn’t sound enthusiastic.

Yalda was puzzled. “One large field here would make things easier, wouldn’t it?” They needed the extra space for the crops that they’d gain by cutting through the intervening rock, but in any case she’d have thought it would be more convenient to work a single expanse of soil.

“I heard you were going to put explosives here,” Erminia said, “to blow out any fire that starts below us. If that’s what it comes to, I’d rather we lost as little of the crop as possible.”

It was a fair point, but Yalda didn’t reply; she didn’t want to confirm the plan in a casual conversation, let alone start debating the pros and cons of individual section boundaries.

The rumors were already spreading, though. The longer she delayed dealing with them, the weaker her position would be.

She said, “Can you spread the word to all your friends and colleagues: there’ll be a meeting at the summit, five days from now, on the third bell.”

“A meeting about what?” Erminia asked.

Good question, Yalda thought. Why you should be perfectly relaxed about the prospect of your wheat fields exploding beneath your feet?

“We’ve fixed the crops,” she said. “Now we need to talk about what we’re going to do to avoid going the way of Gemma.”

Yalda waited outside the meeting hall, counting the people as they entered while she rehearsed two speeches in her head.

One speech was about the time the crew had spent working together on the slopes, with their lives in each other’s hands and the fate of the Peerless in the hands of everyone. She’d been rescued from a near-fatal accident herself, but they all had their own stories of their friends’ courage and ingenuity. After that, why would they imagine that they needed a rule of fear to keep them safe? One weak-willed farmer with starving children had been persuaded to commit one dangerous act. But Nino had repented and been punished, and he had no reason to try to harm anyone again. He did not need to die, either for the sake of his own crimes or for the sake of the Peerless’s future. Letting him live would not be an act of weakness; it would be an affirmation of everyone’s mutual trust.

The other speech she had ready, in case her first one went badly, concerned the equipment and protocols that could be developed to limit access to the charges, without rendering the fire response so slow as to be useless. And if she grew desperate enough, she was prepared to start talking up the prospects of contingency plans to rescue anyone who ended up outside the mountain in the event of an unplanned breach of the walls.

Palladia emerged from the hall. “Who are we waiting for?” she asked Yalda.

Yalda checked the roll. “Isidora and three others; I think they were all on lookout shifts.” The shifts ended precisely on the bell, but even if they’d forgotten about the meeting and worked through to the usual time, they were later than she would have expected. “I’ll wait until four chimes past, then we’ll have to start without them.”

“You don’t think someone…?” Palladia asked anxiously.

“Snapped a rope?” Yalda had been too distracted to even think of such a thing, but the pang of horror at the thought passed quickly. “The others would have sent for help by now.” The lookouts had already completed one shift safely with the newly-strengthened designs, but in any case the protocols were clear: if someone had ended up adrift in the void, the other lookouts did not try to retrieve their colleague themselves, they returned to the mountain immediately to raise the alarm.

“What’s the mood in there?” Yalda asked. She’d greeted everyone as they’d arrived, but they’d all been equally polite to her. When even Babila and Delfina congratulated her on the success of the spin-up, she could hardly trust anyone’s words or demeanor to reveal their true plans.

“You should take a look for yourself,” Palladia suggested.

Yalda dragged herself over to the entrance. There was plenty of room in the hall for people to spread out comfortably, and many had done just that, but about a third of the crew were clustered together toward the front, clinging to the support ropes that held them up against the weak gravity, jostling each other excitedly, buzzing and chirping.

In the center of this pack was Frido, dispensing his wisdom. She couldn’t hear what he was saying, but the enthusiastic responses were deafening. She’d heard all this noise from out in the corridor, but she’d imagined it was down to boisterous groups of friends rejoicing in their achievements, not one man charming the crowd.

Who was she fooling? She was not a politician or an orator; no one would listen to her words about building the future on trust. If she’d wanted to defeat Frido, she should have started poisoning people against him long ago—making up some story about him having forced his runaway daughter back to her co. Either that, or listened to Nino’s advice from the sagas and just had him killed.

She returned to Palladia. “If you went to him and offered a deal from me, do you think he’d listen?”

“What kind of deal?”

Yalda said, “I’ll stand aside, I won’t oppose him at all, if he promises to let Nino live. Let him threaten hypothetical future saboteurs with any octofurcating thing he likes—just let him respect the decisions I made in my own time, and leave Nino be.”

“What if he says no?” Palladia asked. “You’ll have weakened your position for nothing.”

Yalda could hear the mirth surging in the hall again. “What else can I do? Ask him. Please.”

Reluctantly, Palladia pulled herself back along the rope toward the entrance.

“Yalda! Good news!”

Yalda turned. It was Isidora who’d called out; she and the other three lookouts were approaching in the distance.

Palladia hesitated. “So everyone’s safe?”

“Well, there they all are,” Yalda said.

“And that’s the good news?” Palladia was confused. “Of course it’s good, but…”

Yalda was about to reply that she couldn’t think of any other possibility, but something in Isidora’s tone gave her pause.

Palladia made a move toward the entrance again. Yalda said, “Wait.” She turned and called down the corridor to Isidora, “What good news?”

The expression of joyous bafflement on the woman’s face started Yalda’s skin tingling before she said a word.

“No impacts!” Isidora shouted back. “Two shifts, nooooo impacts!”

Yalda waited in silence until they were close enough to speak properly.

“Two shifts?” she asked Isidora.

“I was going to tell you after the first shift,” Isidora explained, “but you were so busy, and I thought the observers might just be confused by the new setup. We reconfigured the lookout posts… I know it makes no sense, that couldn’t explain a null count, but I had to be sure. I had to see it for myself before I made a fuss about it.”

Palladia said, “No impacts since the spin-up? You’re serious?”

Prospera, who was one of the other lookouts, said, “Staring at dark rock for four bells, the miracle is I didn’t start hallucinating flashes. Zero means zero.”

Palladia turned to Yalda. “How? You think we’ve just passed out of the dust?”

“Do you believe in that kind of coincidence?” Yalda replied.

“What else could explain it?” Palladia countered.

Yalda exchanged glances with Isidora, and let her speak. “The spin-up,” Isidora replied. “Whatever’s been making the flashes, whatever’s been striking the surface, the centrifugal force must be enough to cast it off before it can heat the rock.”

Palladia was incredulous. “To a dust particle with infinite velocity, that force is nothing, it’s completely irrelevant!” She addressed Yalda imploringly. “You agree with me, don’t you? Or is everyone going mad?”

Isidora nodded to Yalda: your turn.

Yalda said, “I agree with you completely—which means the flashes can’t be coming from anything moving so fast. They must be coming from orthogonal dust… I mean dust that was orthogonal to us before the launch, not now.”

Palladia blinked. “Hurtlers? Original Hurtlers?”

“How else could it make sense?” Yalda replied. “Whatever it is that’s been causing the flashes must be moving so slowly relative to the Peerless that our spin is enough to brush them away. Well, we always knew that our trajectory would tame the Hurtlers.”

Palladia grimaced. “But if we’ve tamed them, what caused the flashes? How could something striking us so slowly turn the rock white-hot?”

“I have absolutely no idea,” Yalda confessed, “but if it wasn’t kinetic energy that was heating the rock, all I can think of is some chemical process—and the dust must have needed to stay on the rock long enough to react with it in some fashion. Now that the slopes are unable to hold on to debris… no more flashes.”

Palladia was angry now. “You’re telling me that Hurtlers are made of… what? A liberator for calmstone? The dust from the orthogonal worlds that fills the void here isn’t actual rock, it’s a refined substance people extract from plants with the express purpose of causing fuel to burn?”

Yalda said, “Not the sarcastic bit at the end, but whatever’s been hitting us must act as a liberator for calmstone. Don’t ask me how—but if you don’t believe that, tell us how else you can explain the sudden cessation of the flashes.”

Palladia glared back at her in silence, then she said, “I have no idea. But you’re right, it can’t be a coincidence. The spin is protecting us. So whatever’s been striking us, it’s not high-velocity material.”

“Which means there’s no reason, now, to believe that even a piece dozens of times larger than those that caused the flashes could set the slopes on fire,” Yalda suggested.

“No reason at all,” Palladia agreed.

“So our plan for exploding walls is superfluous?”

Palladia hesitated. “Absolutely. We just need some sacrificial cladding near the axis—at the summit and the base, where the centrifugal force offers no protection…” She stopped speaking; she was trembling with relief.

Yalda put a hand on Palladia’s shoulder then turned to Isidora. “I think we should all go in there and share the good news with Frido and his friends.”

19

“Be warned,” Yalda said, “that I’m going to be spending most of my time telling you about things I don’t understand. Along the way I’ll offer you a few facts and a few guesses—but then I’ll explain why those facts aren’t quite enough and why those guesses can’t quite be right.”

She looked out across the room. Many of the faces were familiar to her, young women and men whose education she’d been following from the start. But there were half a dozen students she barely recognized, too, which was even more encouraging. Once they put the old barbarities behind them, everyone on the Peerless could live the life of the mind. One day they’d all be doing rotational physics with their eyes closed, thinking about the symmetries of four-space as naturally as they moved their limbs.

“What don’t I understand?” she continued. “I don’t understand why solids are stable. I don’t understand why gases aren’t sticky. And I don’t understand why the gentlest contact with the dust that surrounds us can turn rock white-hot.”

“I thought we proved that solids were stable,” Ausilia interjected. “In our last class with Severa.”

Yalda said, “What you did, I think, was show that there are several geometries that an array of luxagens can assume in which Nereo’s force, acting between them, would hold them in place. Is that right?”

“That’s how I understood it,” Ausilia replied.

“So how does that work?”

“Every luxagen is surrounded by hills and valleys of potential energy,” Ausilia said. “If you have a number of them, you can drop them all into their neighbors’ valleys, making a nice, neat pattern in which they should all prefer to stay put.”

“That’s perfectly true,” Yalda said. “But there are a couple of problems Severa wouldn’t have raised, to avoid confusing you while you were still learning the fundamentals.”

She sketched a basic one-dimensional example.

“A luxagen can sit in its neighbor’s energy valley,” she said. “And I’ve put it in the first valley, rather than any of the more distant ones, which are shallower than the first. But is that really the deepest place there is?”

There was silence for a few pauses, then Prospera offered, “The pit would be deeper.”

“Of course,” Yalda replied. “The pit centered on the luxagen itself is bottomless, though I’ve only drawn it going down a short way. Once they’re close enough, two positive luxagens just keep attracting each other ever more strongly, until they collide. So why don’t all the luxagens in a piece of rock simply end up falling into each other’s energy pits, until the whole rock has shrunk down to a speck?”

“Isn’t that like asking why the world doesn’t crash into the sun?” Fatima suggested. “If there’s any sideways motion, the two luxagens wouldn’t actually collide. If they started outside the pit, they’d just skim around it and end up outside again. And even if they had the right amount of energy to stay in the pit, wouldn’t they just orbit each other, like Gemma and Gemmo?”

“You’re right,” Yalda said. “But if two luxagens end up orbiting each other, there’s something more to think about: a luxagen moving back and forth makes light. If the luxagens create light, they need to provide true energy to do that. But to provide true energy—to lose it themselves and turn it into light—they need to gain kinetic or potential energy. So why don’t they end up moving faster and faster, and breaking the whole solid apart?”

Silence again. Then Giocondo—a young man Yalda could only name from his tag—said, “What if the luxagens are moving too fast to make light?”

Yalda waited a pause to let the other students ponder that. “Go on,” she said.

“There’s a maximum frequency of light,” Giocondo began tentatively. “In the equation for light, the sum of the squares of the frequencies in the four directions must equal a fixed number—so none of the individual frequencies can have squares that are bigger than that number. If a luxagen is moving back and forth with a greater frequency than that… it can’t create light in step with its motion, because there’s no such thing.”

Yalda said, “That’s correct. And eventually we’ll work through the calculations for the amount of true energy that an oscillating luxagen passes to the light field, and we’ll show that when the frequency crosses the threshold Giocondo’s just described, the energy flow drops to zero.”

“Then why is there still a problem?” Ausilia asked. “Oh… why don’t all the luxagens end up orbiting in one single energy pit?”

Prospera said, “Because the peaks around the pit keep getting higher. Maybe a few luxagens are orbiting in the same pits, but the more of them you throw in together, the higher the energy barrier around them becomes.”

“Right,” Yalda said. “The more luxagens you have, as long they’re sitting in each other’s pits or valleys, the potential just keeps adding up: all the valleys become deeper and all the peaks get higher. So eventually the pit would become inaccessible, because it’s surrounded by insurmountable peaks.”

Fatima said, “So that keeps all the luxagens from falling together completely, and the rest just end up in each other’s valleys rather than each other’s pits?”

“Go on,” Yalda pressed her.

“I suppose they’d roll around in the valleys too, just like they orbit around the pits,” Fatima mused.

“And if they roll fast enough in the valleys,” Giocondo added, “they’d be stable there too. They wouldn’t emit light and end up tearing the solid apart.”

Yalda was delighted. “Bravo, everyone! A few lapses into the class, and we have solids rendered almost solid again.”

Ausilia said, “Almost? What’s the catch?”

“The idea that Giocondo raised is very appealing,” Yalda said, “and as far as our measurements can guide us, it seems to be true. The energy pits and valleys in real solids seem to be shaped in such a way that the natural frequencies of motion for the luxagens are greater than the maximum frequency of light.

“The only trouble is: if a luxagen isn’t going to make any light at all, there can’t be any wobbles to its orbit in the pit, or its rolling around in the valley. If there was even the tiniest imperfection in its motion that progressed at a sufficiently low frequency, then that would start to generate light.”

“Which would make the imperfection stronger,” Ausilia realized. “So it would lose true energy faster, grow stronger even faster… and the whole thing would get out of control.”

Yalda said, “Exactly. And the thing is, the shape of the potential energy that we get from Nereo’s equation doesn’t allow for perfect orbits, or perfect rolling in the valleys. The main cycle can have a high enough frequency to avoid creating light, but the potential has built-in flaws that guarantee that there’ll be lower-frequency motion as well. It seems to be unavoidable.”

“But solids don’t blow themselves apart,” Fatima proclaimed irritably. “Not without a liberator.”

“Of course,” Yalda said. “So although we seem to have most of the story, although it almost adds up… there must be something we’re missing, something that nobody yet understands.”

She let them ponder that for a moment, but then moved on swiftly. Being told that you’d reached the point where you could only make progress by breaking new ground was a daunting thing to hear for the first time.

“The second mystery,” Yalda continued, “is the structure of particles of gas. There are plenty of symmetrical polyhedrons where putting a luxagen at every vertex gives you a mechanically stable configuration—which seems to make them good candidates for the little balls of matter of which we expect a gas to be comprised. But those polyhedrons share the problem solids have: the luxagens rolling in their energy valleys will always have some low frequency components to their motion, so they ought to give off light and blow the whole structure apart.

“There’s another problem as well, though: tiny, pure fragments of solids are sticky, as Sabino’s experiments have shown. But the gases that make up air don’t seem to be sticky at all; it’s as if the field around them has somehow canceled out, almost completely.

“A young friend of mine back home, Valeria, showed that a spherical shell of luxagens of the right size would have no external field, so you might think that a polyhedron of a similar size could get close to that perfect cancellation. The trouble is, the need for mechanical stability gives you one size for the polyhedron, and the need to cancel the external field gives you a different size. It seems to be impossible to meet both criteria at once.”

Some of the students were beginning to look dismayed. Proving the mechanical stability of an icosahedron built out of luxagens had not been an easy exercise, and now they had to accept that all that hard work had been nothing but the first step into a larger, unknown territory.

“The third mystery,” Yalda said, “is the strangest, and the most dangerous. The Peerless is surrounded by fine dust that we believe is the same kind of material that we saw back home as Hurtlers, when it burned up in the solar wind at close to infinite velocity. But we’ve more or less matched its velocity now… so why should it behave any differently toward us than any other dust?”

Tamara, another near-stranger to Yalda, had heard the theory that had begun circulating a few days after the news that the spin of the Peerless had stopped the impact flashes. “The luxagens are swapped,” she said. “Any that would be positive in our own materials will be negative for that dust, and vice versa.”

“Can you say why?” Yalda pressed her.

“It’s come to us… around the cosmos,” Tamara struggled, tracing out a loop with one hand.

“And why does that matter?” Yalda persisted. “How does that swap the luxagens?”

“I don’t know,” Tamara confessed.

Yalda sketched out the general idea.

She said, “Suppose the orthogonal stars, the orthogonal worlds, are fragments that broke off the primal world backward. They’ve come full circle around the cosmos, and we’re moving alongside them with our arrows of increasing entropy in agreement. We know that those arrows agree, because otherwise the orthogonal stars would be invisible to us.

“But Nereo’s equation ties the field around a luxagen to a vector that points along its history—and there’s no reason for that arrow to have anything to do with entropy; it should simply stay the same along the luxagen’s entire history. That vector determines whether the luxagen is positive or negative: if we meet a luxagen with the vector pointing into our future, we call it positive; if the vector’s pointing into our past, we call it negative.”

“So who drew the arrows on the luxagens?” Fatima joked.

“Well, exactly,” Yalda conceded. “No one really knows what this vector means. Still, we ought to be able to tell when two luxagens have different signs. Close up, a negative luxagen will repel a positive one, and the whole pattern of potential energy seen by a positive luxagen around a negative one will be upside down: all the usual peaks will become valleys, all the usual valleys will become peaks.”

“Which would cause havoc if you mixed the two,” Prospera suggested.

“Not necessarily,” Yalda replied. “You can’t replace a positive luxagen with a negative one in exactly the same location in a solid, but the negative one wouldn’t want to be there anyway—it sees the potential energy curve upside down, so it would prefer to be at a peak rather than in a valley. And if it’s located at a peak, it won’t disrupt the original pattern, it will reinforce it.”

“So it’s not really clear why a speck of dust with its luxagens swapped should cause any more damage when it collides with ordinary rock than an ordinary speck of dust traveling at the same speed. But then… we don’t really know how plant-derived liberators work, nor do we understand why rocks don’t simply burst into flames all by themselves. So we’re a very long way from determining what will or won’t set any given solid on fire.”

Yalda paused to take in the students’ expressions, to see who was beginning to look burdened by the uncertainty they were facing, and who was exhilarated by the prospect of searching for something entirely new.

“I don’t have the answers,” she said. “All I can do is give you some tools that will help you probe these mysteries, then stand back and see what you discover.”

“Yalda, can I speak with you?”

Yalda looked up from her notes to see Lavinio on the ropes at the entrance to her office. “Of course.”

As he approached, the solemnity of his demeanor became apparent. “Don’t tell me it’s the wheat,” Yalda begged him.

“The wheat’s fine,” Lavinio assured her. “But there’s blight in some of the goldenrod.”

“Some of it?”

“Not every plant is showing signs of infection,” he said, “but there are infected plants in all four gardens.”

“How could that happen?” The gardens were worked by different staff, and even Lavinio refrained from visiting all of them. An infection in one should not have spread easily to the others.

“We can’t know for sure.”

“Can’t we make a guess, to try to stop it happening again?” If the protocols for disease limitation were flawed, they needed to be corrected urgently.

Lavinio said, “It was probably all the re-planting work, just before the spin-up. All that dust in the air was impossible to contain; it would have spread throughout the mountain.”

That did make sense—and if the hazard had not been avoidable, at least there was a chance it would never be repeated.

Yalda braced herself. “So, how do things stand?”

“We’ve taken three cuttings of goldenrod from each garden and started growing them in a dozen new locations,” Lavinio said. “The transfer was done with all possible care; two separate couriers who’d never been in any of the gardens took each cutting part of the way, and I’ve recruited new people to look after the plants. But realistically, we can’t expect them all to stay free of the blight.”

“No.”

“We can’t risk harvesting petals from the cuttings at all, until they’re established,” Lavinio continued. “And it might be unwise to take too many from the original plants either, right now; we don’t want to weaken them unduly before we know that at least some of the new ones have ended up in good condition.”

“I understand.” For the next few stints there would not be much holin produced; that was unavoidable.

But Lavinio had done everything possible to safeguard their future supplies. With a bit of luck, the shortage need not be severe or long-lasting.

Yalda said, “Let me know if anything changes.”

In the pharmacy, Sefora checked the stock of holin tablets. “We have enough for about seven stints at our current usage,” she said. “There are some petals still being processed, but that will only add a day or two to the supply.”

Since the launch, every woman on the Peerless had been taking a regular dose of holin that depended on her age, using tables Daria had drawn up that erred on the side of caution. Until now, the gardens had been providing more than enough goldenrod petals to keep the stores replenished; the real limit on building up a larger stockpile had been the shelf life of the drug.

“Can you draw up new dosage tables?” Yalda asked.

“On what basis?”

“We need to stretch out the stockpile—but there’s no point letting the holin sit and go bad.”

“So… stretch it out how long?” Sefora pressed her.

“It’s hard to say,” Yalda admitted. “It’s not clear when the gardens will be producing again.”

“How much are you willing to cut the dosages?”

“How much can I, without putting people at risk?”

“No one has those numbers,” Sefora replied. “Holin’s efficacy has never been properly studied, never quantified. All we’ve ever had are anecdotal reports: if you heard of a woman of a given age who wasn’t protected by the dose you heard she’d been taking, you assumed it would be wise to take more.”

On the Peerless, that uncertainty was meant to have been subsumed by a constant surplus of the drug—and with four bountiful gardens, that should have been possible.

“I suppose it’s too late to start testing it on arborines,” Yalda lamented.

“That would take years,” Sefora agreed.

“Not to mention some actual holin we could spare.”

Sefora said, “I’ll draw up dosage tables that will let the stockpile last ten stints. Any longer than that, and I wouldn’t trust the quality. Do you want me to make any exceptions?”

“Exceptions?”

“If we cut the older women’s doses in strict proportion to everyone else’s,” Sefora explained, “I can’t promise that they won’t face a greater increase in risk.”

Yalda said, “You mean a drop of three parts in ten could be enough…?” She pictured Tullia, lying motionless on the floor of her apartment. No one had the numbers, but they all had their fears.

“I could always leave the dosage for the senior women unchanged,” Sefora said. “If we made an exception for everyone older than a dozen years and ten, that’s less than a sixth of the female population. Spread out between all the younger cohorts, the difference would scarcely be noticeable.” She was not quite as old as Yalda, but she would fall into the same category.

Yalda considered the suggestion. Wouldn’t it be fairer, to protect the most vulnerable members of the crew? It would certainly be prudent: they couldn’t risk having the most experienced women taken without warning, leaving the Peerless drifting aimlessly through the void.

She said, “I think you should do that.”

“If a particle is moving in an energy valley that takes the form of a parabola,” Yalda told the class, “it will repeat the same harmonic motion over and over, with a frequency that depends only on a single number describing the parabola’s shape.”

“Like a weight oscillating on the end of a spring?” Prospera suggested.

“Or a pendulum under gravity?” Fatima added.

“In idealized versions, yes,” Yalda agreed, “though in reality both those systems experience friction, and both have small deviations from a parabolic potential.

“Still, in the absence of friction—or light generation—the particle’s energy will be conserved, and if it’s moving back and forth in just one dimension, even if the valley isn’t shaped like a parabola the particle will always return to its starting point. So its motion will be perfectly cyclical, and it won’t have any harmonics with a lower frequency than that cycle.

“But in two or more dimensions, things start to get more complicated. Even when energy is conserved, a particle need not retrace its path exactly. If the shape of the valley is a perfect paraboloid it will do so—” Yalda sketched an example:

“But that’s not the case for the energy valley in a solid, due to Nereo’s potential. There, the cross-section isn’t exactly parabolic, and it will be shaped a bit differently when you slice it in different directions.”

“So instead of moving back and forth with a single, pure frequency, a luxagen in this energy valley will follow a path that we need to describe with a multitude of different frequencies, all present in different amounts.”

“Like describing the strengths of all the colors in a flame?” Ausilia asked.

“Very much like that,” Yalda said. “What we’re going to be doing, eventually, is trying to predict both the colors of solids in ambient light and the spectrum of the light that we’d expect them to emit. And then the question will be: why are our predictions so wrong? Why does it actually take some kind of disruption for solids to start emitting any light at all?”

After the lesson, half the class moved to the food hall, unwilling to end the discussion. Yalda watched the young women among them swallowing their two tablets of holin with their loaves; each cube was smaller on its side than the old ones by just one part in six, which made the rather greater reduction in its volume almost invisible. But she’d taken her own dozen full-strength cubes in the privacy of her apartment, ashamed of the discrepancy even though it was unlikely that anyone would have noticed it.

Yalda listened to the students’ excited chatter and answered their questions with care. Who else could have taught them harmonic analysis of Nereo’s potential, if not her? Who else could have set them on the path to a future where everything the Peerless would need in order to survive its exile and return in triumph was finally understood?

Isidora. Sabino. Severa. Perhaps a dozen people in all. She was not indispensable.

Fatima lingered after the others in the group had gone. “Have you thought any more about Nino?” she asked Yalda.

“You know I’d be happy to free him,” Yalda replied. “But to do that, I need to be in a position of strength. I’m sure Nino understands that.”

Fatima was unmoved. “You rescued the crops and swatted away the orthogonal dust with the same hand! Everyone knows that they owe their lives to you. How much stronger do you think you’ll ever be?”

“This goldenrod problem—” Yalda protested.

“That’s hardly your fault.”

“Whether it is or it isn’t, people won’t be happy until it’s resolved.” Suddenly self-conscious, Yalda looked around the hall with her rear gaze, but no one was paying them any attention.

Fatima said, “There’ll always be something. If you just saw Nino, if you spoke with him—”

“Anyone else would have gotten rid of him by now,” Yalda declared irritably.

Fatima regarded her with disbelief, then lapsed into a reproachful silence.

Yalda said, “I didn’t mean that. I’m sorry. When things have improved, I’ll look at his situation again.”

“You were in prison once, weren’t you?” Fatima replied. It was a rhetorical question; she knew the answer. “Waiting for someone to set you free?”

“I won’t abandon him,” Yalda said. “I promise you that. Just let me find the right time.”

“Ten of the goldenrod cuttings are infected with blight,” Lavinio announced. “The other two appear to be healthy. But those two cuttings are all we have now; the plants in the four main gardens are lost.”

Yalda absorbed the news, and tried to think through the consequences calmly. They would not be able to harvest any petals from the cuttings until they’d grown larger, or they’d risk killing the plants. It could be as long as half a year before any more holin was being produced—and after that, it could take another year or two for the rate of supply to return to normal.

“What if you split each cutting—after a few stints—and grew the halves separately?” she suggested.

“That would just delay the time until they were strong enough to survive harvesting,” Lavinio explained. “The most important thing is keeping those two plants strong and uninfected.”

“I understand.”

“We’re lucky we haven’t lost the goldenrods completely,” Lavinio said bluntly. “If we’re not careful, it might yet end that way.”

When he’d left, Yalda clung to the ropes beside her desk, fighting a growing sense of helplessness. Word of how serious the problem had become would not take long to spread; if she failed to deal with it swiftly there’d be chaos.

Rationing the stored holin more severely wouldn’t help; there was no point eking it out so slowly that it began to lose its potency. The only way she could survive the wait until production started again was to commandeer enough of what remained to increase her dose as time went on, to compensate for the drug’s deterioration.

But then even when the holin was fresh again, there would not be enough to go around.

She could ask Sefora to draw up a plan to save the oldest women, leaving the others to take their chances. No one on the Peerless was a child, though; no one would be immune to the risk. The shortage would take its toll across the mountain—while the drugs that kept each old woman alive could protect half a dozen of their younger crew-mates.

Yalda struggled to clear her mind. How was she meant to weigh up the choices and reach the right decision? Eusebio had given her Frido to share the burdens of leadership, but she’d destroyed any chance of trust between them, any hope of getting honest advice from him.

She dragged herself along the ropes to the front of the office and pulled the doors closed. She let her body relax completely, then she felt herself begin to shiver and hum.

How close had she come to snatching a few more years for herself, by risking the futures of all the young women who still had their lives ahead of them? How close had she come to stealing the hard-won promise of Prospera, Ausilia and Fatima—Fatima who’d never shown her anything but loyalty, who’d had the love and courage to pluck her from the void?

What had she imagined her own role would be? To see the journey through to the end? To return to Zeugma to share the triumph with Eusebio, and join in the celebrations with all her lost friends? She’d made her choice: she’d been vain enough to believe that the Peerless needed her. But it needed her only to set its course; everything else belonged to the generations to follow.

Yalda composed herself. Once her body was still again she felt calm and lucid.

She’d played her part, and it was almost over. But now she knew what needed to be done.

Isidora’s co worked in the pharmacy, and he’d done the same job for eight years back home. Yalda met him to gauge his loyalties. While Sefora was in charge he would follow her instructions, but he accepted Yalda’s right to replace her. And he did not want his own co to lose control over her body.

Yalda picked a dozen young women to accompany her. They made their move a bell before the main shift began; none of the junior pharmacists put up any serious resistance, and by the time Sefora came on duty Yalda’s team had the holin store surrounded.

“Are you going to punish me for doing what you asked?” Sefora demanded angrily. She looked to her colleagues for support, but they wouldn’t meet her gaze; they were backing the new guard.

“I’m not punishing you at all,” Yalda replied. “You served the Peerless well, but now this job needs someone new. You can retire to a life of ease.”

“Really?” Sefora emitted a mirthless buzz. “Is that what you intend doing yourself?”

Yalda said, “You can hear about my plans at the meeting, along with everyone else.”

Yalda surveyed the faces of the assembled crew. “I wish we had holin for everyone in the mountain,” she said, “but that’s beyond our control now. So the time has come for the women like myself who would use the most of it to step aside, and leave what remains to those who have the most to lose.”

She listed the replacements for a dozen senior positions. A trace of discontent rippled through the crowd, but she could see expressions of acceptance, too. There was no painless way through the shortage, but any other scheme would have ended in insurrection.

“On the question of who should take my place as leader,” she said, “everyone knows there is an obvious choice.” Yalda stretched out an arm toward Frido, who was clinging to a rope near the front of the hall. “But before I appoint my successor, I need to ask him if he’s willing to meet some conditions.”

Frido said, “Tell me what you want.”

“When I step down,” Yalda said, “I want the right to choose my own co-stead. And when I’m gone, I want my family to be left unharmed. I want my co-stead and my children to be given your respect and protection, and to suffer no revenge.”

Frido regarded her with an expression of wounded horror. “What kind of monster do you take me for? Yalda, you have the love and respect of everyone here. No one will harm your family.”

“You give me your word, before the whole crew?” she insisted.

“Of course. Everything you’ve asked for, I promise it will be done.”

Yalda had no idea what was going through his mind, but what else could he have said? She’d just granted the young runaways the best prospects they could have hoped for to make it through the holin shortage. If Frido had so much as hinted that he expected to assert some bizarre, paternalistic right to veto her choice of co-stead, they would have torn him apart.

She said, “Then it’s done. I resign the leadership in your favor. If the crew accepts you, the Peerless is in your hands.”

Frido moved forward, toward the stage. Behind him, half the crew began chanting Yalda’s name—affirming her decision, not rejecting her successor, but it still made Frido flinch.

Watch your back, Yalda thought. Get used to it. That’s what your life is going to be like now.

20

Fatima moved ahead of Yalda down the center of the stairwell, pausing now and then to allow her to catch up. Yalda didn’t mind being hurried along this way; if they’d been traveling side by side they would have had to pass the time discussing the reason for their journey.

When they came to the first radial tunnel, Fatima let herself free-fall most of the way, only snatching at the rope ladder when she began to veer away from it. Yalda declined to follow her example, and descended slowly, rung by rung. The locked doors they encountered along the way did not appear marked, let alone damaged. No one had been sufficiently motivated to try to assassinate the half-forgotten saboteur.

In the abandoned navigators’ post above the second-tier engines, Yalda waited outside the cell. Nino trusted Fatima, so it was best that he hear most of this from her. But after a few lapses, she invited Yalda in.

“Hello, Yalda.” Nino hung in the center of a sparse network of ropes. He was much thinner than she remembered him, and he kept his eyes averted as he spoke.

“Hello.” The cell was crowded with books and papers. As in Yalda’s own apartment their not-quite-weightless state would make them difficult to manage, but the place had been kept scrupulously tidy.

“Fatima explained your proposal. But she wasn’t able to say what would happen if I refused you.”

“Nothing is by force,” Yalda said. “Whatever you choose, I’m willing to take you to the summit and do my best to protect you.”

“I don’t know if I could look after myself up there,” he said. “Let alone… anyone else.”

Fatima said quietly, “I’ll help.”

Nino seemed paralyzed, unable to reach a decision. How could any of them know what was or wasn’t possible? Yalda surveyed the papers stacked against the rear wall. “We can come back for these later,” she said. “Unless there’s something you need?”

Nino buzzed softly. “I never want to be in the same room as the sagas again.”

Outside the cell he faltered, gawping at the preposterous spaces around him. Had Fatima never broken the rules and let him out during a visit? Perhaps he’d refused, afraid that even a small taste of freedom would make his imprisonment too hard to bear.

On the journey back Fatima was patient, demonstrating to Nino how to negotiate the changing forces on his body. Yalda looked on, trying to be equally encouraging herself, but wondering if she’d made a terrible mistake. Nino might learn to be agile again, but what had she done to his spirit? When she’d been teaching him, she’d had no doubt that his memories of his children were keeping him sane. But he’d spent more than three years excluded from any kind of normal life—and she still didn’t know if he’d be accepted back into the community of the Peerless.

When they left the central stairwell in the academic precinct, Nino blinked and squinted at the lamps around them as if he’d been thrust into the searing blaze of noon. When the first passerby looked their way he stopped moving and clutched the ropes tightly with four hands, his posture growing cowed and defensive. Yalda watched the woman’s expression change from confusion to recognition, then from shock to comprehension. As she passed them on the opposite ladder she glanced at Yalda with what might have been an acknowledgment of her audacity, but exactly what fate she wished for the happy couple was impossible to discern.

Fatima took Nino with her everywhere, introducing him to friends, fellow students and acquaintances without a trace of self-consciousness, as if he were a long-lost uncle who’d just arrived in their company by some mysterious alternative route. At first Yalda took this as some kind of unspoken reproach for her own reticence at the task, but then she realized that it was nothing of the kind. People put up with a very different attitude from Fatima, as Nino’s advocate, than they would have from the woman they blamed for the fact that he was still alive at all. Fatima was utterly partisan on her friend’s behalf, but there was no reason for anyone to think of her as self-serving.

Every day, Yalda tagged along as Fatima showed Nino the food halls, the workshops, the classrooms. He was getting reacquainted with places he hadn’t seen since before the launch, and roaming far enough from the axis to grow familiar with the changing centrifugal force. Some of the people they encountered were brusque, but no one started screaming threats or accusations. And even those who had no particular respect for Yalda, or Fatima, or for Frido’s oath of protection, might have been given pause by the realization that Yalda’s choice of co-stead was the bluntest possible assertion of a woman’s right to decide when, and with whom, she had children. With holin scarce, with pharmacology failing them, any purely cultural force in favor of autonomy was all the more precious.

Isidora and Sabino took turns teaching Yalda’s old class. Yalda sat in and listened, watching Nino struggling to extract some sense from all the arcane technicalities as Fatima whispered explanations to him. This was his world now, not the wheat fields, and whatever role he played in it he’d have to learn some of its language and customs.

Yalda made a bed for him in her apartment, and he accepted that intimacy without complaint or presumption. The first night he was with her she could barely sleep; she did not expect him to wake her and demand what she had offered him, but his presence made it impossible for her to forget the ending she had chosen for herself. Better that than to be taken by surprise, like Tullia. Her only other choice would have been to launch herself into the void again and wait for her cooling bag to run out of air, leaving her to cook in her own body heat. Because whatever she might have wished for in a moment of weakness, however strong the urge to renege might have become, the holin that could have bought her a year or two more was now irrevocably out of her hands.

Nino clutched the rope at the edge of the observation chamber and peered down at the countless tiny color trails fixed above the rocky slope.

“Those are the orthogonal stars?”

Yalda said, “Yes.”

He grimaced. “They look just like the stars back home. But now you’re saying that their worlds could kill us with a touch, if we so much as set foot on them?”

“That’s how it seems,” Yalda replied. “But then, who knows what will happen down the generations? We might even find a way to mine their rock, to render it harmless.”

Nino looked skeptical. He still found it difficult to accept that the Peerless had a future at all.

“Look at what we’ve survived already,” Yalda said. “Harder tests than any you gave us at the launch.”

“If those stars lie in the future,” he said, “why can’t you just search among them with your telescopes and see if they strike the world, or not?”

“Light from that part of their history can’t reach us here,” Yalda explained. “When we looked out at the ordinary stars, back home, we saw them as they were many years ago. The same is true of these stars—but ‘many years ago’ by our measure, now, means far from the world, far from any collision that might happen.”

“But if they continue as we see them—?”

“Then the world will end up in the thick of them,” Yalda said. “That much is clear.”

Nino was silent. Yalda said, “What we’re doing has the chance to help your children, far more than Acilio’s money ever could. Don’t you want to be a part of that?”

“It’s worth trying,” he conceded. “Better than rotting in that cell. And if you really can trust me with your own flesh—”

“Why wouldn’t I?” Yalda did her best to silence her doubts. “You’ve been a good father before. Just promise you won’t force the sagas down their throats.”

“I might tell them a couple of the old stories,” Nino said. “But the rest would be about the flying mountain whose people learned to stop time.”

He reached over and put his hand on Yalda’s shoulder. Nature dulled her fears, lulling her into a sense of rightness at the thought of what lay ahead. If she waited, if she asked for time to say her farewells, that would only make it harder. This was her last chance at the closest thing to freedom: her will, her actions, and the outcome in the world could all be in harmony.

Yalda said, “I want you to name our children Tullia and Tullio, Vita and Vito.” For all that she’d cared for Eusebio, if he was going to outlive her his name could look after itself. “If there’s a solo, call her Clara.”

Nino dipped his head in assent.

“Love them all, educate them all.”

“Of course,” Nino promised. “And you’ll be no stranger to them, Yalda. What I don’t know about you, your friends will tell them. Fatima will tell them a dozen stories of you a day.”

He’d meant to reassure her, but Yalda shivered with grief. A mountain could fly through the void, but she could not see her own children.

She fought against her sadness; if she succumbed to it now and stopped what they’d begun it would only be twice as painful the next time.

Yalda took hold of the ropes with three of her hands; with the fourth she drew Nino’s body closer. The color trails of the old stars were splayed out above them. His chest pressed against hers, innocently at first, but then their skin began to adhere. Yalda twitched, panic-stricken, picturing herself tearing free, but then she stifled her fear and let the process continue. When she looked down, a soft yellow glow could be seen passing through their conjoined flesh, its message older than writing.

Her eyelids grew heavy, and a sense of peace and reassurance suffused her thoughts. There was no need for words now. They were sharing light, and the light carried Nino’s promise to protect what she would become.

APPENDIX 1

UNITS AND MEASUREMENTS

APPENDIX 2

LIGHT AND COLORS

The names of colors are translated so that the progression from “red” to “violet” implies shorter wavelengths. In the Orthogonal universe this progression is accompanied by a decrease in the light’s frequency in time. In our own universe the opposite holds: shorter wavelengths correspond to higher frequencies.

The smallest possible wavelength of light, λ_min, is about 231 piccolo-scants; this is for light with an infinite velocity, at the “ultraviolet limit”. The highest possible time frequency of light, ν_max, is about 49 generoso-cycles per pause; this is for stationary light, at the “infrared limit”.

All the colors of light arise from the same pattern of wavefronts, rotated into different orientations in four-space.

In the diagram above, AB is the separation between the wavefronts in four-space, which is fixed regardless of the light’s color. AD is the light’s wavelength (the distance between wavefronts at a given moment) and BE is the light’s period (the time between wavefronts at a fixed location).

The right triangles ACB and ABD are similar triangles, because the angles at A are the same. It follows that AC/AB = AB/AD, and:

AC = (AB)²/AD

Also, the right triangles ACB and EAB are similar, because the angles at B are the same. It follows that BC/AB = AB/BE, and:

BC = (AB)²/BE

Pythagoras’s Theorem, applied to the right triangle ACB, gives us:

(AC)² + (BC)² = (AB)²

Combining these three results yields:

(AB)⁴/(AD)² + (AB)⁴/(BE)² = (AB)²

If we divide through by (AB)⁴ we have:

1/(AD)² + 1/(BE)² = 1/(AB)²

Since AD is the light’s wavelength, 1/AD is its spatial frequency, κ, the number of waves in a unit distance. Since BE is the light’s period, 1/BE is its time frequency, ν, the number of cycles in a unit time. And since AB is the fixed separation between wavefronts, 1/AB is the maximum frequency of light, ν_max, the frequency we get in the infrared limit when the period is AB.

So what we’ve established is that the sum of the squares of the light’s frequencies in space and in time is a constant:

κ² + ν² = ν_max²

This result assumes that we’re measuring time and space in identical units. But in the table above we’re using traditional units that pre-date Yalda’s rotational physics. The data Yalda gathered on Mount Peerless showed that if we treat intervals of time as being equivalent to the distance blue light would travel in that time, the relationship between the spatial and time frequencies takes the simple form derived above. So the appropriate conversion factor from traditional units to “geometrical units” is the speed of blue light, v_blue, and we have:

(v_blue × κ)² + ν² = ν_max²

The values in the table are expressed in a variety of units that have been chosen so that the figures all have just two or three digits. When we include a factor to harmonise the units, the relationship becomes:

(78/144 × κ)² + ν² = ν_max²

Now, the velocity, v, of light of a particular color is simply the ratio between the distance the light travels and the time in which it does so. If we take the pulses of light in our first diagram, they travel a distance AC in a time BC, giving v = AC/BC. If we then use the relationships we’ve found between AC and AB and the spatial frequency κ, and between BC and BE and the time frequency ν, we have:

v = κ/ν

Again, we can only use this formula with traditional units after applying the appropriate conversion factor:

v = (v_blue × κ)/ν

which, if we’re taking frequencies from the table above, becomes:

v = (78/144 × κ)/ν

The velocity we’ve been describing so far is a dimensionless quantity, related to the slope of a line tracing out the history of the light pulse on a space-time diagram. (The way we draw our diagrams, with the time axis vertical and the space axis horizontal, it’s actually the inverse of the slope.) Multiplying the dimensionless velocity by a further factor of 78, the speed of blue light in severances per pause, gives us the values in traditional units that appear in the table.

AFTERWORD

Much of what we know about the physics of our universe can be understood in terms of the fundamental symmetries of space-time. If you imagine any experiment that can be fully contained on a floating platform out in space, then orienting the platform in different directions or setting it in motion with different velocities will have no bearing on the outcome of the experiment. The particular directions in space and in time with which the platform is aligned make no difference.

However, in our universe the laws of physics distinguish very clearly between directions in space and directions in time. While you’re free to travel through space precisely due north if you wish, as you do so you will also be moving forward in time (as measured, say, by GMT). Expecting to be able to depart from Accra at 1:00:00.000 GMT and arrive at Greenwich to see the clocks showing exactly the same time—because you’d arranged to move “purely northwards” without any of that annoying progress through other people’s idea of time—is not just a tad optimistic, it’s physically impossible. “North” is a “space-like” direction (whatever else might be merely conventional about it), while “the future” is a “time-like” direction (however much it might differ from person to person traveling at relativistic speeds). No amount of relative motion can transform space-like into time-like or vice versa.

The underlying physics of Orthogonal comes from erasing that distinction between time and space—giving rise to an even more symmetrical geometry—and then applying a similar kind of reasoning to that which links the abstract geometry of space-time to the tangible physics of our own universe.

Does every last phenomenon described in the novel follow with perfect mathematical rigor from this process? Of course not! Centuries of effort by people far more able than I am has still not put the physics of our own universe on such a rigorous footing, and to reconstruct everything under different axioms—with no access to experimental results—would be a massive undertaking. So while I’ve tried to be guided throughout the novel by some well-established general principles, at times the finer details are simply guesswork.

That said, the most striking aspects of the Orthogonal universe—the fact that light in a vacuum will travel at different speeds depending on its wavelength; the fact that the energy in a particle’s mass will have the opposite sense to its kinetic energy; the fact that like charges will attract, close up, but then experience a force that oscillates with distance between attraction and repulsion; the existence of both positive and negative temperatures; and the fact that an interstellar journey will take longer for the travelers than for the people they left behind—are all straightforward consequences of the novel’s premise.

My initial thoughts about the Orthogonal universe were clarified by the discussion of the consequences of different numbers of space and time dimensions in Max Tegmark’s classic paper, “Is ‘the Theory of Everything’ Merely the Ultimate Ensemble Theory?” (Annals of Physics 270, pp 1-51, 1998; online at arxiv.org/abs/gr-qc/9704009). Tegmark classifies universes with no time dimension as “unpredictable” (p 34). However, he appears not to have considered cases where the underlying space-time is a compact manifold, making the universe finite. As discussed in the novel, finite universes with the right topologies can exhibit physical laws that support predictions—albeit imperfect ones if the data available spans less than the entire width of the universe. But this isn’t all that different from the situation under Newtonian physics, which also allows the possibility that an object with an arbitrarily high velocity might unexpectedly enter the region whose future you’re trying to predict.

Readers with a background in physics might be aware of a mathematical technique known as Wick rotation, in which equations that apply in our own universe are converted to a form with four spatial dimensions, as part of a strategy for solving the original equations. It’s worth stressing, however, that these “Wick-rotated” equations are not the same as those governing the physics in Orthogonal; there are some additional changes of sign that lead to very different solutions.

Supplementary material for this novel can be found at www.gregegan.net.

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APPENDIX 1

APPENDIX 2

AFTERWORD