Dichronauts

by Greg Egan

  «It’s not too late to join me in retirement,» Theo replied.

  «Yeah, we’ll just walk back to Baharabad from here.»

  «We could go back to the basket and wait for the retrieval. Or we could kill some time on the terrace first, to avoid being cooped up for too long.»

  Seth was no longer entirely sure that he was joking. «That’s tempting, but no. I want to see what’s down here with my own eyes.»

  Theo was silent. Seth took another step, bringing his foot down with infinite care, waiting for the slope to tell him which way it was trying to make him fall.

  they stopped while they still had an hour or so of good light remaining and managed to find suitable crevices for a dozen spikes, to which they attached safety ropes and the wedged slings they hoped to sleep in. With their movements so restricted they had less privacy than ever, but Seth took hold of the longest axial safety rope and went south for a while out of everyone’s view, giving him a chance to defecate in peace.

  He’d volunteered for the first shift keeping watch, and though only Theo could observe the spikes in the dark, Seth took his own role seriously, attending to the tension that reached him through the network of ropes supporting his sleeping colleagues. If a spike came loose, he was sure he had as good a chance of noticing it as Theo.

  When Sarah and Judith took over, he climbed back up north and lay down in the empty sling, with his stilt uncollapsed beside him. He’d never felt more vulnerable, but he was exhausted, and as weariness fought fear, a part of him declared, however insincerely, that it might not be such a bad way to go: sliding away in the night, barely waking before friction with the slope tore his body apart.

  He woke before anyone else, except for Ada, who was still on watch. He ate some bechelnuts and took a few swigs of water; it was so cool now that he rarely felt thirsty during the day. As it grew lighter, he could feel the others stirring, sending tremors through the linked slings.

  the effort required to move safely across the slope never lessened, but by their third day off the terrace, the sheer accomplishment of traveling so far without mishap gave Seth a renewed sense of vigor. It rained that night, but it was a light drizzle, just enough to let them refill their canteens. They’d brought tents, but no one thought it was worth trying to set up shelter, and Seth felt safer sleeping in the rain than if he’d been shielded by a structure that restricted Theo’s vision and might hinder his movements in an emergency.

  The real downside came the next morning, when the slope remained too slippery to traverse until the twilight’s noon. But that was a small inconvenience, and Seth took the whole episode as an encouraging sign. The chasm offered up the most difficult terrain he’d ever faced, but so far it had not proved implacably hostile or impossible to traverse. He felt sure now that they stood a good chance of completing their task safely and returning to the surface with a clear verdict on the obstacle’s longitudinal extent and its significance for the migration.

  Two days later, they saw the river.

  Amina had already heard some kind of susurration from the west, though she hadn’t been prepared to commit to any particular interpretation of the sound. But if she’d ever thought that it might have been nothing more than the patter of distant rain, over the course of the morning a shimmering smudge in the distance resolved itself into an unmistakable torrent, streaming over the slope beneath a cloudless sky.

  As they drew closer to the river, it became apparent that it was flowing north—and the fact that water could cross their intended path in either axial direction proved that the transition line they’d been following would soon come to an end. Even if they’d arrived here at a time when the riverbed was dry and they could march straight across it, the hope of being guided to a second terrace would have been just as dead.

  “We’ll follow it north and see what becomes of it,” Raina decided. Seth had no reason to argue: if they traveled north at least the Siders would be able to ping the terrain ahead of them.

  They kept their distance from the damp slickness of the riverbank, and ascended slowly, struggling to master the new gait. Seth’s hips began to ache incessantly from the need to raise his legs higher than usual to give them clearance from the tilted ground. It was a welcome change not to have to tip his head to the west, though.

  “Why couldn’t they give us a separate expedition to the east?” he joked with Sarah.

  “And the return journey . . .?”

  “Maybe we would have stumbled on a river that let us circumnavigate the habitable zone.”

  Theo couldn’t resist the opportunity to return to his second-favorite gripe. “More chance of that than doing it with balloons.”

  Andrei said, “The bridge of balloons is just a gesture. No one really believes that’s going to work, but they need to be seen to be trying everything.”

  For the first time since they’d left the basket, Seth had Andrei’s face in front of him as he spoke, but he’d missed the opportunity to give him a warning glance to steer him away from the subject.

  “Trying everything?” Theo was incensed. “Except the one idea with a real chance of success!”

  “They didn’t just dismiss the possibility of trapping the sun,” Andrei replied. “Nicholas and I were on the committee that examined it.”

  Theo took a moment to absorb this revelation. Finally he asked, “So did you reject it yourself, or were you voted down?”

  Andrei said, “I was positively disposed to your plan at the start, but after the committee had spent nine days examining it, our own calculations led to a consensus. Your basic idea was perfectly sound: if we could erect mountains in the formation you proposed, they’d put a torque on the sun’s orbit that would balance whatever’s causing the current tilting. But the logistics turned out to be insurmountable.”

  “‘Logistics’!” Theo retorted. “You mean political will?”

  Andrei remained calm. “No. Politics wasn’t even part of our brief. We took it as our starting point that the diplomats could clear the way to assembling the largest construction workforce imaginable: eighty percent of the adult population of the habitable zone, with the remainder either farming, transporting food, or looking after children. But building the necessary structure in any given location would still have taken longer than the time in which that location remained in the habitable zone. Any dawdling child can keep up with the migration, but not even a million adults could build mountains high enough to trap the sun before it moved on. You might as well try to catch an axis lizard by weaving a cage out of grass as it runs past you.”

  Theo fell silent.

  Seth wasn’t sure how to take the news himself. If it meant that he’d heard the last of Theo’s diatribes against the feeble-minded cowards who’d rejected his plan, that would be bliss . . . but the fact that they’d treated the proposal with the utmost seriousness, only to rule it out even as a measure of last resort, was a little disquieting. If Andrei’s opinion on the bridge of balloons was equally well informed, then finding a way past the chasm might be the only remaining option.

  “Why couldn’t someone tell me this earlier?” Theo demanded.

  “You’re not known for your discretion,” Andrei replied bluntly. “The concern was that if you were told about the assessment, you’d dispute the results aggressively, while publicizing them in a way that spread panic.”

  Theo said, “Don’t you think people are entitled to know all this?”

  Andrei’s face took on an expression of mild incredulity. “Most people have never even heard about your plan, and most of those who have thought it was both impractical and entirely unnecessary. Telling the world that surveyors from ten cities were so worried about the future of the migration that they’d seriously contemplated trying to halt the sun would not inspire people’s confidence. When we have a plan that we’re sure will work, that will be the time to spread the word about it.”

  after they’d pursued the river north for three days, even the Walkers could hear
that there was a change ahead. Rising above the steady babbling from the west, the sound of water pounding rock presaged a dramatic change in the flow. Though they were north of the point where the basket had been set down, they were still well south of the cliffs, but the incline would only need to increase by a few degrees to start opposing the water’s northwards passage.

  As a mist of fine droplets began to spray down on them, they veered east in search of dry ground. The spray became thinner, but did not die away entirely; perhaps the wind was dispersing it, though it did not seem to come and go with any palpable breeze. Theo could ping a fair distance upslope, but not far enough to reveal the source of the droplets. Facing east, away from the presumed waterfall, Seth thought he could see a distant glistening in his peripheral vision, but even turning his gaze as far to the north as he dared in his precarious state of balance couldn’t make the smudge of light entirely convincing.

  The next day, by mid-morning, the view was clearer, and Raina confirmed his hunch. “It looks like a flooded terrace,” she agreed.

  If the river had reached a terrace and branched in two, the flow on their side would be traveling east. Even if it was tame enough to ride, it would carry them in the wrong direction. Their only real hope of continuing west would be to cross this branch of the river and resume their journey from the northern bank.

  By the time they stopped to set up camp for the night, the twilight shimmering off the surface of the distributary was unmistakable, whether they looked for it west-north-west or east-north-east. Farther to the west, Seth could see hints of the waterfall at the river’s bifurcation point, with the spray rising high into the air around a vortex comparable to the one in Thanton’s jungle. The eastern branch of the river seemed placid in comparison; the topography at the junction must have directed most of the flow to the west.

  In the morning they advanced slowly; there was no avoiding rock dampened by the spray, so they minimized the risk by scrupulously checking every spot where they placed a stilt or wedge. Seth barely lifted his gaze from the slope until Sarah announced in a tone of relief, “That looks navigable to me.”

  They were only a few dozen paces from the river now. Between Theo’s pings of the bank directly above them, and his own view of the adjacent stretch to the east, Seth could discern a steady flow beneath the mist of droplets escaping from the water’s surface. The spray here wasn’t coming from some turbulent collision of currents, or a jagged outcrop obstructing the flow. Although the slope at the southern bank was less than forty-five degrees, pushing the water uphill, the steeper angle of the as-yet invisible northern bank was blocking its passage, leaving the bulk of it piled up with nowhere to go but east. But it didn’t take more than a faint gust of wind to disturb the surface, pushing parts of it beyond the critical angle and sending droplets cascading down—fast enough for the lightest to bounce off into the air.

  Raina instructed them to start assembling the boat. The small tents that each of them carried in their packs were made of a fabric sufficiently tough and waterproof to serve a second purpose: with a few different choices in the way the panels were tied together and braced, the final structure could form a rigid enclosed volume, with its lower surface seamless and impermeable. Each of the five modules was able to float independently, but the safest configuration would be to bind them all up into the hull of a single vessel.

  They’d rehearsed the assembly process more than a dozen times on the practice ramp, though Seth recalled that only the last attempt had been successful: every other time, either the boat or a member of the team had ended up falling from the ramp into the hay. He proceeded to unpack and unfold the material, switching his attention methodically between the act of ensuring that his balance and footing were secure whenever he needed to shift his weight, and the precise manipulations required to bring the panels of fabric a step closer to their final state.

  The sheer size of that final state made it unwieldy, but for the last few stages he found he could rest at least one corner of the structure on the slope, letting the rock take some of the strain off his wrists. When he was done, he had a shallow, open-topped box with hollow walls and a hollow base. He prodded it cautiously, checking that he hadn’t left any strings loose or struts unanchored. In one of the rehearsals, he’d managed to do everything perfectly, except that the whole surface was inverted, with all the joins where the panels were merely laced together—closing off the would-be tent’s entrance flap—appearing on the craft’s underside, rather than tucked into the edges above, where they’d have a chance to remain dry.

  Andrei said, “Give me flat ground, one dead tree, and five days to carve it into a canoe.”

  “While you’re dreaming, just give us a balloon,” Judith retorted.

  “Actually,” Nicholas replied, “if there’s a second expedition, they should find a way to make that work. Think of the ground we could have covered by now.”

  Judith said, “Only if ‘by now’ excludes however many hundreds of days it would take to set up the infrastructure to get the thing into the air in the first place.”

  When the five modules were complete, they set about the finicky task of binding them together. Four of the boxes had corners cut out of their rectangular bases, allowing them to fit snugly around the fifth. Raina and Sarah started with their modules, lacing them together along a shared wall, then they lowered the northern edges of the pair onto the rock, and Ada came in from the south to attach her own, central component. Seth found it odd that the expedition leader hadn’t been granted that position in the middle of the boat, but apparently the seating plan had been chosen by some anxious bureaucrat in the Office of Surveyors, worried by Ada’s relative inexperience, and the diplomatic consequences if she came to any harm.

  When Ada was done, Seth moved in, with everyone whose hands were free now standing guard around him to be sure the work in progress didn’t go sliding upslope. The trickiest parts were the bottom joins between two surfaces facing north and south, where he had to tip his own piece of the hull southwards to give himself access to the lacing points. He worked slowly and meticulously, planning every movement in advance, trusting his colleagues to perform their supporting roles but taking care to do nothing that took them by surprise.

  Finally, Andrei had his turn, giving the boat its south-east corner. As Seth watched, he had to force himself to stop thinking about the way he would have gone about the job himself, and concentrate entirely on being prepared to halt the boat if it went sliding up the slope.

  Andrei tightened the last length of string and knotted the end. “We have . . . something,” he said, torn between relief at the task’s completion and disdain for the inelegant result. “A floating biscuit tray?”

  “We’re not finished,” Raina reminded him. “We need to attach the runners.” One by one, the four of them in corner positions took the small, stiff boards from their packs and fixed them to the outside of the hull.

  They stood and rested for a while. Seth ached all over; he hadn’t needed to apply any great force during the construction, but to maintain precise control over every movement seemed to require a heightened muscular tension that took its own kind of toll.

  Raina looked around for any crevices in the rock where they could anchor a safety rope and tie up the boat, but there were none in sight. The only way they’d get respite now would be to carry their awkward burden all the way to the river and commence the crossing.

  Seth exchanged a glance with Sarah. “Rivers have always been lucky for us, haven’t they?” she joked. Seth hoped, a little cruelly, that Ada would innocently ask why—but then, Ada had probably heard the whole story of their escape from Thanton long before she’d joined the expedition.

  They set off up the slope toward the water’s edge, moving more slowly than ever as they tried to coordinate their steps. Seth wasn’t sure whether the ten-legged creature they comprised was more stable than a lone Walker, thanks to its numerous points of contact with the ground, or less, given t
hat the south-west leg risked losing track of what the north-east leg was doing.

  At the riverbank, the water fizzed and spluttered, raining down across the slope and flowing back in shallow rivulets. Seth was less afraid now of slipping upslope into the water than of striking the treacherous surface in such a way that he’d be thrown off and dashed against the rock.

  Raina told Ada to climb into the boat. Seth set aside his irritation as he shouldered the shifting weight, treating it as a test of the craft’s robustness. If it could hold together with a passenger in the middle, unsupported from below, that would prove that all the seams were in good shape.

  They continued north until Raina and Sarah were standing in the churning water. They paused to assess the depth, but judged the river too shallow, and advanced again, bringing Seth and Andrei in up to their northern ankles. The surface of the water was still far from horizontal; the flooded terrace was at least a dozen paces away. Seth could feel the slickness of the riverbed: the tiny, experimental forces he applied to the wedge that supported his foot were met with almost no resistance.

  Raina said, “If we keep going like this and someone slips, we’re going to be scattered in five directions.”

  She had them lower the boat until its northern edge was surrounded by water, with the bottom resting on the rock below. Then she and Sarah clambered in, leaving Seth and Andrei holding the boat level. The two new passengers tied their stilts to the east and west sides of the hull, near the halfway point, then Seth and Andrei began sidling again, very slowly, worried as much by the risk of scraping the edge of the hull on the rock as the risk of losing control.

  Finally, Seth felt the boat being buoyed up along its northern edge. Raina said, “See if you can get in.”

  Seth glanced at Andrei, who said, “You first.” The hull came to just above Seth’s waist; he tied his own stilt to the side without taking his weight off it, then he slithered over the edge, face-first, into the snug compartment.