Dichronauts

by Greg Egan

  If he gave in to the urge to inhale, he was dead. Theo began inspeaking gibberish; Seth replied in kind, losing himself in the simple rhythm of the exchange. When the pain in his lungs intruded, he shouted it away: Not yet, not yet, not yet.

  Theo said, «Flerdibyll graznisniff?»

  Seth responded effortlessly: «Mulpeneresh, sockulee!»

  When he burst to the surface, he drew in air while water was still streaming over his face. He spluttered and choked until he’d driven the offending liquid out, then he lay still for a while, gasping, waiting to get his strength back.

  He sat up and began assessing the situation. His section of the hull was full of water, but the hollow walls were apparently keeping it afloat. He could see Andrei sitting in front of him, bedraggled but very much alive, and as Theo cleared the water from his pingers he revealed Ada’s arm in motion near the side of her compartment. She was scooping water out with her cupped hand.

  But the rest of the boat was gone.

  Seth tried to remain calm: he had no more reason to fear that his friends had come to harm than they’d have to believe the same of him.

  The current was carrying their half-boat west, much more rapidly than it had ever moved on the first flooded terrace. Looking back toward the waterfall, Seth could see no trace of his missing companions, but if anything, they’d probably ended up with a more suitable craft: this one was only staying level because the two water-logged southern compartments were keeping Ada’s from overbalancing.

  “Should we try to reconfigure?” he asked Andrei.

  “I wouldn’t risk it,” Andrei replied. “We’re stable enough as we are—and if we hit another axial branch, at least we’ll be prepared.”

  “Where do you think the others are?”

  Nicholas said, “I’ve been calling to them, but I’ve had no reply. They must be on the other side of the falls.”

  “You mean you think they’ve gone east?”

  “Yes.”

  Seth absorbed this. “We need to get back on land, as quickly as possible.” The only way the separated groups would have a chance to reunite was if they both headed for the river’s southern shore.

  “With what?” Andrei gestured toward the boat’s east side. “My runner’s gone. What about yours?”

  Seth checked. “The same.”

  Theo said, “If the water’s shallow enough, maybe you could do something with the stilts?”

  Seth fished around at the bottom of his flooded module. Amazingly, the stilt was still there, strapped to the floor along the northern side. He brought it up, extended it, then lowered it into the river and tied the top to the side of the hull. As he tilted it to increase its reach the shifting currents tugged at it erratically, but he couldn’t feel resistance from anything solid, and when he glanced back at Andrei, who’d been making his own attempts, it was clear that he’d fared no better.

  Ada said, “Can you see that?” She was gazing west.

  “What?” Seth was so low in the water that his own view was pitifully curtailed.

  “I think we’re about to turn again.”

  Seth leaned north as far as he dared, raising his head almost to the same height as Ada’s. They were approaching a point where the river veered west-south-west, spilling down the slope along an ordinary gradient, not an axial one. It was difficult to judge whether the current would do anything helpful at the bend, but the river appeared to grow broader as it turned, which had to decrease its depth.

  He lowered himself and braced against the sunken hull, gripping the stilt tightly. His hope was to get enough purchase to allow him to take turns with Andrei forcing the boat to pivot around their respective ends, with the current doing most of the work, zigzagging the vessel toward the shore.

  As the river spread out and veered down across the slope, Seth tipped the stilt to the south, sending the far end shorewards and ever deeper into the water. In air, the torque at such an extreme angle might have snapped his wrists, but in water the wood’s buoyancy took some of the load.

  The stilt began to shudder; it wasn’t catching on the riverbed, but it was scraping over it. Seth cried out to Andrei, “I’ve got contact!” Between the two of them, surely they could halt the boat.

  “Same here,” Andrei replied. “But it’s not sticking.”

  Seth’s forearms were burning. He clung on, swinging the stilt back and forth, desperate for anything that might work in their favor. “All we need is a rut, a depression in the rock . . .”

  At the sound of churning water, he glanced up to see what was making the noise, but they were traveling almost south-west now, leaving them blind to any changes ahead. Suddenly, the stilt refused to move with the boat; it felt as if it was jammed into a crevice. Seth’s jubilation turned to alarm as the rear of the boat swung far to the north, widening the hull across the flow. Andrei fought to snag the riverbed at a second spot—but he had no luck, and in a matter of seconds the force of the current had freed them, and they were adrift again.

  Seth wiggled the stilt, preparing for a second attempt, then realized that it was taking much less effort to move it than before. He brought it upright, untied it and pulled it out of the water. The bottom two-thirds had snapped off.

  There was a spare in his pack, but before he could retrieve it the boat lurched, slamming him against the hull, and the churning water around him became a torrent flooding straight down the slope.

  The transition was smoother than the last time they’d gone south, and though Seth was not inured to the wild acceleration, once it was clear that the boat remained stable his thoughts turned to the struggle the others would face, trying to keep their own vessel level if their eastward journey took the same turn.

  “When does it stop?” Ada asked. The question might have sounded plaintive if she hadn’t had to shout to be heard.

  Andrei said, “The river must split up, and every branch must dry out eventually.”

  That claim sounded absurd when the words were almost lost in the noise of rushing water, but Seth clung to the fact that they’d already seen two bifurcations of the original, north-bound flow. So long as that process continued, and no other tributary joined them along the way, anyone riding the river would have to end up in a shallow stream somewhere. It was growing harder to feel sure that he had any idea of how long it might take the two halves of the expedition to find their way back to the basket, but for now he’d be ready to rejoice at the prospect of merely standing on dry land again.

  He waited, tensed, for the next violent dunking, glancing now and then at the blur of the eastern shore. The wind had begun to split his skin in places; he could see the same wounds on Andrei’s face.

  Theo asked Ada, “Do you still have your barometer?”

  “No. It’s lost in the river somewhere.” She hesitated. “But if we go much deeper, maybe we could judge our depth from gravitational effects.”

  “That would depend on what we’re doing,” Theo replied. “How we’re moving, what other forces are in play.” He sounded peeved that he’d been drawn into a conversation with the enemy, but having initiated the exchange he could hardly cut it short. “If we were walking down the slope, the most striking effect would be the gravitational force changing direction.”

  “How much would it change?”

  “How deep do you want to go? Eventually it would cross the cone and turn axial.”

  Ada took a while to think that through. “Then on a slope like this, where water flows down under normal conditions . . . by the time you reach a point where gravity is axial and ‘up’ points south, the slope will have switched from more than forty-five degrees to less, measured from the new horizontal, and water will flow upslope. But upslope will mean south. So the flow won’t change direction along the slope. Gravity alone won’t turn the river around and send us back the way we came.”

  “That’s right,” Theo agreed.

  Nicholas said, “But we’re sure to end up traveling north again, for other reasons.”


  “Why?” Ada pressed him.

  “If it’s down to chance, how many times would the river branch the same way?”

  Ada wasn’t satisfied. “How can we be sure that it’s down to chance? If the whole slope tilts a certain way over a wide enough area, the direction might not be random at all.”

  Nicholas had no reply.

  Seth said, “Then we could just reach the bottom of the chasm.” With nothing visible to the south to contradict Theo’s edge-of-the-world nonsense, they’d started taking it far too seriously. “If the river doesn’t carry us north, we could find ourselves on level ground.” No more stilt-walking, no more waterfalls. So long as they had some shelter from the sun, they could live like civilized people for a while, resting and recovering their strength before facing the task of finding a way back.

  as night fell, the river still hadn’t deviated from its southward course. Seth’s concentration had been wavering; he didn’t think he’d lapsed into sleep, but at times his consciousness had detached from his surroundings, distancing him from the relentless battering of the wind, the juddering of the boat, the landscape hurtling by.

  Once it was dark, it became much harder to judge the passage of time. Whatever inner sense of duration he possessed had already grown addled from the daylong twilight, and the boat’s interminable fall confounded it further. Nothing in ordinary experience could drop faster than a stone and not hit bottom by a count of ten, and Seth suspected that whatever fragment of his mind played time-keeper had lost confidence in the meaning of its silent count as it waited for the impact that never came.

  The hull had never made a comfortable resting place, but as the night wore on it only felt worse. The force of the wind and the motion of the boat shoved him against the northern wall, pinning his arm, and he couldn’t find any way to adjust his body that relieved the pressure. When Theo dozed off, Seth tried imagining the others escaping the river, trekking up the slope, and finding a way to complete the survey. The chasm had proved itself absurdly deep, but it might yet turn out to be narrow enough for the migration to detour around it.

  Just as the eastern sky showed a hint of gray, a light rain began to fall. The boat was moving south so rapidly that Seth could feel the drizzle striking at an angle, with the south side of the hull sheltering half his body. The sky grew brighter, then a dazzling bead of light appeared on the horizon: true dawn. He closed his eyes for a minute or two, only opening them again when the rain stopped.

  The shower had moved on, but it was still visible in the east. Seth watched the distant rain glistening in the sunlight. That it was sloping down from the south seemed perfectly sensible to him at first: that was how he’d experienced it, so why shouldn’t it look that way?

  But the rain was tilted against the landscape itself; nothing about the boat’s motion could explain that. For a moment he wondered if the slope could have changed its gradient sufficiently to explain what he was seeing, but it would have needed to have leveled out and started rising to the south—without the river slowing, or their lopsided boat tipping over.

  Theo complained indignantly, “We crossed over, and you didn’t even wake me?”

  Andrei stirred, sighing, noticed Seth’s perplexed gaze and tipped his head to the east. The rainstorm was receding; Seth willed it to vanish entirely, taking the whole disturbing illusion with it.

  Nicholas said, “Could it just be the wind?”

  Andrei maneuvered his canteen out of his pack and held it in one hand, to the south of his body. “That’s not blocking much more air than my arm alone, but I can feel the extra weight of it—pointing more to the north than it’s pointing in the direction that used to be down.”

  Ada said, “I felt the change, halfway through the night. But I thought I must be dreaming.”

  Seth tried to sit up, but as his head rose above the side of the boat, the northward force—whatever its source—threatened to send him toppling over the edge, and he retreated.

  The sun had risen fully now, and the heat was growing fierce. Seth had no doubt that if they’d been at the same latitude up on the surface they would have crossed into absolute summer; it was only their depth that had kept them clear of the cone.

  “The river has to turn sometime,” he declared. Unless the chasm was literally bottomless, the far end would be more like a ceiling than a floor, and at some point the approach would become far too steep for the river to climb.

  Andrei said, “Let’s hope so. Walking on stilts was bad enough; I don’t even want to try with one hand and one foot.”

  Seth wasn’t sure if he was joking. But however strange their circumstances, halfway to the other hyperboloid was still only halfway.

  the sun at noon was merciless, hovering over the source of the now ascending river, painting the churning water at the northern edge of Seth’s vision with a streak of white fire that kept pace with their retreat. He huddled into the boat, but however he squeezed and contorted himself he couldn’t fit entirely in its meager patch of shade.

  Now that he’d acknowledged the shifting vertical, he tracked the change with a dismal fascination. His body was receiving all the cues it needed to tell him just how far his right shoulder had risen above the left, but the situation was so remote from anything he had experienced before that it was more like taking a measurement from a complicated instrument than intuiting his posture in any ordinary way.

  “We need to be prepared,” Theo said. “If we end up on south-facing land, we need to be ready to assemble something that will help us move.”

  “There are some axial struts in the boat that should bear our weight,” Andrei replied reluctantly. “More than one for each Walker. They could give us balance, I suppose. But we’d still be crawling around on our sides, scraping the ground with half our bodies. Unless it’s just a few paces from wherever we’re dumped to a north-flowing river, I don’t think anything could make that journey survivable.”

  “We should have planned for this,” Theo said bitterly. “We planned for the slope, we planned for the rivers.”

  “And what should we have built?” Nicholas demanded. “Forget about the weight we’d be lugging around; what would actually be useful? Wheels won’t work there, in any direction. I suppose you could dream up a contraption with enough levers and pulleys to let a Walker control a set of axial legs beneath some kind of platform or harness . . . and then hope that the terrain was all as flat and firm as a paved road.”

  “I don’t have any answers,” Theo admitted. “Before we hit the river, the most I ever imagined was that we might find some kind of indirect evidence that the slope went all the way down. I wasn’t contemplating a personal visit.”

  Seth kept quiet. He believed that they were far more likely to die in the chasm’s final waterfall than face any of these insoluble problems, but nobody needed to hear that.

  “Runners will still work though, won’t they?” Ada asked, shouting up at them through the walls of her compartment.

  “Yes,” Nicholas agreed. “And in any direction. If we end up in calm water, we might be able to improvise something.”

  “And then hope that there’s a route to a river north that doesn’t involve dry land,” Theo added. “That might be optimistic, but it’s not unthinkable.”

  when the sun finally disappeared behind the slope, it took some dispassionate geometric reasoning for Seth to accept that the day now past, interminable as it had seemed, must have been shorter than any he’d experienced on the surface. Every ordinary cue around him had shifted its meaning, but he held fast to the lofty, impersonal image of the sun in its orbit, the cliffs as he’d seen them from the balloon, the world-piercing chasm as a line on a diagram in his mind’s eye, slanting across the page from one hyperboloid toward the other, whether or not it reached all the way. Geometry might well kill them in the end, but only a rigorous understanding of its principles could make their situation intelligible, let alone survivable. He recalled an old schoolmate, who’d laughed when
Seth had enthused over his plans to study surveying. “What use is all that academic nonsense? Every child knows in their bones how geometry works!” And no doubt they did, until they found themselves in a land where it was geometrically impossible to place both feet on the ground.

  «Where do you think Sarah and the others are now?» Theo asked.

  «I don’t know. I just hope they had more luck getting out of the water.» Seth didn’t want to turn every possibility over in his head, trying to select the most likely outcome. Wherever they were, he couldn’t help them; whatever he told himself would make no difference to their fate.

  «They had much more rope than we do,» Theo noted.

  «So that’s what would have turned things around for us,» Seth replied. «More rope.»

  «No, but I’ve been thinking through some different scenarios. If we end up on a mudflat, and there are trees nearby, maybe we could snag a branch and haul ourselves along.»

  Seth laughed. «And if we end up in a bustling market—even though we have no money, and even if we did it would probably be no good there—maybe we can barter our theodolites for all the rope, and food, and axial stilts we could possibly need.»

  Theo lapsed into silence. After a while, Seth said, «You’re right, though: we should try to think things through.» He struggled to find a way to join the game. «If we end up on ice, we might be able to slide the sections of the boat across the surface. There’d be no need to lasso trees, so long as we can push against the ice itself.»

  Theo spoke aloud. “There’s a change in the flow ahead. I can hear it.”

  Nicholas was less certain, but after a minute or two he concurred. And then Seth heard it too: a violent new hissing from the water above them, cutting through the noise of the wind and the creaking of the boat.