Phoresis

by Greg Egan

  She turned the glider cautiously farther to the left, and felt an unmistakable tug as the air and ice-dust pushed harder against the panels. She settled into the flow, leaving her body almost weightless, then tried again, feeling her way into a faster current. There was nothing to see, nothing to guide her, but she found the right direction, over and over, until there was nowhere left to go.

  The haze darkened; the sun had gone behind Tvíbura. The ocean below was being forced up into the chasm with as much pressure as it ever would be. Rosalind took a moment to inhale the sweet, rich air. On the ocean floor, the tiniest living creatures fed on an entirely different gas, created by nothing but water and hot minerals, and then exhaled this beautiful waste. If they hadn’t existed, living out their strange lives in that hidden realm, nor would she.

  The air grew thinner, and the haze dispersed. Rosalind watched the shadow of her home world racing across the ground. As far as she could tell, she was still ascending; the geyser had given her all it had to offer, and the only question now was whether that had been enough.

  She thought about her mother, waiting for news of the first hint of Tvíburian crops. What would she think, when she learned of a very different discovery, closer to home? It might not be apparent who the messenger had been, but there were people who would recognize the smallest quirks in the style of any glider. Then again, that much of the structure didn’t need to survive the landing; only the message itself had to arrive intact.

  By the time the shadow sped away below her, the whole of Tvíburi had shrunk to the kind of disk she’d seen from the top of the tower. Inasmuch as she could discern her own motion at all, she was traveling west far faster than she was ascending; though she’d shared the speed of the rotating ground when she’d departed, this high up she would have needed a much greater eastward velocity to remain above the same spot. But fleeing to the west also meant fleeing Tvíbura. She might have crossed the line where it would still capture her; it might be slowing her escape at an imperceptible rate that would still be sufficient in the end. Or she might be destined to return to Tvíburi, far from the geyser, with nothing to do but start the long walk back to the village.

  It was impossible to tell. She’d done her best, but her trajectory was out of her hands now. If she glided down onto the ice field, as smoothly as she’d first arrived, would she reproach herself for failing or would she rejoice that she’d been spared? Even with her brothers gone and her mind untainted, how could she want anything but to live?

  Below her, the landscape kept changing, but the cycle of day and night seemed held in abeyance. She couldn’t quite be outracing the sunset, but as time stretched on the disk of Tvíburi remained fully lit. Rosalind closed her eyes and pictured herself suspended in the sunlight forever, perfectly balanced between every hope she’d held for herself, and every hope she’d held for her people. Never falling, never coming down.

  She had not done everything. The runners were the heaviest parts of the glider, and they could only serve their purpose if she failed. She should never have brought them with her.

  She opened her eyes and reached down with both hands, feeling for the clips that held the runners to the frame. There were four clips on each side; she loosened them all, then worked the runners free.

  Before she could talk herself out of it, she summoned all her strength and flung the runners away, sending them back down toward Tvíburi. She watched them retreating, trying to guess their speed, hoping that the small impetus she’d given the glider would make a difference.

  She was naked now; wherever she landed it would be unsurvivable. She was every bit as dead as her brothers had been the moment she’d pushed in the knife. The horror of it only grew stronger, refusing to fade away. But what had she expected? To be at peace?

  The glider had begun to rotate slowly; she’d thrown more powerfully with one arm than the other. Tvíbura came into view, still far from the zenith. She looked away, up at the underside of the glider as the sunlight fell on the panels. On each one, she’d written the same words, repeated half a dozen times. She read and re-read the message, clinging to it as the glider turned, until she saw dusk begin to creep across the limb of Tvíburi. Her home world hadn’t let her escape; it was pulling her back to the east.

  Rosalind reached up and gripped the handle bars, trying to stop herself from shaking so she could gather her thoughts and strengthen her resolve. She was not an inert piece of cargo; she still needed to do her best to keep the glider stable as she came into Tvíbura’s thin atmosphere, and to steer as close as she could toward a place where the wreckage would be found. The panels would be shredded and scattered across the ice field, but it was up to her to ensure that someone would eventually stumble upon the fragments.

  Although we can’t grow the usual crops here, we are well-fed and safe. Do not be afraid to join us if you need to.

  11

  Petra lay harnessed to her sled, tense in the airless silence, stealing quick glances to either side to reassure herself that her mute companions hadn’t taken their eyes away from their telescopes. The catapult would only be released if Kirsi and Rada tugged on the two levers at exactly the same time, in a unanimous vote that was meant to spare them all the consequences of an ill-timed attempt. But if any kind of error was possible, that included a lack of consensus that would prevent her from being launched at all. Waiting six days for the next opportunity would be unbearable.

  She was spared the anti-climax: the sled shot forward and she was in the void. She turned and looked back toward the tower, elated. The rope trailing behind her lay in an almost perfect straight line, but the tug of the harness on her shoulders was as gentle as she could have wished. Ingrid and Lena were working the crank so smoothly that the rope they unwound was neither slack nor tense, neither holding her back nor looping dangerously ahead of her.

  She quickly returned her gaze to the view ahead, determined not to be taken by surprise. However accurate the predictions of the libration had become, the team’s ability to aim and calibrate the catapult remained the greatest source of uncertainty. The target might appear on her left or her right, outracing her or lagging behind. The only thing she was sure she could rule out was a head-on collision; they couldn’t have achieved that themselves if they’d tried, and their errors were hardly going to conspire to make it happen.

  A star blinked, then another, and another. Petra registered the events but couldn’t yet extract any real sense of the occluding object’s position and motion. Kirsi had spent more than a hundred nights staring through her telescope before she’d managed to record a sufficient number of occultations to be sure that the old tower was even standing; until then, for all anyone knew it might have crumbled to the ground. Petra had been in awe of her then, but now that she was actually approaching the thing and still couldn’t interpret the fleeting evidence it offered for its existence, her admiration only increased. She would never have had that much patience.

  Slowly, a dim gray thread emerged against the blackness, slightly to the left of the sled’s forward bearing. From this distance, it seemed as remote and unreal as it did through the telescopes: more like a flaw in the lens than a solid object. But now there was no lens to bear the flaw, and when Petra closed each of her eyes in turn, the blemish remained.

  As she watched, the thread thickened slightly, but it was also moving away to the left. She needed to change her trajectory, but acting too soon or too late would ruin the encounter. She estimated the angular speed of the target, then waited and did it again. Waited again. Estimated.

  She pushed the lever on her own small catapult and sent a rock flying into the void to her right, at the same time loosening the brake on her rope spool. The sled responded with a brief, satisfying jolt, and the extra rope began playing out beside her. When the spool was half unwound, she tightened the brake again, and the right-angle bend she’d created began to deform. There was a huge amount of rope laid out along her original trajectory, and the rock she’d eje
cted hadn’t possessed anything like the momentum needed to realign it all neatly behind her, but if she’d judged the timing correctly she’d have a good chance to make it to the tower before too much of the rope got the message that she’d changed direction.

  The thread was a gray sliver now, slowly growing wider. Petra could discern a sharp boundary at one end—the “bottom,” by her parochial reckoning—but when she followed it “up” toward Tvíbura, it just narrowed and dimmed until it had no visible effect on the stars behind it.

  The tower passed by on her left, but they weren’t done with each other yet. She looked back anxiously and watched the gray sliver cross over to her right. She’d overshot it twice now in different directions, but it was speeding up in the first direction, as it moved away from the closest point in the libration cycle, so it was only a matter of time before it overtook the rope she’d laid across its path—

  The sled shuddered alarmingly, swung from side to side, then settled. When Petra looked back along the rope, the part she could see was pointing straight toward the tower. If everything had gone to plan, she was effectively tethered to her target now, with the tower’s own acceleration keeping the rope pressed taut against it. And so long as nothing slipped or broke, she would keep on circling it, drawn ever closer as the rope wound her in.

  She could feel a gentle tension in the rope, conveyed to her as a slight pressure from the harness. The stars wheeled slowly across the sky, but even when they’d come full circle it was hard to feel any sense of progress. It was only by the time she’d completed half a dozen orbits that she was able to convince herself that the cycles were growing shorter.

  When Rada had suggested this method for the crossing, Petra’s first instinct had been that conservation of angular momentum would impose an ever greater velocity on the traveler as she spiraled inward, which would either lead to the rope snapping from centrifugal tension, or deliver her to her destination with a speed she’d have no hope of countering or surviving. But that hunch had proved to be misplaced. The curious geometry of the spiral involute meant that the pull of the rope, while slowly changing the direction in which she was moving, would never increase her speed. The time it took her to loop around the tower was only shrinking because she was drawing closer—and rather than feeding her energy, the tower and Tvíbura were helpfully draining away her unwanted angular momentum.

  She needed to be patient now, but vigilant too; with no real idea of her precise distance from the tower when it had snagged the rope, counting orbits wouldn’t tell her much. All she knew for sure was the general character of motion along the involute: the distance from the center would change at an almost constant rate for a considerable time, only to start falling precipitously at the end.

  The turning of the stars, with the same familiar constellations rolling into view over and over, risked lulling her into a state of dreamy torpor, but the slow revelation of the tower itself held her attention, however frustrating the pace. The walls of her own world’s newer version were hardly perfect geometrical forms, but the old tower looked almost as if it had melted and flowed at times, or at least that its underlying vegetative skeleton had started to rebel and go wild. Petra did not expect anyone to have tended to it for a very long time, but she tried not to extrapolate too far from these undeniable signs of neglect. One forsaken structure was no fair measure of the state of a whole world.

  When the rush finally came she was more than ready for it. The tower seemed to spiral in toward her, spinning as it approached, revealing ever richer details in its warped and pitted surface but moving too fast for her to dwell on any one feature. The extent of her motion “upward” toward Tvíbura had seemed quite small to her before, but now she could see that the helix she’d wound was pitched more steeply than she’d expected, and when she tried to find the point where the rope had first touched the wall, it was too distant to discern.

  She’d already rewound her catapult and prepared it for its final task. Before the relentless spiraling left her disoriented, she ejected a second rock to oppose her motion, and suddenly the sky was all but still.

  The sled had not been perfectly halted: it continued upward along the length of the tower, while also trying to pull away from it. Then the rope caught it and stopped it, briefly, before it rebounded inward and swung toward the wall. Petra was ready; she raised the baffle at the side of the sled, and when it struck the wall there was a thud she could feel through her bones, but no pain, and no apparent damage.

  She waited for a while, prepared for the worst: for the helix to start unraveling. But gravity here was still negligible; it was not as if the coiled rope was having to support any real burden to stay in place. And in the tests on her own tower, the rope of tangler fibers had ended up not just wrapped around the ice, but bonded to it in places, the friction of the encounter having partly melted it.

  Petra checked that her pack was secure, then untied her harness from the frame of the sled, leaving it joined only to the rope on a second spool. She released the brake on the spool—allowing it to turn almost freely, but with a governor to slow it if it spun too quickly—then clambered off the sled and floated beside the wall of the tower.

  Now that she was no longer swayed by the notion of lying on her stomach on the sled, her sense of the vertical changed. The bulk of the tower, the part beyond the helix, was clearly below her. Tvíbura was the only way down.

  She gave the sled a firm upward push, which had little effect on it but sent her body downward at the speed of a brisk walk. She watched the bumpy, mottled wall of the tower move past her.

  She knew that the balcony couldn’t be far, but it was a relief when she finally spotted it below her. She waited for it to come closer, then reached out and placed a gloved hand against the wall, careful not to apply pressure and push herself away. The friction wasn’t much, but nor was her weight. She bent her knees as she struck the floor of the balcony; she bounced up again, but clawed at the wall and halted her ascent. The encounter pushed her away from the wall, but downward as well, and the balcony’s outer, protective wall stopped her and sent her inward again. Petra forced herself to stay calm as she bounced from surface to surface; so long as she did nothing to gain energy, and nothing that sent her over the balcony, she’d have to come to a halt eventually.

  When she was still, she lay on the floor for a while, grateful and amazed. The bridge might be far from complete, but the first strand was in place. The worlds would be joined, for anyone to cross between them at will. They had proved that it was possible.

  The ice was gone around the seals of the entrance from the balcony into the tower, which made it easy to get through the six doors, but meant at least one level of the tower would be devoid of air. Petra was unfazed; anything less would have left her resentful that she’d wasted so much time training in the void. The seals could be repaired eventually, the whole journey made infinitely easier, but the world would not begrudge her the satisfaction of being among the few prepared to go first.

  Inside, the staircase looked almost familiar in design, but hallucinatory in detail. The Yggdrasil roots had had their way, retreating and advancing, laying down mounds of ice in one place, resorbing it in another. The stairs were still traversable, but the endless variation of bumps and pits rendered every footfall a surprise.

  Petra descended as quickly as she could, taking advantage of the low gravity. She would be cautious once a fall might actually injure her, but if she dawdled now that would only make her more impatient later, and more tempted to take foolish risks. Apart from the breach at the entrance, the walls themselves appeared to have retained their integrity; for all the strange distortion and dimpling she encountered, there were no outright holes in the ice. As the sun dropped lower and came directly through the walls, it cast strange bright spots and twisted caustics over the central column. It was like the dappled light in one of the shallower tunnels through the tanglers.

  She reached the bottom of the level just as night fell,
and found the seals of the connecting chamber not just intact, but blocked by outgrowths of ice. She took the chisel from her pack, but then thought better of trying anything in the fading light. If she ruined the seals and lost all the air from another level, that would be unforgivable. So she spread her blanket on the floor beside the chamber, and slept.

  Standing in the open air at the top of the final staircase, with the wind and dust of the ancestral world blowing in her face, Petra felt her brothers begin to stir.

  The stairs she could see in front of her weren’t any more malformed and bulbous than the thousands of helical ones she’d already negotiated, but the unbroken line from the landing to the still distant ground did induce a new kind of vertigo: if she slipped and fell, it wasn’t clear how far she might descend before she came to a stop. The swollen ice on the side walls had spat out all the railings to which she might have hooked a safety rope; all she could do was keep her hands on the walls to steady herself, and take scrupulous care with where she placed her feet.

  As she prepared to step off the landing, she felt her brothers rebel, all but paralyzing her with fear. She only had one more pessary left, and whatever progress her friends might have made on the rope bridge by the time she came back, she was fairly sure that her need to exercise unfettered judgment would be even greater then. “What would you have me do?” she muttered. “Stay here forever and starve to death?”

  She still had food in her pack, of course. And whether her brothers were aware of that or not, she wasn’t really arguing with them, so much as with a kind of conspiracy they’d formed with her own, more ancient instincts. Ancient Petra certainly knew about the food.