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.