Schild's Ladder (33 page)

He watched the Planck worms as they reached the partition; this time, they appeared to be trapped. However many mutations were part of the throng, they couldn't include an exhaustive catalog of all the possibilities. The toolkit was X-raying each gate and designing the perfect key as they approached; that strategy had to win out some of the time.

If not always by a wide margin. Tchicaya was just beginning to picture the
Sarumpaet
streaking ahead triumphantly, when the second barrier fell to the Planck worms.

He addressed the toolkit. “Is there anything we can throw in their way? Anything we can scribe that would act as an obstacle?”

“I could trigger the formation of a novel layer population. But that would take time, and it would only stretch across a single vendek cell.” However long the artificial barrier held, the Planck worms would still percolate down along other routes.

They glided through a dozen more cells, maintaining a tenuous lead. Even when they appeared to be widening the gap, there was no guarantee that they wouldn't plunge into a cell to find that the Planck worms had reached the same point more quickly by a different route.

The honeycomb stretched on relentlessly; the
Sarumpaet
gained and lost ground. After eight hours of nominal ship time, they'd crossed a thousand cells. In near-side terms, they were a millimeter beneath the point where the border had last rested, and the chase had gone on for mere picoseconds. The Planck worms had spent more than two hours diversifying before they'd learned to penetrate these catacombs, but having found the basic trick they appeared to be unstoppable. So much for the strategy of burning away one vendek population and the predators trapped within it; that would have been like trying to cure a victim of bubonic plague by sterilizing a single pustule.

Tchicaya said, “If this goes on for a hundred kilometers, I'm going to lose my mind.”

“We could go into Slowdown,” Mariama suggested. “We wouldn't risk missing anything; the ship could bring us up to speed in an instant.”

“I know. I'd rather not, though. It just feels wrong.”

“Like sleeping on watch?”

“Yeah.”

Three days later, Tchicaya gave in. The honeycomb could prove to be a centimeter thick, or a light-year; the probes could barely see half a micron ahead. They had no decisions to make; until something changed, all they were doing was waiting.

“Just don't go dropping out on your own,” he warned Mariama.

“To do what?” She gestured at the spartan scape. “This makes Turaev in winter look exciting.”

Tchicaya gave the command, and the honeycomb blurred around them, the palette of false colors assigned to the vendeks—already recycled a dozen times to take on new meanings—merging into a uniform amber glow. It was like riding a glass bullet through treacle. Above them, the Planck worms retreated, crept forward, slipped back again. The
Sarumpaet
inched ahead, but in fast motion the race looked even closer than before, their advantage even more tenuous.

As the Slowdown deepened, their progress grew smoother. After a full nanosecond of near-side time, they appeared to be leaving the Planck worms behind. After a microsecond, the worms slipped back out of range of the probes, and there was nothing to be seem but the
Sarumpaet
itself, and the honeyed esophagus down which it was gliding.

At sixty microseconds, the toolkit signaled an alarm and the ship dragged them back to full speed.

The
Sarumpaet
had stopped moving, in the middle of a cell of pale blue vendeks. “The probes can't go any deeper,” the toolkit explained. “We've reached a new kind of boundary: whatever's behind it is qualitatively different from all the vendek mixes we've encountered so far.”

Tchicaya glanced down into the darkness, as if his eyes could reveal something that the probes, responsible for the entire scene, had missed.

Mariama frowned. “Different how?”

“I have no idea. The probes don't even scatter back from the boundary. I've tried redesigning them, but nothing works. Anything I send down simply vanishes.” For all its knowledge and speed, the toolkit had never been intended to act as much more than a repository of facts. It couldn't begin to cope with novelty in the manner of the people who'd contributed to it.

They sat and discussed the possibilities. Tchicaya had learned quite a bit from his faction's experts, and Mariama even more, but they needed a bigger group; on the
Rindler
, everyone's ideas had sparked off someone else's.

For weeks, they argued and experimented. They took turns sleeping for an hour each; even without any fixed, bodily need to recuperate, their minds were still structured to function best that way. The toolkit diligently analyzed vast lists of possibilities, sorting through the quantum states that might be swallowing all their probes without a trace, hunting for a new design that would avoid that fate and return with solid information.

Nothing worked. The darkness beneath them remained inscrutable.

They had no way of knowing how long it would be until the Planck worms came flooding down after them. On bad days, Tchicaya consoled himself with the thought that when they died, the Planck worms might be buried with them. On worse days, he faced the possibility that brute mutation would find a way through, where all their passion and borrowed ingenuity had failed.

On the thirty-seventh day, Tchicaya woke and looked around the scape. They'd tried all manner of distractions for the sake of inspiration, but no stroll through a forest, no mountain hike, no swim across a sunlit lake had led them to the answer. So they'd stopped ransacking their memories for places to camp, and returned to the unpalatable truth. They were stranded in an ugly, barren cave in the pockmarked rind of an alien universe, waiting to be corroded into noise by a billion species of ravenous sludge.

Mariama smiled encouragingly. “Any revelatory dreams?”

“I'm afraid not.” He'd dreamed he was a half-trained Sapper from the legend, suddenly confronted by a new kind of bomb, falling beside it toward a landscape of shadows that might have been anything from a desert to a vast metropolis.

“My turn, then. Come on, get up.”

“I will. Soon.” She could just as easily conjure up a bed of her own, but taking turns with one imposed a kind of discipline.

Tchicaya closed his eyes again. Sleep had lost all power to assuage his weariness, but it was still an escape while it lasted. He'd understood from the start that their struggle was quixotic, but he'd never imagined such a dispiriting end. They'd spend their last days writing equations on paper planes, and tossing them into an abyss.

As he drifted back toward sleep, he pictured himself gathering up a mountain of crumpled paper and heaving it out of the
Sarumpaet
into the darkness below. If by chance some scrap went wafting through into another world, he'd never even know that he'd succeeded.

He opened his eyes. “We launch all our paper planes at once. Then we throw a message back, and use it to clear away all the garbage.”

Mariama sighed. “What are you ranting about?”

Tchicaya beamed at her. “We have a list of the kind of states the region below us might be in, and we have strategies for dealing with them all. But we still haven't found a probe that will cross through and return—giving us a definite answer, letting us know which strategy to use. Fine. We put the
Sarumpaet
into a superposition of states, in which it tries them all simultaneously.”

Mariama was speechless. It took Tchicaya several seconds to interpret this response; he had rarely surprised her, and he had certainly never shocked her before.

She said, “Who cares about quantum divergence, if one world out of every quadrillion is the best of all possible worlds? That sounds like some desperate fatalist nonsense from the last days before the Qusp.”

Tchicaya shook his head, laughing. “I know!
But it's not!
Answer me this: a quantum computer does a search for the solution to an equation, testing a few trillion candidates simultaneously. In how many worlds does it fail?”

Mariama scowled. “None, if there's a solution at all. But that's different. The divergence is all internal and contained; it doesn't split the environment into branches halfway through the calculation.” A flicker of uncertainty crossed her face. “You don't think we could—”

Tchicaya said, “We're not in the near side anymore. Coherence is nowhere near as fragile here. Whatever this gulf is that we're facing, there's no fundamental reason why we shouldn't be able to stretch a single quantum computer all the way across it. And if we handle all the strategies with sufficient care, we ought to be able to manipulate the whole coherent system so that the failures cancel out.”

She nodded slowly, then broke into an astonished grin. “We reach out and swallow the problem; we internalize it completely. Then we can bludgeon our way through by trial and error, without the world ever seeing a single mistake.”

They spent three days refining the idea, thrashing out the details with the toolkit and the ship. It was a complex maneuver, and it would require precise control over the ship's environment, both before and after it crossed through the boundary. The toolkit had had plenty of time to study the surrounding vendeks, and it understood the physics of this obscure
cul-de-sac
as thoroughly as that of the near-side vacuum itself. The second half of the problem could not be dealt with by direct observation, but that didn't mean they'd be taking a leap into the dark. Each strategy for making the crossing relied on a set of assumptions about the other side. Once they put the ship into a superposition of strategies, each component would know the kind of place it would end up in, if it ended up anywhere at all.

Tchicaya snapped awake, knowing the reason instantly. He'd been summoned to alertness by the tug of a trip wire that he'd installed, back on the near side, when he'd worked with the toolkit to construct a software container to sit between their minds and the raw quantum gates of the ship's processor.

Mariama was seated a short distance away, gazing out into the vendek cell. Tchicaya said, “Do you want to tell me what you're doing?”

She turned to him, frowning slightly. “Just rearranging a few things internally. I didn't realize I had so little privacy.”

“I own this whole setup,” he said. “You knew that when you came into it.”

Mariama spread her arms. “Fine. Rummage through my memories; see if I care.”

Tchicaya sat up on the edge of the bed. “What were you trying to expel into the environment?” At the border of the simulated Qusp in which her mind was cocooned, he'd replaced some of the more arcane facilities of the standard hardware—things she'd have no good reason to want to use, under the circumstances—with fakes that merely rang alarm bells. It had been a last-minute decision; the toolkit would have happily simulated the Qusp in its entirety, as the simplest means of guaranteeing that everything worked smoothly when it was piped through.

“Nothing,” she said. “It was a mistake. I didn't even realize you'd put me in a cage, so I brshed against the bars by accident.” She waved a hand at him irritably. “Go back to sleep.”

He rose to his feet. “Are you going to tell me, or am I going to have to look for myself?” In an ordinary Qusp, the owner of the hardware could freeze the whole program and inspect its state at leisure. But the quantum gates here were implemented at too low a level; there was no room for that approach. All he could do was send in a swarm of utility algorithms to search for anything suspicious, while shuffling her working mind aside. That would do no lasting damage, but he had no idea how she would experience it. It could be extremely unpleasant.

Mariama regarded him calmly. “You do whatever you think you have to. I've already been flayed once.”

Tchicaya hesitated. He did not want to hurt her, and if he was wrong, he'd never be able to look her in the eye again. There had to be another way to call her bluff.

“There's no need,” he said. “I know exactly what you were trying to do.” He wasn't certain of anything, but of all the possibilities he could imagine, one stood out sharply.

“Really? Do you want to enlighten me?”

“You brought in a stock of qubits entangled with the near side. You had to get rid of them now, or they would have shown up tomorrow when we prepared the ship.” Anything that interacted with an entangled qubit would have its phase irretrievably scrambled. To a pure quantum system they'd be poison. They'd have to be carefully isolated, locked away somewhere inside her mind.

“You're right,” she admitted. The expression on her face barely changed, as if this amounted to a minor clarification of her original story. “But I wasn't trying to use them. I was trying to get rid of them.”

“Why don't you use them right now? Kill us both, right now?” However many she was carrying, she could not have imagined they'd be enough to do real harm to the far side. So the poison could only have had one target.

“I don't want to do that, Tchicaya. I want to go with you. Deeper in. As far as we can.”

“
Why
?” Why had she dragged him down here at all? To give his version at the border an excuse to give up? Once he was also deep in the far side, battling the Planck worms like a valiant Lilliputian, it would be far easier to feel that he'd done all he could.

“To see what's there,” she said. “To help protect it, if it's worth it.”

“And help destroy it, if it isn't?”

“I never lied about that,” she insisted. “I never told you that I'd fight for some exotic wasteland, over the lives of real people.”

That was true. She'd told him exactly what she believed, and he'd still wanted her beside him.

Tchicaya sagged to his knees. He had the means to kill her, or to leave her behind for the Planck worms. The ship's processor would do whatever he asked. But nothing she had done was unforgivable. In her place, fighting for the same stakes, he would have lied, too, armed himself, too. How could he accuse her of betraying anything? For all he knew, if they'd taken different turns the last time they'd parted, they might have ended up in each other's shoes.