Schild’s Ladder

The first boundary was almost upon them, but they’d probed this one thoroughly in advance. As the ship crossed through the glistening membrane — an act portrayed as a simple mechanical feat, but which amounted to redesigning and rebuilding the entire hull — a motion within the scape caught Tchicaya’s eye.

Mariama turned to him with a triumphant smile. "That’s what I call an amphibious vehicle: glides smoothly from microverse to microverse, whatever their dynamic spectra."

He stared at her. "You weren’t — "

"Complete? Ninety-three percent should be good enough. I packaged myself very carefully; don’t take that decapitated progress icon literally." She looked up. "Oh, shit. That wasn’t meant to happen."

Tchicaya followed her gaze. The Planck worms had already crossed the boundary. Some freeloading mutation, useless against the earlier obstacles, must have finally proven its worth. Their adversary was not dispersing, weakening as it spread; it was like an avalanche, constantly building in strength. If the Planck worms retained every tool they tried out, whether or not it was immediately successful, their range of options would be growing at an exponential rate.

"You have to hand it to Birago," Mariama observed begrudgingly. "The killer twist was his, not Tarek’s or mine. We were too hung up on the notion of mimicking natural replicators — as if nature ever made plagues that were optimized for destroying anything."

"Humans did. He might have had some tips from the anchronauts."

They crossed into another cell of the honeycomb, as smoothly as before. Tchicaya wasn’t entirely sure what would happen if the Sarumpaet failed to negotiate a population transition, but whether it was the Planck worms or some hostile strain of vendeks that rushed in and consumed them, they wouldn’t have much time to dwell on their fate before they blinked out of existence. As local deaths went, he’d had worse.

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.