Грег Иган - Schild’s Ladder

Mariama remained seated at the front of the shuttle as Tchicaya clambered out of the airlock. He nodded a greeting, and tried to smile. Her Exoself would be discouraging her from doing anything to interfere with her body’s healing, by means both gentler and more precise than a blanket of agony; extrapolating from the raw pain of the minor burns he’d willingly experienced as a child was absurd. Still, the sight of her weeping, blistered skin made his guts tighten.

He said, "Hitchhiking in space isn’t so bad. I’ve waited longer for a ride, on land."

Mariama replied through the IR link. "Try showing more flesh. That always works wonders."

On their way back to the Left Hand, Tchicaya received the first good news he’d heard since the moratorium vote. The horizon had stopped falling. The Left Hand was no longer seeing new stars creeping into view.

That in itself didn’t fix the depth of the lost region everywhere, but the particular geometry was suggestive. The new horizon was exactly where it would have been if the Planck worms had failed to penetrate the signaling layer, where the vendek population changed abruptly, a hundred kilometers into the far side.

As they approached the Left Hand, the news became even better. The fireflies had finally begun to vanish, and the timing of their deaths confirmed the best possible scenario: the border had retreated to the signaling layer, and no further.

Tchicaya was elated, but Mariama said, "Don’t assume this is the new status quo . Birago wasn’t exactly confiding in me toward the end, but if what he’s done here bears any resemblance to the work I was involved in with Tarek, the Planck worms won’t have given up at the first obstacle."

"Meaning what?"

"They’ll mutate. They’ll experiment. They’ll keep on varying themselves, until they find a way to break through."

"You knew how to do that? You had it all worked out?"

"No," she admitted. "But as soon as you showed us the vendeks themselves, they provided an awful lot of inspiration. Tarek and I didn’t pursue that, but don’t expect Birago to have passed up the opportunity."

They docked with the Left Hand, and carried it down to the point where the fireflies were disappearing.

Regaining alignment with the border took almost an hour, as a cycle of increasingly delicate adjustments brought the stylus into range. Once that was achieved, Tchicaya scribed a series of probes that would spread out laterally as well as moving straight in, improving their chances of gaining a comprehensive picture of the Planck worms. Unsurprisingly, now that the signaling layer was infected with Planck worms and exposed to vacuum, it was no longer vibrating, no longer tapping out primes. Tchicaya longed to discover the mechanism that had driven it, but he had to stay focused; trying to dissect the far-siders' ruined SETI equipment — if that was what it was — had to take second place to dealing with the plague the beacon had been unable to deter on its own.

As he launched the last probe, he turned to Mariama. "If you gave me all the details of the work you did with Tarek, there’d be no need for you to hang around."

She emitted a disgusted wheezing noise, the first real sound he’d heard her make. "Is that some kind of childish comeback, because I didn’t want to waste fuel on making you cozy?"

"No. But I’m the one who came to the Rindler to protect the far side. There’s no reason for you to keep crawling over broken glass for the sake of someone else’s agenda."

Mariama searched his face. "You really don’t trust me, do you?"

"To do what? To betray your own ideals? You always wanted to wipe this thing out."

"I never thought that would involve genocide."

"We’re still not certain that it would."

She sighed, bodily. "So you’re afraid that if we find a natural explanation for the signaling layer, my presence might suddenly become embarrassing?"

"Wouldn’t it?"

"I voted for the moratorium," she said. "I voted to do nothing but look for signs of life, for a full year. Whatever happens, I’ll honor that commitment."

Tchicaya experienced a twinge of shame, but he didn’t back off. He said, "Make up your mind. Are you here to protect the far side? Or are you here to relaunch the Planck worms in a year’s time, if the far side proves to be sterile?"

Mariama shook her head. "Why do I have to choose? If there are sentient creatures in there, they deserve our protection. If there’s nothing but an exotic ocean full of different kinds of Planck-scale algae, then the sooner it’s rendered safely back into vacuum, the better. Is that distinction really so hard to grasp? What did I ever do to get lumped in with the rebels, in your head? When’s the last time I displayed nineteenth-century morality?"

"Twenty-third."

"That just shows how little history you know. Most people who left Earth in that era did so precisely because they were out of step with contemporary mores. In this case, I’d say they were about four centuries behind the times."

Tchicaya looked away. Was she protesting too much? But she was just as entitled as he was to be contemptuous of the anachronauts' views. Being wise after the fact about the complexity of the far side, and the unwitting genocide the Preservationists might have committed, was like blaming the Mimosans for failing to anticipate the failure of the Sarumpaet rules.

The probes began returning. The Planck worms they revealed were dauntingly complex structures, at least as elaborate as the vendeks themselves. And Mariama had been right: they’d begun to mutate, to try out variations. The software counted thousands of strains.

Even if they were capable of adaptation, though, they were too simple to achieve it through anything but trial and error. Their designer had left them to fend for themselves, and in the end that would leave them as vulnerable as any other dumb pathogen.

Tchicaya addressed the toolkit, allowing Mariama to listen in. "Find a graph we can scribe that will wipe these things out — without moving deeper and damaging the native vendeks." As he spoke the words, this sounded like a breathtakingly optimistic request, but the Planck worms themselves had been seeded from a single point, so there was no reason why the antidote couldn’t be introduced the same way.

There was a perceptible delay while the toolkit explored the problem. "I don’t believe that’s possible," it declared. "The Planck worms are exploiting the ordinary vacuum behind them: they set up correlations across the border that cause the vendeks to decohere. I’m unable to find a method of attacking the Planck worms that wouldn’t also destroy the whole vendek population in which they’re immersed."

Mariama said, "What if the vendek population changes, deeper in?"

"Anything might be possible then, but until I know the details, there are no guarantees."

Tchicaya scribed probes to look deeper.

The second change swept the border as swiftly as the first. Through the windows of the shuttle, they saw the smooth gray plain transformed into a complex, striated pattern of dozens of bright hues. Tchicaya’s heart raced; it was like watching a pool of acid eat its way down through featureless rock, exposing thousands of delicately layered sediments.

Mariama said, "The border must be motionless again, or we’d see the pattern changing. So the Planck worms have hit more obstacles. We might have killed them off, if we’d burnt away this whole layer first."

"Including whatever it contained," Tchicaya countered. "We have no idea what might have been there."

Mariama replied flatly, "Whatever was there, it’s gone now anyway."

Tchicaya said nothing, but she was right. If he’d acted more swiftly, they might have cauterized the wound. If he was going to refuse to make decisions with imperfect knowledge, he might as well give up intervening and simply leave the far-siders to protect themselves.