Schild’s Ladder

He got the three devices out: three small, dark spheres chained together with optical cables. The Mediator and the Exoself both bore a fuzz of fine gray wires that had tapped into the body’s nervous system.

Tchicaya consulted his own Mediator; it wasn’t a great resource compared to the Rindler's library, but it knew all about its own design. Given a disembodied version of the same hardware, with the radio transceiver fried, how could he reestablish contact?

His Mediator described the specialized hardware that could do this. The shuttle was carrying nothing even remotely similar.

Tchicaya contemplated the bloodied parts in his hand. He’d asked her once to leave him, so he could complete this task alone. Now he appeared to have had his request granted.

"There are no other ways to make contact?" he asked his Mediator.

"Not if the device remains disembodied."

He couldn’t grow her a new body from scratch; there was no time. And the cells of the old one had already done their best; they would not be coaxed back into operation.

Tchicaya said, "What if it was inside someone else’s flesh? Inside a body with another Mediator?"

"Where, exactly?"

"Where would it have to be?"

"Inside the skull. Or very close to the spinal cord."

That was the solution, then. Tchicaya steeled himself. He still wasn’t certain where her loyalties finally lay, but he was even less certain that he could go on without her.

He stripped of his bloodied clothes, and peeled away his suit. Then he asked his Exoself to guide him. It knew the position of every nerve and blood vessel in his body, and it could move his hands with great precision.

The stylus came into alignment with the border. Tchicaya launched a swarm of probes, then instructed the toolkit to start work automatically as soon as the echoes began returning: designing a replicator that would burn away all the current strains of Planck worms, whatever the cost to the vendeks around them.

Mariama spoke. "What’s happening?"

Tchicaya said, "You’re behind my right kidney. My nervous system’s just managed to link up with your Mediator."

This revelation only fazed her for a moment. "I didn’t even think about communication. That body failed so suddenly, I didn’t have time to make plans."

"Are you okay?"

"Absolutely."

"What are you simulating?"

"Nothing, yet. I’ve just been thinking in the dark."

"Do you want to share my senses?" It was what he would have asked for, himself, if their roles had been reversed: anything to anchor his mind to reality, even if it was secondhand.

Mariama hesitated. "I’d like access, thanks, but I’ll make myself an icon with a viewpoint in a scape, and put your vision up on a screen. I don’t want to start pretending that I’m inhabiting your body. Since I can’t actually control it, that would just make me feel trapped."

"Right." Tchicaya felt a frisson of anxiety, but the notion that he’d invited in a guest who could mount a coup was pure fantasy. Every connection between his nerve cells and her Mediator was entirely under the control of his Exoself; right down to the molecular level, this body would only take instructions from the matching hardware.

"Keep talking while I do that," she said. "What’s the situation with the border?"

Tchicaya brought her up to date.

Mariama was puzzled. "You’re not scribing the interface?"

"What’s the use?" he replied. "That would only tie up the stylus. We’re better off trying to kill the Planck worms from the outside. That way, we can use their own trick against them: we can correlate them with the vacuum, make them decohere. It’s a simpler problem. All we have to do is scribe something aggressive enough to take them on, but with a dead-end design that will fail completely at the next change of vendeks."

"You might be right," she conceded. "I hope it is that simple."

Tchicaya looked out across the rainbow-hued landscape. Everything that happened here — all the destruction wrought by the Planck worms, and by their putative remedy — would spread out at the speed of light across the entire border. The vendeks' diversity seemed to have acted as an effective barrier so far, but there could be gaps in that defense, threads or channels of identical populations running deep into the far side. He was gambling on a dizzying scale, like some dilettante ecologist in Earth’s colonial era, trying to balance one introduced predator against another.

The toolkit spoke. "I’m afraid the Planck worms have been sneakier than I expected. The need to attack a new mix of vendeks hasn’t filtered out any of the old mutations; they’ve all hitched a ride down with their successful cousins. So there are more than ten million different variants now. I can scribe seeds for individual replicators that would wipe out all of them, but that’s going to take more than nine hours."

"Start doing that immediately," Tchicaya said, "but also start thinking about a single seed that could do the same job."