Schild’s Ladder

Tchicaya pondered this. "So everything ends up on the same footing as rest mass? It’s as if speeding past an electron fast enough could make it look like any other particle at all — but in its own reference frame, it’s still an electron?"

"That’s right."

Suljan shouted triumphantly, "We have an echo!"

Tchicaya turned to face the screen. It showed a simple blip, the plot of a returning pulse. Suljan’s method had coarser resolving power than Yann and Branco’s, but that was what allowed it to penetrate further: his signal wouldn’t reflect back from the middle of a vast sea of vendeks repeating the same population mix, so any return at all meant that it had encountered a larger-scale change.

Hayashi was beside Suljan at the console. "There must be a layer population, like Umrao predicted," she said. "Some ten-to-the-forty nodes from the border."

Rasmah leaned toward Tchicaya and whispered, "A hundred kilometers, in good old reactionary language."

Umrao was pleased. He said, "I wish we could tell exactly what the border mix changed into, though." He looked around the table. "Come on, there’s a challenge for you. Range and resolution. How?"

Rasmah joked, "I’m sure using the Right Hand as well would do wonders."

Tchicaya said, "They’ll be getting echoes, too, right now, won’t they?" The two Hands themselves were about a hundred kilometers apart, so it was plausible that the scatter could reach them.

"Only if they know precisely what to look for." Rasmah raised her hands defensively. "Don’t say it: I’m the one who believes in spies."

A sense of anticlimax had descended on the room; the result was important, but it didn’t compare to their first glimpse of the Planck-scale structure of the far side. That there was macroscopic structure, too, was encouraging, but extracting further detail would be difficult. A hundred kilometers of solid rock would be no barrier to investigation, but a shift of vendeks was not like a change from crust to mantle, refracting and scattering seismic waves in a simple, predictable fashion. It was more like the boundary between two distinct ecosystems, and the fact that remnants of their expedition had straggled back intact after crossing a wide savanna didn’t mean the adjoining jungle would be so easily probed.

Suljan said, "I think it’s moving." Successive pulses were coming back with slightly different delays. The reflective layer was more or less keeping pace with the expanding border, but the signal showed it drifting back and forth. "Vibrating, maybe?"

Rasmah replied, "It’s probably something changing in the border region, messing with the propagation speed." That explanation made more sense to Tchicaya; the signal was crossing a vast tract with potentially variable conditions, so it was more economical to attribute any delay to the vendeks it encountered along the way.

Suljan gave her a withering look. "More expert commentary from the peanut gallery. The returns are too clean, and too sharp; that much variation in propagation speed would broaden them detectably."

"Hmm." Rasmah didn’t argue, but her eyes glazed over; she was checking something. When she emerged, she said, "Okay, you’re right. And the changes are too fast and too regular; the source of the variation would have to be fairly localized, so it must be the reflector, not the medium."

Tchicaya turned to Umrao. "Any ideas?"

"I didn’t see anything like this in the simulations," he said. "But then, I just remixed the vendeks from the border region. This layer might hold completely different ones."

The vibrations stopped.

Yann stared at the plot on the screen. "Just like that? No decay curve?"

The vibrations resumed.

Tchicaya looked around the room. Several people had left; apparently, the ringing of the far side’s equivalent of a planetary ionosphere was of no interest to them. Anything that influenced signal propagation was of crucial importance, though, and if this layer could move, it might even break up and reveal something deeper.

The vibrations halted again, only to restart a few seconds later. "One hundred and thirty-one oscillations," Yann noted.

Rasmah said, "What’s that going to tell us?"

Yann tapped his fingers against the table, one hand in time with the returning pulses, the other beating out the rhythm of the reflecting layer itself. Tchicaya resisted an urge to tell his Mediator to stop rendering Yann’s icon; the constant drumming was annoying, but he’d never edited anyone from his sensory map before, and he wasn’t about to start.

"One hundred and thirty-seven," Yann announced.

Tchicaya said, "You think there’s some longer-period cyclic process, modulating the faster one?"

Yann smiled enigmatically. "I have no idea."

Suddently, Rasmah groaned. "I know what you’re thinking!"

"What?" Tchicaya turned to her, but she wasn’t giving anything away.

She said, "I’ll bet you anything that you’re wrong."

Yann shook his head firmly. "I never gamble."

"Coward."

"We have no mutually beneficial assets."

"Only because you threw yours away," she retorted.