Schild's Ladder

by Greg Egan

  “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.

  Umrao said, “I'm completely lost. What are you people talking about?”

  “One hundred and thirty-seven,” Yann counted. “One hundred and thirty-eight. One hundred and thirty-nine.”

  He fell silent. The vibrations had stopped.

  Tchicaya said, “The slower cycle is varying, a little. Maybe lengthening. What does that tell us?”

  Rasmah had turned pale. At the console, Suljan, who'd been paying no attention to the conversation at their table, suddenly leaned into a huddle with Hayashi. Tchicaya couldn't hear what they were whispering about, but then Suljan let out a long, loud string of obscenities. He turned to face them, looking shocked but jubilant.

  “You know what we've got here?” he asked.

  Umrao smiled. “I just worked it out. But we shouldn't jump to conclusions.”

  Tchicaya pleaded, “What conclusions?”

  “Three consecutive primes,” Suljan explained.

  The vibrations had resumed, and Yann was calmly tapping them out again. Tchicaya calculated the next number in the sequence, and thought about trying to quantify the odds of the first three occurring by chance, but it would be simpler just to wait for the pattern to be broken or confirmed.

  “One hundred and forty-seven. One hundred and forty-eight. One hundred and forty-nine.”

  On cue, the vibrations halted.

  Yann said, “I wouldn't rule out nonsentient processes. We don't know enough about the kinds of order that can arise in this system.”

  Umrao agreed. “There's no reason evolution couldn't have stumbled on something useful about primes in the far-side environment. For all we know, this could be nothing more than an exotic equivalent of cicada calls.”

  “We can't rule out anything,” Suljan conceded. “But that has to cut both both ways. It has to include the possibility that someone is trying to get our attention.”

  Chapter 12

  “It looks as if the Colosseum is about to welcome us in,” Rasmah said. “You first.”

  “I don't think so.” Tchicaya held up his hand; it was shaking. They'd spent almost two hours sitting in the corridor outside the impromptu amphitheater where the Preservationists were meeting, and now the blank, soundproof wall in front of them was beginning to form a door.

  “Turn down your adrenaline,” she advised him.

  “I don't want to do that,” he said. “This is the right way to be. The right way to feel.”

  Rasmah snorted. “I've heard of traditional, but that's ridiculous.”

  Tchicaya bit back an irritated reply. If he was going to harness his body's natural agitation, he could still keep his behavior civilized. “I don't want to be calm,” he said. “This is too important.”

  “So I get to be the rational one, and you get to be impassioned?” Rasmah smiled. “I suppose that's as good a strategy as any.”

  It had taken Tchicaya six days of arguing to push a motion through the Yielders' convoluted decision-making process, authorizing disclosure of the recent discoveries to the opposition, and he had hoped that it would be enough
. The Preservationists would repeat the experiments, see the same results, reach the same conclusions. He'd set the chain of events in motion, and it would have an unstoppable life of its own.

  Then the Preservationists had announced that two Yielders would be permitted to address them before they made their decision on a moratorium, and he'd found himself volunteering. Having worked so hard to create a situation where they were apprised of the facts and prepared to listen, it would have been hypocritical to back out and leave this last stage to someone else.

  The door opened, and Tarek emerged, looking worse than Tchicaya felt. Whatever the body did in times of stress could be ameliorated at will, but Tarek had the eyes of someone whose conscience was robbing him of more than sleep.

  “We're ready for you,” he said. “Who's first?”

  Rasmah said, “Tchicaya hasn't smeared himself in goat fat yet, so it'll have to be me.”

  Tchicaya followed her in, then hung back as she approached the podium. He looked up at the tiers of seats that almost filled the module; he could see stars through the transparent wall behind the top row. There were people here that he knew well, but there were hundreds of complete strangers, too; the ranks of the Preservationists had been swelled by new arrivals.

  The audience was completely silent. There was an expression of stony resentment on some faces, an unambiguously hostile gaze, but most people just looked tired and frayed, as if the thing they hated most was not the presence of Yielders bearing unpalatable revelations, but the sheer burden of having to make an invidious choice. Tchicaya could relate to that; part of him longed for nothing more than a turn of events that would render all further effort irrelevant, one way or another, so he could curl up and sleep for a week.

  Rasmah began. “You've seen the results of our recent experiments, and I'm going to assume that you've replicated them successfully. Perhaps someone will correct me if that's wrong, and the raw data is in dispute.”

  She paused. Sophus called out, “That's not in dispute.” Tchicaya felt a small weight lifting; if there'd been a technical hitch, or some elaborate bluff in which the Preservationists claimed that they'd seen nothing, the whole discussion would have bogged down in recriminations immediately.

  Rasmah said, “Good. You've also seen Umrao's simulations, and I hope you've performed some of your own. We could sit here for a week debating whether or not the structures we've called ‘vendeks’ deserve to be described as living creatures, but it's plain that a community of them—or a mixture, if you prefer a more neutral term—forms a completely different backdrop than the vacuum we're familiar with, or anything else most of us imagined we'd find behind the border when we made our way here.

  “We've all pinned states with exotic dynamic laws to the border. We've seen tens of thousands of samples from the whole vast catalog of vacuum-based physics. But the far side's natural state, the closest it can come to emptiness and homogeneity, has access to all of those possibilities at once.

  “I came here expecting to see physics written in a different alphabet, obeying a different grammar, but conforming to the same kind of simple rules as our own. It was Sophus who first realized how myopic that expectation was. Our vacuum isn't just devoid of matter; our universe isn't simply sparse, in a material sense. What lies behind the border is neither physics in a different language, nor an amorphous, random Babel of every possibility jumbled together. It's a synthesis: a world painted in hues so rich that everything we've previously imagined as a possible universe begins to seem like a canvas filled from edge to edge with a single primary color.

  “We've seen hints, now, that there might be organisms far more sophisticated than the vendeks, just behind the border. There's probably nothing I can say that will influence your interpretation of the evidence. I'm not certain what it means, myself. It could be anything: sentient creatures longing for contact; a mating song between animals; an inanimate system constrained by far-side physics to lie in a state more ordered than our instincts deem likely. I don't know the answer, nor do any of you.

  “Maybe there is no far-side life worth speaking of. Maybe there are just different pools of vendeks, all the way down. We can't tell yet. But imagine for a moment that the signal we're seeing comes from a creature even as complex as an insect. If life of that sophistication can arise in just six hundred years, then the far side must be so amenable to structure, and order, and complexity that it's almost inconceivable that we'd be unable either to adapt to it, or to render parts of it hospitable.

  “Suppose we were handed a galaxy's worth of planets, all so near to Earthlike that we could either terraform them easily, or tweak a few genes of our own in order to flourish on them. What's more, suppose they came clustered together, so close that the time it took to travel between them was negligible: days or weeks, instead of decades or centuries. If we migrated to these worlds, it would mean an end to our fragmentation, an end to the rule that says: yes, you can see how other cultures live, but the price you pay will be alienation from your own.

  “On top of this, imagine that interspersed among these Earthlike worlds was another galaxy's worth of planets, all dense with a riotous variety of alien life. On top of that, imagine that these worlds were immersed in a new kind of physics, so rich and strange that it would trigger a renaissance in science that would last ten thousand years, transform technology, reinvigorate art.

  “Is that what the far side really is offering us? I don't know, and neither do you. Maybe there are some of you for whom it makes no difference: whatever lies behind the border, it can't be worth the price of even one more planet lost, one more people scattered. But I hope that many of you are willing to pause and say: Mimosa has brought tragedy and turmoil, and that has to be stopped, but not at any cost. If there is a world behind the border that could bring new mysteries, new knowledge, and ultimately a new sense of belonging to billions of people—a place that could mean as much to our descendants as our home worlds mean to us—then it can't be unimaginable that the balance could ever tip in its favor.

  “People left families and nations behind them on Earth. They'd swum in rivers and walked on mountains that they would never see again. Were they all traitors, and fools? They didn't destroy the Earth in their wake, they didn't force the same sacrifice on anyone else, but they did put an end to the world as it had been, when humanity had been connected—when the speed of light was a phrase that meant instant contact, instant collisions of cultures and values, not a measure of your loss if you tried to achieve those things.

  “I don't know what lies behind the border, but possibilities that seemed like castles in the air a year ago are now a thousand times less fanciful. Everything I've talked about might yet turn out to be a mirage, but if so, it's a mirage that we've all seen with our own two eyes now, hovering uncertainly in the heat haze. A few more steps toward it will tell us, once and for all, whether or not it's real.

  “That's why I'm asking for this moratorium. Whether you recoil from the vision I've painted, or merely doubt its solidity, don't make a decision in ignorance. Give us one more year, work beside us, help us find the answers—and then make your choice. Thank you.”

  Rasmah took half a step back from the podium. Someone in the audience coughed. There was no polite applause, but no jeering either. Tchicaya didn't know how to read the indifferent silence, but Rasmah had been fishing for converts rather than searching for a compromise, and if anyone had been swayed by her message that would probably not be a response they'd wish to broadcast.

  Tarek said, “We'll take questions when Tchicaya has spoken.”

  Rasmah nodded and walked away from the podium. As she passed Tchicaya, she smiled encouragingly and touched his arm. He was beginning to wish he'd gone first, and not just because she was a hard act to follow. Before a gathering of Yielders, a speech like the one she'd just delivered would have fired him up, filling him with confidence. Watching it received with no visible effect by the people who counted was a sobering experience.
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  Tchicaya reached the podium and looked up at the crowd, without fixing his eyes on any one face. Mariama would be here, somewhere, but he counted himself lucky that he hadn't spotted her, that her certain presence remained an abstraction.

  “There is a chance,” he said, “that there is sentient life behind the border. We have no proof of this. We lack the depth of understanding we'd need even to begin to quantify the odds. But we do know that complex processes that would have been inconceivable in a vacuum—or in the kind of hot plasma present in our own universe, six hundred years after its birth—are taking place right now on the far side. Whether or not you count the vendeks as living creatures, they reveal that the basic structure of this region is nothing at all like empty space.

  “None of us arrived here armed with that knowledge. For centuries, we'd all pictured the ‘novo-vacuum’ as the fireball from some terrible explosion. I came here myself in the hope that we might gain something from the challenge of learning to survive inside that fireball, but I never dreamed that the far side could harbor life of its own.

  “Life does not arise easily in a universe of vacuum. Apart from the Earth, there are just four quarantined planets strewn with single-celled organisms, out of almost a million that have been explored. For twenty thousand years, we've clung to a faint hope that the Earth would not be unique as the cradle of sentience, and I don't believe that we should abandon that hope. But we're now standing at the border, not between a desert with rare oases on one side, and a lake of molten lava on the other, but between that familiar desert and a very strange ocean.

  “This ocean might be a desert, itself. It might be turbulent, it might be poisonous. All we know for certain is that it's not like the universe we know. But now we've seen something fluttering beneath the surface. To me, it looks like a beacon, a declaration of intelligence. I concede that this interpretation might be completely wrong. But if we'd ever spotted something a tenth as promising on a planet, wouldn't we be shouting with joy, and rushing to investigate?