Dichronauts

  Copyright © 2017 by Greg Egan

  All rights reserved. No part of this book may be reproduced in any manner without the express written consent of the publisher, except in the case of brief excerpts in critical reviews or articles. All inquiries should be addressed to Night Shade Books, 307 West 36th Street, 11th Floor, New York, NY 10018.

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  Library of Congress Cataloging-in-Publication Data

  Names: Egan, Greg, 1961- author.

  Title: Dichronauts / Greg Egan.

  Description: New York : Night Shade Books, [2017]

  Identifiers: LCCN 2016038952 | ISBN 9781597808927 (hardback)

  Subjects: | BISAC: FICTION / Science Fiction / High Tech. | FICTION / Science Fiction / General. | FICTION / Science Fiction / Adventure. | GSAFD: Science fiction.

  Classification: LCC PR9619.3.E35 D53 2017 | DDC 823/.914--dc23

  LC record available at https://lccn.loc.gov/2016038952

  Print ISBN: 978-1-59780-892-7

  Cover illustration by Justinas Vitkus

  Cover design by Claudia Noble

  Printed in the United States of America

  Five steps forward

  Three steps right

  Are four steps off the path

  But keep you safe in sight

  Three steps forward

  Five steps right

  Are four steps into Sider Land

  Out of the light

  —Children’s rhyme

  PART ONE

  1

  On the day the movers zigzagged the museum, Seth’s whole family woke before dawn. They ate a quick breakfast then set out across the city to join the crowd gathering to watch the spectacle.

  By the time they arrived the sky was bright, and all the vantage points to the east and west of the museum were taken, occupied by a good-natured but impenetrable throng. So they sidled north into the border precincts thinned by the encroaching summer—and found themselves on a deserted hill overlooking the building itself.

  “Not from the north!” Elena complained. Irina responded indignantly, “I can make it twice as clear as you can!”

  «What do you think?» Theo inspoke. «I’m not even pinging the scene myself—there’s so much bouncing around from the crowd that it wouldn’t make any difference.»

  «This is fine,» Seth replied. He didn’t know if Elena was just being spiteful, or if she and Irina had a real problem sharing views.

  Seth’s mother said firmly, “We’ll watch from here, it’s as good a spot as any.”

  The movers had finished their preparations inside the building, and were assembling into four rope teams outside: two near the south-west corner, two at the north-east. There must have been fifty Walkers in each team, but a part of Seth’s mind still refused to accept that they stood any chance of shifting the huge structure. He’d seen the wheels and pivots under the floor once, in a tour of the ancient building that had treated it as an exhibit in its own right, but the sheer size of those hidden components, however well-designed they were to aid the movers’ task, had only made it seem more fanciful that anyone could set them in motion.

  “There’s a career for you, Seth,” Theo’s father suggested.

  “Maybe.” Seth wasn’t sure if he was joking or not. “I think Theo would get bored, though.”

  “That’s not important. He can still enjoy his own work. It’s like that with all of us: give and take.”

  «What is it with your father?» Seth asked.

  «He wants me to study business law—so he’s hoping you’ll pick something that he thinks I’d hate, as much as he thinks you’d hate his choice for me. If we’d both be suffering equally, it would be harder for you to complain.»

  «Do you want to study law?» Seth’s gut tightened at the thought of spending half his life, even as a scribe, on anything so soul-sappingly tedious.

  «No.» Theo sounded amused that he’d needed to ask. «Do you want to be a mover?»

  «I don’t know. I doubt I’ll ever be strong enough.»

  The teams had taken hold of their ropes and were beginning to strain against the impossible load. The two groups at each corner were pulling at slightly different angles, but all four directions required an awkward stance. Seth noticed that a few of the movers at the north-east corner were west-facers, which had to make their work even harder.

  «You see that?» he asked Theo.

  «If they were upside-down, I’d be impressed.»

  Seth laughed and did a half backflip, landing on his hands, west-facing himself now. His own vision was undisturbed—the world itself no more seemed upside-down than when he tipped his head back to see westward—but his arms were blocking Theo’s view, and the thought of trying to maneuver them out of the way made him fear he’d lose his balance, risking a dangerous uncontrolled fall. He hadn’t hand-walked since he was a kid; it had seemed easy then, but he was seriously out of practice. He completed the flip, and brushed the soil from his palms.

  “What are they doing?” Elena asked irritably. “Nothing’s happening!”

  “The hardest part is getting the wheels moving,” Seth’s father explained. “But once they start—”

  The museum began to turn. It stretched out along the grand avenue that cut diagonally across the city, growing ever more slender, taking up ever less of the width of the road. The center of the building, the pivot, stayed fixed, while the opposing teams of movers ran ahead of their ropes’ anchor points: half of them heading north-east, half south-west.

  “See anything now?” Irina asked sarcastically. Elena said nothing, but when Seth glanced her way she seemed rapt.

  «Are you sure you don’t want to be a mover?» Theo teased him. Seth could feel his own face contorted into an expression of delighted stupefaction. The parallelogram below them was insanely skinny now; he’d turned pebbles about as far as this, but nothing larger.

  «Only if I get to ride inside the buildings, and see all the rooms remain normal while the city turns around me.»

  «You can always turn yourself and get the same view.»

  «You think so? Who’s going to clear the streets for me?»

  The museum’s far corner, needle-thin, slid past the bakery, the grain store, the bath house. It crossed the bridge over the brook and pushed on into the northern residential precinct. The movers themselves were barely visible now, stretched out along the road into surreal slivers of flesh. Despite what he’d said to Theo’s father, Seth doubted that their Siders were feeling bored. Strength and agility were not the only skills at play here: to execute a maneuver like this, you’d need the clearest possible sense of your surroundings in all directions, demanding a flawless collaboration between Walker and Sider.

  The corner itself became too narrow to discern, but then Seth noticed some of the movers reorienting their bodies and heading back down the avenue. They’d turned the massive building as far as they needed for the first step, so now the central pivot would be raised and a new one, almost at the corner, lowered, as the wheels were reconfigured to facilitate a counter-turn that would end with the museum one-eighth of the way to its destination. It would take the whole day to bring it to its new location at the city’s southern edge.

  Seth’s father handed out the stone-fruit he’d brought, and everyone sa
t on the dusty ground of the abandoned lot, chewing the sweet, heavy pulp while they waited for the second act to start.

  Theo said, “We should halt the sun, not move the cities.”

  His mother laughed. “One day, maybe.”

  “I’m serious.”

  Seth’s mother tipped her head back and scowled, unable to resist taking Theo at his word. “And survive on what? Do you think you could farm the same plots of land forever?”

  “And what about quarries?” Seth added. “Whatever happened with the crops, you couldn’t dig up the same stone twice.”

  “How much new stone would we actually need,” Theo countered, “if we weren’t leaving a trail of rubble behind?”

  “Hmm.” Seth paused to give the question due consideration. Theo had a habit of blurting out these absurd, dream-like propositions and then defending them with such sincerity that everyone’s skepticism began to waver.

  “And how exactly do we halt the sun?” Theo’s father asked.

  “There’s a mover for every job, isn’t there?” Theo replied. “So surely there’s an un-mover for this one.”

  This assertion was so strange that it hung in the air unchallenged.

  Seth looked down across the city. Much as he admired the movers, the truth was that the best he could hope to be if he joined them was some kind of minor assistant, checking ropes or carrying tools. He was healthy enough to be a stonemason or a farm laborer, but he did not have the strength to fling buildings around like toys.

  Theo inspoke, «You know who goes before the movers?»

  Seth wasn’t sure he understood the question. «Road-builders?» Theo clearly wanted to escape the law, but there had to be limits to what he would put up with.

  «Before the road-builders.»

  Privately, Seth ran through a list of ever-less-appealing preparatory trades—then as he pictured the untamed ground that the very first of these workers would confront, a wilderness untouched by leveler or plow, he finally understood what Theo had been hinting at. Why hadn’t he thought of it himself? He’d always wanted to work outdoors, and there was no job less confining.

  «You’d be happy with that?» he asked. «We’d do it together?»

  «Why not?» Theo replied. «We could study something real, and do something useful.»

  «While we wait for the un-movers to catch the sun?»

  «Of course. And not a statute, a contract, a lease, a trust, or a lien in sight.»

  Seth’s mother was facing away from him, but he caught his west-facing father’s eye.

  “We know what we’re going to do,” he announced. “Both of us.”

  “Is that right?” Theo’s father asked warily.

  «Fuck, yes,» Theo inspoke. Seth struggled to keep a straight face, waiting for his Sider to find the courage to express the same sentiment out loud, but then he found himself unable to keep silent any longer.

  “We’re going to be surveyors,” he said. “Movers are indispensable—but even they need someone going ahead of them, to find the best route for the city to take.”

  2

  “The possibility that the world might be infinite was recognized by people as far back as the Siseans.” Maria’s Walker, Samira, wrote the name beside the timeline she’d sketched; Seth copied it, wondering abashedly if he was the only person in the class who’d never heard of this entire civilization before. “But many other cultures assumed that the migration was ultimately cyclic. There were two main variants of that mythology: one in which constant southwards travel would eventually take you back to where you’d started—as if north and south were no different than east and west—and another, more elaborate, version, in which the advance of the sun regularly slowed, halted, then went into reverse, allowing the migration to change direction. Of course, the last occasion on which the reversal was meant to have happened was always taken to be so remote as to be the stuff of legend rather than history, and the absence of any signs of past habitation required the proponents to believe in summers so fierce as to transform buildings into sand, while mysteriously permitting the same kind of stone in its natural state to survive intact.”

  Seth resisted the urge to share a silent joke about the halting sun with Theo. There was no prohibition on inspeech during lectures, but if they didn’t discipline themselves and limit its use to helping each other follow the material, they’d end up constantly distracted.

  “Later,” Maria continued, “as geodesy began to develop into a true science, doubts arose as to whether an infinite assembly of matter was physically possible. The sun was understood to be orbiting the world under the influence of the same gravitational attraction that we feel here on the ground—but then the sun itself, a finite body, was seen to exert a tidal pull on the water in every river. If the world was infinite in extent, how could its gravitational pull fail to be infinite too?”

  Seth contemplated this disturbing notion. If you could measure the tides raised by the sun, and then measure the tug of the world upon your body, hadn’t you, by implication, measured the world itself and discovered its limits?

  But Maria had a far less radical solution. “In the generations following Siméon’s Treatise on the Tides, geodesists reached a consensus on this question. There is, in fact, a simple shape the world could take that would allow it to be infinite, while giving rise to a finite gravitational force.” Samira sketched four hyperbolas on the drawing board, symmetrically arranged around a central point. “Imagine that these curves go on forever. Take the area between them and spin it around the north-south axis to make a solid. The one-sheeted hyperboloid that wraps around the axis is the ground on which we stand. The two dish-shaped surfaces at the north and south are unreachable to us, but if anyone lived in those sunless places they would feel the same kind of gravity holding them to the ground as we do.”

  Samira began writing a familiar equation below the sketch, while Maria reminded the students of its role. “Siméon’s Law of Gravity says that if you add the second rates of change of potential energy along the two ordinary directions, then subtract the same quantity along the axial direction, the result will be proportional to the density of matter. Though this shape has an infinite volume and, in its entirety, an infinite mass, you can satisfy Siméon’s Law while still producing a finite force everywhere—in fact, a force that’s of equal strength everywhere on the surface.

  “Your task, to be completed by tomorrow, is to find precise expressions for the potential energy in each distinct region. You may assume, for the sake of simplicity, that the density of the rock is the same everywhere.”

  «This gets better every day!» Theo enthused as they headed out of the classroom. «All the things they never explained properly in elementary school are starting to make sense now.»

  «I’m going to need your help with this,» Seth confessed. Geodesy delighted him, too, but some of the technicalities were frightening.

  «Remember the formula they gave us for the point mass?»

  «Vaguely.»

  «You wrote it down,» Theo assured him. «I think we’ll just have to . . . spread that out over a larger volume.»

  «An infinite volume!»

  «Yes,» Theo conceded. «But the shape is so symmetrical that there’s bound to be some kind of trick to make the problem simpler than it sounds.»

  Seth went to the study hall and sat scribbling formulas, trying out Theo’s suggestions. In the empty space around a point mass, the potential energy was proportional to the inverse of the true distance from the point, with the force the inverse squared. But when matter was spread out at a uniform density, there was a solution that was even simpler, with the potential energy proportional to the square of the true distance from some point, and the force proportional to the distance.

  «Some point?» Seth could follow all the algebra, but the symbols meant nothing until he knew what they were measuring. «Which point?»

  «Whatever makes things simplest,» Theo proposed.

  Seth liked that r
ule. «The center?»

  When they followed through with detailed calculations, everything began to fall into place. On the ground, and in the space above it, the gravitational pull of the entire, infinite world was no different from that of a certain finite mass at the world’s center. It was as if all the vast plains and mountain ranges that stretched on forever to the north and south had been replaced by a single pebble—an absurdly heavy one, but not boundlessly so—that acted as their gravitational proxy. But there was a further, equally astonishing twist: although an idealized point mass produced infinite forces along its cones—where the true distance from the point fell to zero—in the solid rock between the hyperboloids, those forces weren’t merely hidden, they were tamed: reduced to finite quantities. The pebble at the center of the world was a useful fiction, from a sufficient distance, but if you could actually burrow down into the rock, instead of being torn apart as you approached the cone, you’d find the force of gravity decreasing.

  «Well done, Surveyor Theo.»

  «Well done, Surveyor Seth.»

  Walking home, they passed a playground where half a dozen kids were sledding up a northwards ramp. Seth paused, tempted but a little embarrassed.

  «Go ahead,» Theo encouraged him. «We’re not that ancient yet.»

  The group were friendly and one of the kids let Seth take a turn on her sled. He lay on his stomach on the polished stone, gripping the worn handles. Three Walkers sidled up the start of the ramp, pushing him to get him started. Seth could feel the changing forces in his gut as his helpers’ task grew easier, then superfluous, and as they stepped away he cried out in abandon at the rush of acceleration. The sled shot up the slope; Theo showed him the edge drawing closer. Then they were truly flying, off the ramp, through the air, down into the straw heap.

  «Isn’t gravity glorious?» Theo’s words sounded muddy and his view appeared blurred; the jolt of landing must have shifted him a little. Seth waited for him to squirm back into place before replying, «It is.»

  He rose to his feet and picked up the sled, sidling back to return it to its owner.