The Clockwork Rocket

by Greg Egan

  Yalda was momentarily lost for words. If Nereo was offering her ways to improve her methodology, that meant he was taking the whole thing seriously.

  “So you think it’s a worthwhile experiment?” she asked.

  “Absolutely,” Nereo assured her. “And better you than me on Mount Peerless! I’ve grown used to the presence of certain decadent comforts, such as air.”

  Giorgio buzzed amusement.

  Yalda had never been to the observatory, but she didn’t care what hardships it entailed. “You’ll let us borrow the light comb, sir?” The glittering key to the secrets of wavelength, bought with his patron’s incomparable wealth, would be borne up the mountain’s slope to meet the starlight… in her hands?

  “I’ll let you borrow it for eight chimes,” Nereo replied, “before I have to leave to catch my train. That should be long enough for you to calibrate your best prism against it.”

  “Prism? But—”

  Nereo said, “Everything in your methodology should work just as well with a prism used to recombine the star trails; all you need in order to make that worthwhile is a conversion table that translates between the angles at which the same hue emerges from the different devices. Do you think you can manage that, before I depart?”

  In the optics workshop a young student was using the heliostat for an experiment in polarization, but when Yalda asked if he could take an early lunch he obliged without a moment’s hesitation.

  From the storage room, she chose a prism that she’d used before; the sides had been polished to near-perfect flatness, and were unblemished by chips or scratches. Equally, the clearstone from which it had been cut appeared to contain no internal flaws. She knew that it would separate the colors smoothly, however mysterious its method.

  With the prism in place in the beam of sunlight that was brought down from the clockwork-driven mirror on the roof, Yalda locked Nereo’s comb onto a platform that could swivel through the emerging fan of colors, along with a slit for selecting a narrow range of the prism’s light. The slit could not be set too fine, though, or it would itself diffract the beam.

  She placed a white screen half a stride beyond the comb, and set about recording the pairs of angles for a succession of hues: the angle at which the light had been bent by the prism, and the angle at which it was subsequently bent by the comb.

  Yalda worked with scrupulous care, but after a while the process became mechanical, automatic. She glanced at the polarisers she’d taken off the bench: slabs of an exotic form of clearstone from Shattered Hill. Place one of them in a beam of light, and the beam’s brightness was diminished by one third. A second polariser aligned identically with the first had no effect, but if the two were “crossed”—their axes arranged at right angles to each other—the original brightness was diminished by a further third.

  Giorgio had sought to explain this in terms of the wave doctrine. An elastic solid could experience shear waves, in which the medium suffered distortions perpendicular to the direction of the wave’s motion. A polariser, he argued, must somehow be inhibiting light’s equivalent of such waves when they lined up with the stone’s special axis. A horizontal polariser could rid a beam of sunlight of its left-to-right vibrations; a second, aligned vertically, would rid it of all waves that vibrated up and down.

  A mystery remained, though. Along with shear waves, every solid carried pressure waves, which were much like the sound waves in air. The velocities of the two kinds of wave were due to distinct properties of the material, and pressure waves always traveled faster than shear waves. It would require both a truly bizarre material and an absurd coincidence to force the two to share the same speed.

  When two crossed polarisers were held up to a star trail, if the light that emerged had traveled from the star at a different speed than the light that was blocked, some portion of the trail’s spread of velocities should have been favored over the rest. But in fact what was seen was a perfectly uniform dimming of the entire trail. Light waves that lacked polarity—supposedly the equivalent of a solid’s pressure waves—were no faster or slower than the rest.

  Yalda could not believe that this was a coincidence—a perfect conspiracy of elastic moduli. Rather, what it suggested was that the whole analogy was flawed. Whatever carried light between the stars wasn’t actually being squeezed and stretched and sheared. Nereo had pinned down light’s wavelength, the distance at which each cycle repeated, but the truth was that no one yet had an answer to the question: cycles of what?

  When Yalda had a full range of measurements, she sprinkled dye onto her chest and made three copies of the figures on paper: one for Giorgio, one for Nereo—not much use to him, since the numbers were tied to a particular slab of clearstone, but an appropriate gesture nonetheless—and one to keep in the workshop alongside the prism.

  Nereo was waiting at the university’s southern gate, an ornate stone archway encircled by violet-flowered vines that had been bred to open their blossoms even in daylight. Yalda thanked him profusely, and almost offered to carry his luggage to the station, but Rufino and Zosimo were already grappling with the cases, and she’d learned not to wound their pride with gratuitous displays of physical prowess.

  When they’d left, Giorgio upbraided her sternly before finally conceding, “I suppose it was worth it in the end. You’re not much of a diplomat, but this could yield interesting results.”

  The understatement was insulting, but Yalda didn’t push her luck. “I hope so,” she said.

  Giorgio regarded her with weary affection. “And I hope you’re ready to try displaying a bit more tact.”

  “Of course!” Yalda said, genuinely chastened now. “The next visitor, I promise—”

  Giorgio hummed, annoyed. “Forget about the next visitor! You want to use the telescope, don’t you?”

  “Yes.” Yalda was bewildered; did he mean that she’d probably be up on the mountain, unable to cause any more embarrassment when his next guest came to speak?

  Then she understood.

  “The next unallocated slot at the observatory begins in seven stints,” Giorgio said. “If you want that slot for your wavelength measurements, you know who you’re going to have to deal with.”

  A motif of two entwined helices was carved into the wall outside Ludovico’s office. The curves represented the motion of Gemma and Gemmo, co-planets that circled a common center every eleven days, five bells, nine chimes and seven lapses. Of course, they also traveled around the sun, and their distance from the world rose and fell substantially during each six-year orbit. Before Yalda had even been born, Ludovico had noticed that as Gemma and Gemmo drew farther away, the precise clockwork of their mutual circling seemed to slow, very slightly: the observed times when one planet crossed in front of the other slipped behind the predictions of celestial mechanics. But Ludovico had realized that the laws of gravity were not at fault; the light was just taking a little longer to arrive. With this insight, his observations had allowed him to compute the first reliable figures for the speed of light, averaged across the colors.

  By the time Ludovico called Yalda into his office, the sun had set. He’d lit a firestone lamp, which sat sputtering and sizzling on a corner of his grand, paper-strewn desk. Standing before him, eyes respectfully downcast, Yalda summarised her proposal quickly. Her aim, she declared, was to correlate the angles of separation in a star trail with the angles of deflection produced by a clearstone prism; there was no need to mention Nereo’s device at all. “If I can find the formula that links the prism’s effect with the light’s velocity, that might lead to some insight into the mechanism of chromatic deflection.” In fact the data she gathered would be perfectly suited to that purpose; she was not really being dishonest.

  When she’d finished speaking, Ludovico emitted a muted hum, the tone signifying gratitude that a tedious ordeal had finally ended.

  “I’ve never had much time for you, Yalda,” he said. “Not because you hail from the benighted eastern provinces, with your qua
int dialect and bizarre customs; that can be endearing, and even correctible. And not because you’re a woman—or almost a woman, or something that might have been a woman if nature had taken its proper course.”

  Yalda looked up, startled. She hadn’t been insulted in quite such an infantile fashion since she’d left the village school.

  “No, what I find objectionable is your arrogance and your utter inconstancy. You hear of an experiment, you read of some research, and whatever ideas you’ve supported in the past fly out the window. You simply trust in your own infallible powers of reasoning to guide you to the truth, as you swerve this way and that.” Ludovico held up a hand and made a zigzagging motion. “Well, I’ve heard of all the same experiments, I’ve read all the same research. I suppose I must not share your hubris, though—because I’m not driven to the same undignified series of self-contradictory declarations and endless changes of allegiance.”

  Yalda said nothing, but she struggled to recall what she might have done to earn this tirade. At her admission interview, where Ludovico had sat on the panel, she’d professed some sympathy for the particle doctrine; that had been before Giorgio’s double-slit experiment. But at a debate half a year ago, she’d taken the side of the wave doctrine, and expressed the flaws in the opposing view quite forcefully. Why not? The evidence had mounted up, and she’d found it increasingly compelling. But apparently it was a kind of arrogance, to trust her feeble powers of reasoning to bring her to that conclusion.

  Ludovico reached down to a shelf below his desk and lifted up a bulky stack of paper. In fact it was a book, Yalda realized, though the binding was in a terrible state.

  “Have you ever read Meconio on the theory of luminous corpuscles?” he demanded.

  “No sir,” Yalda admitted. Meconio had been a philosopher in the ninth age; she’d heard that he’d made some minor contributions to the study of rhetoric, but his grasp of natural phenomena had been less than impressive.

  “If you can produce a halfway perceptive, three-dozen-page essay on Meconio within the next two stints, I’ll allow you to use the observatory.” Ludovico held out the tattered book; Yalda reached across and took it carefully. “A little exposure to a truly great mind might finally endow you with a trace of humility.”

  “Thank you, sir. I’ll do my best.”

  Ludovico hummed irritably. “If you can’t manage a commentary that’s worthy of my attention, leave the book with my assistant and don’t ever waste my time again.”

  Yalda left his office and trudged down the dark hallway toward the exit. Two bells ago, she’d been euphoric; now she just felt hopeless. This man had set her an impossible task; even if Meconio’s tome was strewn with dazzling insights worth praising to the sky, she would never be able to plow through so much fusty ninth-age language in time to write anything sensible about his ideas.

  “Are you all right?”

  Yalda turned, startled; someone had just emerged from one of the unlit rooms opening into the hallway. The voice had come from nearby, but all she could see was a faint outline in the darkness.

  “I was measuring floral spectra,” the woman explained—work best done at night, without a lamp, the better to observe the plant’s luminescence. “My name’s Tullia.”

  “I’m Yalda. Pleased to meet you.” Yalda couldn’t keep the despondency from her voice, but Tullia’s solicitousness had come before she’d said a word. “Why did you ask me—?”

  “I could tell you’d had one of those Ludo moments,” Tullia confessed. Her outline was growing sharper as Yalda’s eyes adjusted to the gloom. “There’s actually a distinctive effect on people’s gait as they come down the hall. Sadistic belittlement is his specialty, so I know exactly how that sounds. But if he starts to get you down, just remember: everything he says comes from halfway up his anus.”

  Yalda struggled to stifle her response, lest the sound echo all the way back to his office. “That’s remarkably flexible for someone his age,” she observed.

  “Flexibility is not a quality I associate with Ludikins,” Tullia replied. “I expect his tympanum’s been stuck there for the last dozen years. Let me get my things.”

  Tullia ducked back into the workshop, then they walked together out into the night. As they crossed the starlit courtyard, she said, “I see he’s given you his favorite reading material.”

  “I have to write an essay on it in the next two stints,” Yalda lamented.

  “Ah, Meconio!” Tullia chirped sardonically. “Proof that it is possible to fill five gross pages with scholarific assertions about the world, without bothering to test even one of them. Don’t worry, though, we’ve all had to do the essay. I’ll give you an old one, with enough tweaks to disguise it.”

  Yalda didn’t know whether to be scandalized or grateful. “You’d do that?”

  “Of course. Why wouldn’t I?” Tullia was mocking her; she’d picked up the undertone of disapproval. “It’s not as if I’m helping you cheat in some serious assessment; this is just Ludi’s ludicrous self-indulgence. Well… octofurcate him at every opportunity, that’s my policy. What’s the favor you needed from him, anyway?”

  Yalda explained her wavelength-velocity project.

  “That’s an elegant idea,” Tullia declared. “It’s tough up on the mountain, though, so remind me to give you a few tips before you go. It’s easy to overheat there.”

  “You’ve been to the observatory?”

  “Six times.”

  Yalda was impressed—and not only by the woman’s physical stamina. “What are you working on?”

  “I’m looking for life on other worlds.” Tullia’s tone made this endeavor sound entirely practical, as if it were no different from looking for weeds in a wheat field.

  “You think we could see signs in the spectra?” Yalda was skeptical, but the notion was delightful.

  “Of course,” Tullia replied. “If someone observed our world from a distance, its light trail would be very different from any star’s. Plants create a wide variety of colors, but they come in discrete hues. When rock burns, the fuel itself has a distinctive color in its emissions, but the spectrum from the hot gas is continuous.”

  “But how can you know what the plants on other worlds will be like?”

  “The detailed photochemistry might be different,” Tullia conceded, “but I bet life would still show up as discrete bands of color. I mean, do you know of any method for getting energy out of rocks without producing light along the way? And if it’s not staged and controlled, if it’s not confined to specific channels the way plants do it… that’s a world on fire. That’s a star.”

  Yalda had become so caught up in the conversation that she’d barely noticed them leaving the campus. She looked around to get her bearings.

  Tullia said, “I’m going to meet some friends in the south quarter. You’re welcome to come along if you like.”

  “Are you sure?”

  “Absolutely.”

  They turned into the avenue that led south toward the Great Bridge. Yalda liked the evenings in Zeugma; light spilt from the windows of restaurants and apartments to reflect off the cobblestones, but you could see the star trails clearly too. Families and couples were out walking, lost in their own concerns; no one looked twice at her bulky form. If she hadn’t run into Tullia she would have been pacing through the city alone now, waiting for the beauty of the streets and the sky to overcome her anger at Ludovico’s patronizing diatribe. Belatedly, she formed a pocket and slipped Meconio’s treatise into it for safe-keeping; if she lost the book itself, not even the most ingratiating tribute to his genius would be enough to save her.

  Halfway across the Great Bridge, they stopped to stare down into the blackness of the crevasse that divided the city. Several ages ago the ground here had been full of firestone; the first settlements had grown up around shallow mines. Later an elaborate system of tunnels had been built, plunging deep into the stone. But early in the eleventh age there’d been an accident in the mines and t
he whole deposit had been ignited. Half the city had been destroyed, and every trace of fuel had been burned away. All that remained was this jagged abyss, a taunting geochemical map in reverse: here’s what you could have had, now that it’s gone.

  “I think every world probably started out with much the same mixture of minerals,” Tullia said. “Maybe they were all part of a single, primal world, eons ago. But I suspect that, whatever its origin, there are only three things that can happen to a world: it remains dark, like Gemma and Gemmo; it catches fire, like the sun and the stars; or life comes along to perform the same kind of chemistry in a more controlled way.”

  Yalda gazed into the hole left behind by the Great Ignition. “This place makes me think that those possibilities need not be mutually exclusive.”

  “True enough,” Tullia replied. “In fact, for all we know that might be a universal truth. Maybe the stars didn’t just burst into light; maybe they started out covered in plants, which grew too productive for their own good. All the liberators we’ve discovered so far are plant extracts, after all. And maybe it’s just a matter of time before the same thing happens here—either the plants do it, or the honor goes to someone in the chemistry department.”

  “Now I’m worried,” Yalda said, only half-joking. “If anyone can set calmstone on fire, it’s a chemist.”

  They continued across the bridge into the south quarter. “I used to work in that restaurant,” Tullia said, pointing to a brightly lit building on a crowded side street. “When I was a student.”

  “Is that where we’re going?”

  “Only if you want to drop in for a spot of arson.”

  “You sound nostalgic.”

  “The clientele were mostly Councilors’ sons and their entourages,” Tullia said. “How could I not have fond memories?”

  She led the way to another restaurant, one that Yalda had passed many times before, but instead of taking the front entrance they slipped into a winding alley that ran behind the kitchen. Tullia exchanged shouted greetings with a woman Yalda glimpsed working inside, but they continued down the alley until they reached an unlit flight of stairs. It took Yalda a moment to orient herself; the stairs led back to a second story over the restaurant.