The Clockwork Rocket

instead of a tearing sound and springs flying everywhere, from fuel you get light and hot gas.”

Eusebio was bemused. “That’s a charming image, but I don’t see how it helps in any practical way.”

“Ah, but it does!” Yalda insisted. “By reacting various chemicals together in sealed vessels—which trap all the products, and turn all the light into heat—people have built up tables showing how much potential energy different substances have, relative to each other. Fuel and liberator are like something on the tenth floor of this tower, while the gases they produce are on the ground floor. The difference in chemical energy manifests itself as pressure and heat, just as the difference in gravitational energy, if you dropped a book from that height, would manifest itself in the book’s velocity.”

Eusebio was growing interested now. “And it all works out? Chemical energy is like a kind of accounting, it’s as simple as that?”

Yalda realized that she might have oversold the idea, just slightly. “In principle it should work, but in practice it’s hard to get accurate data. Think of it as a work in progress. But if you ever go out to the chemistry department—”

Eusebio buzzed amusement. “I’m not suicidal!”

“You can always watch their experiments from behind the safety walls.”

“You mean the ‘safety walls’ that need to be rebuilt three or four times a year?”

The truth was, Yalda had only visited Amputation Alley once herself. She said, “All right… be content to reap the benefits from a distance.”

“You say it’s a work in progress,” Eusebio mused. “Fatal explosions aside, what’s the hitch?”

“I’m no expert in their methodology,” Yalda admitted. “I suppose there’s room for errors to creep in when they measure temperature and pressure, and I expect it’s also hard to trap all the light. We can measure the energy in heat, but if there’s light emitted we don’t know how to account for that.”

“So how exactly do you know that they’ve made mistakes?” Eusebio pressed her. “What is it that tells you that their data is wrong?”

“Ah.” Yalda hated to disillusion him, but she had to be honest about the magnitude of the problem. “Someone showed that the values in the last table they published could be used, indirectly, to derive the result that pure, powdered firestone and its liberator contained only slightly more chemical energy than the gases they produce— nowhere near enough to explain the high temperature of the gases. But that extra thermal energy can’t just fall out of the sky; it has to come from a change in chemical energy. And that’s before you even start worrying about the energy carried off by the light.”

“I see,” Eusebio announced cynically. “So ‘chemical energy’ is a beautiful theory… but after all that risk and toil, the results show that it’s actually nonsense?”

Yalda preferred a different interpretation. “Suppose I told you that a friend of a friend of mine had seen a pebble drop from a third floor window, but you knew that the pebble in question had hit the ground with a deafening crash, and made a crater two strides deep. Would you throw out the whole idea of conservation of energy… or would you doubt my third-hand account of the height from which the pebble had fallen?”

Yalda squeezed into the lecture theater just as the guest speaker, Nereo, began ascending to the stage. There were only about four dozen people in the audience, but the venue had been chosen for its facilities, not its capacity, and the optics classes that were given here usually attracted just a couple of dozen students. Her late entry brought some resentful glares, but at least her height gave her the advantage of not needing to jostle for position—and when she realized that she was blocking the view of the young man behind her, she quickly changed places with him.

“My thanks to the scientists of Zeugma for their generous invitation to speak here today,” Nereo began. “I am delighted to have this opportunity to discuss my recent work.” Nereo lived in Red Towers, where his research was supported by a wealthy patron. With no university there, he had no colleagues around him to challenge or encourage him, though perhaps the whims of a rich industrialist were less onerous to deal with than Zeugma’s academic politics.

“I am confident,” Nereo continued, “that this learned audience is intimately familiar with the competing doctrines regarding the nature of light, so I will not spend time recapitulating their strengths and weaknesses. The wave doctrine rose to favor over the particle doctrine more than a year ago, when our colleague Giorgio showed that two narrow slits in an opaque barrier, illuminated with light of a single color, cast a pattern of alternating bright and dark regions—as if waves emerging from the two slits were slipping in and out of agreement with each other. The geometry of this pattern provided a means of estimating the light’s wavelength—and the measurements suggested a wavelength for red light about twice that for violet.”

Yalda looked around for Giorgio, her supervisor; he was standing near the front of the audience. She’d found his experiments persuasive, though many long-time proponents of the particle doctrine were unmoved. Why invoke some fanciful notion of “wavelength”, they argued, when every child who’d ever glanced up at the stars could see that what distinguished one color of light from another was simply its speed of travel?

“With all respect to my colleague, though,” Nereo said, “the double-slit pattern has often proved difficult to work with. The pattern is faint and the features that we wish to locate precisely can be indistinct, leading to considerable uncertainty in the measurements. In the hope of remedying these problems, I have investigated a natural extension of Giorgio’s idea.

“Suppose we obtained a large number of identical sources of any vibration, and arranged them in a line in a regular fashion, with a spacing roughly comparable to, but exceeding, the wavelength of the vibration itself.”

An image appeared on Nereo’s chest.

“If we ask in what direction the wavefronts from all these sources will come into agreement,” he said, “the answer is that, firstly, they will agree if you move orthogonally away from the line on which they lie. However, that’s not the only case. They will also agree at another angle, at a particular inclination to the central direction on either side.”

“Unlike the first direction, though, this one will depend on the wavelength of the vibration: as the wavelength grows, the angle from the center grows too.”

“The precise relationship between wavelength and angle is a simple trigonometric formula that will be familiar to all of you from Giorgio’s work; he dealt with two sources, and I am merely extending that idea. But increasing the number of sources does yield a powerful advantage: the passage of more light delivers a brighter, clearer pattern.”

Nereo gestured to an assistant, who pulled on a control rod for the blinds covering the skylight, plunging the theater into darkness. Before Yalda’s eyes had time to adjust, three brilliant patches of light appeared on a screen behind the now-invisible speaker. She recognized the central one as an almost unmodified image of the sun, captured by the heliostat on the theater’s roof. On either side of it were two dazzling streaks of color, distorted echoes of the primary image. Their inner rims, closest to the center of the screen, were deep violet, and they progressed in a rich, clear spectrum all the way to red. They were like star trails for the sun.

Nereo spoke from the darkness. “With the aid of my benefactor’s best machinists, I constructed a system of pantographs to etch precisely-spaced apertures in a sliver of calmstone: more than two dozen gross per scant. My measurements imply that vibrations of violet light come six dozen gross to the scant; the reddest light about three and a half dozen.” This was broadly in agreement with Giorgio’s results: a refinement, not a contradiction.

Yalda had seen a similar spectacle produced many times before, with clearstone prisms, but beyond the sheer beauty of Nereo’s crisper version she understood its significance. No one could give a detailed account of the underlying process by which a prism split light into its individual colors, so the angles at which different hues