Greg Egan - The Clockwork Rocket

“What does your family think about all this?” she asked him. Yalda had said nothing to Lidia and Daria, or any of her colleagues; Eusebio had asked her to hold off mentioning the project to anyone until he’d tied up various “administrative loose ends”.

“I’ve managed to convince my father that it’s worth risking the money,” he said. “Even if the world’s in no danger from the Hurtlers, anything that the travelers invent on their journey could easily double our fortune.”

“What about your co?”

“She thinks I’m crazy. But I’ve told her—and my father—that I can’t countenance bringing children into the world until this rocket’s been launched to vouchsafe their future… which seems to have made both of them happy.”

“Why?”

Eusebio buzzed amusement. “She’s glad the day’s so far away. He’s glad it’s going to be so soon.”

Yalda said nothing. Eusebio turned to her and added, “Just to be clear, she can actually wait as long as she wishes.”

Yalda fought down the urge to reply sarcastically: How generous of you. If Eusebio really was supporting his co against their father’s nagging, it wasn’t worth starting a fight with him just because he sounded smug about it.

“Are you sure this isn’t going to lose you your fortune?” she asked. “What’s the going price for a mountain these days?”

Eusebio said, “I’ve already bought mining licenses from all the Councils that claim jurisdiction. It wasn’t cheap, but it didn’t ruin me.”

Mount Peerless was almost equidistant from five different cities; the only way to ensure an undisputed title would be to pay them all off. Yalda said, “Doesn’t a mining license include some deal about a share of the profits?”

“Of course. And if I do make any money from the project, I’ll give the Councils their cut.”

“But they all think you’re planning to dig out the sunstone and sell it?”

“I haven’t disabused them of that assumption.” The truck shuddered on the stony track. “But do you really want me to try lecturing my fellow Councilors on rotational physics? My father was willing to take my word for it—having paid so much for my education—but I can’t see Acilio and his cronies patiently following the trail of evidence from the velocity-wavelength formula to the passage of time for fast-moving travelers.”

“No.” The mention of Acilio reminded Yalda of someone else she’d been trying not to think about. “How do things stand with the university?”

“I’m negotiating a payment for them to relocate the observatory,” Eusebio said. “It’s not finalized, but given the amount we’ve been discussing they’ll be able to build a new telescope twice the size.”

“But not at the same altitude.”

“You can’t have everything. Don’t you think this is more important?”

“It’s not me you’ll have to convince,” Yalda warned him. “Have you ever heard of a man called Meconio?”

“ Meconio? I thought he was long dead.”

“Not in spirit.” Perhaps the university would take Eusebio’s money and accept the deal before Ludovico discovered the connection between this “mining project” and the loathsome subject of the new physics.

“How much of the mountain do you think is sunstone?” she asked.

“Maybe two-thirds, by mass.”

Yalda did some quick calculations on her back. “That might be enough for one quarter-turn in four-space, but there’s no chance at all that it will cover the whole voyage.”

Eusebio glanced at her, surprised. “You expect the yield to stay the same, after half an age working on improvements?”

“Maybe not, but if there’s barely any sunstone left over from the acceleration stage… what kind of yield are you hoping for?”

“I don’t expect the travelers to burn sunstone for the later stages,” Eusebio replied.

Yalda was startled. “You want them to turn hardstone and calmstone into fuel?”

“Either that,” he said, “or move beyond the need for fuel entirely.”

Yalda waited for a sign that he was joking; none came. “So you’re counting on this rocket riding the Eternal Flame? Is that what you told your father to expect?”

Eusebio hunched his shoulders defensively. “Just because Ninth Age charlatans wrote a lot of nonsense about a similar idea doesn’t mean it’s actually impossible.”

“A flame that consumes no fuel?”

“Tell me why it can’t exist!” he demanded. “Not the version the philosophers imagined: some magic stone that would sit on your shelf, creating light and nothing else—that would violate conservation of energy. But if light and kinetic energy are created together there’s no reason they couldn’t balance each other precisely, without any change in chemical energy to plug the gap. Fuel doesn’t need to be consumed; that’s just the way it works with the kinds of fuel we have right now.”

Yalda had no argument about the energy balance, and while she couldn’t calculate the relevant entropies on the spot, creating light generally meant an increase. In conventional flames the hot gas formed by the spent fuel also contributed to the rise in entropy, but there was no reason to think it was essential. On the face of it, then, a slab of rock could create a beam of light—balancing the energy and momentum of the beam by recoiling in the opposite direction, but suffering no other change—without violating any principle she could name.

Accepting that statement of theory was one thing. Being stranded in the void with an infinite velocity, exiled from your home until you conjured the Eternal Flame into existence, was a different proposition.

“I can’t tell you it’s impossible,” Yalda conceded, “but you still need to ensure that there’s a useful amount of sunstone left after the acceleration—even if you have to throw away half the rest of the mountain to eliminate some dead weight. Give them something they can make more efficient, not a choice between bringing a Ninth Age myth to life, or never coming back!”

“Let’s see what the detailed surveys tell us,” Eusebio said, trying to sound conciliatory. “Two thirds was just a conservative guess.”

Yalda stared out across the desert. Who was going to volunteer to ride on this folly if it looked harder to survive than the Hurtlers?

She said, “Please tell me you’re not expecting the travelers to invent their own means of dealing with waste heat.”

“Of course not.”

“So…?”

“I’m planning to divert some of the exhaust gas,” Eusebio said. “Letting it expand and drive a piston while it’s thermally isolated will cool it down and supply some useful energy—then decompressing it further while it’s circulating around the habitation will draw in heat. Most of it will then be released into the void, but some will be used to maintain the pressure in the habitation, which would otherwise decline over time as the original atmosphere leaks out.”

“So you’ll be burning some sunstone for these purposes, even when the rocket isn’t in use?”

“Yes—though compared to the amount used for propulsion it won’t be much.”

Yalda couldn’t fault this scheme, or suggest any obvious refinements, but that wasn’t good enough. “Now that you’ve proved that you have no fear of explosions,” she said, “how about a detour to Amputation Alley?”

Eusebio regarded her suspiciously. “Why?”

“There’s a man there called Cornelio who knows more about heat than either of us. You should ask his advice on this.”

“Can he keep a confidence?”

“I have no idea,” Yalda replied, irritated. Cornelio had always treated her honourably, but she wasn’t going to vouch for his willingness to go along with Eusebio’s whims.