Greg Egan - The Eternal Flame

“And exploiting them for your own ends,” Carla replied.

Carlo was still annoyed with Silvano, but he wasn’t feeling quite so cynical. “For three generations, we’ve had to make do with what we carried with us from the home world,” he said. “When the Object came along, everyone expected something from it. Silvano was hoping we could farm there, and he’s already had to give up on that. A universal liberator sounds dangerous—but the idea of mastering it still makes us feel powerful. Now you want everyone to forget about the Object and just trust you to pull energy out of thin air.”

“No one has to take this on trust,” Carla replied. “If it works, it will work right in front of their eyes.”

Carlo said, “But they still need to trust you that it’s worth the resources to build this thing you want to put before their eyes.”

“So do you believe there’s any chance that it will work?” Carla didn’t seem hurt by his lack of confidence; if anything, she sounded glad to have someone skeptical whom she could interrogate on the matter.

Carlo was honest with her. “I don’t know. All this shuffling of luxagens between energy levels sounds a bit like sleight of hand.”

“The light source we’ve already made was all about shuffling luxagens between energy levels,” Carla protested. “The Council had no problem approving that idea.”

“Because you shine a stonking great sunstone lamp on it!” Carlo replied. “It doesn’t offend anyone’s common sense to think that you can put light in and get light out.”

“All right.” Carla thought for a while. “Forget about the details of the device, then. Just look at what it claims to do.”

She made a quick sketch on her chest.

“The Peerless has a certain energy-momentum vector before we use this device,” she said. “It’s just an arrow whose length is the mass of the whole mountain, and we draw it vertically in a reference frame in which we start out at rest.”

“Right.” Carlo wasn’t so tired that he couldn’t follow that much.

“The photon rocket emits a pulse of light,” Carla continued. “Ultraviolet, preferably, so it’s moving rapidly and its energy-momentum vector is a long way from vertical. To obey the conservation laws, the total energy-momentum has to be the same, before and after that pulse is emitted. Before, there’s just the original energy-momentum of the Peerless : the vertical arrow. After the pulse is emitted, there’s the light’s vector, plus the new vector for the Peerless , whatever that might be. The sum of those last two vectors has to equal the first, so if you join up their arrows base to tip they form a closed triangle with the original vector.”

She paused inquiringly. “I’m still with you,” Carlo said.

“If the rocket had no effect on the mountain other than giving it a push,” she continued, “the new vector would need to have exactly the same length as before: the same mass. That would be ideal—but I’m not even claiming to be able to do that! Instead, if we allow for some waste heat raising the temperature of the Peerless , the extra thermal energy lowers the mass of the mountain, very slightly. But even that doesn’t ruin the geometry—there are still vectors for the light and the accelerated mountain that add up, exactly as they need to.”

Carlo gazed at the closed triangle on her chest. “I can see that it’s not physically impossible, in those terms,” he conceded. “But everything else that makes light needs some kind of fuel, some kind of input.” He gestured at the walls. “Even moss has to have rock to feed on.”

“Because moss isn’t interested in just making light! Its real concern is growth and repair; it can’t do that without inputs.”

“We need to make repairs, too.”

“Of course,” Carla agreed. “Even if this device works perfectly, it won’t make us self-sufficient for eternity. We’ll still be using up all our limited supplies, including sunstone for cooling and other purposes. It won’t buy us another eon to contemplate the ancestors’ plight. The most I’m hoping it can do is get us back to them—with an idea worth trying for the evacuation.”

“So you’re saying that our grandchildren might see the home world?” Carlo joked.

“Maybe our great-grandchildren,” Carla replied. “We’re not going to be firing up these engines tomorrow.”

The note of caution only made her sound more serious. Carlo turned the idea over in his mind; it was shocking just how alien it felt. This would be their purpose, finally fulfilled. No one was prepared for that.

“If we could see ourselves returning within a few generations…” He faltered.

“You don’t sound too happy about it,” Carla complained.

“Because I don’t know how people would take it,” he replied. “Would it make it easier to keep the population stable, if we knew there was an end to the restraint in sight? Or would it be harder to stay disciplined, if we could tell ourselves that there won’t be enough time for a little growth to do as much damage?”

“I don’t know either,” Carla said. “But as problems go, aren’t these the kind worth having?”

Carlo reorganized the shifts so that Amanda and Macaria could work beside him on the playback experiments. The changes they’d be looking for might be subtle, so they’d need as many eyes on the subject as possible.

Eyes and hands. With Benigna tranquilized and locked to her plinth, he decided, they could palpate her body safely enough. How else would they be able to characterize small changes in the arborine’s flesh?

Their record of the onset of Zosima’s fission had been ruined by the torn tape, but Carlo had identified a subsequent pause in the activity and isolated the first unbroken set of instructions that followed. He had considered playing back the tapes from the individual probes one at a time, but when he wound the recordings from the three lower probes through the viewer together it was clear from all their shared, synchronized motifs that these signals were acting in concert. If he disregarded this, he risked unbalancing the light’s effect to a point where it was merely pathological. He wouldn’t have tried to decipher a piece of writing by throwing out two symbols in every three before offering it to a native speaker, and if his intuitive sense of the structure of this language meant anything, the three probes’ recordings—over a period of about a lapse and half—constituted the smallest fragment with any chance of being intelligible to the arborine’s body.

On the morning of the experiment Carlo arrived early and started administering the tranquilizer to Benigna. The drug he pumped into her gut through the plinth was far milder than the paralytic in the darts, and would not act anywhere near as quickly. Benigno didn’t take long to notice the effects, though: he let out a series of low hums, and his co’s steadily weakening replies did not reassure him.

“I’m sorry,” Carlo muttered. He’d planned to put up curtains to block Benigno’s view of the procedure—and Zosimo’s too, for good measure—but he hadn’t thought to do it so early.

Macaria turned up as he was finishing the task. He tied the last corner of the fabric into place, then dragged himself over to Benigna. Her eyes were still open, but when he tugged on her arms the muscles were slack. Macaria joined him, and they set about establishing a baseline for the arborine’s anatomy. Carlo had decided not to make any exploratory incisions; as informative as they might have been, there was too much risk of disrupting the very effects they were hoping to measure.