Greg Egan - The Eternal Flame

It sounded absurd, but what principle did it violate? Source strength would be conserved, since the total was zero both before and after. Energy would be conserved, so long as each photon had the energy of a single luxagen. Momentum would be conserved if the photons were traveling in opposite directions, making the total zero before and after.

Is this charity?Ivo asked her.

Carla was taken aback. He thought she’d concocted the whole theory just to get him off the hook with Ada and Tamara. Of course not!

Luxagens vanish?Ivo’s face made it clear that he found this no more plausible than a conjuror’s claim to make a vole disappear from a sealed box.

Only in pairs,Carla replied, as if that were enough to make the idea respectable.

But no wonder it sounded preposterous: where else could they have seen positive and negative luxagens come together, with any hope of understanding the result? Not in the fleeting, uncontrollable events on the hull of the Peerless . Not in the light from the Hurtlers that menaced the ancestors; they hadn’t even known the luxagen’s mass or grasped the link between energies and frequencies.

Only here. Wherever this beautiful new physics carried them, it could only have begun here.

By the time the Gnat was fully re-pressurized and Carla had removed her cooling bag, she’d given Ada and Tamara a version of the Mite ’s misadventures that never even raised the possibility of contamination in the air tanks.

“Air sets orthogonal rock on fire?” Tamara sounded every bit as skeptical as Ivo had been. “Are you sure there wasn’t some other—?”

“Air is made of positive luxagens,” Carla interjected. “Just like any other ordinary matter. That’s all it takes to set orthogonal matter on fire.”

She offered an illustration.

“The length of each line is the mass of the particle, and its height is the particle’s energy. Nereo’s arrow agrees with our time axis for positive luxagens, and opposes it for negative ones.”

Ada said, “Doesn’t a positive luxagen repel a negative one, close up?”

“It does,” Carla agreed.

“And the force pushing them apart goes to infinity as they get closer,” Ada added. “So how do they ever get to touch?”

“Luxagen waves don’t respect energy barriers absolutely,” Carla replied. “The wave for two luxagens with opposite signs will lie mostly in the energy valley where they’re far enough apart for Nereo’s force to become attractive—but it won’t be entirely confined to that valley, and it will allow some probability for the two luxagens to make contact. The fact that the probability is so small is why the process is relatively slow—why we had measurable delays before the flashes when we dropped the projectiles. But slow or not, once it happens, it happens.”

Ada looked dubious. Carla said, “Let me show you another process that’s worth thinking about.”

Ada stared at the new diagram. “A photon comes in from the left, a positive luxagen comes in from the right. They collide and bounce off each other.”

“Nothing too strange in that?”

“No,” Ada conceded.

“This picture is the same as the last one,” Carla said. “I just rotated it by a quarter of a turn. If a photon and a luxagen can bounce off each other like this, the version of events where two luxagens turn into two photons must be possible as well. It’s exactly the same thing, seen from a different viewpoint.”

Ada looked annoyed, but Tamara gave a chirp of delight.

“It’s an audacious theory,” Tamara said. “But where does it leave us? If we can’t even touch orthogonal rock with air, how are we going to get a sample to calibrate the reaction?”

“We can’t get a sample,” Carla replied. “But if these pictures are right, they tell us most of what we need to know. The UV line outshines everything else in the spectra, and if we dump a few hefts of calmstone onto the Object almost every luxagen in that heap of gravel will end up suffering the fate I’ve drawn. We know the energy and momentum produced by that reaction, so we can calibrate everything using that as our first guess.”

Tamara turned to Ivo. “What do you think?”

Ivo had been quiet since they’d returned to the Gnat , letting Carla give her version of events without comment.

“I don’t know what to make of this hypothesis,” he said. “But if we drop enough material to have a measurable effect on the Object’s trajectory—by Carla’s calculations—then we’ll have a chance to see how well her prediction bears out. If we’re going to be forced to work by trial and error, we might as well make the first trial count for something.”

Carla computed the total mass that needed to be flung onto the surface, but left the details of the orbit that would deliver it to Ada and Tamara. When Ivo had checked her arithmetic—and had her justify every assumption behind the numbers—she took on the purely physical task of winding the large catapult. Whatever damage the hyperthermia had wrought on her body, as she struggled against the wheel the pain and tenderness began to leave her.

Ivo loaded the catapult’s chamber, working the levers inside the hull that shifted measured scoops from the calmstone store. Compared to the tiny pellets they’d dropped before, this new bombardment was like a declaration of war. Carla had tried to balance the likelihood that some proportion of the material would be blown clear of the surface, unconsumed, against the possibility of an unanticipated process amplifying the whole effect. Though she’d never set eyes on Gemma, she’d heard the tale of the dark world that became a star repeated endlessly since childhood.

But Gemma had been ignited by a Hurtler, traveling at an infinite velocity relative to the rock of the planet. Sheer momentum would have carried the Hurtler deep below the surface before the annihilation began, trapping much of the heat produced and rendering it far more damaging. She did not believe that an explosion that was open to the void would start a wildfire.

Ada wore the blindfold this time, but she followed a clock with her fingertips and called out the command to launch. When Ivo released the catapult, Carla could see the pile of brown rubble tumbling away in the starlight, receding almost as slowly as the Mite when it began its journey. But in five bells’ time the rubble would take a tighter curve around the Object than she and Ivo had done, and find a wall of orthogonal rock in its way. By then, the Gnat would be diametrically opposite the point of impact, shielded from the blast.

The wait was as tense as the Mite ’s descent, but at least they could converse normally. “Who wants to break the news to Silvano?” Ada joked. “I don’t think he’ll be farming much wheat here.”

“Or mining much fuel,” Tamara added, “unless we can think of a way to handle it.” She turned to Ivo. “Is orthogonal rock a fuel, or a liberator?”

“There is no right word for it,” Ivo said. “Chemistry is about the rearrangement of matter. If matter disappears, that’s something else entirely.”

Half a bell before the impact, Carla handed out loaves, trying not to think about the fast she’d have to go through when she needed to return to her old mass. What mattered now was that they kept themselves alert, prepared to respond to any more surprises.