Greg Egan - The Eternal Flame

“If we had sufficiently accurate clocks,” Carla mused, “we could find our distance from each beacon using the time it takes for the light to reach us.”

Tamara buzzed with mirth. “Accurate to what, a piccolo-pause? While you’re at it, why not use the geometric frequency shift to compute our velocity?”

“Who knows?” Ada interjected. “If people end up shuttling back and forth between the Peerless and the Object, do you think they’ll still be navigating like this after a dozen generations?”

“There’s only so much you can do to make a clock keep better time,” Tamara replied. “We’re already close to the limits of engineering.”

“But nature’s full of systems with their own rapid, regular cycles,” Carla countered. “Light itself, among others.”

“Very practical,” Tamara retorted. “Once you filter out a single pure hue from a lamp, the beam will still be made up of lots of short wave trains: a few cycles at a time, all with random phases compared to each other. Even if you had some way to count the cycles, it would be like listening for the ticking from a vast heap of clocks that started up at random moments, ran for a few pauses and then died.”

“That’s true,” Carla said. “But why not look for better ways to use the same clocks? The light given off by tarnishing mirrorstone as it spits out each luxagen ought to be in phase with the light that stimulated the emission in the first place. If you bounced that emitted light back onto the mirrorstone, looping the whole process around, you might be able to build a source that remained in phase over much longer periods than any kind of natural light.”

“Light that elicits light that elicits still more light?” Ada joked. “That’s starting to sound like the Eternal Flame.”

“Not so eternal,” Carla said ruefully. “The tarnishing would use up the mirrorstone just as surely as any flame consumes its fuel.”

“And you count the cycles… how?” Tamara pressed her.

Carla said, “I’ll have to get back to you on that.”

Tamara felt the dials at her fingertips reach the configuration she’d been waiting for. The beacon’s flash came a moment later—almost certainly from her own clock running slightly fast. But this wasn’t Carla’s brave new world yet, and it was the position of the beacon against the stars that mattered most, not the timing. She recorded the angles on her forearm, then turned the theodolite toward her second target.

“First sighting acquired,” she announced. “Well within the expected region.”

Ivo hummed with pain. “I’m sorry, I’m going to have to take the bag off. Just partly, along the right side.”

Tamara said, “Carla, can you assist him?”

“Of course.”

Tamara watched them without leaving her post. It was a simple enough maneuver, and even if the Gnat chose this moment to split apart and spill them all into the void, Ivo would still have his bag, helmet and two air cylinders with him like the rest of them.

Ivo chirped with relief as he resorbed his right arm, then he spent a lapse rearranging the flesh internally before extruding a new limb. Carla helped him refit it to the bag, running some air through as a test.

“Thank you,” Ivo said. “I think I can check the equipment myself now.”

“There’s no pain at all?” Tamara asked him.

“None. The new arm’s perfect.”

“All right.” With anyone else she wouldn’t have fretted over such a minor injury, but Ivo’s dexterity would soon be crucial.

Tamara turned back to the theodolite in time to catch the flash of another beacon. “Second sighting acquired,” she said. “Within the expected region.” Each flash, observed against the background of the stars, placed the Gnat on a line that passed through the beacon in question. Had the craft been stationary it could have been pinned down at the intersection of two such lines, but even in motion three sightings would be enough to determine its trajectory, and any more would improve the accuracy of the solution. That was assuming that all the errors in the measurements were random, but she and Ada could compare their results as a check against any systematic bias.

“Still nothing?” Tamara asked Ada, puzzled that her co-navigator had yet to report a single sighting. Each beacon only flashed once a bell, but the times were staggered so that one beacon or another was visible every lapse.

“Either my first target’s died, or something on the window obscured it,” Ada explained.

“Why didn’t you tell me?” Tamara was confused. They needed to communicate everything clearly, but Ada knew that perfectly well. In all of the drills, she’d been scrupulous.

“Ivo was telling us about his injuries, I didn’t want to interrupt him.”

“But once he’d stopped—”

“I know,” Ada said. “I apologize.” Her tone was even, with no trace of resentment, but Tamara still felt awkward to be reprimanding her.

There were contingency plans for all the observations to be performed out on the hull if there was serious damage to the windows from an encounter with orthogonal dust, but that was an extreme measure that would make the whole procedure much more arduous. Stray particles from the Gnat ’s own exhaust might have left a smattering of subtle defects in the clearstone, but Tamara had never really thought through the proper response to such a minor vitiation.

“Once we’ve finished with the beacons, we should do a systematic check for pitting,” she decided.

“Good idea,” Ada said. A moment later she added, “Ah, first sighting acquired! Within the expected region.”

An ache in Ivo’s arm. A few flaws on a window pane. Tamara had no intention of becoming complacent, but these were the kind of problems she could live with. In the drills, they’d rehearsed the complete disintegration of the Gnat ’s cabin, flailing around the mock-up in their cooling bags until they’d learned to use their air cylinders as rockets to bring themselves together on the engine module, ready to make the flight home without a single wall to protect them. She should not be unsettled by anything less.

When they’d both computed their estimates of the Gnat ’s trajectory, Tamara’s agreed with Ada’s to within the error bounds. The results implied that they would need to fire the engines again, briefly, in order to aim the Gnat squarely at the rendezvous point, but they could refine their measurements even further by waiting a few bells before repeating them.

To quantify the pitting on the windows, they each made observations of two gross stars that should have been visible from their respective posts, checking for any images that were obscured or distorted. Tamara found two cases where she could see a faint, blurred oval of light in place of a portion of one of the star trails—and by shifting the theodolite sideways while retaining its direction, she could move this aberration across the field, a sure sign that the flaw was in the window itself.

Ada found three. It was not a bad rate. And when the beacon Ada had missed was due to repeat, they were able to use the trajectory data to anticipate its location in the sky much more precisely. This time it did show up, in the dead center of the targeted star field. Whatever the original problem had been, the navigational procedure itself was proving to be as robust as Tamara had hoped.

Carla fetched four loaves from the storage cupboard and the whole crew ate together. The women had agreed to double their usual food intake; they could return to fasting once the mission was over, but for now nothing mattered more than a clear head.

“I’ve been thinking about your luxagen waves,” Ivo told Carla. “They’re not confined entirely to the energy valleys, are they?”