Greg Egan - The Clockwork Rocket

“He’s in Zeugma helping Eusebio with something.”

“I never thought this would work,” Yalda admitted.

Benedetta chirped gleefully. “I know! That makes it so much better!”

Yalda was still having trouble believing it. She was holding in her hands a sheet of paper that had left the world behind, crossed the void faster than anything but a Hurtler, turned around and come back… and then traveled here by post from Mount Respite.

“Which stage was it taken in?” she asked.

Benedetta pointed to the serial number.

“Meaning…?” Yalda had forgotten what the numbers signified.

“Odd numbers were for the first stage of the journey, when the probe was traveling away from us.”

“Good,” Yalda managed numbly. She thought for a while. “I think you should come and tell my recruits what you’ve found.”

“Of course.”

Yalda introduced Benedetta to the group, then recounted some of the background to the problem. Years before, she had managed to identify a slight asymmetry in the Hurtlers’ light trails, demonstrating that their histories were not precisely orthogonal to the world’s. This had finally made it possible to say which direction they were coming from; until then, their trails might have marked a burning pebble crossing the sky in either direction. But it had revealed nothing about the Hurtlers’ own arrow of time.

Doroteo was confused. “Why doesn’t their arrow of time just point from their origin to their destination?”

Yalda said, “Suppose you drive toward a railway crossing, and you notice that the track doesn’t make a perfect right angle with the road you’re on; it comes in from the left as you approach the crossing. You might think of the ‘origin’ of the track as being a station that lies behind you, to the left—but assuming that this track is only used in one direction, you still have no reason to believe that the trains will actually be traveling from your left to your right.”

Doroteo grappled with the analogy. “So… we can map the geometry of a Hurtler’s history through four-space as a featureless line, but we can’t put an arrow on it. We can’t assume that the tilt you discovered means the Hurtler’s arrow is pointing slightly toward our future; it might as easily be pointing slightly toward our past.”

“Exactly,” Yalda said. “Or at least, that was how things stood until now.”

Benedetta was shy before the strangers, but with Yalda’s encouragement she took over the story.

The probes had been launched two and a half years earlier: six dozen rockets fuelled by sunstone from the mountain’s excavations, fitted with identical instruments and sent out like a swarm of migrating gnats in the hope that one of them would complete its task and find its way home. Their flight plan had been a less ambitious variation on that of the Peerless , reaching just four-fifths the speed of blue light before decelerating and reversing, with literally just a couple of bells spent in free fall along the way. Compressed air, clockwork and cams controlled the timing of the engines, with opposing pairs of thrusters built into the design to avoid the need to rotate the craft. The aim of the project had been to get an imaging device moving as rapidly as possible, parallel to the Hurtlers’ path, first in one direction and then the opposite.

“This paper was made sensitive to ultraviolet light, about one and a half times as fast as blue light,” Benedetta explained, holding up the travel-worn sheet. “The orthogonal stars all lie in our future, so we can’t expect to see them, or image them under ordinary conditions. But the whole meaning of ‘past’ and ‘future’ depends on your state of motion.”

She sketched the relevant histories on her chest.

“With the probe traveling at four-fifths the speed of blue light, infrared light from the orthogonal stars would have reached it at an angle in four-space corresponding to ultraviolet light from the past.” Benedetta held up the evidence again. “So we’ve managed to record an image of these stars—which to us still lie entirely in the future—by giving the probe a velocity that placed part of their history in its past.”

Fatima said, “How do you know those are images of orthogonal stars, not ordinary ones?”

Benedetta gestured at her chest. “Look at the angle between the light and the histories of the ordinary stars. To them, the light’s traveling backward in time! Only the orthogonal stars could have emitted it.”

Yalda added, “And if the orthogonal stars’ future had pointed the other way, then the probe could only have imaged them once it had reversed and was coming back toward us. So we know the direction of the arrow now—not from the Hurtlers themselves, but from the light of these stars whose origin the Hurtlers share.” This finally put to rest Benedetta’s old fear: that the Peerless might be headed straight toward the orthogonal cluster’s past, requiring the rocket to function while opposing the entropic arrow of its surroundings. Now it was clear that it would not face that challenge until the return leg of the journey.

“And how far away are they?” Fatima asked. “These orthogonal stars?”

Benedetta looked down at the images. “We can’t be sure, because we don’t know how bright they are. But if they’re about as luminous as our own stars, the nearest could be no more than a dozen years away.”

The group absorbed this revelation in silence. Five Hurtlers were spreading lazily across the morning sky, and while Gemma itself was below the horizon, here was a promise of interlopers vastly larger than the baubles that had set that world on fire.

Just when Yalda was beginning to think that the stark new threat might push some of the waverers into making a commitment, Leonia broke the mood. “Six dozen probes went up,” she said, “and this is the only one that’s been recovered? What happened? Did all the others end up as craters in the ground?”

“That’s possible,” Benedetta conceded. “Landings are difficult to automate. But the real problem was returning the probes to this tiny speck of rock from such a distance. The world is a very small target; the tolerances required for attitude and thrust control were close to the limits of what any of us believed was feasible. We were lucky to get even one back.”

“But the Peerless will be traveling much farther,” Serafina noted anxiously.

“With people inside it to navigate,” Yalda replied. “It won’t be down to clockwork to get them home.”

Leonia was unswayed. “Be that as it may, when you rehearsed your great project—on a much smaller scale—you only had one success in six dozen. And you were hoping to impress us by juggling voles! ”

The demonstration launch Yalda had arranged would carry six of the animals above the atmosphere, then bring them back down again—hopefully still alive. While clearly no surrogate for the flight of the Peerless , this was not a trivial achievement—and some people did find it reassuring to see that Eusebio’s rockets no longer exploded on launch, or cooked their passengers with the engine’s heat.

“What would persuade you, then?” Yalda demanded irritably. “A full-scale rehearsal, where we send up Mount Magnificent with a crew of arborines?”

Leonia responded to this sarcastically grandiose proposal with a much more modest alternative. “It might mean something if you went up yourself, instead of the voles.”

Before Yalda could reply, Benedetta said, “I’ll do it.”

Fatima emitted an anxious hum. “Are you serious?”