Incandescence

scatter the fungus as widely as possible, but the fluid dynamics is all wrong for any plausible stellar wind that might have blown into this system.”

The way ahead was blocked by a cloud of rubble again. Rakesh braked and let himself float in the middle of the tunnel.

He said, “This thing has no engine. How do you think they got it clear of the planet when the neutron star came through?”

Parantham shrugged. “They might have taken it up in small pieces, and used the fungus to weld them together.”

“That’s assuming they had rockets at all,” Rakesh said.

“Well, yes. If they didn’t, they could have simply built it on the ground and let the tidal force lift it. That would have been a very risky strategy, though.”

“Where would the safest launching site be?” Rakesh glanced at a model and answered his own question, “The point furthest from the neutron star at the moment when its tidal force canceled the planet’s gravity. Assuming that it survived the quakes, the ark would simply drift up into space.”

Parantham said, “It wouldn’t have had much of a head start, though, before all the rocks came tumbling after it. Collisions between the debris would redistribute its momentum, creating some fragments that would outrace the pack. You couldn’t avoid a serious peppering, at the very least.”

“I suppose they could have made more than one ark, to improve the odds,” Rakesh suggested. “The others might have been destroyed by debris, or captured by the neutron star.”

Parantham let out a long, reproachful moan. “Captured by the neutron star?”

Rakesh was bemused. “You don’t think that’s possible?”

“Of course it’s possible. And that’s exactly what they wanted! This one’s the failure, the one that was left behind!”

“How is it a failure to get left behind?”

“The giants’ stellar wind,” she said, “has a greater energy density than middle-aged starlight, but there’s something that would give it even more oomph: the gravitational field of a neutron star. The neutron star would have drawn the wind into an accretion disk around it, far richer in energy than anything else in sight. The Arkmakers saw this monster coming, and thought: if it’s going to pulverize our home, better to learn to drink from that whirlpool than skulk around in the ruins waiting for the next disaster.

“This ark, and everything in it, was designed to survive in an accretion disk. The asymmetrical flow-through would have given it a kind of buoyancy, pushing it back out into larger orbits if it ever sank in too deep.” Parantham ran a model, and piped the output to Rakesh. “The wind in the disk would have been strong enough to keep the fungus alive almost everywhere, to support the food chain throughout the ark.”

Rakesh absorbed the model’s results. Parantham’s conclusions were hard to dispute.

“So this place was starved from the beginning?” he said. “When they missed the neutron star, they had no hope?” The children of the Arkmakers, designed to escape the fate of their planet-bound parents, had found themselves stranded with the wrong biology, trapped inside an ingenious machine for extracting energy from an exotic new source that was receding into the distance at a few hundred kilometers a second.

Parantham said, “No hope for themselves. But I can’t believe this was the only ark. There could have been a dozen, there could have been a thousand. If they really saw no prospect of fleeing from the neutron star, every resource on the planet would have been used to maximize the chances of hitching a ride.”

Rakesh looked around at the ruins of this desperate strategy, and tried to picture the same tunnels teeming with life while the hot wind from a neutron star’s accretion disk whistled through the walls. Perhaps the extraordinary gamble could have paid off, if they’d repeated it a sufficient number of times.

“If they hitched a ride, where did it take them?” Rakesh asked. When he and Parantham had first realized what it was that had created the asteroid belt, they had run dynamical models and checked the maps, but they’d been unable to locate the neutron star that had done the deed. The only thing that had been clear was the general direction of its motion.

“Toward the center,” Parantham replied. “Deeper into the core.”

12

As the work team gathered in the Calculation Chamber, Roi caught sight of Neth and proclaimed hopefully, “Sixth time brings success!”

“Sixth?” Neth replied. “Surely this is the third?”

“It’s one task to frame a hypothesis, then another to test it,” Roi insisted. “So that’s six separate acts.”

Neth was too polite to object, and perhaps too serious to understand that Roi was only joking. If the proverb was worth anything, it certainly wasn’t worth taking literally. It did encourage persistence, though, and Roi had a feeling that their persistence was finally going to be rewarded.

Since Neth’s discovery that orbits around the Hub might become unstable, a dozen or so members of Zak’s original team had left to educate hatchlings into the secrets of weight and motion, and a dozen more had headed for the sardside, with the even more ambitious aim of recruiting a new team to build Bard’s tunnel. The task of those who remained was to find a geometry for space and time that satisfied Zak’s principle, in the hope of learning more about the dangers the Splinter would face in the future.

Tan had refined his ideas for characterizing geometry to the point where he could calculate the natural paths—the closest things to straight lines—on any curved surface. The vital step that remained, though, was to find the correct way to move from the geometry of space alone to a version that included time.

When Tan analyzed a path on a curved surface, he broke it up into a multitude of tiny, straight line segments of equal length. These small straight lines acted as markers for the direction of the curve. The geometry of the surface could then be embodied in a simple mathematical rule that Tan called a “connection”. The connection allowed you to take a direction at one point and shift it to another, nearby point, in a manner that respected the geometry of the surface. If a curve was a natural path, then when you broke it up into line segments and used the connection to shift them all one step forward, the shifted segments would coincide with the originals: shifting the first segment one step along the curve would give you the original direction of the second segment, and so on. If the curve was not a natural path, then the directions would fail to agree, and the resulting discrepancies would be a measure of how much the curve swerved unnecessarily, as opposed to merely following the geometry.

That the curves were broken into line segments of equal length was a crucial part of the recipe, because the analysis had to yield the same verdict if the surface was picked up and rotated, or if two people were viewing it from different angles. If you decreed that the curve should be broken up some other way, such as into segments that spanned equal horizontal distances, then different people would be left arguing over which direction was “horizontal”. Nobody would argue as to whether two successive segments were of equal length. With the connection respecting this rule—preserving the lengths of the segments as it moved them from point to point—everything worked smoothly, and everyone agreed on which paths constituted natural motion and which did not.

What happened, though, when you considered the path of a tossed stone, moving forward in time as well as through space? Anyone could draw a picture in which some chosen direction represented time, and the path of a moving object slanted across the skin, but how could people ever agree on the correct scale for such a diagram? Whether one heartbeat, one shift, or one lifetime passed from the top of the skin to the bottom was a completely arbitrary choice.

Nevertheless, suppose you settled on a scale. What would happen if you divided the path of a stone into segments of equal length? To Roi, who tossed a stone forward across the Null Chamber at one span per heartbeat, the path she drew would slant across the skin. If Zak happened already to be moving at the same pace in the same direction, the stone would be motionless to him, so he would draw a line that stretched solely in the time direction. Suppose that after five heartbeats, the stone hit an obstacle. Zak’s line would be “five heartbeats” long, whatever the scale of the picture made that. Roi’s line, though, would have to be longer: it would stretch five heartbeats in the time direction, but it would also cross five spans of space. The accounts of the two