Incandescence

Zak said, «Think about the orbit of that stone. The stone was always on the sardside of the Null Line, so its whole orbit was larger than the orbit of the center of the Splinter. Larger orbits have longer periods, so the stone took longer than we did to go around the Hub. That's why it drifted backward. It wasn't as fast as us.»

«But it started out faster,» Roi protested.

«That's true. At the same distance from the Hub, where its orbit touched ours, it was faster than us. That's why we weren't constantly outpacing it, and it didn't move backward all the time. But over a complete orbit, we were faster.»

This made sense, but Roi still wasn't satisfied. «Why didn't the stones you launched go backward? Was that because they were moving faster than mine?»

«Definitely not!» Zak was emphatic. «The only difference was, they hit the target before they could swing around and go into reverse. If we had taken the target away — and the walls of the chamber too, if necessary — then those stones would have followed exactly the same kind of path as your one. The fact that they were moving faster made their paths larger, and we only saw a small part of each path, but other than that everything about them was the same.»

«I see.» The whole point of Zak's version had been to concentrate on the very start of the motion, before the garm-sard weight could complicate things. «Can I try something else?»

«Anything,» Zak said.

She detached the spring-shot, then reattached it pointing in the opposite direction: sharq along the Null Line. Now the stone would be moving backward from the start, making it slower than the Splinter at the point where their orbits touched.

Its path followed exactly the same pattern as before, except that rarb became sharq, and sard became garm. After leaving the spring-shot the stone veered garmwards; halted its leisurely sharqwards progress and went rapidly into reverse; reached a maximum distance garmwards from the Null Line and started back toward it; then, close to the Null Line, performed a small loop that took it back into the same cycle, albeit many spans rarb of where it had begun.

Roi said, «Its orbit was smaller than ours, so it was racing ahead of us?»

«Yes.»

«And the way it moved away from the Null Line and then back again, that's because the orbit wasn't a perfect circle?»

«Right,» Zak said. «We remain a constant distance from the Hub, but there are orbits like this that draw closer to the Hub and then move away again.»

Roi contemplated this. «What if we could put a stone into an orbit that wasn't a perfect circle, but was still the same size as ours, overall? With the same period?»

Zak didn't reply immediately, but his posture made it clear that he was intrigued by her suggestion. «That could be very useful,» he said eventually. «We ought to see it execute a fixed, cyclic motion instead of running away across the chamber.»

Roi detached the spring-shot from the Null Line again, and attached it pointing sardwards: perpendicular to the Null Line, «halfway between» the two directions she'd already tried. She was acting purely on instinct, and even as she tightened the clips she wondered if launching the stone away from the Hub meant she'd be putting it into an orbit that would keep it perpetually on one side of the Null Line. But then, shooting the stone along the Null Line itself, which seemed more symmetrical in that respect, certainly didn't work, so what she was trying made as much sense as anything.

She squeezed the plunger one notch, then released it.

The stone veered sideways as it emerged, but not as sharply as it had in the previous two experiments. As it moved, it picked up pace, but nowhere near as rapidly as before. Roi was surprised; she'd half expected the sardwards weight to take over and drag the stone into a frenzied spiral as the weight of motion twisted its ever-quickening flight. Instead, the stone continued to turn in a smooth, shallow arc, still progressing sardwards while swinging around ever more to the sharq.

Eventually, its sardwards motion leveled off, about two spans from the Null Line. It was moving perhaps three times faster now than it had when she'd launched it. It continued to swing around gently, coming back toward the Null Line, while its sharqwards speed lessened.

As it approached the Null Line, Roi tensed. It was no longer traveling sharqwards, but it would probably perform the same annoying little loop as the others, and then it would be lost to them, drifting away across the chamber.

It didn't. It crossed straight over the Null Line, at about the same speed as it had left the spring-shot, and veered to the rarb. The symmetry was unmistakable: it was performing exactly the same kind of motion as it had when she'd fired it, only with garm in place of sard and rarb in place of sharq. If that symmetry held true, there was only one place where it could cross the Null Line again.