He led her across the web, to a tube anchored firmly to several wires. “The axis of this cylinder runs from shomal to junub; the midpoint is precisely on the Null Line.” He took two stones from his carapace, then placed one gently in the mouth of the tube, and the other beside the tube’s midpoint. Both floated where he left them, showing no immediate evidence of motion.
“What do you think will happen?” he asked Roi.
She thought carefully. “The stone that’s a little bit shomal will have a little bit of weight, slowly pulling it toward the Null Line. So given time, it will fall down to the Null Line.”
“So let’s wait and see.”
To help pass the time, Zak asked Roi to recount the details of her journey, and they chatted about the different work teams she’d seen, the changes in vegetation from place to place, the rumors of food shortages. As they talked, the first stone did indeed gradually descend into the tube, while the second one remained where Zak had placed it.
As the moving stone approached the fixed one at the midpoint of the tube, Roi said, “I was right, wasn’t I?”
“Keep watching,” Zak insisted.
The stone didn’t stop at the center. It kept traveling slowly down the tube, away from the Null Line.
“But there is no weight at the Null Line!” Roi said. “If you’re exactly at the Null Line, you should go nowhere. You don’t start falling junub!” She gestured at the other stone, which continued to float where Zak had left it.
“Move away from the Null Line, and throw a stone across it,” Zak suggested. He pointed to the slender wire that marked the invisible line, then handed her a projectile.
Roi complied, bracing herself on a cross-wire. The stone didn’t quite touch the wire, but it came close before sailing smoothly past it.
“It kept going,” she mused. That didn’t surprise her; she hadn’t really expected the Null Line to magically rob the missile of its velocity. So why had she been surprised that the stone that had fallen under its own weight, rather than being tossed, had also kept moving?
She went back to watching the stone in the tube. Eventually, it reached the mouth opposite the one where Zak had placed it. She waited for it to emerge from the tube, but again her guess proved ill-founded. Moving just as slowly as it had at the start, it now reversed its motion and began to fall back into the tube.
“A little junub,” she said, “means a little weight back toward the Null Line again. And somehow, it all balances out. When the stone first crosses the Null Line, the weight begins to act in reverse, but not enough to halt it completely, only to begin to slow it down. Only when it reaches as far junub as it began shomal does it halt completely. And then it starts the very same motion again, in reverse.”
“Right,” said Zak. “But where does this beautiful pattern come from? The weight, the motion, two stones coming together and moving apart?”
“I have no idea,” Roi confessed.
“What comes back to itself, over and over? What repeats itself, endlessly?”
Roi was blank for a moment. “A circle?”
“Yes.”
“I don’t see any circle.”
Zak rummaged in his right cavity and fished out a loop of wire. “When we throw a stone here, it looks as if it follows a straight line. But how can we really be sure? Before long it hits the wall of the chamber, and we can’t know where it would have gone if it hadn’t been stopped. So imagine that the way things move when there’s nothing to interfere with them isn’t always a perfectly straight line. Suppose that if they’re tossed in the right direction, they travel around and around, in a very big circle.”
Roi was perplexed. “How big? As big as the Splinter?”
“Much bigger than that. Imagine a circle so big that you could follow a part of it from one side of the Splinter to the other and not even notice the curve, not even see that it was not a straight line.”
Roi’s mind reeled. Crossing from one side of the Splinter to the other. then stretching out far into the Incandescence?
She said, “I understand that we couldn’t tell the difference from a straight line. But even if it’s true, how would we ever know? Why should we believe it?”
Zak said, “Because of this.”
He produced a second loop of wire and held it together with the first, in such a way that the two circles shared a common center. They did not coincide completely, though; a small angle separated them, so they only actually touched at two points. “Imagine two stones, moving along two circles like these. They come together, they pass each other, they move apart, then they come together again. Over and over.”
Roi pictured it, following two points around the two loops. It was half true: the
“How do you know that?” Zak countered.
“Because they’re right in front of me! They’re not moving away from me!”
“And how do you know you’re not moving, yourself?”
This was becoming surreal. Roi replied patiently, “Because then I’d slam into the wall of the chamber.”
“And how do you know that wall isn’t moving? How do you know the whole Splinter isn’t moving?”
Roi tensed her limbs ready to reply, then found she had nothing to say.
Zak said, “I believe the Splinter is moving in a very large circle, around a point far away in the Incandescence. When we let two stones, displaced from each other along the shomal-junub axis, move without interference, they come together, then move apart, just as if they were following two such circles inclined at a small angle to each other. And I believe that the time it takes for the stones to complete their cycle tells us how long it takes for the Splinter to make one orbit around that distant point.”
Roi looked back at the stones. The first one, the “moving” one, had almost returned to the midpoint of the tube. If Zak was right, though, then they were both moving in exactly the same way. There really was no difference between them.
She said, “Then what’s the Null Line? Why is it special?”
“The Null Line is a piece of the circle that the center of the Splinter traces out as it moves,” Zak replied.
“But why are things weightless only
Zak gestured at the two stones. “Both of these are equally weightless, because both are free to move along their natural paths. The only real difference between them is that the one at the Null Line has the whole Splinter keeping step with it, so we think of that one as ‘fixed’ and the other as ‘falling’.
“When you’re shomal of the Null Line, your unhindered path would be just like the path of the ‘falling’ stone, but the floors of the tunnels and chambers won’t permit you to follow that path, and their refusal is what you feel as weight. The further shomal you are, the greater the difference between your preferred and actual motion, the harder the rock has to push up on you to keep you shomal, and so the greater your weight.”
Roi turned these propositions over in her mind, torn between skepticism and amazement. She was not yet convinced that Zak was right, but she was beginning to see how his grand vision of weight and motion might fit together.
“What about garm and sard?” she said. She struggled to picture Zak’s extraordinary cosmology: shomal and junub pointed out of the plane of the circle that the Splinter followed, and the directions along the Null Line, rarb and sharq, pointed along that circular path. “Is the center on the garmside or the sardside?” she asked.
“The garmside.”
So garm and sard pointed toward, and away from, the center of the circle. “If I’m garm of the Null Line,” Roi mused, “but not shomal or junub, the Splinter will still carry me in a circle around the same point. And the same is true if I’m sard. So why can’t those circles count as natural motion, just like moving along the Null Line?”
“This is where things become trickier,” Zak admitted. “I claim that following a circle around one special point comprises natural motion, but I believe that’s only true if you’re traveling at the right speed. That speed depends on how large the circle is.
“The center of the Splinter must be following its natural path—otherwise objects at the Null Line would not
