Roi understood that at least some of this was a logical necessity, not a matter of coincidence. The two points of view, one tied to the rock of the Splinter, the other taking a grand cosmic perspective, were describing exactly the same reality, so of course they had to agree with each other, once you knew how to translate between them. Nonetheless, she couldn’t accept that the fact of the Splinter’s rotation could be both crucial and completely invisible, impalpable, and immeasurable.
She said, “When I throw a stone in the Null Chamber, why can’t I see its path being bent?”
“It’s a subtle effect,” Zak said. “I’ve made some crude measurements of it, but it’s hard to detect just by looking.”
“You’ve measured the Splinter’s rotation!” Roi was astonished. “Why didn’t you tell me that before?”
“I wouldn’t say I’d measured the rotation. My measurements show that the rotation exists, but I haven’t come close to quantifying the rate of spin.”
“But you’ve seen the effect itself?”
“Absolutely,” Zak said.
“Can you show me?”
They went to the Null Chamber, and Zak fished out a device he called a spring-shot from one of the storage clefts. It was a tube fitted with a spring-loaded plunger that could be cocked to varying degrees of compression and then released, propelling a stone from the tube. The projectile emerged in a reasonably predictable direction, with a choice of velocities.
He attached the spring-shot to the wire that marked the Null Line. Then he prepared a “target board”, a flat sheet of cuticle that he coated first with resin, then with a kind of powder. If you pushed a stone against the surface of the target, however gently, the powder sank into the resin, changing the way the two of them scattered the light, leaving a visible record of the point of contact.
He fixed the target board to the Null Line with a bracket, six spans or so rarb of the spring-shot.
“We’re sending this stone straight down the Null Line, so according to the map it should feel no weight at all,” Zak said. “First I’ll make it move as fast as possible, and we’ll see what happens.”
He squeezed the plunger as tightly as it would go, then released it. The stone flew out rapidly, more or less following the wire that marked the Null Line, and struck the target. When they went to examine the board, it was, unsurprisingly, marked at the edge, just beside the wire.
“Now we reduce the speed.”
Roi said, “Now I’m confused. I thought this weight was supposed to increase with speed.”
“It does. But greater speed also gives it less time to act before the stone hits the target. By making the stone move more slowly, the weight that bends its path becomes weaker, but the extra time the stone spends in flight more than compensates for that.”
Zak was right. With the spring half-compressed, the stone emerged less rapidly, but the mark it made was shifted to the sard side of the Null Line by about twice the width of the stone itself. In a third experiment, with the compression reduced further, the sardwards shift was even more pronounced.
Roi could picture what was happening very clearly, now. While the stone was in flight, the Splinter was turning, carrying the wire and the target board a small way garmwards, leaving the un-rotated path of the stone to strike the target askew.
She said, “Why can’t we use this to measure the speed of the rotation?”
“It’s a crude experiment,” Zak insisted. “If I release the stone several times in succession with the spring compressed by exactly the same amount, the point where it strikes the target still varies. And how can I know how fast the stone is moving? It’s traveling too quickly for me to time its motion accurately.”
“Let it move more slowly, then. You’ll get a larger effect as well.”
“There’s a catch,” Zak said ruefully. “The more the stone gets pushed away from the Null Line, the more it comes under the sway of the ordinary garm-sard weight as well. What we’re measuring will no longer be one simple thing. When you combine that with the uncertainties in the aim and the velocity, I don’t think there’s much hope of getting a meaningful number out of the results.”
Roi could see how daunting the complications were, but she wasn’t ready to give up. “Can I try it? Slowly? Just to see what happens?”
“Of course.”
She squeezed the plunger down to the first notch, the smallest compression possible, then released it. The stone emerged at an absurdly leisurely pace, and even as she watched, it veered visibly sardwards. By the time it had traveled less than half a span toward the target its path had turned sideways, and not long after that it had swung around so far that it was level with the spring-shot again, albeit some distance sard of it. While continuing to move sardwards, its velocity along the Null Line had been completely reversed.
Roi said, “This is not what I’d expected.”
“It’s just following the rules,” Zak said.
Roi moved aside to let the stone pass her. Eventually, its sardwards drift seemed to level out, and it was simply moving backward, parallel to the Null Line, much faster than it had been moving when it had left the tube. Its direction continued to change, though; the relentless sideways tug of the weight of motion was stronger for this stone than it was for the wind, and it began to veer back toward the Null Line.
As the stone approached the Null Line, the sharqwards part of its motion slowed, leveled out, and reversed, so it was now heading back toward the spring-shot. This didn’t last long, though. When it reached the Null Line itself, almost grazing it, the stone executed a small loop that took it first sardwards, and then—in a replay of its original manoeuvre upon being launched—swung it around sharqwards once more. It was far behind the spring-shot, let alone the target, and showed no sign that it would ever come close to either again. Rather, it seemed to be cycling back and forth between the Null Line and a certain distance sardwards, while it drifted—mostly, though not constantly—ever more sharqwards.
Roi approached Zak. “What’s the simple explanation? I suppose I can accept that the sardwards weight combined with the weight of motion did all of that, but there must be an easier way to understand it.”
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.