“What makes you think they could stop us if we want to go?” asked Umbo.

“Getting onto a human starship isn’t the same as going through the Wall,” said Rigg.

“We can do things with time,” said Param, “but we can’t fly.”

“Maybe we could use the Odinfolder technology to put something on board their ship,” said Umbo. “A plague, maybe. Something that kills them all. But we show the Visitors who are on Garden what happened to their ship, and then we take them back in time before we implanted the plague, so that they’ll understand that we could kill them but we chose not to.”

“How would that make them not want to destroy us?” asked Loaf. “That’s the point I’m not getting. Because I think that’s a sure way to guarantee that they send the Destroyers.”

Umbo shrugged and turned away, a little angry. Rigg was so tired of the way Umbo took offense at any slight, while he felt no compunction about slighting Rigg at every opportunity. The only thing that had kept them from open quarrels during these many months was the fact that they were able to avoid each other most of the time.

“It’s not a stupid idea,” said Olivenko. “We just need to refine it.”

“We can’t use any version of it,” said Rigg. “As soon as the expendables realize what we’ve done, the orbiters destroy our wallfold. We aren’t allowed to develop weapons.”

“It’s a disease,” said Umbo, “not a weapon.”

“If we send it to their ship in order to kill people, it’s a weapon, and we get blown to smithereens,” said Rigg.

“You’re such an expert on how the ships’ computers think?” said Umbo.

“No, you are,” said Rigg.

Umbo’s lips tightened, but he didn’t argue with Rigg’s point. Umbo knew more than anyone about how the original starship worked, and in fact the computers would not be fooled by a sophistry like, It’s a disease, not a weapon.

“Maybe we just need to study more,” said Param.

“No,” said Umbo. “We have a deeper problem than the fact that if we went to Earth, we couldn’t travel back in time to when we were on Garden. We don’t even know if our time skills even work off the surface of Garden.”

“Why wouldn’t they?” asked Olivenko.

“Think about it,” said Umbo. “We don’t understand anything about how we’re able to travel back in time—or how Param can make microjumps into the future, skipping the moments in between. But we do know some obvious things about the rules of time-shifting. It’s absolutely tied to the surface of the planet.”

“It worked fine when we flew to the Wall with Vadesh,” said Param.

“Really? Did you try any time-skipping in flight?” asked Umbo.

Param bristled. “We jumped off a rock once, if you remember.”

“We were never more than two meters from solid stone,” said Umbo.

“It’s a good question,” said Rigg, “but the flyer isn’t a real test, anyway, because it’s still tied to the gravity well of Garden. The real problem is this: Garden is flying through space as it orbits our sun. The whole solar system is also moving rapidly through space. Say we travel back in time by six months. In that amount of time, Garden has moved completely around the sun to the opposite side. Yet we travel back, not to where we were in absolute space, which would kill us instantly, but to where we were in relation to the surface of Garden. Our time-shifting is tied to the planet. So Umbo’s asking, what happens if we leave the surface of Garden and go to another planet? Do we even have time-shifting ability there? Or is our time-shifting still relative to the surface of Garden? If we’re on Earth, in a certain position millions of kilometers away from Garden, and travel back in time, do we end up in exactly that position relative to Garden? Because Earth and Garden move so differently from each other, that we’d end up in cold deep airless space if we’re still tied to Garden.”

Umbo glared at him. Rigg couldn’t imagine why. Hadn’t Rigg just defended Umbo’s argument? There was no figuring out what made anybody work. But now Rigg had a whole bunch of new stories to help him understand. Among the Mongols, Temujin and Jamuka had been blood brothers, but they became bitter enemies on the way to Temujin becoming Khan and taking the name Genghis, or Chinggis. It was part of human nature that best friends could easily become rivals and then deadly foes. Rigg would count himself successful if he could keep it at the level of rivalry without ever letting Umbo become his enemy.

“I think it’s obvious,” said Olivenko, “that it’s tied to whatever planet you’re on.”

“I don’t think anything’s obvious,” said Rigg. “Whatever we decide, we’re betting our lives on it. All the paths I can see are actually views into the past—I see the actual people and animals going through all the movements of their lives, and they’re tied to Garden. But they’re all people who were born here, who lived their whole lives here. And think of when we went downriver, Loaf, Umbo—when I was a prisoner in the cabin of that boat, I tried to catch on to the paths of previous travelers, and I couldn’t, because their paths hung in the air over open water, and I could only reach them for a moment or two as our boat passed under them. It might work that way no matter how far we get from Garden—paths just hanging there in space, long after the ship is gone.”

“But the original pilot, Ram Odin,” said Umbo, “he had time gifts. That’s where all our abilities come from. And he did time stuff when he was in a ship in space.”

“Yet the ship was displaced nineteen times,” said Rigg. “Doesn’t that tell you something? During the microseconds when the ship’s nineteen computers were separately calculating and activating the jump, the whole ship had moved far enough in space that Ram’s unconscious time-jump reached nineteen different places. We can’t go into space and use our time-shifting ability, or we’ll just create duplicate ships.”

“We don’t know that,” said Olivenko.

“But we don’t know it won’t happen. Or worse,” said Rigg. “Please remember that when I suggested going back to Earth with the Visitors, I wasn’t counting on the idea that we could keep ourselves out of trouble by using time-jumping. For all we know, that’ll be a sure ticket to our deaths. My idea was to go back, to make the attempt, and if they kill us, then they kill us.”

“Well, there’s this,” said Olivenko. “Even if that happened, and we couldn’t save ourselves—or rather, you couldn’t save us with your time-shifting—the Odinfolders would send another book into the past and tell them—and us—that having us get on the Visitors’ ship was a very bad idea and we shouldn’t do it.”

Param laughed. “So the fact that they haven’t already received such a Future Book proves that we succeeded?”

“Or that we decided not to do it,” said Rigg.

“Or that we did it, and failed, but the Odinfolders decided not to show us the Future Book that resulted, and just went on to try something else.”

“Or they gave up,” said Param, “and just decided to die.”

“No matter how much we learn,” said Umbo, “we never know enough.”

“All we can do is what we’ve always done,” said Rigg. “Make a try at something, and then if it doesn’t work, go back and try again. But we can’t always go back.”

Umbo leapt to his feet, “Right, there are things that stay terrible. For instance, Rigg, that you’ve completely forgotten about going back to save my brother Kyokay’s life.”

It struck Rigg like a knife, that this was part of what Umbo still held against him. “We already decided that we can’t, because if we did, then we’d never have gotten together to learn how to manipulate time.”

“But now we know more, we have more control, we could figure out a way to catch him partway down maybe, or—”

“Maybe we can,” said Rigg. “Maybe we can put in a net to catch him, or train a giant bird to snatch him out of the air, or a huge puff of air to blow him out to sea. But we’ll do it later—go back and save one boy after we’ve figured out how to save the whole world.”