“No,” said Rigg. “Not even close. He can speed up his own perceptions, so time seems to go slower, and he can speed up other people’s. He did that to me—that’s how I first learned that all these paths were really people. The facemask does it for me now—but it can’t speed up anybody else’s perceptions. And that business with just appearing to people? That’s Umbo. I latch on to a person’s path and there I am. He sends a message somehow. He’s the really powerful one, if he only learned to master it, but who can help him? What he does, nobody else does.”

“But once he gives a warning, doesn’t he have to be sure to give that same warning when he gets to that point in the future?” asked Ram.

“Hasn’t an expendable explained that to you?” asked Rigg. “Because of Umbo’s warning, we never get to that point in the future. It doesn’t exist. Everything that happened down that road—the network of cause and effect—it’s gone. It never happens.”

“So he never sends the message,” said Ram Odin, shaking his head.

“I remember thinking the same way,” said Rigg. “But what we’ve learned by experience is that an effect can’t undo its own cause. So the old future just disappears. Never happens. And I can never time-shift to a lost time because it leaves no paths in our past.”

“So why didn’t you just disappear when you warned yourself—or I should say, warned Noxon?”

“Because I didn’t warn Noxon. I’m not Umbo. I couldn’t just appear, I came. The moment I changed the course of events, Noxon would no longer become the me who had killed you. But that wouldn’t erase me—because I had caused the change in Noxon’s path, so I had to remain.”

“I hope I’m not the only one to whom this makes no sense.”

“It’s causality. Conservation of causality. Umbo’s messages can cause things in the past to change in such a way that it destroys the future in which he sends those messages. Like the messages from the Odinfold time-senders—because they’re sending an object, not going themselves. But a shifter like me and Noxon and Umbo, too, when he actually travels—I can’t go to the past and change it and destroy myself, because I am the causer, still in that past, persisting into the change I made. It makes sense, really.”

“No, it doesn’t,” said Ram Odin. “But if you think you understand it, that’s good enough for me.”

They rested a little while on the lumpy bed—the master bed of the house, the best bed, thought Rigg—long enough to make it convincing. Then they came back out of the room. “I know where the button is,” said Rigg. “Let’s go get it.”