“Our best calculation,” said the expendable, “is that as ­hydrogen dust is consumed in the engines of the outbound ship, it powers the movement of both ships, the outbound and the inbound.”

“So matter is crossing over.”

“No,” said the expendable. “It remains entirely in the normal timeflow.”

“Then energy is crossing.”

“No,” said the expendable. “Energy and matter are the same thing, in a fusion engine.”

“Something is crossing over,” said Ram Odin impatiently.

“As near as we can tell,” said the expendable, “and that’s not very near, because there is no way to measure this, the only thing crossing over is momentum.”

“Is momentum actually a thing, in physics?” asked Noxon.

“It is not,” said the expendable, “at least not the way we’re using the term. Our hypothesis is that there’s some unknown force binding the subatomic particles to each other, forward and backward.”

“You know what?” asked Ram Odin. “I don’t actually care how it works. I only know that when and if we do break free and jump back into ordinary time, we’ll be hurtling toward Earth in reverse. That’s another argument in favor of making the switch before we come too close to Earth.”

“Yet if we try it too far out, we can’t bring the ship with us,” said Noxon.

“That’s all we were doing,” said a mouse. “Trying to figure these things out in advance.”

“I believe you completely,” said Noxon to the mice.

“No you don’t,” said the mouse.

“Apparently you don’t believe me,” said Noxon.

“I will alert you by standing exactly here and raising my arm like this when we’re approaching Earth but are still well back from it. Maybe a week or two out. Would that be good?”

“Yes,” said Noxon. “Make it three weeks. I want to see when I start getting a sense of paths that are tied to Earth rather than to spaceships. And that means I need to start looking well before the point of no return.”

“So shall we slice time?” asked Ram Odin. “I’m looking forward to my first actual time-shift.”

“You just had one.”

“Only an hour or so. And I didn’t feel anything.”

“You won’t feel anything. Nothing in the ship will change, unless he moves around. We’ll be moving, of course, but if I do this right, we won’t complete even a single step.”

“Then let’s do it,” said Ram Odin. “We have nothing to lose but seven years of mind-numbing boredom.”

“All right,” said Noxon, and he took a step as he sliced forward.

Chapter 15: Building a House

CHAPTER 15

Building a House

Singhfold wasn’t all up-and-down, mountains and valleys—there was a coastal plain, and some high plateaus. But the level ground was in the rain shadow of the mountains, and those who lived there scratched out a living by damming the occasional streams and laboriously irrigating the fields.