"Well," said Kyle, "there's no recorded case of anyone ever being killed by a meteor falling on them, but it could could happen. So, is there a universe in which I was killed that way yesterday? Another one in which I was killed that way the day before? A third in which I was killed that way the day before that? A fourth, fifth, and sixth in which it was my brother, not me, who was killed? A seventh, eighth, and ninth in which both of us were killed on those days by meteor impacts?" happen. So, is there a universe in which I was killed that way yesterday? Another one in which I was killed that way the day before? A third in which I was killed that way the day before that? A fourth, fifth, and sixth in which it was my brother, not me, who was killed? A seventh, eighth, and ninth in which both of us were killed on those days by meteor impacts?"

Cheetah did not hesitate. "No."

"Why not?"

"Because meteors have no volition-in every universe, precisely the same meteors. .h.i.t the Earth."

"All right," said Kyle, "but say one crashes today in-I don't know-say in Antarctica. Now, I've never been to Antarctica, and I never intend to go there, but is there some parallel universe in which I did go, and in which I happened to be killed by that meteor? And then aren't there seven billion times as many universes, accounting for all the people alive who might instead have gone to Antarctica?"

"It does seem rather an awful lot of parallel universes, doesn't it?" said Cheetah.

"Exactly. In which case there must be some sort of filtration process-something that distinguishes between conceivable universes and plausible ones, between those that we simply can imagine and those that have some reasonable chance of actually existing. That could explain why we only got one other factor back in the experiment."

"I suppose you're right and-oh."

"What?" said Kyle.

"I see what you're getting at."

Kyle was surprised; he wasn't sure he himself knew what he was getting at. "And that is?"

"The ethics of the many-worlds interpretation."

Kyle considered. "You know, I guess you're right. Say I find a wallet that contains an unlocked SmartCash card with a thousand dollars on it. Say the wallet also has a driver's license in it; I've got the rightful owner's name and address right there."

Cheetah had a cross-shaped pattern of LEDs on his console. He could activate the vertical column of them or the horizontal row to simulate either nodding or shaking his head. He did his nod.

"Well," said Kyle, "according to the many-worlds interpretation, anything that can possibly go two ways does does go two ways. There's a universe in which I return the money to the person who lost it, but there's also a universe in which I keep it for myself. Now, if there are bound to be two universes, then why the heck go two ways. There's a universe in which I return the money to the person who lost it, but there's also a universe in which I keep it for myself. Now, if there are bound to be two universes, then why the heck shouldn't shouldn't I be the guy who keeps the money?" I be the guy who keeps the money?"

"An intriguing question, and without impugning your character, such a dilemma does seem within the realm of possibility. But I suspect your moral concerns run deeper: I suspect you're wondering about you and Rebecca. Even if in this universe you didn't molest her, you're wondering if there is some conceivable universe in which you did." did."

Kyle slumped back in his chair. Cheetah was right. For once, the G.o.dd.a.m.ned machine was right.

It was an insidious thing, the human mind. The mere accusation was enough to get it working, even against itself.

And was was there such a universe? A universe where he really could creep into his own daughter's room after midnight and do those horrible things to her? there such a universe? A universe where he really could creep into his own daughter's room after midnight and do those horrible things to her?

Not here, of course. Not in this this universe. But in another one-one, perhaps, where he hadn't got tenure, where his control over life had slipped away, where he drank more than he should, where he and Heather were still fighting to keep the wolf from the door-or where they had divorced early on, or he was a widower, and his own s.e.xuality was finding no normal outlet. universe. But in another one-one, perhaps, where he hadn't got tenure, where his control over life had slipped away, where he drank more than he should, where he and Heather were still fighting to keep the wolf from the door-or where they had divorced early on, or he was a widower, and his own s.e.xuality was finding no normal outlet.

Could such a universe exist? Could Becky's memories, although false in this universe, be a true reflection of another reality? Could she now have access, through some quantum aberration, to those memories from a parallel world, just as a quantum computer accesses information from other timelines?

Or was the very notion that he'd abuse his daughter utterly outlandish, impossible, unthinkable-a meteor conking him on the head in the Antarctic?

Kyle stood up and did something he'd never done before. He lied to Cheetah.

"No," he said. "No, you're completely wrong about that."

He left the lab, the lights shutting off automatically as he did so.

Maybe, some thought, the Centaurs had simply skipped one day for a holiday on their homeworld, or to indicate some sort of punctuation in the overall message. If that were the case, the next message would come in at 6:36 P.M. the following day, Friday, July 28.

Heather had spent much of the thirty-one intervening hours dealing with reporters; overnight, the alien messages had gone from being of no general interest to front-page news worldwide. And now the CBC was doing a live remote feed from Heather's office.

The news crew had provided a large digital clock, which was attached to the top of Heather's monitor with masking tape. They'd brought three cameras: one was kept trained on Heather, another on the clock, and the third on her monitor screen.

The clock was counting down. It was now two minutes to the scheduled time for the next message.

"Professor Davis," said the black female reporter, who had a pleasant Jamaican accent, "what are you thinking? What are you feeling as we wait for another message from the stars?"

Heather had done five other TV appearances over the last thirty-one hours, but she'd yet to come up with an answer she was happy with. "I don't really know," she said, trying to follow the reporter's instruction not to look directly into the camera. "I feel like I've lost a friend. I never did know what he was saying, but he was there, every day. I could count on him. I could trust him. And now that's shattered."

As she said that, she wondered if Kyle was watching.

"Twenty seconds," said the reporter.

Heather turned to look at the computer monitor.

"Fifteen."

She raised her left hand, fingers crossed.

"Ten."

It couldn't be finished.

"Nine."

It couldn't have come to an end.

"Eight."

Not after all this time.

"Seven."

Not after a decade.

"Six."

Not without an answer.

"Five."

Not without the key.

"Four."

Not with it still remaining a mystery.

"Three."

Her heart was pounding.

"Two."

She closed her eyes and astonished herself to find that she was thinking a silent prayer.

"One."

Heather opened her eyes, focused on the screen.

"Zero."

Nothing. It was over.

11.

Heather pushed the door buzzer outside Kyle's lab. There was no response. She touched her thumb to the scanning plate, wondering for a moment whether he'd delisted her from the index. But the door slid aside, and she entered the lab.

"Is that you, Professor Davis?"

"Oh, h.e.l.lo, Cheetah."

"It's been some time since you've dropped by. It's good to see you."

"Thanks. Is Kyle around?"

"He had to go down to Professor Montgomery's office; he said he would be back shortly."

"Thanks. I'll wait, if that's- Good grief, what's that?"

"What's what?" asked Cheetah.

"That poster. It's Dali, isn't it?" The style was unmistakable, but it was a Dali she'd never seen before: a painting of Jesus nailed to a most unusual cross.

"That's right," said Cheetah. "Dr. Graves says it's been exhibited under several names, but it's best known as 'Christus hypercubus.' Christ on the hypercube."

"What's a hypercube?"

"That is," said Cheetah. "Well, actually it's not a real hypercube. Rather, it's an unfolded one." One of the monitors on Cheetah's angled console lit up. "Here's another picture of one." The screen displayed this: [Picture A]

"But what the heck is it?" asked Heather.

"A hypercube is a four-dimensional cube. It's sometimes also called a tesseract."

"What did you mean a moment ago when you said it was 'unfolded'?"

Cheetah's lenses whirred. "That's an intriguing question, actually. Dr. Graves has told me about hypercubes. He uses them in his first-year computing cla.s.s; he says it helps students learn to visualize problems in a new way." Cheetah's cameras swiveled as he looked around the room. "See that box on the shelf there?"

Heather followed Cheetah's line of sight. She nodded.

"Pick it up."

Heather shrugged a little, then did so.

"Now that's a cube," said Cheetah. "Use your fingernail to pull the tab out of the slot. See it?"

Heather nodded again. She did as Cheetah asked, and the box started to come apart. She continued to unfold it, then laid it out on the tabletop: six squares forming a cross-four in a row, plus two sticking off the sides of the third one.

"A cross," said Heather.

Cheetah's LEDs nodded. "Of course, it doesn't have to be-there are eleven fundamentally different ways you can unfold a cube, including into a T shape and an S shape. Well, not that that cube-it's cut and scored for unfolding in that particular way. Anyway, that's an unfolded cube-a flat, two-dimensional plan that can be folded through the third dimension to make a cube." Cheetah's eyes swiveled back toward the Dali painting. "The cross in the painting consists of eight cubes-four making the vertical shaft, and four more making the two mutually perpendicular sets of arms. That's an unfolded tesseract: a three-dimensional plan that could be folded through the fourth dimension to make a hypercube." cube-it's cut and scored for unfolding in that particular way. Anyway, that's an unfolded cube-a flat, two-dimensional plan that can be folded through the third dimension to make a cube." Cheetah's eyes swiveled back toward the Dali painting. "The cross in the painting consists of eight cubes-four making the vertical shaft, and four more making the two mutually perpendicular sets of arms. That's an unfolded tesseract: a three-dimensional plan that could be folded through the fourth dimension to make a hypercube."

"Folded how? In what direction?"

"As I said, through the fourth dimension, which is perpendicular to the other three, just as height, length, and width are perpendicular to each other. In fact, there are two two ways to fold up a hypercube, just as you could fold that two-dimensional piece of cardboard either up or down-up resulting in the shiny, white side of the cardboard making up the outside, and down resulting in the dull, plain side making up the outside. All dimensions have two directions: length has left and right; depth has forward and backward; height has up and down. And the fourth dimension, it has ways to fold up a hypercube, just as you could fold that two-dimensional piece of cardboard either up or down-up resulting in the shiny, white side of the cardboard making up the outside, and down resulting in the dull, plain side making up the outside. All dimensions have two directions: length has left and right; depth has forward and backward; height has up and down. And the fourth dimension, it has ana ana and and kata." kata."

"Why those terms?"

"Ana is Greek for up; is Greek for up; kata kata is Greek for down." is Greek for down."

"So if you fold a group of eight cubes like those in the Dali painting in the kata kata direction, it makes a hypercube?" direction, it makes a hypercube?"

"Yes. Or in the ana ana direction." direction."

"Fascinating," said Heather. "And Kyle finds this kind of thinking helps his students?"

"He thinks so. He had a professor named Papineau when he was a student here twenty years ago-"

"I remember him."

"Well, Dr. Graves says he doesn't recall much of what Papineau taught him, except that he was always finding ways to expand his students' minds, giving them new ways of looking at things. He is trying to do something similar for his students today, and-"

The door slid open. Kyle walked in. "Heather!" he said, clearly surprised. "What are you doing here?"

"Waiting for you."

Without a word, Kyle reached over and flicked Cheetah's SUSPEND switch. "What brings you by?"

"The alien messages have stopped."

"So I'd heard. Was there a Rosetta stone at the end?"