Watch

“No. When you die, you’re gone.”

“So does DoubleAllC, or even just plain old Always Cooperate, really make sense for a Unitarian—for someone who doesn’t believe there’s a reward to be had in an afterlife? I mean, DoubleAllC and AllC can’t win unless they’re playing against people using the same strategy. And you obviously aren’t —not in the scenario described: you’ve been struck on one cheek first, so you know you’re playing against someone who defects at least part of the time. In what game-theory way does turning the other cheek make sense? I mean, presumably the other guy is just going to hit you again.”

Her mother lifted her eyebrows. “Ah, but see, you’re missing something. The easiest games to model are two-person games, but real life is an n-person game: it involves a large and variable number of players. You might lose a lot to one person, but gain more than you expected from someone else. Person A might be cruel to you, but person B, seeing that, might be even more kind to you because of it. And when you’re playing with a lot of people, the game goes on indefinitely— and that makes a huge difference. The examples in the Old Testament couldn’t be endlessly iterated: an eye for an eye can only go two rounds—after that, you’re out of eyes. Even a tooth for a tooth ends after a maximum of thirty-two rounds.”

Caitlin took a sip of milk, and her mother went on. “That’s the problem with two-person iterated games: they eventually come to an end. Sometimes they end because, like with the dollar auction, players just give up because it’s become ridiculous. And sometimes they end because the players run out of time.

“In fact, there was a famous case of a game theorist being brought in to IBM to do some management- training exercises. He divided the managers into teams and had them play games in which cooperating was the best strategy—which was the point he wanted them to learn.

“Everything worked fine until just before 4:00 p.m., when the seminar was scheduled to end. Suddenly, one of the teams turned on the other and kept defecting. That team won, but the first team felt so betrayed, IBM had to send its members off for therapy, and it was months before they’d work at all with members of the other team again.”

“Wow,” said Caitlin.

“But if you take the whole of humanity as the field of players, then your interaction doesn’t end even if any one specific player drops out. That’s why reputation is so important, see? You’ve bought things on eBay, right? Well, that’s a perfect example: how you’ve treated other people shows up in your Feedback rating. The world knows if you defect. We’re all interconnected in a…”

“…a worldwide web?” said Caitlin.

She smiled. “Exactly.” She gobbled the last of her sandwich. “Speaking of which, we should get back upstairs…”

“All right,” said Tony Moretti, pacing down one side of the control room at WATCH. “Reports. Shel, you first.”

Shelton Halleck was leaning forward in his chair, his arms crossed in front of him on the workstation, the one with the snake tattoo on top. He was plainly exhausted. “We’ve been through everything Caitlin Decter has written with a fine-toothed comb,” he said. “And everything Malcolm and Barbara Decter and Kuroda have written, too, but there’s nothing about how Exponential actually works—nothing that contradicts what Decter told the CSIS agents, but nothing that confirms it, either.”

“All right,” said Tony. “Aiesha, what have you got?”

She looked more awake than Shel, but her voice was raw. “Maybe something, maybe nothing,” she said. “Caitlin set up a webcam chat with an Internet cartographer at the Technion a while ago: Anna Bloom is her name.” A dossier came up on the middle of the three big screens, showing a picture of an elderly gray-haired woman. “We weren’t monitoring Caitlin’s traffic back then, so we don’t have a recording of the video chat—but I can’t think of any reason for a girl in Canada to talk to a Web scientist in Israel except to discuss the structure of Exponential.”

“We could get the Mossad to speak to this Bloom,” said Tony. “The Technion is in Jerusalem?”

“No, Haifa,” Aiesha said. She turned and looked at the series of digital clocks on the back wall. “It’s almost 11:00 p.m. there.”

“There’s no time to waste,” Colonel Hume said. “Let me call her directly—one computer expert to another. It’s time to cut through all the bull.”

Caitlin’s instant messenger bleeped and the words Mind-Over-Matter is now available popped up. She felt her heart racing.

Hi, she typed.

Hey! Matt replied. How was your day?

Fine, ty.

I’ve got the stuff from your locker, he replied. OK if I come by?

Caitlin was surprised to find her heart pounding. She paused, trying to think of something suitably witty or sexy to say, but then she mentally kicked herself for hesitating, because poor Matt must have been on tenterhooks. Sure! she wrote, and then, to take the sting out of her delay, she added a trio of smiley faces.

W00t! he wrote. ’Bout half an hour, OK?

This time she replied immediately: Yes.

Heading out. *poof *

Caitlin crossed the hall to speak to her mother, who was typing away with Webmind in her study.

“A friend’s coming over,” Caitlin said.

Her mother looked up from her keyboard. “Who is it?”

Caitlin found herself slightly embarrassed. “They were in my math class.”

But the pronoun obfuscation did not get past her mom. “It’s a boy,” she said at once.

“Um, yes.”

“Is it Trevor?”

“No! Don’t worry, Mom. He won’t be back.”

“Well, okay,” she said, and—

And there it was, that look she’d seen before: her mother trying to suppress a grin. “But, sweetheart,” she added, “you might want to clean yourself up a bit.”

Cripes! She’d been so intent on Webmind that she hadn’t brushed her hair today, and she looked down now and saw that she was wearing just about the rattiest T-shirt she owned. And— gak!—she hadn’t showered for two days. She hurried down the hall to the bathroom.

thirty-five

The doorbell rang, and Caitlin found herself running to it. She was now wearing a silky blue shirt—one her mother said was too low-cut for school. But she was not going to school anymore; she was pleased with her impeccable logic. Her shoulder-length brown hair was still wet, but at least she’d brushed it.

She opened the door. “Hi, Matt!”

And—wow!—boy’s eyes really did do that. She’d read about it, but hadn’t yet seen it: straight to the boobs, and only apparently with an effort of will coming up to the face.

His voice cracked. It was so cute! “Hi, Caitlin!”

He had a—a sack, or something in his right hand. “Here’s your stuff,” he said, setting it down on the tiled floor.

“Thanks!”