Neel cuts in: “Where’d you grow up?””Palo Alto,” she says. From there to Stanford to Google: for a girl obsessed with the outer limits of human potential, Kat has stayed pretty close to home. Neel nods knowingly. “The suburban mind cannot comprehend the emergent complexity of a New York sidewalk.””I don’t know about that,” Kat says, narrowing her eyes. “I’m pretty good with complexity.””See, I know what you’re thinking,” Neel says, shaking his head.”You’re thinking it’s just an agent-based simulation, and everybody out here follows a pretty simple set of rules”– Kat is nodding–“and if you can figure out those rules, you can model it. You can simulate the street, then the neighborhood, then the whole city. Right?””Exactly. I mean, sure, I don’t know what the rules are yet, but I could experiment and figure them out, and then it would be trivial–” “Wrong,” Neel says, honking like a game-show buzzer. “You can’t do it. Even if you know the rules– and by the way, there are no rules–but even if there were, you can’t model it. You know why?”My best friend and my girlfriend are sparring over simulations. I can only sit back and listen. Kat frowns. “Why?””You don’t have enough memory.””Oh, come on–“”Nope. You could never hold it all in memory. No computer’s big enough. Not even your what’s-it-called–“”The Big Box.””That’s the one. It’s not big enough. This box–” Neel stretches out his hands, encompasses the sidewalk, the park, the streets beyond–“is bigger.”The snaking crowd surges forward.