Lloyd tried to make his voice sound gentle. "There seems to be a lot of supporting evidence from other people's visions that you really are dead in 2030, Theo."

Theo opened his mouth, as if to protest, but then he closed it. A moment later, he spoke again. "You're right. You're right. Sorry."

Lloyd nodded; he hadn't really realized before just how hard all of this must be on Theo. He turned and looked at della Robbia. "Well, Franco, if the visions weren't of our future, then what did they portray?"

"An alternative timeline, of course. That's completely reasonable, given MWI." The many-worlds interpretation of quantum physics says that every time an event can go two ways, instead of one or the other way happening, both happen, each in a separate universe. "Specifically, the visions portray the universe that split from this universe at the moment of your LHC experiment; they show the future as it is in a universe in which the time-displacement effect did not occur."

But Lloyd was shaking his head. "You don't still believe in MWI, do you? TI demolishes that."

A standard argument in favor of the many-worlds interpretation is the thought experiment of Schrodinger's cat: put a cat in a sealed box with a vial of poison that has a fifty-fifty chance of being triggered during a one-hour period. At the end of the hour, open the box and see if the cat is still alive. Under the Copenhagen interpretation — the standard version of quantum mechanics — until someone looks in, the cat is supposedly neither alive nor dead, but rather a superposition of both possible states; the act of looking in — of observing — collapses the wave function, forcing the cat to resolve itself into one of two possible outcomes. Except that, since the observation could go two ways, what MWI proponents say really happens is that the universe splits at the point at which the observation is made. One universe continues on with a dead cat; the other, with a living one.

John G. Cramer, a physicist who had often done work at CERN, but was normally with the University of Washington, Seattle, disliked the Copenhagen interpretation's emphasis on the observer. In the 1980s, he proposed an alternative explanation: TI, the transactional interpretation. During the nineties and aughts, TI had become increasingly popular amongst physicists.

Consider Schrodinger's hapless cat at the moment it is sealed in the box, and the observer's eye, at the moment, an hour later, that it looks upon the cat. In TI, the cat sends out an actual, physical "offer" wave, which travels forward into the future and backward into the past. When the offer wave reaches the eye, the eye sends out a "confirmation" wave, which travels backward into the past and forward into the future. The offer wave and the confirmation wave cancel each other out everywhere in the universe except in the direct line between the cat and the eye, where they reinforce each other, producing a transaction. Since the cat and the eye have communicated across time, there is no ambiguity, and no need for collapsing wave fronts: the cat exists inside the box exactly as it will eventually be observed. There's also no splitting of the universe into two; since the transaction covers the entire relevant period, there's no need for branching: the eye sees the cat as it always was, either dead or alive.

"You would like TI," said della Robbia. "It demolishes free will. Every emitted photon knows what will eventually absorb it."

"Sure," said Lloyd, "I admit that TI reinforces the block-universe concept — but it's your many-worlds interpretation that really demolishes free will."

"How can you possibly say that?" said della Robbia, with expressive Italian exasperation.

"There's no hierarchy among the many worlds," said Lloyd. "Say I'm walking along and come to a fork in the road. I could go left, or I could go right. Which one do I choose?"

"Whichever one you want!" crowed della Robbia. "Free will!"

"Nonsense," said Lloyd. "Under MWI, I choose whichever one the other version of me didn't choose. If he goes right, I have to go left; if I go right, he has to go left. And only arrogance would lead one to think that it was always my choice in this universe that was considered, and that it was always the other choice that was simply the alternative that had to be expressed in another universe. The many-worlds interpretation gives the illusion of choice, but it's actually completely deterministic."

Della Robbia turned to Theo, spreading his arms in an appeal for common sense. "But TI depends on waves that travel backward in time!"

Theo's voice was gentle. "I think we've now abundantly demonstrated the reality of information traveling backward in time, Franco," he said. "Besides, what Cramer actually said was the transaction occurs atemporally — outside of time."

"And," said Lloyd, warming to the fight now that he had an ally, "your version of what happened is the one that demands time travel."

Della Robbia looked stunned. "What? How? The visions simply portray a parallel universe."

"Any parallel MWI universes that might exist would surely be moving in temporal lockstep with ours: if you could see into a parallel universe, you'd still see today, April 26, 2009; indeed, the whole concept of quantum computing depends on parallel universes being precisely in lockstep with ours. So, yes, if you could see into a parallel universe, you might see a world in which you'd gone over to sit down with Michael Burr, over there, instead of with me and Theo, but it'd still be now. What you're suggesting is adding contact with parallel universes on top of seeing into the future; it's hard enough to accept one of those ideas without also having to accept the other, and — "

Jake Horowitz had appeared at their table. "Sorry to interrupt," he said, "but there's a call for you, Theo. Says it's about your posting on the Mosaic web site."

Theo hurried away from the table, abandoning his half-eaten kebab. "Line three," said Jacob, trailing behind him. There was an empty office just outside the lunch room; Theo ducked in. The phone's caller ID simply said "Out of Area." He picked up the handset.

"Hello," he said. "Theo Procopides here."

"My God," said the male voice, in English, at the other end of the phone. "This is weird — talking to somebody you know is going to be dead."

Theo didn't have any response for that, so he simply said, "You have some information about my murder?"

"Yes, I think so. I was reading something about it in my vision."

"What did it say?"

The man recounted the gist of what he'd read. There were no new facts.

"Was there anything about survivors?" asked Theo.

"How do you mean? It wasn't a plane crash."

"No, no, no. I mean, did it say anything about who survived me — you know, about whether I had a wife or kids."

"Oh, yeah. Let's see if I can remember… "

See if I can remember. His future was all incidental; nobody really cared. It wasn't important, wasn't real. Just some guy they'd read about.

"Yeah," said the voice. "Yeah, you'll be survived by a son and by your wife."

"Did the paper give their names?"

The person blew air into the mouthpiece of his phone as he thought. "The son was — Constantin, I think."

Constantin. His father's name; yes, Theo had always thought he might name a son that.

"And the boy's mother? My wife?"

"I'm sorry. I don't remember."

"Please try."

"No, I'm sorry. I just don't remember."

"You could undergo hypnosis — "

"Are you crazy? I'm not going to do that. Look, I called you up to help you out; I figured I'd do you a good turn, you know? I just thought it'd be a nice thing to do. But I'm not going to be hypnotized, or pumped full of drugs, or anything like that."