I turned to Sadiq. "You did some mine clearance once, didn't you?"

"Years ago." Before I could say anything more, he'd caught my drift. "You're thinking that might have been more satisfying? Bang, and it's gone, the proof is there in front of you?"

"One less mine, one less bomblet," I said. "However many thousands there were to deal with, at least you could tick each one off as a definite achievement."

"That's true. It was a good feeling." He shrugged. "But what should we do? Give up on this, because it's harder?"

He took the truck down the slope, then supervised the students as they attached the wisps of polymer to the specialized winch they'd built. Ha.s.san and Rashid were in their 20s, but they could easily have pa.s.sed for adolescents. After the war, the dictator and his former backers in the west had found it mutually expedient to have a generation of Iraqi children grow up malnourished and without medical care, if they grew up at all. More than a million people had died under the sanctions. My own sick joke of a nation had sent part of its navy to join the blockade, while the rest stayed home to fend off boatloads of refugees from this, and other, atrocities. General Mustache was long dead, but his comrades-ingenocide with more salubrious addresses were all still at large: doing lecture tours, running think tanks, lobbying for the n.o.bel peace prize.

As the strands of polymer wound around a core inside the winch's protective barrel, the alpha countrose steadily. It was a good sign: the fine particles of uranium oxide trapped by the Wall had remained bound to the polymer during dehydration, and the reeling in of the net. The radiation from the few grams of U-238 we'd collected was far too low to be a hazard in itself; the thing to avoid was ingesting the dust, and even then the unpleasant effects were as much chemical as radiological. Hopefully, the polymer had also bound its other targets: the organic carcinogens that had been strewn across Kuwait and southern Iraq by the apocalyptic oil well fires. There was no way to determine that until we did a full chemical a.n.a.lysis.

We were all in high spirits on the ride back. What we'd plucked from the wind in the last six weeks wouldn't spare a single person from leukemia, but it now seemed possible that over the years, over the decades, the technology would make a real difference.

I missed the connection in Singapore for a direct flight home to Sydney, so I had to go via Perth.

There was a four-hour wait in Perth; I paced the transit lounge, restless and impatient. I hadn't set eyes on Francine since she'd left Basra three months earlier; she didn't approve of clogging up the limited bandwidth into Iraq with decadent video. When I'd called her from Singapore she'd been busy, and now I couldn't decide whether or not to try again.

Just when I'd resolved to call her, an email came through on my notepad, saying that she'd received my message and would meet me at the airport.

In Sydney, I stood by the baggage carousel, searching the crowd. When I finally saw Francine approaching, she was looking straight at me, smiling. I left the carousel and walked toward her; she stopped and let me close the gap, keeping her eyes fixed on mine. There was a mischievousness to her expression, as if she'd arranged some kind of prank, but I couldn't guess what it might be.

When I was almost in front of her, she turned slightly, and spread her arms. "Ta-da!"

I froze, speechless. Why hadn't she told me?

I walked up to her and embraced her, but she'd read my expression. "Don't be angry, Ben. I was afraid you'd come home early if you knew."

"You're right, I would have." My thoughts were piling up on top of each other; I had three months'

worth of reactions to get through in 15 seconds. We hadn't planned this. We couldn't afford it. I wasn't ready.

Suddenly I started weeping, too shocked to be self-conscious in the crowd. The knot of panic and confusion inside me dissolved. I held her more tightly, and felt the swelling in her body against my hip.

"Are you happy?" Francine asked.

I laughed and nodded, choking out the words: "This is wonderful!"

I meant it. I was still afraid, but it was an exuberant fear. Another ocean had opened up before us.

We would find our bearings. We would cross it together.

It took me several days to come down to Earth. We didn't have a real chance to talk until the weekend; Francine had a teaching position at UNSW, and though she could have set her own research aside for a couple of days, marking could wait for no one. There were a thousand things to plan; the six-month UNESCO fellowship that had paid for me to take part in the project in Basra had expired, and I'd need to start earning money again soon, but the fact that I'd made no commitments yet gave me some welcome flexibility.

On Monday, alone in the flat again, I started catching up on all the journals I'd neglected. In Iraq I'd been obsessively single-minded, instructing my knowledge miner to keep me informed of work relevant to the Wall, to the exclusion of everything else.

Skimming through a summary of six months'worth of papers, a report in Science caught my eye: "An Experimental Model for Decoherence in the Many-Worlds Cosmology." A group at Delft University in the Netherlands had arranged for a simple quantum computer to carry out a sequence of arithmetic operations on a register which had been prepared to contain an equal superposition of binary representations of two different numbers. This in itself was nothing new; superpositions representing up to 128 numbers were now manipulated daily, albeit only under laboratory conditions, at close to absolutezero.

Unusually, though, at each stage of the calculation the qubits containing the numbers in question had been deliberately entangled with other, spare qubits in the computer. The effect of this was that the section performing the calculation had ceased to be in a pure quantum state: it behaved, not as if it contained two numbers simultaneously, but as if there were merely an equal chance of it containing either one. This had undermined the quantum nature of the calculation, just as surely as if the whole machine had been imperfectly shielded and become entangled with objects in the environment.

There was one crucial difference, though: in this case, the experimenters had still had access to the spare qubits that had made the calculation behave cla.s.sically. When they performed an appropriate measurement on the state of the computer as a whole, it was shown to have remained in a superposition all along. A single observation couldn't prove this, but the experiment had been repeated thousands of times, and within the margins of error, their prediction was confirmed: although the superposition had become undetectable when they ignored the spare qubits, it had never really gone away. Both cla.s.sical calculations had always taken place simultaneously, even though they'd lost the ability to interact in a quantum-mechanical fashion.

I sat at my desk, pondering the result. On one level, it was just a scaling-up of the quantum eraser experiments of the '90s, but the image of a tiny computer program running through its paces, appearing "to itself" to be unique and alone, while in fact a second, equally oblivious version had been executing beside it all along, carried a lot more resonance than an interference experiment with photons. I'd become used to the idea of quantum computers performing several calculations at once, but that conjuring trick had always seemed abstract and ethereal, precisely because the parts continued to act as a complicated whole right to the end. What struck home here was the stark demonstration of the way each calculation could come to appear as a distinct cla.s.sical history, as solid and mundane as the shuffling of beads on an abacus.

When Francine arrived home I was cooking dinner, but I grabbed my notepad and showed her the paper.

"Yeah, I've seen it," she said.

"What do you think?"

She raised her hands and recoiled in mock alarm.

"I'm serious."

"What do you want me to say? Does this prove the Many Worlds interpretation? No. Does it make it easier to understand, to have a toy model like this? Yes."

"But does it sway you at all?" I persisted. "Do you believe the results would still hold, if they could be scaled up indefinitely?" From a toy universe, a handful of qubits, to the real one.

She shrugged. "I don't really need to be swayed. I always thought the MWI was the most plausible interpretation anyway."

I left it at that, and went back to the kitchen while she pulled out a stack of a.s.signments.

That night, as we lay in bed together, I couldn't get the Delft experiment out of my mind.

"Do you believe there are other versions of us?" I asked Francine.

"I suppose there must be." She conceded the point as if it was something abstract and metaphysical, and I was being pedantic even to raise it. People who professed belief in the MWI never seemed to want to take it seriously, let alone personally.

"And that doesn't bother you?"

"No," she said blithely. "Since I'm powerless to change the situation, what's the use in being upset about it?"

"That's very pragmatic," I said. Francine reached over and thumped me on the shoulder. "That was a compliment!" I protested. "I envy you for having come to terms with it so easily."

"I haven't, really," she admitted. "I've just resolved not to let it worry me, which isn't quite the same thing."

I turned to face her, though in the near-darkness we could barely see each other. I said, "What gives you the most satisfaction in life?" "I take it you're not in the mood to be fobbed off with a soppy romantic answer?" She sighed. "I don't know. Solving problems. Getting things right."

"What if for every problem you solve, there's someone just like you who fails, instead?"

"I cope with my failures," she said. "Let them cope with theirs."

"You know it doesn't work like that. Some of them simply don't cope. Whatever you find the strength to do, there'll be someone else who won't."

Francine had no reply.

I said, "A couple of weeks ago, I asked Sadiq about the time he was doing mine clearance. He said it was more satisfying than mopping up DU ; one little explosion, right before your eyes, and you know you've done something worthwhile. We all get moments in our lives like that, with that pure, unambiguous sense of achievement: whatever else we might screw up, at least there's one thing that we've done right."

I laughed uneasily. "I think I'd go mad, if I couldn't rely on that."

Francine said, "You can. Nothing you've done will ever disappear from under your feet. No one's going to march up and take it away from you."

"I know." My skin crawled, at the image of some less favored alter ego turning up on our doorstep, demanding his dues. "That seems so f.u.c.king selfish, though. I don't want everything that makes me happy to be at the expense of someone else. I don't want every choice to be like...fighting other versions of myself for the prize in some zero-sum game."

"No." Francine hesitated. "But if the reality is like that, what can you do about it?"

Her words hung in the darkness. What could I do about it? Nothing. So did I really want to dwell on it, corroding the foundations of my own happiness, when there was absolutely nothing to be gained, for anyone?

"You're right. This is crazy." I leaned over and kissed her. "I'd better let you get to sleep."

"It's not crazy," she said. "But I don't have any answers."

The next morning, after Francine had left for work, I picked up my notepad and saw that she'd mailed me an e-book: an anthology of cheesy "alternate (sic) history" stories from the '90s, ent.i.tled My G.o.d, It's Full of Tsars! "What if Gandhi had been a ruthless soldier of fortune? What if Theodore Roosevelt had faced a Martian invasion? What if the n.a.z.is had had Janet Jackson's ch.o.r.eographer?"

I skimmed through the introduction, alternately cackling and groaning, then filed the book away and got down to work. I had a dozen minor administrative tasks to complete for UNESCO, before I could start searching in earnest for my next position.

By mid-afternoon, I was almost done, but the growing sense of achievement I felt at having buckled down and cleared away these tedious obligations brought with it the corollary: someone infinitesimally different from me-someone who had shared my entire history up until that morning-had procrastinated instead. The triviality of this observation only made it more unsettling; the Delft experiment was seeping into my daily life on the most mundane level.

I dug out the book Francine had sent and tried reading a few of the stories, but the authors'

relentlessly camp take on the premise hardly amounted to a reductio ad absurdum, or even a comical existential balm. I didn't really care how hilarious it would have been if Marilyn Monroe had been involved in a bedroom farce with Richard Feynman and Richard Nixon. I just wanted to lose the suffocating conviction that everything I had become was a mirage; that my life had been nothing but a blinkered view of a kind of torture chamber, where every glorious reprieve I'd ever celebrated had in fact been an unwitting betrayal.

If fiction had no comfort to offer, what about fact? Even if the Many Worlds cosmology was correct, no one knew for certain what the consequences were. It was a fallacy that literally everything that was physically possible had to occur; most cosmologists I'd read believed that the universe as a whole possessed a single, definite quantum state, and while that state would appear from within as a mult.i.tude of distinct cla.s.sical histories, there was no reason to a.s.sume that these histories amounted to some kind of exhaustive catalog. The same thing held true on a smaller scale: every time two people sat down to a game of chess, there was no reason to believe that they played every possible game. And if I'd stood in an alley, nine years before, struggling with my conscience? My subjective sense of indecision proved nothing, but even if I'd suffered no qualms and acted without hesitation, to find a human being in a quantum state of pure, unshakable resolve would have been freakishly unlikely at best, and in fact was probably physically impossible.

"f.u.c.k this." I didn't know when I'd set myself up for this bout of paranoia, but I wasn't going to indulge it for another second. I banged my head against the desk a few times, then picked up my notepad and went straight to an employment site.

The thoughts didn't vanish entirely; it was too much like trying not to think of a pink elephant. Each time they recurred, though, I found I could shout them down with threats of taking myself straight to a psychiatrist. The prospect of having to explain such a bizarre mental problem was enough to give me access to hitherto untapped reserves of self-discipline.

By the time I started cooking dinner, I was feeling merely foolish. If Francine mentioned the subject again, I'd make a joke of it. I didn't need a psychiatrist. I was a little insecure about my good fortune, and still somewhat rattled by the news of impending fatherhood, but it would hardly have been healthier to take everything for granted.

My notepad chimed. Francine had blocked the video again, as if bandwidth, even here, was as precious as water.

"h.e.l.lo?"

"Ben? I've had some bleeding. I'm in a taxi. Can you meet me at St. Vincent's?"

Her voice was steady, but my own mouth went dry. "Sure. I'll be there in 15 minutes." I couldn't add anything: I love you, it will be all right, hold on. She didn't need that, it would have jinxed everything.

Half an hour later, I was still caught in traffic, white-knuckled with rage and helplessness. I stared down at the dashboard, at the real-time map with every other gridlocked vehicle marked, and finally stopped deluding myself that at any moment I would turn into a magically deserted side-street and weave my way across the city in just a few more minutes.

In the ward, behind the curtains drawn around her bed, Francine lay curled and rigid, her back turned, refusing to look at me. All I could do was stand beside her. The gynecologist was yet to explain everything properly, but the miscarriage had been accompanied by complications, and she'd had to perform surgery.

Before I'd applied for the UNESCO fellowship, we'd discussed the risks. For two prudent, well-informed, short-term visitors, the danger had seemed microscopic. Francine had never traveled out into the desert with me, and even for the locals in Basra the rates of birth defects and miscarriages had fallen a long way from their peaks. We were both taking contraceptives; condoms had seemed like overkill. Had I brought it back to her, from the desert? A speck of dust, trapped beneath my foreskin? Had I poisoned her while we were making love?

Francine turned toward me. The skin around her eyes was gray and swollen, and I could see how much effort it took for her to meet my gaze. She drew her hands out from under the bedclothes, and let me hold them; they were freezing.

After a while, she started sobbing, but she wouldn't release my hands. I stroked the back of her thumb with my own thumb, a tiny, gentle movement.

2020.

"How do you feel now?" Olivia Maslin didn't quite make eye contact as she addressed me; the image of my brain activity painted on her retinas was clearly holding her attention.

"Fine," I said. "Exactly the same as I did before you started the infusion."

I was reclining on something like a dentist's couch, halfway between sitting and lying, wearing a tight-fitting cap studded with magnetic sensors and inducers. It was impossible to ignore the slight coolness of the liquid flowing into the vein in my forearm, but that sensation was no different than it had been on the previous occasion, a fortnight before.

"Could you count to ten for me, please." I obliged.

"Now close your eyes and picture the same familiar face as the last time."

She'd told me I could choose anyone; I'd picked Francine. I brought back the image, then suddenly recalled that, the first time, after contemplating the detailed picture in my head for a few seconds-as if I was preparing to give a description to the police-I'd started thinking about Francine herself. On cue, the same transition occurred again: the frozen, forensic likeness became flesh and blood.

I was led through the whole sequence of activities once more: reading the same short story ("Two Old-Timers" by F. Scott Fitzgerald), listening to the same piece of music (from Rossini's The Thieving Magpie), recounting the same childhood memory (my first day at school). At some point, I lost any trace of anxiety about repeating my earlier mental states with sufficient fidelity; after all, the experiment had been designed to cope with the inevitable variation between the two sessions. I was just one volunteer out of dozens, and half the subjects would be receiving nothing but saline on both occasions. For all I knew, I was one of them: a control, merely setting the baseline against which any real effect would be judged.

If I was receiving the coherence disruptors, though, then as far as I could tell they'd had no effect on me. My inner life hadn't evaporated as the molecules bound to the microtubules in my neurons, guaranteeing that any kind of quantum coherence those structures might otherwise have maintained would be lost to the environment in a fraction of a picosecond.

Personally, I'd never subscribed to Penrose's theory that quantum effects might play a role in consciousness; calculations dating back to a seminal paper by Max Tegmark, 20 years before, had already made sustained coherence in any neural structure extremely unlikely. Nevertheless, it had taken considerable ingenuity on the part of Olivia and her team to rule out the idea definitively, in a series of clear-cut experiments. Over the past two years, they'd chased the ghost away from each of the various structures that different factions of Penrose's disciples had anointed as the essential quantum components of the brain. The earliest proposal-the microtubules, huge polymeric molecules that formed a kind of skeleton inside every cell-had turned out to be the hardest to target for disruption. But now it was entirely possible that the cytoskeletons of my very own neurons were dotted with molecules that coupled them strongly to a noisy microwave field in which my skull was, definitely, bathed. In which case, my microtubules had about as much chance of exploiting quantum effects as I had of playing a game of squash with a version of myself from a parallel universe.

When the experiment was over, Olivia thanked me, then became even more distant as she reviewed the data. Raj, one of her graduate students, slid out the needle and stuck a plaster over the tiny puncture wound, then helped me out of the cap.

"I know you don't know yet if I was a control or not," I said, "but have you noticed significant differences, with anyone?" I was almost the last subject in the microtubule trials; any effect should have shown up by now.

Olivia smiled enigmatically. "You'll just have to wait for publication." Raj leaned down and whispered, "No, never."

I climbed off the couch. "The zombie walks!" Raj declaimed. I lunged hungrily for his brain; he ducked away, laughing, while Olivia watched us with an expression of pained indulgence. Die-hard members of the Penrose camp claimed that Olivia's experiments proved nothing, because even if people behaved identically while all quantum effects were ruled out, they could be doing this as mere automata, totally devoid of consciousness. When Olivia had offered to let her chief detractor experience coherence disruption for himself, he'd replied that this would be no more persuasive, because memories laid down while you were a zombie would be indistinguishable from ordinary memories, so that looking back on the experience, you'd notice nothing unusual.

This was sheer desperation; you might as well a.s.sert that everyone in the world but yourself was a zombie, and you were one, too, every second Tuesday. As the experiments were repeated by other groups around the world, those people who'd backed the Penrose theory as scientific hypothesis, rather than adopting it as a kind of mystical dogma, would gradually accept that it had been refuted.

I left the neuroscience building and walked across the campus, back toward my office in the physics department. It was a mild, clear spring morning, with students out lying on the gra.s.s, dozing off with books balanced over their faces like tents. There were still some advantages to reading from old-fashioned sheaves of e-paper. I'd only had my own eyes chipped the year before, and though I'd adapted to the technology easily enough, I still found it disconcerting to wake on a Sunday morning to find Francine reading the Herald beside me with her eyes shut.

Olivia's results didn't surprise me, but it was satisfying to have the matter resolved once and for all: consciousness was a purely cla.s.sical phenomenon. Among other things, this meant that there was no compelling reason to believe that software running on a cla.s.sical computer could not be conscious. Of course, everything in the universe obeyed quantum mechanics at some level, but Paul Benioff, one of the pioneers of quantum computing, had shown back in the '80s that you could build a cla.s.sical Turing machine from quantum mechanical parts, and over the last few years, in my spare time, I'd studied the branch of quantum computing theory that concerned itself with avoiding quantum effects.

Back in my office, I summoned up a schematic of the device I called the Qusp: the quantum singleton processor. The Qusp would employ all the techniques designed to shield the latest generation of quantum computers from entanglement with their environment, but it would use them to a very different end. A quantum computer was shielded so it could perform a mult.i.tude of parallel calculations, without each one sp.a.w.ning a separate history of its own, in which only one answer was accessible. The Qusp would perform just a single calculation at a time, but on its way to the unique result it would be able to pa.s.s safely through superpositions that included any number of alternatives, without those alternatives being made real. Cut off from the outside world during each computational step, it would keep its temporary quantum ambivalence as private and inconsequential as a daydream, never being forced to act out every possibility it dared to entertain.

The Qusp would still need to interact with its environment whenever it gathered data about the world, and that interaction would inevitably split it into different versions. If you attached a camera to the Qusp and pointed it at an ordinary object-a rock, a plant, a bird-that object could hardly be expected to possess a single cla.s.sical history, and so neither would the combined system of Qusp plus rock, Qusp plus plant, Qusp plus bird.

The Qusp itself, though, would never initiate the split. In a given set of circ.u.mstances, it would only ever produce a single response. An AI running on the Qusp could make its decisions as whimsically, or with as much weighty deliberation as it liked, but for each distinct scenario it confronted, in the end it would only make one choice, only follow one course of action.

I closed the file, and the image vanished from my retinas. For all the work I'd put into the design, I'd made no effort to build the thing. I'd been using it as little more than a talisman: whenever I found myself picturing my life as a tranquil dwelling built over a slaughter house, I'd summon up the Qusp as a symbol of hope. It was proof of a possibility, and a possibility was all it took. Nothing in the laws of physics could prevent a small portion of humanity's descendants from escaping their ancestors' dissipation.

Yet I'd shied away from any attempt to see that promise fulfilled, firsthand. In part, I'd been afraid of delving too deeply and uncovering a flaw in the Qusp's design, robbing myself of the one crutch that kept me standing when the horror swept over me. It had also been a matter of guilt: I'd been the one granted happiness, so many times, that it had seemed unconscionable to aspire to that state yet again. I'd knocked so many of my hapless cousins out of the ring, it was time I threw a fight and let the prize go to my opponent instead.

That last excuse was idiotic. The stronger my determination to build the Qusp, the more branches there would be in which it was real. Weakening my resolve was not an act of charity, surrendering the benefits to someone else; it merely impoverished every future version of me, and everyone they touched.