Snow turned and watched her for a long time. He remembered how it had been in the tent as she, he realized, discovered that she was still human.

"You know, I will never grow old and die," she said.

"I see."

She tilted her head questioningly and awaited his answer.

A slow smile spread across his face. "I'll come with you," he told her. He put his drink down and reached out to take hold of her hand. There was still blood under her fingernails and the tear duct in her left eye was not working properly. It didn't matter.

Singleton

GREG EGAN.

Greg Egan (www.nets.p.a.ce.net.au/~gregegan), who lives in Perth, West Australia, hit his stride in the early 1990s, and became one of the most interesting new hard SF writers of the decade. He is internationally famous for his stories and novels. His early fiction was supernatural horror, and his first novel ( An Unusual Angle -not SF) was published in 1983, but his writing burst into international prominence in 1990, with several fine SF stories that focused attention on his writing. His SF novels to date are Quarantine (1992), Permutation City (1994), Distress (1995), Diaspora (1997), Teranesia (1999), and Schild's Ladder (2002); his short story collections are Our Lady of Chern.o.byl (1995), Axiomatic (1995), and Luminous (1999). He quit his job as a computer programmer to write full time in 1992. As of 2000, he had become the flagship hard SF writer of the younger generation.

"Singleton," from Interzone , is a hard SF quantum computer story. Egan a.s.sumes an Everett-Wheelertype quantum mechanics-usually referred to as the Many Worlds Interpretation-but meticulously sets up a mechanism by which an individual AI could subvert the branching. But it is also a philosophical story on a human scale about making choices and accepting consequences. A married couple, a physicist and a mathematician, want to have a child, and they have one never before imagined in SF. There is enough material for a novel compressed into this provocative novella.

2003.

I was walking north along George Street toward Town Hall railway station, pondering the ways I might solve the tricky third question of my linear algebra a.s.signment, when I encountered a small crowd blocking the footpath. I didn't give much thought to the reason they were standing there; I'd just pa.s.sed a busy restaurant, and I often saw groups of people gathered outside. But once I'd started to make my way around them, moving into an alley rather than stepping out into the traffic, it became apparent that they were not just diners from a farewell lunch for a retiring colleague, putting off their return to the office for as long as possible. I could see for myself exactly what was holding their attention.

Twenty meters down the alley, a man was lying on his back on the ground, shielding his bloodied face with his hands, while two men stood over him, relentlessly swinging narrow sticks of some kind. At first I thought the sticks were pool cues, but then I noticed the metal hooks on the ends. I'd only ever seen these obscure weapons before in one other place: my primary school, where an appointed window monitor would use them at the start and end of each day. They were meant for opening and closing an old-fashioned kind of hinged pane when it was too high to reach with your hands.

I turned to the other spectators. "Has anyone called the police?" A woman nodded without lookingat me, and said, "Someone used their mobile, a couple of minutes ago."

The a.s.sailants must have realized that the police were on their way, but it seemed they were too committed to their task to abandon it until that was absolutely necessary. They were facing away from the crowd, so perhaps they weren't entirely reckless not to fear identification. The man on the ground was dressed like a kitchen hand. He was still moving, trying to protect himself, but he was making less noise than his attackers; the need, or the ability, to cry out in pain had been beaten right out of him.

As for calling for help, he could have saved his breath.

A chill pa.s.sed through my body, a sick cold churning sensation that came a moment before the conscious realization: I'm going to watch someone murdered, and I'm going to do nothing. But this wasn't a drunken brawl, where a few bystanders could step in and separate the combatants; the two a.s.sailants had to be serious criminals, settling a score. Keeping your distance from something like that was just common sense. I'd go to court, I'd be a witness, but no one could expect anything more of me.

Not when 30 other people had behaved in exactly the same way.

The men in the alley did not have guns. If they'd had guns, they would have used them by now. They weren't going to mow down anyone who got in their way. It was one thing not to make a martyr of yourself, but how many people could these two grunting slobs fend off with sticks?

I unstrapped my backpack and put it on the ground. Absurdly, that made me feel more vulnerable; I was always worried about losing my textbooks. Think about this. You don't know what you're doing.

I hadn't been in so much as a fist fight since I was 13. I glanced at the strangers around me, wondering if anyone would join in if I implored them to rush forward together. But that wasn't going to happen. I was a willowy, unimposing 18-year-old, wearing a T-shirt adorned with Maxwell's Equations. I had no presence, no authority. No one would follow me into the fray.

Alone, I'd be as helpless as the guy on the ground. These men would crack my skull open in an instant. There were half a dozen solid-looking office workers in their 20s in the crowd; if these weekend rugby players hadn't felt competent to intervene, what chance did I have?

I reached down for my backpack. If I wasn't going to help, there was no point being here at all. I'd find out what had happened on the evening news.

I started to retrace my steps, sick with self-loathing. This wasn't kristallnacht. There'd be no embarra.s.sing questions from my grandchildren. No one would ever reproach me.

As if that were the measure of everything.

"f.u.c.k it." I dropped my backpack and ran down the alley.

I was close enough to smell the three sweating bodies over the stench of rotting garbage before I was even noticed. The nearest of the attackers glanced over his shoulder, affronted, then amused. He didn't bother redeploying his weapon in mid-stroke; as I hooked an arm around his neck in the hope of overbalancing him, he thrust his elbow into my chest, winding me. I clung on desperately, maintaining the hold even though I couldn't tighten it. As he tried to prize himself loose, I managed to kick his feet out from under him. We both went down onto the asphalt; I ended up beneath him.

The man untangled himself and clambered to his feet. As I struggled to right myself, picturing a metal hook swinging into my face, someone whistled. I looked up to see the second man gesturing to his companion, and I followed his gaze. A dozen men and women were coming down the alley, advancing together at a brisk walk. It was not a particularly menacing sight-I'd seen angrier crowds with peace signs painted on their faces-but the sheer numbers were enough to guarantee some inconvenience. The first man hung back long enough to kick me in the ribs. Then the two of them fled.

I brought my knees up, then raised my head and got into a crouch. I was still winded, but for some reason it seemed vital not to remain flat on my back. One of the office workers grinned down at me.

"You f.u.c.kwit. You could have got killed."

The kitchen hand shuddered, and snorted b.l.o.o.d.y mucus. His eyes were swollen shut, and when he laid his hands down beside him, I could see the bones of his knuckles through the torn skin. My own skin turned icy, at this vision of the fate I'd courted for myself. But if it was a shock to realize how I might have ended up, it was just as sobering to think that I'd almost walked away and let them finish him off, when the intervention had actually cost me nothing. I rose to my feet. People milled around the kitchen hand, asking each other about first aid. I remembered the basics from a course I'd done in high school, but the man was still breathing, and he wasn't losing vast amounts of blood, so I couldn't think of anything helpful that an amateur could do in the circ.u.mstances. I squeezed my way out of the gathering and walked back to the street. My backpack was exactly where I'd left it; no one had stolen my books. I heard sirens approaching; the police and the ambulance would be there soon.

My ribs were tender, but I wasn't in agony. I'd cracked a rib falling off a trail bike on the farm when I was twelve, and I was fairly sure that this was just bruising. For a while I walked bent over, but by the time I reached the station I found I could adopt a normal gait. I had some grazed skin on my arms, but I couldn't have appeared too battered, because no one on the train looked at me twice.

That night, I watched the news. The kitchen hand was described as being in a stable condition. I pictured him stepping out into the alley to empty a bucket of fishheads into the garbage, to find the two of them waiting for him. I'd probably never learn what the attack had been about unless the case went to trial, and as yet the police hadn't even named any suspects. If the man had been in a fit state to talk in the alley, I might have asked him then, but any sense that I was ent.i.tled to an explanation was rapidly fading.

The reporter mentioned a student "leading the charge of angry citizens" who'd rescued the kitchen hand, and then she spoke to an eye witness, who described this young man as "a New Ager, wearing some kind of astrological symbols on his shirt." I snorted, then looked around nervously in case one of my housemates had made the improbable connection, but no one else was even in earshot.

Then the story was over.

I felt flat for a moment, cheated of the minor rush that 15 seconds' fame might have delivered; it was like reaching into a biscuit tin when you thought there was one more chocolate chip left, to find that there actually wasn't. I considered phoning my parents in Orange, just to talk to them from within the strange afterglow, but I'd established a routine and it was not the right day. If I called unexpectedly, they'd think something was wrong.

So, that was it. In a week's time, when the bruises had faded, I'd look back and doubt that the incident had ever happened.

I went upstairs to finish my a.s.signment.

Francine said, "There's a nicer way to think about this. If you do a change of variables, from x and y to z and z-conjugate, the Cauchy-Riemann equations correspond to the condition that the partial derivative of the function with respect to z-conjugate is equal to zero."

We were sitting in the coffee shop, discussing the complex a.n.a.lysis lecture we'd had half an hour before. Half a dozen of us from the same course had got into the habit of meeting at this time every week, but today the others had failed to turn up. Maybe there was a movie being screened, or a speaker appearing on campus that I hadn't heard about.

I worked through the transformation she'd described. "You're right," I said. "That's really elegant!"

Francine nodded slightly in a.s.sent, while retaining her characteristic jaded look. She had an undisguisable pa.s.sion for mathematics, but she was probably bored out of her skull in cla.s.s, waiting for the lecturers to catch up and teach her something she didn't already know.

I was nowhere near her level. In fact, I'd started the year poorly, distracted by my new surroundings: nothing so glamorous as the temptations of the night life, just the different sights and sounds and scale of the place, along with the bureaucratic demands of all the organizations that now impinged upon my life, from the university itself down to the shared house groceries subcommittee. In the last few weeks, though, I'd finally started hitting my stride. I'd got a part-time job, stacking shelves in a supermarket; the pay was lousy, but it was enough to take the edge off my financial anxieties, and the hours weren't so long that they left me with no time for anything but study.

I doodled harmonic contours on the notepaper in front of me. "So what do you do for fun?" I said.

"Apart from complex a.n.a.lysis?"

Francine didn't reply immediately. This wasn't the first time we'd been alone together, but I'd never felt confident that I had the right words to make the most of the situation. At some point, though, I'dstopped fooling myself that there was ever going to be a perfect moment, with the perfect phrase falling from my lips: something subtle but intriguing slipped deftly into the conversation, without disrupting the flow. So now I'd made my interest plain, with no attempt at artfulness or eloquence. She could judge me as she knew me from the last three months, and if she felt no desire to know me better, I would not be crushed.

"I write a lot of Perl scripts," she said. "Nothing complicated; just odds and ends that I give away as freeware. It's very relaxing."

I nodded understandingly. I didn't think she was being deliberately discouraging; she just expected me to be slightly more direct.

"Do you like Deborah Conway?" I'd only heard a couple of her songs on the radio myself, but a few days before I'd seen a poster in the city announcing a tour.

"Yeah. She's great."

I started thickening the conjugation bars over the variables I'd scrawled. "She's playing at a club in Surrey Hills," I said. "On Friday. Would you like to go?"

Francine smiled, making no effort now to appear world-weary. "Sure. That would be nice."

I smiled back. I wasn't giddy, I wasn't moonstruck, but I felt as if I was standing on the sh.o.r.e of an ocean, contemplating its breadth. I felt the way I felt when I opened a sophisticated monograph in the library, and was reduced to savoring the scent of the print and the crisp symmetry of the notation, understanding only a fraction of what I read. Knowing there was something glorious ahead, but knowing too what a daunting task it would be to come to terms with it.

I said, "I'll get the tickets on my way home."

To celebrate the end of exams for the year, the household threw a party. It was a sultry November night, but the back yard wasn't much bigger than the largest room in the house, so we ended up opening all the doors and windows and distributing food and furniture throughout the ground floor and the exterior, front and back. Once the faint humid breeze off the river penetrated the depths of the house, it was equally sweltering and mosquito-ridden everywhere, indoors and out.

Francine and I stayed close for an hour or so, obeying the distinctive dynamics of a couple, until by some unspoken mutual understanding it became clear that we could wander apart for a while, and that neither of us was so insecure that we'd resent it.

I ended up in a corner of the crowded backyard, talking to Will, a biochemistry student who'd lived in the house for the last four years. On some level, he probably couldn't help feeling that his opinions about the way things were run should carry more weight than anyone else's, which had annoyed me greatly when I'd first moved in. We'd since become friends, though, and I was glad to have a chance to talk to him before he left to take up a scholarship in Germany.

In the middle of a conversation about the work he'd be doing, I caught sight of Francine, and he followed my gaze.

Will said, "It took me a while to figure out what finally cured you of your homesickness."

"I was never homesick."

"Yeah, right." He took a swig of his drink. "She's changed you, though. You have to admit that."

"I do. Happily. Everything's clicked, since we got together." Relationships were meant to screw up your studies, but my marks were soaring. Francine didn't tutor me; she just drew me into a state of mind where everything was clearer.

"The amazing thing is that you got together at all." I scowled, and Will raised a hand placatingly. "I just meant, when you first moved in, you were pretty reserved. And down on yourself. When we interviewed you for the room, you practically begged us to give it to someone more deserving."

"Now you're taking the p.i.s.s."

He shook his head. "Ask any of the others."

I fell silent. The truth was, if I took a step back and contemplated my situation, I was as astonished as he was. By the time I'd left my home town, it had become clear to me that good fortune had nothing much to do with luck. Some people were born with wealth, or talent, or charisma. They started with anedge, and the benefits s...o...b..lled. I'd always believed that I had, at best, just enough intelligence and persistence to stay afloat in my chosen field; I'd topped every cla.s.s in high school, but in a town the size of Orange that meant nothing, and I'd had no illusions about my fate in Sydney.

I owed it to Francine that my visions of mediocrity had not been fulfilled; being with her had transformed my life. But where had I found the nerve to imagine that I had anything to offer her in return?

"Something happened," I admitted. "Before I asked her out."

"Yeah?"

I almost clammed up; I hadn't told anyone about the events in the alley, not even Francine. The incident had come to seem too personal, as if to recount it at all would be to lay my conscience bare. But Will was off to Munich in less than a week, and it was easier to confide in someone I didn't expect to see again.

When I finished, Will bore a satisfied grin, as if I'd explained everything. "Pure karma," he announced. "I should have guessed."

"Oh, very scientific."

"I'm serious. Forget the Buddhist mystobabble; I'm talking about the real thing. If you stick to your principles, of course things go better for you-a.s.suming you don't get killed in the process. That's elementary psychology. People have a highly developed sense of reciprocity, of the appropriateness of the treatment they receive from each other. If things work out too well for them, they can't help asking, 'What did I do to deserve this?' If you don't have a good answer, you'll sabotage yourself. Not all the time, but often enough. So if you do something that improves your self-esteem-"

"Self-esteem is for the weak," I quipped. Will rolled his eyes. "I don't think like that," I protested.

"No? Why did you even bring it up, then?"

I shrugged. "Maybe it just made me less pessimistic. I could have had the c.r.a.p beaten out of me, but I didn't. That makes asking someone to a concert seem a lot less dangerous." I was beginning to cringe at all this unwanted a.n.a.lysis, and I had nothing to counter Will's pop psychology except an equally folksy version of my own.

He could see I was embarra.s.sed, so he let the matter drop. As I watched Francine moving through the crowd, though, I couldn't shake off an unsettling sense of the tenuousness of the circ.u.mstances that had brought us together. There was no denying that if I'd walked away from the alley, and the kitchen hand had died, I would have felt like s.h.i.t for a long time afterward. I would not have felt ent.i.tled to much out of my own life.

I hadn't walked away, though. And even if the decision had come down to the wire, why shouldn't I be proud that I'd made the right choice? That didn't mean everything that followed was tainted, like a reward from some sleazy, palm-greasing deity. I hadn't won Francine's affection in a medieval test of bravery; we'd chosen each other, and persisted with that choice, for a thousand complicated reasons.

We were together now; that was what mattered. I wasn't going to dwell on the path that had brought me to her, just to dredge up all the doubts and insecurities that had almost kept us apart.

2012.

As we drove the last kilometer along the road south from Ar Rafidiyah, I could see the Wall of Foam glistening ahead of us in the morning sunlight. Insubstantial as a pile of soap bubbles, but still intact, after six weeks.

"I can't believe it's lasted this long," I told Sadiq.

"You didn't trust the models?"

"f.u.c.k, no. Every week, I thought we'd come over the hill and there'd be nothing but a shriveled-up cobweb."

Sadiq smiled. "So you had no faith in my calculations?"

"Don't take it personally. There were a lot of things we could have both got wrong."

Sadiq pulled off the road. His students, Ha.s.san and Rashid, had climbed off the back of the truck and started toward the Wall before I'd even got my face mask on. Sadiq called them back, and made them put on plastic boots and paper suits over their clothes, while the two of us did the same. We didn'tusually bother with this much protection, but today was different.

Close up, the Wall almost vanished: all you noticed were isolated, rainbow-fringed reflections, drifting at a leisurely pace across the otherwise invisible film as water redistributed itself, following waves induced in the membrane by the interplay of air pressure, thermal gradients, and surface tension. These images might easily have been separate objects, sc.r.a.ps of translucent plastic blowing around above the desert, held aloft by a breeze too faint to detect at ground level.

The further away you looked, though, the more crowded the hints of light became, and the less plausible any alternative hypothesis that denied the Wall its integrity. It stretched for a kilometer along the edge of the desert, and rose an uneven 15 to 20 meters into the air. But it was merely the first, and smallest, of its kind, and the time had come to put it on the back of the truck and drive it all the way back to Basra.

Sadiq took a spray can of reagent from the cabin, and shook it as he walked down the embankment.

I followed him, my heart in my mouth. The Wall had not dried out; it had not been torn apart or blown away, but there was still plenty of room for failure.

Sadiq reached up and sprayed what appeared from my vantage to be thin air, but I could see the fine mist of droplets strike the membrane. A breathy susurration rose up, like the sound from a steam iron, and I felt a faint warm dampness before the first silken threads appeared, crisscrossing the region where the polymer from which the Wall was built had begun to shift conformations. In one state, the polymer was soluble, exposing hydrophilic groups of atoms that bound water into narrow sheets of feather-light gel. Now, triggered by the reagent and powered by sunlight, it was tucking these groups into slick, oily cages, and expelling every molecule of water, transforming the gel into a desiccated web.

I just hoped it wasn't expelling anything else.

As the lacy net began to fall in folds at his feet, Ha.s.san said something in Arabic, disgusted and amused. My grasp of the language remained patchy; Sadiq translated for me, his voice m.u.f.fled by his face mask: "He says probably most of the weight of the thing will be dead insects." He shooed the youths back toward the truck before following himself, as the wind blew a glistening curtain over our heads. It descended far too slowly to trap us, but I hastened up the slope.

We watched from the truck as the Wall came down, the wave of dehydration propagating along its length. If the gel had been an elusive sight close up, the residue was entirely invisible in the distance; there was less substance to it than a very long pantyhose-albeit, pantyhose clogged with gnats.

The smart polymer was the invention of Sonja Helvig, a Norwegian chemist; I'd tweaked her original design for this application. Sadiq and his students were civil engineers, responsible for scaling everything up to the point where it could have a practical benefit. On those terms, this experiment was still nothing but a minor field trial.