Permutation City

by Greg Egan

  Elizabeth.

  She raised a cup to his lips. He took a sip, spluttered and choked, but then managed to force some of the thin sweet liquid down.

  She said, “You’re going to be fine. Just take it easy.”

  “Why are you here?” He coughed, shook his head, wished he hadn’t. He was touched, but confused. Why had his original lied – claiming that she wanted to shut him down – when in fact she was sympathetic enough to go through the arduous process of visiting him?

  He was lying on something like a dentist’s couch, in an unfamiliar room. He was in a hospital gown; there was a drip in his right arm, and a catheter in his urethra. He glanced up to see an interface helmet, a bulky hemisphere of magnetic axon current inducers, suspended from a gantry, not far above his head. He thought: fair enough, to construct a simulated meeting place that looked like the room that her real body must be in. Putting him in the couch, though, and giving him all the symptoms of a waking visitor, seemed a little extreme.

  He tapped the couch with his left hand. “What’s the message? You want me to know exactly what you’re going through? Okay. I’m grateful. And it’s good to see you.” He shuddered with relief, and delayed shock. “Fantastic, to tell the truth.” He laughed weakly. “I honestly thought he was going to wipe me out. The man’s a complete lunatic. Believe me, you’re talking to his better half.”

  Elizabeth was perched on a stool beside him. She said, “Paul. Try to listen carefully to what I’m going to say. You’ll start to reintegrate the memories gradually, on your own, but it’ll help if I talk you through it all first. To start with, you’re not a Copy. You’re flesh and blood.”

  Paul coughed, tasting acid. Durham had let her do something unspeakable to the model of his digestive system.

  “I’m flesh-and-blood? What kind of sadistic joke is that? Do you have any idea how hard it’s been, coming to terms with the truth?”

  She said patiently, “It’s not a joke. I know you don’t remember yet, but … after you made the scan that was going to run as Copy number five, you finally told me what you were doing. And I persuaded you not to run it – until you’d tried another experiment: putting yourself in its place. Finding out, first hand, what it would be forced to go through.

  “And you agreed. You entered the virtual environment which the Copy would have inhabited – with your memories since the day of the scan suppressed, so you had no way of knowing that you were only a visitor.”

  “I—?”

  “You’re not the Copy. Do you understand? All you’ve been doing is visiting the environment you’d prepared for Copy number five. And now you’re out of it. You’re back in the real world.”

  Her face betrayed no hint of deception – but software could smooth that out. He said, “I don’t believe you. How can I be the original? I spoke to the original. What am I supposed to believe? He was the Copy? Thinking he was the original?”

  “Of course not. That would hardly have spared the Copy, would it? The fifth scan was never run. I controlled the puppet that played your ‘original’ – software provided the vocabulary signature and body language, but I pulled the strings. You briefed me, beforehand, on what to have it say and do. You’ll remember that, soon enough.”

  “But … the experiments?”

  “The experiments were a sham. They could hardly have been performed on a visitor, on a physical brain – could they?”

  Paul shook his head, and whispered, “Abulafia.”

  No interface window appeared.

  He gripped the couch and closed his eyes, then laughed. “You say I agreed to this? What kind of masochist would do that? I’m going out of my mind. I don’t know what I am.”

  Elizabeth took hold of his arm again. “You’re disoriented – but that won’t last long. And you know why you agreed. You were sick of Copies baling out on you. You had to come to terms with their experience. Spending a few days believing you were a Copy would make or break the project: you’d either end up psychologically prepared, at last, to give rise to a Copy who’d be able to cope with its fate – or you’d gain enough sympathy for their plight to stop creating them.

  “The plan was to tell you everything while you were still inside, after the third experiment. But when you went weird on me in there, I panicked. All I could think of was having the puppet playing your original tell you that it was going to pause you. I wasn’t trying to frighten you. I didn’t think you’d take it so badly.”

  A technician came into the room and removed the drip and catheter. Paul propped himself up and looked out through the windows of the room’s swing doors; he could see half a dozen people in the corridor. He bellowed wordlessly at the top of his lungs; they all turned to stare in his direction. The technician said mildly, “Your penis might sting for an hour or two.”

  Paul slumped back onto the couch and turned to Elizabeth. “You wouldn’t pay for reactive crowds. I wouldn’t pay for reactive crowds. It looks like you’re telling the truth.”

  #

  People, glorious people: thousands of strangers, meeting his eyes with suspicion or puzzlement, stepping out of his way on the street – or, more often, clearly, consciously refusing to.

  The freedom of the city was so sweet. He walked the streets of Sydney for a full day, rediscovering every ugly shopping arcade, every piss-stinking litter-strewn park and alley, until, with aching feet, he squeezed his way home through the evening rush-hour, to watch the real-time news.

  There was no room for doubt: he was not in a virtual environment. Nobody in the world could have had reason to spend so much money, simply to deceive him.

  When Elizabeth asked if his memories were back, he nodded and said, of course. She didn’t grill him on the details. In fact, having gone over her story so many times in his head, he could almost imagine the stages: his qualms after the fifth scan; repeatedly putting off running the model; confessing to Elizabeth about the project; accepting her challenge to experience for himself just what his Copies were suffering.

  And if the suppressed memories hadn’t actually reintegrated, well, he’d checked the literature, and there was a two point five per cent risk of that happening; electronically censoring access to memories could sometimes permanently weaken the neural connections in which they were encoded.

  He even had an account from the database service which showed that he’d consulted the very same articles before.

  He reread and replayed the news reports that he’d accessed from inside – and found no discrepancies. He flicked through encyclopedic databases – spot-checking random facts of history, geography, astronomy – and although he was surprised now and then by details which he’d never come across before, there were no startling contradictions. The continents hadn’t moved. Stars and planets hadn’t vanished. The same wars had been lost and won.

  Everything was consistent. Everything was explicable.

  And yet he couldn’t stop wondering about the fate of a Copy who was shut down, and never run again. A normal human death was one thing – woven into a much vaster tapestry, it was a process which made perfect sense. From the internal point of view of a Copy whose model was simply halted, though, there was no explanation whatsoever for its demise – just an edge where the pattern abruptly came to an end.

  But if the insight he’d gained from the experiments was true (whether or not they’d ever really happened) – if a Copy could assemble itself from dust scattered across the world, and bridge the gaps in its existence with dust from across the universe … then why should it ever come to an inconsistent end? Why shouldn’t the pattern keep on finding itself?

  Or find a larger pattern into which it could merge?

  The dust theory implied a countless number of alternative worlds: billions of different possible histories spelled out from the same primordial alphabet soup. One history in which Durham did run Copy number five – and one in which he didn’t, but was persuaded to take its place as a visitor, instead.

  But if the visito
r had been perfectly deceived, and had experienced everything the Copy did … what set the two of them apart? So long as the flesh-and-blood man had no way of knowing the truth, it was meaningless to talk about “two different people” in “two different worlds.” The two patterns of thoughts and perceptions had effectively merged into one.

  If the Copy had been allowed to keep on running after the visitor had learned that he was flesh-and-blood, their two paths would have diverged again. But the Copy had been shut down; it had no future at all in its original world, no separate life to live.

  So the two subjective histories remained as one. Paul had been a visitor believing he was a Copy. And he’d also been the Copy itself. The patterns had merged seamlessly; there could be no way of saying that one history was true and the other false. Both explanations were equally valid.

  Once, preparing to be scanned, he’d had two futures.

  Now he had two pasts.

  #

  Paul woke in darkness, confused for a moment, then pulled his cramped left arm out from under the pillow and glanced at his watch. Low power infrared sensors in the watch face detected his gaze, and flashed up the time – followed by a reminder: DUE AT LANDAU 7 AM. It was barely after five, but it hardly seemed worth going back to sleep.

  Memories of the night before came back to him. Elizabeth had finally confronted him, asking what decision he’d reached: to abandon his life’s work, or to forge ahead, now that he knew, firsthand, what was involved.

  His answer seemed to have disappointed her. He didn’t expect to see her again.

  How could he give up? He knew he could never be sure that he’d discovered the truth – but that didn’t mean that nobody else could.

  If he made a Copy, ran it for a few virtual days, then terminated it abruptly … then at least that Copy would know if its own pattern of experience continued.

  And if another Paul Durham in one of the countless billions of alternative worlds could provide a future for the terminated Copy – a pattern into which it could merge – then perhaps that flesh-and-blood Durham would repeat the whole process again.

  And so on, again and again.

  And although the seams would always be perfect, the “explanation” for the flesh-and-blood human believing that he had a second past as a Copy would necessarily grow ever more “contrived”, less convincing … and the dust theory would become ever more compelling.

  Paul lay in bed in the darkness, waiting for sunrise, staring into the future down this corridor of mirrors.

  One thing nagged at him. He could have sworn he’d had a dream, just before he woke: an elaborate fable, conveying some kind of insight. That’s all he knew – or thought he knew. The details hovered maddeningly on the verge of recollection.

  His dreams were evanescent, though, and he didn’t expect to remember anything more.

  Chapter 17

  (Remit not paucity)

  April 2051

  Maria shifted in her seat to try to get her circulation flowing, then realized it wasn’t enough. She stood up and limped around the room, bending down to massage her cramped right calf.

  She said, “And you claim you’re the twenty-third?” She was almost afraid to sound too skeptical; not because she believed that Durham would take offense, but because the story was so strangely entrancing that she wasn’t sure she wanted to deflate it, yet. One hint of mockery and the floodgates would open. “You’re the twenty-third flesh-and-blood Paul Durham whose past includes all those who came before?”

  Durham said, “I may be wrong about the exact number. I may have counted this last version more than once; if I’m capable of believing in twenty-three incarnations, some of them might be false. The whole nature of the delusions I suffered contributes to the uncertainty.”

  “Contributes? Isn’t that a bit of an understatement?”

  Durham was unflappable. “I’m cured now. The nanosurgery worked. The doctors pronounced me sane, and I have no reason to question their judgment. They’ve scanned my brain; it’s functioning impeccably. I’ve seen the data, before and after. Activity in the prefrontal cortex—”

  “But don’t you see how absurd that is? You acknowledge that you were deluded. You insist that you’re cured now. But you claim that your delusions weren’t delusions—”

  Durham said patiently, “I’ve admitted from the outset: my condition explains everything. I believed – because I was mentally ill – that I was the twenty-third generation Copy of another Paul Durham, from another world.”

  “Because you were mentally ill! End of story.”

  “No. Because I’m certifiably rational now – and the logic of the dust theory makes as much sense to me as ever. And it makes no difference whether my memories are true, false, or both.”

  Maria groaned. “Logic of the dust theory! It’s not a theory. It can’t be tested.”

  “Can’t be tested by whom?”

  “By anyone! I mean … even assuming that everything you believe is the truth: you’ve ‘been through’ twenty-three separate experiments, and you still don’t know what you’ve proved or disproved! As you say: your condition accounts for everything. Haven’t you heard of Occam’s Razor: once you have a perfectly simply explanation for something, you don’t go looking for ever more complicated ways of explaining the very same thing? No dust theory is required.” Her words reverberated in the near-empty room. She said, “I need some fresh air.”

  Durham said firmly, “After twenty-three ambiguous results, I know how to get it right this time. A Copy plus a virtual environment is a patchwork, a mess. A system like that isn’t rich enough, detailed enough, or consistent enough, to be self-sustaining. If it was, when I was shut down, the entire VR world I was in would have persisted. That never happened. Instead – every time – I found a flesh-and-blood human with a reason to believe he shared my past. That explained my pattern of experience far better than VR – even to the point of insanity.

  “What I have to do now is construct a consistent pattern which can only have one past.”

  Maria took a few deep breaths. It was almost too much to bear: Durham’s sad flat, his cosmic visions, his relentless, mechanical logic, grinding away trying to make sense of the legacy of his disease. The doctors had cured him, he was sane. He just didn’t want to disown his delusional past – so he’d invented a flawlessly logical, utterly irrefutable, reason to hang on to it.

  If he’d really told the cops all this, why were they still hounding him? They should have seen that he was harmless and left him alone – and left his moronic clients to fend for themselves. The man wasn’t even a danger to himself. And if he could ever harness a fraction of the energy and intelligence he’d put into this “project” and direct it toward something worthwhile—

  Durham said, “Do you know what a Garden-of-Eden configuration is?”

  Maria was caught blank for a second, then she said, “Yes, of course. In cellular automaton theory, it’s a state of the system that can’t be the result of any previous state. No other pattern of cells can give rise to it. If you want a Garden-of-Eden configuration, you have to start with it – you have to put it in by hand as the system’s first state.”

  Durham grinned at her as if she’d just conceded the whole argument. She said, “What?”

  “Isn’t it obvious? A cellular automaton isn’t like patchwork VR; it’s every bit as consistent as a physical universe. There’s no jumble of ad hoc high-level laws; one set of rules applies to every cell. Right?”

  “Yes, but—”

  “So if I set up a cellular automaton in a Garden-of-Eden configuration, run it through a few trillion clock ticks, then shut it down … the pattern will continue to find itself in the dust – separate from this version of me, separate from this world – but still flowing unambiguously from that initial state. A state which can’t be explained by the rules of the automaton. A state which must have been constructed in another world – exactly as I remember it.

  “The whole prob
lem, so far, has been that my memories are always entirely explicable within the new world. I shut myself down as a Copy – and find myself in a flesh-and-blood body with flesh-and-blood memories which the laws of physics could have produced from earlier states of a flesh-and-blood brain. This world can explain me: only as man whose delusions are unlikely beyond belief – but there’s no denying that I do have a complete extra history, here, that’s not literally, physically impossible. So whatever I prefer to believe, I have to concede that the outcome of the experiment is still ambiguous. I could, still, be wrong.

  “But a cellular automaton can’t provide an ‘extra history’ for a Garden-of-Eden configuration! It’s mathematically impossible! If I find myself inside a cellular automaton universe, and I can track my past back to a Garden-of-Eden configuration, that will be conclusive proof that I did seed the whole universe in a previous incarnation. The dust theory will be vindicated. And I’ll finally know – beyond any doubt – that I haven’t merely been insane all along.”

  Maria felt punch-drunk. At one level, she knew she should stop humoring him, stop treating his ideas seriously. On another, it seemed that if Durham was so wrong, she should be able to point out the reasons why. She shouldn’t have to call him a madman and refuse to listen to another word.

  She said, “Find yourself in a cellular automaton world? You don’t mean the Autoverse—?”

  “Of course not. There’s no prospect of translating a human into Autoverse biochemistry.”

  “Then what?”

  “There’s a cellular automaton called TVC. After Turing, von Neumann and Chiang. Chiang completed it around twenty-ten; it’s a souped-up, more elegant version of von Neumann’s work from the nineteen-fifties.”

  Maria nodded uncertainly; she’d heard of all this, but it wasn’t her field. She did know that John von Neumann and his students had developed a two-dimensional cellular automaton, a simple universe in which you could embed an elaborate pattern of cells – a rather Lego-like “machine” – which acted as both a universal constructor and a universal computer. Given the right program – a string of cells to be interpreted as coded instructions rather than part of the machine – it could carry out any computation, and build anything at all. Including another copy of itself – which could build another copy, and so on. Little self-replicating toy computers could blossom into existence without end.