Permutation City

'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 problem, 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 a 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.

She said, 'Chiang's version was three-dimensional, wasn't it?'

'Much better. N-dimensional. Four, five, six, whatever you like. That leaves plenty of room for data within easy reach. In two dimensions, the original von Neumann machine had to reach farther and farther -- and wait longer and longer -- for each successive bit of data. In a six-dimensional TVC automaton, you can have a three- dimensional grid of computers, which keeps on growing indefinitely -- each with its own three-dimensional memory, which can also grow without bound.'

Maria said numbly, 'Where are you supposed to fit into all of this? If you think translating human biochemistry into Autoverse terms is difficult, how are you going to map yourself into a six-dimensional world designed solely to support von Neumann machines?'

'The TVC universe is one big, ever-expanding processor cluster. It runs a Copy of me --'

'I thought the whole point was to do away with Copies!'

'-- in a VR environment which lets me interact with the TVC level. Yes, I'll be a patchwork Copy, as always -- there's no alternative to that -- but I'll also be linked to the cellular automaton itself. I'll witness its operation, I'll experience its laws. By observing it, I'll make it a part of what has to be explained.

'And when the simulated TVC universe being run on the physical computer is suddenly shut down, the best explanation for what I've witnessed will be a continuation of that universe -- an extension made out of dust.'

Maria could almost see it: a vast lattice of computers, a seed of order in a sea of a random noise, extending itself from moment to moment by sheer force of internal logic, 'accreting' the necessary building blocks from the chaos of non-space-time by the very act of defining space and time.

Visualizing wasn't believing, though.

She said, 'What makes you so sure? Why not another deluded psychiatric patient, who believes he was -- briefly -- a Copy being run on a TVC automaton being run on a processor cluster in another world?'

'You're the one who invoked Occam's razor. Wouldn't you say that a self-contained TVC universe is a simpler explanation, by far?'

'No. It's about the most bizarre thing I can imagine.'

'It's a lot less bizarre than yet another version of this universe, containing yet another version of me, with yet another set of convenient delusions.'

'How many of your clients believed all this? How many think they're coming along for the ride?'

'Fifteen. And there's a sixteenth who, I think, is tempted.'

'They paid -- ?'

'About two million each.' He snorted. 'It's quite funny, the significance the police have attached to that. Some large sums of money have changed hands, for reasons more complex than usual -- so they assume I must be doing something illegal. I mean, billionaires have been known to make donations larger than that to the Church of the God Who Makes No Difference.' He added hastily, 'None of mine.'

Maria was having some trouble with the scale of things herself. 'You found fifteen Copies willing to part with two million dollars after hearing this bullshit? Anyone that gullible deserves to lose their money.'

Durham took no offence. 'If you were a Copy, you'd believe the dust theory, too. You'd feel the truth of it in your nonexistent bones. Some of these people carried out the same experiments as I did -- computing themselves in randomized fragments -- but others didn't need to. They already knew that they could scatter themselves across real time and real space, and they'd still find themselves. Every Copy proves the dust theory to itself a million times a day.'

It suddenly occurred to Maria that Durham might have invented all of this for her sake, alone -- while telling his clients exactly what Hayden had assumed: some fraudulent but utterly non-metaphysical tale of a hidden supercomputer. But she couldn't see what he had to gain by confusing her . . . and too many details made too much sense, now. If his clients had accepted the whole mad vision, the problem of making them believe in a nonexistent supercomputer vanished. Or at least changed from a question of evidence to a question of faith. She said, 'So you promised to fit a snapshot of each of your 'backers' into the Garden-of-Eden configuration, plus the software to run them on the TVC?'

Durham said proudly, 'All that and more. The major world libraries; not quite the full holdings, but tens of millions of files -- text, audio, visual, interactive -- on every conceivable subject. Databases too numerous to list -- including all the mapped genomes. Software: expert systems, knowledge miners, metaprogrammers. Thousands of off-the-shelf VR environments: deserts, jungles, coral reefs, Mars and the moon. And I've commissioned Malcolm Carter, no less, to create a major city to act as a central meeting place: Permutation City, capital of the TVC universe.

'And, of course, there'll be your contribution: the seed for an alien world. Humanity is going to find other life in this universe, eventually. How can we give up hope of doing the same? Sure, we'll have our own software descendants, and recreated Earth animals, and no doubt novel, wholly artificial creatures as well. We won't be alone. But we still need a chance to confront the Other. We mustn't leave that possibility behind. And what could be more alien than Autoverse life?'