Permutation City

'No! You have no right!'

Durham hesitated. Paul felt numb, disbelieving. Some part of him refused to acknowledge any danger -- refused to accept that it could be this easy to die. Being paused wouldn't kill him, wouldn't harm him, wouldn't have the slightest effect. What would kill him would be not being restarted. He'd be passively annihilated, ignored out of existence. The fate that befell his own shit.

Durham reached offscreen.

13

(Remit not paucity)

FEBRUARY 2051

Maria said, 'Recalculate everything up to epoch five, then show me sunrise on Lambert. Latitude zero, longitude zero, altitude one.'

She waited, staring into the blank workspace, fighting the temptation to change her instructions and have the software display every stage of the simulation, which would have slowed things down considerably. After several minutes, a fissured dark plain appeared, raked with silver light. The unnamed sun -- dazzling and swollen, and, so low in the sky, too white by far -- turned a chain of extinct volcanoes on the horizon into black silhouettes like a row of pointed teeth. In the foreground, the surface looked glassy, inhospitable.

Maria raised her viewpoint to a thousand meters, then sent it skimming east. The terrain repeated itself, the eerily symmetric cones of dead volcanoes the only relief from the fractured igneous plains. This specific, detailed scenery was nothing more than a series of computerized 'artist's impressions,' manufactured on demand from purely statistical data about the planet's topography; the simulation itself hadn't dealt with anything so finicky as individual volcanoes. Touring the planet was a wasteful means of finding out anything -- but it was hard to resist playing explorer, treating this world as if its secrets had to be deduced painstakingly from its appearance . . . even when the truth was the exact opposite. Reluctantly, Maria froze the image and went straight to the underlying numerical data. The atmosphere was much too thin, again. And this time, there was almost no aqua at all.

She backtracked through the simulation's history to see when the aqua had been lost, but this version of Lambert had never possessed significant oceans -- or ice caps, or atmospheric vapor. She'd made a slight change in the composition of the primordial gas-and-dust cloud, increasing the proportion of blue and yellow atoms, in the hope that this would ultimately lead to a denser atmosphere for Lambert. Instead, she'd caused more than half of the debris in the Kuiper belt to condense into a whole new stable outer planet. As a consequence, far fewer ice-rich comets from the belt had ended up striking Lambert, robbing it of its largest source of aqua by far -- and much of its atmosphere. Gas released by volcanic eruptions provided a poor substitute; the pressure was far too low, and the chemistry was all wrong.

Maria was beginning to wish she'd kept her mouth shut. It had taken her almost an hour on the phone to persuade Durham that it was worth trying to give Lambert a proper astronomical context, and a geological history that stretched back to the birth of its sun.

'If we present this world as a fait accompli, and say: 'Look, it can exist in the Autoverse' . . . the obvious response to that will be: 'Yes, it can exist -- if you put it there by hand -- but that doesn't mean it's ever likely to have formed.' If we can demonstrate a range of starting conditions that lead to planetary systems with suitable worlds, that will be one less element of uncertainty to be used against us.'

Durham had eventually agreed, so she'd taken an off-the-shelf planetary-system modeling program -- irreverently titled The Laplacian Casino -- and adapted it to Autoverse chemistry and physics; not the deep physics of the Autoverse cellular automaton, but the macroscopic consequences of those rules. Mostly, that came down to specifying the properties of various Autoverse molecules: bond energies, melting and boiling points versus pressure, and so on. Aqua was not just water by another name, yellow atoms were not identical to nitrogen -- and although some chemical reactions could be translated as if there was a one-to-one correspondence, in the giant fractionating still of a protostellar nebula subtle differences in relative densities and volatilities could have profound effects on the final composition of each of the planets.

There were also some fundamental differences. Since the Autoverse had no nuclear forces, the sun would be heated solely by gravitational energy -- the velocity its molecules acquired as the diffuse primordial gas cloud fell in on itself. In the real universe, stars unable to ignite fusion reactions ended up as cold, short-lived brown dwarfs -- but under Autoverse physics, gravitational heating could power a large enough star for billions of years. (Units of space and time were not strictly translatable -- but everybody but the purists did it. If a red atom's width was taken to be that of hydrogen, and one grid-spacing per clock-tick was taken as the speed of light, a more or less sensible correspondence emerged.) Similarly, although Planet Lambert would lack internal heating from radioisotope decay, its own gravitational heat of formation would be great enough to drive tectonic activity for almost as long as the sun shone.

Without nuclear fusion to synthesize the elements, their origin remained a mystery, and a convenient gas cloud with traces of all thirty-two -- and the right mass and rotational velocity -- had to be taken for granted. Maria would have liked to have explored the cloud's possible origins, but she knew the project would never be finished if she kept lobbying Durham to expand the terms of reference. The point was to explore the potential diversity of Autoverse life, not to invent an entire cosmology.

Gravity in the Autoverse came as close as real-world gravity to the classical, Newtonian inverse-square law for the range of conditions that mattered, so all the usual real-world orbital dynamics applied. At extreme densities, the cellular automaton's discrete nature would cause it to deviate wildly from Newton -- and Einstein, and Chu -- but Maria had no intention of peppering her universe with black holes, or other exotica.

In fact, gravity had been seen as an irrelevant side effect of Lambert's original choice of automaton rules -- since running an Autoverse large enough for it to make the slightest difference was blatantly impossible -- and several people had tried to remove the redundancy, while leaving everything else intact. Nobody had succeeded, though; their 'rationalized' versions had always failed to generate anything remotely like the rich chemistry of the original. A Peruvian mathematician, Ricardo Salazar, had eventually proved that they shouldn't have bothered: the Autoverse rules were poised on the border between two radically different levels of algorithmic complexity, and any tinkering in the hope of improved efficiency was necessarily self-defeating. The presence or absence of gravity, in itself, had no bearing on Autoverse chemistry -- but the roots of both phenomena in the simple automaton rules seemed to be inextricably entwined.

Maria was aiming for a star with four planets. Three small worlds, one giant. The seed-world, Lambert, second from the sun -- with a decent-sized moon if possible. Whether or not tidal pools had been a driving force in real-world evolution, life's bridge from sea to land (and even though the sun itself would cause small tides, regardless), it couldn't hurt to make Lambert as generally Earth-like as possible, since Earth was still the only example to turn to for inspiration. With so much about terrestrial evolution still in dispute, the safest policy was to cover every factor which might have been significant. The gravitational effects of the other planets would ensure a reasonably complex set of Milankovitch cycles: minor orbital changes and axis wobbles, providing long-term climate variations, ice ages and interglacials. A belt of comets and other debris would complete the picture; not merely supplying an atmosphere, early on, but also offering the chance of occasional mass-extinctions for billions of years to come.