Diaspora

by Greg Egan

  Paolo was feeling disoriented himself. The Transmuters remained as elusive as ever, but this non-sentient tool of another civilization entirely had just casually answered the question that had driven the Diaspora across three universes.

  Or half the question.

  He summoned up a map of the Milky Way, every star labeled with gestalt tags indicating mass and velocity. “Can you read this?”

  “Yes.” The worm added candidly, “I know what you’re going to ask. What’s the fate of the core?”

  Paolo was suddenly grateful that the thing was non-sentient. Their minds had all been read, they’d all been rendered as naked as they could be to any lover — but unless the worm was lying, it was churning through this information, blindly, to determine the answers they needed, with no more awareness than the polis library.

  “So were the Transmuters right or wrong? Do you agree with their prediction?”

  “Not quite. They were extrapolating a long way into the future, and a galaxy is a complex system. They couldn’t be expected to get everything right.”

  Elena asked, “So how far out were they?”

  The worm said, “As the core collapses, most of its energy will end up as extra-dimensional spin. Energy in that form can’t interact with local gravitons, so the region won’t seal itself off behind an event horizon as rapidly as it otherwise would. And before it does, the energy density will grow high enough to start creating new space-time.”

  “A mini Big Bang?” Karpal moved restlessly away from the girder, as if that could give him a head start in spreading the warning. “A center of creation, in the middle of the galaxy?”

  “Yes.”

  Elena said, “But won’t the new space-time be orthogonal to the old? A bubble perpendicular to the main universe, not intruding into it?” She sketched a rough diagram, a large sphere with a smaller one growing out of it, the two joined only at a narrow neck.

  “That’s correct. But that small, shared region at the galactic core will still reach extreme temperatures before it pinches off to form a black hole.”

  “How extreme?”

  “Hot enough to break up nuclei within a radius of fifty thousand light years. Nothing in the galaxy will survive.”

  Elena fell silent. Paolo thought: There will be no sign of it, here. Not a pinprick of radiance, like a distant supernova, to mark the passing of a hundred billion worlds. The apocalypse would be invisible.

  Paolo knew that the Contingency Handler could feel no compassion for their plight; it could only utter the formalities programmed into it long ago, translated as best it could. But the message it conveyed still managed to bridge time, and scale, and cultures.

  It said, “Bring your people through. They’re welcome here. There’s room enough for everyone.”

  * * *

  Part Eight

  « ^ »

  Yatima liked the way the concentric 3-spheres of color pricked out in the sky by stars of equal Doppler-shift converged on their destination; it seemed so much more emphatic than an ordinary starbow of circular bands. Wrapping the image of Weyl so tightly, it seemed to promise that, this time, the Transmuters would not have slipped away.

  Paolo said, “End of story, I suppose. From that point on, they’ll know the territory better than we do.”

  “Maybe.” Yatima hesitated. “They might still be curious about one thing, though.”

  “What?”

  “You, Paolo. You had all the information you needed. You’d made the whole Diaspora worthwhile. So why did you choose to keep traveling?”

  * * *

  19

  –

  Pursuit

  « ^ »

  Carter-Zimmerman polis, U**

  The polis returned to the singularity in order to cut communications time lags to a minimum. There was some talk in Poincaré C-Z of quarantining themselves from the “infected” second-macrosphere clone, though this made no sense to Paolo; the Contingency Handler had infiltrated the polis by physical manipulation of the hardware on a molecular level, and no mere software sent back through the singularity would be capable of any such feat. But Paolo was happy enough to let the faction reason their way out of paranoia in their own good time; he could interact with Poincaré C-Z as easily as if he was there in person, so he felt no great desire to cross back.

  The message itself had passed through; he wasn’t needed. The moment an independent check of the Handler’s infinite-dimensional Kozuch Theory (carried out in the uncorrupted Poincaré polis) had confirmed its perfect fit to the Lac G-1 data and generated the same dire predictions for the core, Orlando had left by maser to spread the news in person, merging with his Swift self along the way. The entire Diaspora, gleisners included, lay within 250 light years of Swift, so unless they were very unlucky with the timing of another singularity slip, everyone would have the chance to escape. If they didn’t trust the near-omnipotent Star Striders, as the Handler’s creators had come to be called, they could always remain in the first macrosphere. Paolo had no doubt that between Orlando and the Swift versions of Yatima and Karpal, the case would be put forcefully enough to persuade anyone who hadn’t lost touch with the physical world entirely. Even the sequence of the Orphean carpets could be brought through, and re-seeded on another world.

  It was the best they could have hoped for, but Paolo felt frustrated, ashamed, superfluous. He knew he’d willfully denied the meaning of the Transmuters’ map because of Lacerta — because he’d been tired of measuring everything against Orlando’s suffering and Orlando’s loss. Even on Poincaré, it was Orlando who’d made the sacrifice that opened the way to the second macrosphere; Paolo had merely stepped through the singularity, and the truth had fallen into his hands without cost. And now he faced spending the next five hundred years waiting for Orlando to return in triumph, leading the whole Coalition to safety.

  The Handler told Paolo about the galaxy’s six thousand civilizations. There were organic creatures of various biochemistries and body plans, as well as software running in polises and robots, and a spectrum of unclassifiable hybrids. Some were natives of the second macrosphere, some were from as far away as the Star Striders. Twelve had been born in the Milky Way, and either read the Transmuters’ message and followed their path, or reached the same conclusions and invented the same technology themselves.

  So there was an abundance of possibilities to contemplate, here, as models for the Coalition’s future evolution. If the right protocols were followed, most of these cultures would be open to some form of contact with the newcomers, hopelessly backward as they were.

  But the Transmuters had not stayed. They’d entered this universe after the Star Striders, spoken with them briefly, then moved on.

  When Paolo heard of Yatima’s plan, he went straight to Elena. Her current homescape was a verdant jungle on a tide-locked moon of an imaginary gas giant. The banded planet filled a third of the sky.

  She said, “Why? Why follow them? There are people with the same technology here. People with as long a history. Out of six thousand cultures, what’s so special about the Transmuters?”

  “They weren’t just fleeing the core burst. They wanted to do more than escape.”

  Elena gave him a try-harder look. “Most of the people here have nothing to do with the core burst. There are more than a thousand cultures native to this galaxy.”

  “And they’ll all be here when I get back. Will you come with me?” Paolo met her eyes, imploringly.

  She laughed. “Why should I go with you? You don’t even know why you’re going yourself.”

  They argued for kilotau. They made love, but it changed nothing. Paolo felt her tolerant bemusement firsthand, and she understood his restlessness. But it did not draw them closer.

  Paolo brushed the dew from his skin. “Can I hold you in my mind? Just below sentience? Just to keep me sane?”

  Elena sighed with mock wistfulness. “Of course, my love! Take a lock of my mind on your journey, and I’ll carry a lock
of yours on mine.”

  “Your journey?”

  “There are six thousand cultures here, Paolo. I’m not going to hang around the singularity for five hundred years, waiting for the rest of the Diaspora to catch up.”

  “Then be careful.”

  Six thousand cultures. And he wouldn’t have to lose her. For an instant, Paolo almost changed his mind.

  Elena replied placidly, self-contained. “I will.”

  * * *

  20

  –

  Invariance

  « ^ »

  Yatima-Venetti polis, UN*

  Yatima found the sight of the sky in the second macrosphere disturbing; ve kept wondering which combinations of stars were the images of different individual Striders. If the Handler was to be believed, the local computing nodes in each star system were only millimeters wide, and they communicated with the others, light years away, with pulses so weak, so tightly aimed, so unpredictable in wavelength, and so ingeniously encoded that a thousand interstellar civilizations had come and gone without noticing their presence. The Handler had refused to disclose the nature of its own physical infrastructure, but it must have been operating below the femtomachine level to have penetrated the polis defenses. One line of speculation had it that the Striders had woven a computing device into the virtual wormholes of the vacuum throughout the galaxy, and the Contingency Handlers ran on empty space, permeating everything.

  Paolo said, “I’m dropping the seeds.”

  “Okay.”

  He braced himself between two girders of the satellite, and pitched a handful of entry capsules in a counter-orbital direction. Yatima smiled. It was very theatrical. The real capsules were launched in response to the mime, and Yatima couldn’t tell when the scape stopped showing Paolo’s fictitious ones and switched to the genuine external image.

  Kozuch, the planet beneath them, was Mercury-sized and almost as hot. Like Swift, it stood out for hundreds of light years, branded by heavy isotopes; this step of the route, at least, was clear. The capsules’ nanomachines would build a neutron-manipulation system, and then construct a polis in the third macrosphere. The whole procedure was simpler than interstellar flight, once you knew what to do.

  Yatima said, “I hope they repeat the marker they used on Poincaré. If we have to find someone in every six-dimensional universe who remembers them passing, this could be a very slow process.”

  Paolo replied with studied nonchalance, “I’ll bridge with anyone. I’m willing to do that.”

  “That’s nice to know.”

  He said, “We can’t be sure that the Transmuters came from our universe. They left a map of the core burst for the locals to find, but they might have been passing through from a lower level, not fleeing it themselves.”

  “So they could be more at home in six dimensions?”

  Paolo shrugged. “I’m just saying we shouldn’t make any assumptions.”

  “No.”

  A point on the surface of planet Kozuch beneath them was beginning to sprout a giant black disk, a purely metaphorical gate into the next macrosphere. Yatima could remember when no one in C-Z would have dared taint a realistic scape with abstractionism like this. They could see sparse stars in the disk’s blackness, a two-dimensional projection of the new polis observatory’s view.

  Ve stared down into the expanding well. “I’m doing this because of some badly-chosen fields in my mind seed. What’s your excuse?”

  Paolo didn’t reply.

  Yatima looked up. “Well, you should be good company.”

  Ve tugged symbolically downward on a girder of the satellite, and it went plummeting toward the gate.

  The nearest star to the singularity in the third macrosphere held more life than Poincaré, but there was no marker, and no obviously intelligent species to ask for directions.

  The next was barren, or at least too hot and too turbulent for life to have evolved on its thin, fleetingly solid continents. If anything lived in the magma oceans, it was beyond their powers to identify.

  The third star was much older and cooler, with a completely solid crust. It was girded by a system of giant causeways, easily visible from orbit. This hypersurface crisscrossed with roads was like some galactic Roman empire out of ancient fantasy, with all the intervening vacuum removed.

  Yatima said, “This is it. The Transmuters.”

  As they approached, there was no signal from the ground. No imitations of lonG-1ost friends appeared in their scapes to welcome them; no invisible defenses woven into the vacuum burned them from the sky.

  The second wave of probes revealed that whatever cities or structures the causeways had linked were buried deep beneath an almost uniform, star-wide layer of rubble. It looked as if the crust had suddenly contracted, as some nuclear/chemical pathway had switched on or off deep within the star. That the causeways were visible at all was astounding. Nothing else had survived.

  The fourth star showed traces of primitive life, but they didn’t stop to examine the evidence closely. There was a marker slab, the same pure mineral as Poincaré, and this time it was much closer to the polar sphere.

  They named the fourth star Yang-Mills. The Diaspora’s rule in the past had been one person only per astronomical body, but it didn’t seem right to split the famous pair between universes, or to give one the gateway star and the other a less significant memorial.

  Waiting for the long-nucleon facility to be completed, Yatima viewed images, relayed through two singularities, of the first wave of core-burst refugees arriving in U-star C-Z. Blanca was there, and Gabriel twice; some versions of him must have declined to merge. Yatima searched for Inoshiro, but the refugees were all from the Diaspora. No one had yet arrived from Earth.

  In the fourth macrosphere, they carried out remote spectroscopy on the hundred nearest star systems. There was a planet labeled with heavy isotopes, 270 light years away. They named it Blanca. By the time they reached it, the core burst would have annihilated Swift, and the whole migration out of the home universe would be ancient history.

  Yatima had vis exoself freeze ver for the journey. When ve woke, and jumped from vis homescape to Satellite Pinatubo, Paolo said flatly, “We’ve lost contact.”

  “How? Where?”

  “The polis orbiting Yang-Mills can’t communicate with the singularity station. The beacon seems to have vanished from the sky.”

  Yatima’s first response was relief. A malfunction in the station’s communications hardware wasn’t as bad as one of the singularities slipping or decaying. They’d receive no more news from the lower levels, but there was nothing to stop them physically returning, repairing the fallible hardware along the way.

  Unless the station had not only lost contact with the distant polis, it had also lost track of the Planck-sized singularity right beside it. The entire second macrosphere could vanish like a fiber in a haystack.

  Yatima tried to read Paolo’s gestalt. He’d clearly had time to think of the same scenario. “Are you okay?”

  Paolo shrugged. “I knew the risks.”

  “We can turn back anytime you want to.”

  “If the station’s been seriously damaged, we’re already too late. The singularity’s either been lost by now, or it hasn’t; a few thousand years either way before we return won’t make the slightest difference.”

  “Except that we’ll know our fate sooner.”

  Paolo shook his head, with a determined smile. “What if we go back, and find that everything’s working perfectly except for the communications link? We’ll feel like complete idiots. We’ll have wasted centuries for nothing.”

  “We could keep going here, but send clones of ourselves back into the third macrosphere, to ride the polis to the station and check it out.”

  Paolo looked down impatiently at planet Blanca’s cratered surface. “I don’t want to do that. I don’t want to split myself again, just to half turn back. Do you?”

  Yatima said, “No.”

  “Then l
et’s drop the seeds, and move on.”

  Paolo had spent some time awake in the fourth macrosphere, immersing himself in five-plus-one-dimensional physics, and he’d managed to design a vastly improved spectroscope. With this, they located the Transmuters’ marker from the vicinity of the fifth macrosphere’s singularity, on the second-closest star, which they dubbed Weyl.

  The marker was still covering the rotational pole.

  Yatima had vis exoself bring ver out of hibernation at the mid-point of the journey. Ve stood on the 5-space version of Satellite Pinatubo, feeling verself dissolving into the sparse sky. It was meaningless to ask how many universes each handful of vacuum here contained. The Handler’s revelations meant that even in the home universe, there were an infinite number of levels below them.

  Maybe there was life and civilization, star-farers and long-particle engineers in every universe. But even the Striders, even the Transmuters, could only ascend a finite distance. There could be a Diaspora slowly working its way up from a hundred thousand levels below the home universe, which no one born in the Milky Way would ever know about.

  But their own Diaspora had already overlapped with the Transmuters’. The space around them was infinite, but if they clung to the trail they’d never lose them. It was only a matter of time and persistence before they caught up.

  Later, Paolo woke and joined ver. They sat on a girder, planning their meeting with the Transmuters. And the more they talked about it, the more confident Yatima felt that they didn’t have far to go.

  In the sixth macrosphere, there was an artifact drifting freely in space, a billion kilometers from the singularity.

  It was an irregular shape, roughly spheroidal, two hundred and forty kilometers wide — the size of a large asteroid. It was not greatly pitted, but they were a long way from any star system full of debris. The surface was probably one or two million years old.