Diaspora

by Greg Egan

  Gabriel smiled airily. “We’ve proved the basic principle; the rest is just a matter of persistence. Until the neutralization of those positrons, Kozuch-Wheeler wormholes might have turned out to be nothing but a useful fiction: just another metaphor that gave the right predictions at low energies, but fell apart under closer scrutiny.” He paused for a moment, looking slightly scandalized by his own words; it was a risk that the Forge group had rarely mentioned. “But now we’ve shown that they’re real, and that we understand how to manipulate them. So what can go wrong from here ?”

  “I don’t know. When it comes to interstellar wormholes, it might take longer than you think to find one that doesn’t lead straight into the heart of a star, or the core of a planet.”

  “That’s true. But a certain amount of matter in every system has to be in the form of small asteroids, or interplanetary dust — somewhere we can burrow out from easily. And even if our estimates are wrong by a factor of a thousand, it would still only take a year or two to find and enlarge each new usable wormhole. Would you call that failure? When the gleisners are exploring a new system every century and calling it success?”

  “No.” Blanca tried harder. “Okay, what about this? You’ve just proved that you can splice two identical, electron-positron wormholes together, at the electron ends. What if it doesn’t work when you substitute a proton for one of the positrons?” Only primordial electron-proton wormholes offered the chance of an instant short-cut to the stars; the current experiment was using freshly created electron-positron pairs merely for the sake of having both ends of each wormhole accessible. Working exclusively with electron-proton wormholes might have been simpler in theory, but new ones with known endpoints couldn’t be created at a useful rate under anything less than Big Bang conditions.

  Gabriel hesitated, and for a moment Blanca wondered if he’d taken the scenario to heart. “That would be a setback,” he conceded. “But Kozuch Theory clearly predicts that when you hit an electron linked to a proton with another one linked to a positron, the proton will decay into a neutron, the positron will neutralize ... and the final wormhole will be even wider than the one we’ve just made. And there’s no room left, now, for idle speculation about Kozuch Theory being wrong. So —” He thumbed his nose at ver, then jumped to the Forge scape.

  Blanca followed. The schematic ahead of them showed a wire-thin cylinder; the thickness was not remotely to scale, but the length was correctly portrayed, stretching more than ten times wider than Pluto’s orbit. All the planetary orbits were drawn in, but the inner four, Mercury to Mars, were lost in the glare of the tiny sun.

  The Forge was a giant particle accelerator, consisting of over fourteen trillion free-flying components. Each one used a small light-sail to balance the sun’s slight gravitational pull and keep itself locked onto a rigid straight line 140 billion kilometers long. The sails worked off beams sent fanning out from a network of solar-powered UV lasers, orbiting the sun closer than Mercury; they also extracted the energy needed to power the accelerator.

  Most of the components were individual PASER units, lined up one after the other at ten-meter intervals. They re-focused the electron beams, then boosted the energy of each particle passing through them by about 140 microjoules. That didn’t sound like much, but for one electron it was equivalent to 900 trillion volts. PASERs used the Schächter effect: a suitable material was bathed in laser light, raising its atoms into high-energy states, and when a charged particle passed along a narrow channel drilled through the material, its electric field triggered the surrounding atoms into giving up their energy. It was as if the laser primed countless tiny electronic catapults, and then the particle came along and sprung them all, one after the other, getting a small kick forward from each one.

  The energy density maintained within each PASER was enormous, and Blanca had seen a recording of an early test model bursting from radiation pressure. There hadn’t been much of an explosion, though; the PASERs were tiny garnet-like crystals, each one massing less than a gram. Substantial asteroids, hundreds of meters wide, had been mined for the tens of millions of tons of raw materials needed to make the Forge, but even Carter-Zimmerman’s most gung-ho astrophysical engineers would have vetoed any design that required gutting Ceres or Vesta or Pallas.

  Blanca jumped to one end of the Forge, where the scape showed a “live” image of the real equipment, albeit delayed by the 65 hours it took for the signal to reach Earth. At both ends of the linear accelerator, electron-positron pairs were created in small cyclotrons; the positrons were retained in storage rings, while the electrons were fed straight into the main accelerator. The opposing beams met in the center of the Forge, and if two electrons collided head-on, fast enough to overcome electrostatic repulsion, Kozuch Theory predicted that they’d splice wormholes. The electrons themselves would disappear without a trace — locally violating conservation of both charge and energy — but the negative charge lost would be balanced by the neutralization of the positrons at the new wormhole’s far ends, and the energy of the missing electrons would manifest itself as the mass of the two neutral particles which the positrons had become, dubbed “femtomouths” or “FMs” by the Forge group’s theorists, since they were expected to be about a femtometer wide.

  Blanca was remaining cautiously skeptical, but it seemed that the predicted sequence of events had finally taken place. No instruments had witnessed the vanishing act at the center of the Forge; tracking the torrent of electrons and looking for one perfect collision among all the near misses would have been impossible. But neutral particles of exactly the right mass, heavy as specks of dust but smaller than atomic nuclei, had been caught in the laser traps surrounding both storage rings at exactly the same time.

  Gabriel had followed ver, and now they moved together through the hull of the storage ring facility and hovered above the laser trap. The scape merged a camera-based view of the equipment with schematics generated from instrument readings; most unrealistically, they could see the putative FM — a black dot radiating self-important tags — being gently shuffled through the trap by the shifting gradients of luminosity, scattering UV photons just enough to let the lasers nudge it along.

  It would take over an hour for the FM to be delivered from the trap into the next stage. They rushed, though not as quickly as before.

  “Aren’t the rest of the Forge group watching this?” They’d entered the scape privately, invisible and oblivious to any other users; Gabriel had inflected the address that way.

  “Probably.”

  “Don’t you want to be with them at the moment of proof?”

  “Apparently not.” Gabriel pressed his hand inside ver again, deeper this time; pulses of warmth spread out from the center of vis torso. Blanca turned toward him and stroked his back, reaching for the place where the fur became, if he chose, almost unbearably sensitive. C-Z culture had its problems, but in Konishi a simple exchange of pleasure phrased in this manner would have been unthinkable. The two of them were not slavishly embodied; harm remained impossible, coercion remained impossible. But Konishi had sanctified autonomy in the same absurd fashion as the statics had sanctified the pitfalls of the flesh.

  The FM arrived in the gamma-ray chamber, and a series of intense pulsed bombardments began. The gamma-ray photons had wavelengths of around ten-to-the-minus-fifteen meters, roughly the same as the FM’s diameter. A photon’s wavelength had nothing to do with the size of its wormhole mouth, but it did measure how precisely you could constrain its location and aim it at a chosen target.

  Blanca protested, half-seriously, “Why couldn’t you have positioned the Forge so the time lags were equal?” Gamma rays should have been emerging instantaneously from the wormhole’s other mouth, but the far end of the accelerator was three billion kilometers further from Earth than the near end, so it would be another three hours before they’d know what had happened there, 68 hours earlier.

  Gabriel defended himself almost absent-mindedly. “It was a compro
mise. Comets to avoid, gravitational effects to balance ...” Blanca followed his gaze into the flickering gamma-ray glow, and knew at once what he was thinking. What they were witnessing here opened up some very strange possibilities. According to a hypothetical observer flying along the axis of the Forge toward the far end, these photons, transported faster than light, would be coming out of the wormhole before they went in. That peculiar ordering of events was largely academic — the traveler wouldn’t even know about it until photons from both ends had had time to reach ver — but if ve also happened to be carrying a wormhole mouth of vis own, linked to one in the hands of an accomplice in a second spacecraft following behind, then as the traveler flew past the far end of the Forge ve could signal the accomplice to destroy the gamma-ray source at this end ... before the photons ve’d just seen emerging had ever been sent.

  Once they had a second wormhole, the Forge group would be able to make this ancient thought experiment a reality. The most likely solution to the paradox involved virtual particles — the mouths of vacuum wormholes — traveling in a loop that included both the Forge wormhole and the ship-borne one. Virtual particles were constantly streaming along every available path through space-time, and though crossing ordinary space between the mouths of the two wormholes would take them a certain amount of time, moving through the ship-borne wormhole would carry them back into the past, reducing the total time needed to go around the loop. As the two spacecraft neared the point where signaling from future to past became possible, the transit time for the loop would approach zero, and each virtual particle would find an exponentially growing army of doppelgangers hard on its heels: future versions of itself which had already made the trip. As they slipped into perfect phase with each other, their rapidly increasing energy density would make the wormhole mouths implode into tiny black holes, which would then vanish in puffs of Hawking radiation.

  Apart from ruling out time travel, this would have serious practical consequences: once the galaxy was crisscrossed with wormholes, there’d be loops of virtual particles threading them all, and any careless manipulation of the mouths could see the whole network annihilated.

  Gabriel said, “It’s almost time. Shall we ...?” They jumped to the far end of the Forge, where the scape was showing the most recent data available: still a few minutes before the gamma-ray bombardment had begun. The second FM sat in an observation chamber, under the scrutiny of a cylindrical array of gamma-ray detectors, nudged occasionally by UV lasers to keep it perfectly centered. The faint scatter from the lasers was the only sign that the thing was really there; with no electric charge or magnetic moment, it was a far more elusive object than a single atom.

  “Don’t you think we should be with the others?” Blanca had lived with the distant promises of the Forge for so long now that it was hard to be moved by this first, microscopic hint of what lay ahead. But if they really were on the threshold of a change that would shape the history of the Coalition for the next ten thousand years, it seemed like a fair excuse for public celebration.

  “I thought you’d be pleased.” Gabriel laughed curtly, offended. “At the end of eight centuries, we’re together for this moment. Doesn’t that mean anything to you?”

  Blanca stroked his back. “I’m deeply touched. But don’t you think you owe your colleagues —”

  He disengaged from ver angrily. “All right. Have it your way. We’ll join the crowd.”

  He jumped. Blanca followed. As they re-entered the scape in public mode it seemed to expand dramatically; half of Carter-Zimmerman was hovering in the space above the observation chamber, and the image had been re-scaled to fit them all in.

  People recognized Gabriel at once, and flocked around to congratulate him. Blanca moved aside and listened to the excited well-wishers.

  “This is it! Can you imagine the gleisners’ reaction, when they arrive at the next star and find that we’ve beaten them to it?” The citizen’s icon was an ape-shaped cage full of tiny yellow birds in constant flight.

  Gabriel replied diplomatically, “We’ll be avoiding their targets. That was always the plan.”

  “I don’t mean we should explore the system in competition with them. Just leave an unmistakable sign.” Blanca considered interjecting that the first few thousand wormholes they widened would be most unlikely to include any of the gleisners’ immediate destinations, but then thought better of it.

  On jumping to the scape, they’d synched by default to the average rate of its inhabitants, a rush of about a hundred thousand. It was fluctuating, though; some people were growing impatient, while others were trying to prolong the suspense. Blanca let verself drift with the average, enjoying the sense of being jostled through time by the whims of the crowd. Ve wandered through the scape, exchanging pleasantries with strangers, finding it hard to take the vast machinery of the observation chamber seriously so soon after experiencing it all on a scale where there’d barely been room to spread vis arms. Ve spotted Yatima in the distance, deep in conversation with other members of the Forge group, and felt an amusing surge of quasi-parental pride — even if most of the skills ve’d taught the orphan would have been more use to a Konishi Miner than a C-Z physicist.

  As the moment approached, people started chanting a countdown. Blanca searched for Gabriel; he was surrounded by demonstrative strangers, but when he saw ver approaching he broke away.

  “Five!”

  Gabriel took vis hand. “I’m sorry.”

  “Four!”

  He said, “I didn’t want to be with the others. I didn’t want to be with anyone but you.”

  “Three!”

  Fear flashed in his eyes. “My outlook’s programmed to cushion me, but I don’t know how I’ll take this.”

  “Two!”

  “One traversable wormhole, and then the rest is mass-production. I’ve made this my whole life. I’ve made this my whole purpose.”

  “ONE!”

  “I can find another goal, choose another goal ... but then who will I be?”

  Blanca reached up and touched his cheek, not knowing what to say. Vis own outlook was much less focused; ve’d never faced a sharp transition like this.

  “ZERO!”

  The crowd fell silent. Blanca waited for the uproar, the cheers, the screams of triumph. Nothing.

  Gabriel looked down, then Blanca did too. The femtomouth was scattering the lasers’ ultraviolet, as ever, but no gamma rays were emerging.

  Blanca said, “The other mouth must have drifted out of the focus.”

  Gabriel laughed nervously. “But it didn’t. We were there, and the instruments said nothing.” People around them were whispering their own theories discreetly, but their gestalt seemed more tolerantly amused than derisive. After eight centuries of setbacks, it would have been too good to be true if the Forge had delivered the definitive proof of its success at the first opportunity.

  “Then there must be a calibration error. If the mouth drifted, but the instruments thought it was still at the focus, then the whole system needs to be recalibrated.”

  “Yes.” Gabriel ran his hands through the fur of his face, then laughed. “Here I am expecting to fall off the edge of the world, and one more thing goes wrong to save me.”

  “One final screw-up to smooth the transition. What more could you ask for?”

  “Yeah.”

  “And then what?”

  He shrugged, suddenly embarrassed by the whole question. “You said it yourself: linking the Forge is only the start. We haven’t wrapped the universe in wormholes yet. And at this rate, there’ll be screw-ups to smooth the transition for another eight hundred years.”

  Blanca spent half a gigatau exploring vis new imaginary world, fine-tuning the parameters and starting again a thousand times, but never intervening and sculpting the landscape directly. That was wicked — it made it less artful, and more mock-physical — but no one had to know. When ve opened it up to the public, people would marvel at its perfect blend of consistency and spo
ntaneity.

  Ve was sitting on the edge of a deep canyon, watching leaf-green dust clouds flow in around ver like a vivid but ethereal waterfall, when Gabriel appeared. Blanca had spent some time worrying about the problems with the Forge, but within the first megatau it had slipped from vis thoughts completely. Ve knew they’d sort it out, the way they’d sorted out every other obstacle. It was always just a matter of perseverance.

  Gabriel said calmly, “Gamma rays are coming through the far end now.”

  “That’s wonderful! What was the problem? A misaligned laser?”

  “There was no problem. We haven’t carried out any repairs. We haven’t changed a thing.”

  “What, the mouth just drifted back into the focus? Is it oscillating back and forth in the trap?”

  Gabriel dipped his hands into the green flow. “It was always sitting at the focus, perfectly positioned. The gamma rays we’re seeing now are the ones that went in at the start. We coded all the pulses with a time stamp, remember? Well, the first pulses to emerge had the time stamp for the gamma rays sent in five and a half days ago. They’ve taken as long to come out as if they’d crossed the ordinary space between the mouths. Exactly, down to the picosecond. The wormhole is traversable, but it isn’t a short cut. It’s a hundred and forty billion kilometers long.”

  Blanca absorbed this in silence. Asking if he was sure didn’t seem like a good idea; the Forge group would have spent the last few megatau searching frantically for a more palatable conclusion.

  Finally, ve said, “Why? Do you have any ideas?”

  He shrugged. “The only thing we can come up with that makes any sense is this: the total energy of the wormhole depends almost entirely on the size and shape of the mouths. It’s the mouths that interact with virtual gravitons; the wormhole tunnel can be as long or short as you like, and the mouths will still have exactly the same mass.”