by Greg Egan
Within minutes, the microprobes had reported six more eruptions, from underwater sites scattered around the planet. It made no sense to Azar; these gigatonnes of water were rising into orbits about a thousand kilometers up, but if they were meant as weapons, who were they aimed at? The microprobes were much lower down, and Mologhat was a hundred times further away. A direct hit with a solid iceberg could have done a lot of damage to any intruder, but these glistening snowballs weren’t even holding together; Tallulah was just shrouding itself with a tenuous halo of tiny ice crystals.
“This isn’t warfare!” she declared. “They don’t think they’re under attack. They saw the gamma rays, and thought: antimatter. They’re afraid they’re drifting into a cloud of antimatter. The ice is to tell them if there’s any more around.”
Shelma considered this. “I think you’re right. They picked up a flash of annihilation radiation, and jumped to the conclusion that it was a natural source.”
Never mind that there was no natural source of bulk antimatter anywhere in the galaxy; if you’d spent a billion years in space without encountering another civilization, perhaps a small cloud of antihydrogen seemed like a far less extravagant hypothesis than alien visitors using proton-antiproton gamma rays for communications.
“So they still don’t even know we’re here?” Azar wondered. “All those radio messages came to nothing. What do we have to do to get noticed – tattoo the binary digits for pi across the stratosphere?”
Shelma said, “I wouldn’t advise that. But it’s not even clear to me that there’s anyone home; this might just be a non-sentient device that’s outlived the people it’s meant to be protecting.”
The water missiles had stopped; the absence of any answering flash of radiation must have made it clear that if there was antimatter around, it was far too thinly spread to pose any kind of hazard.
Azar tried calling Mologhat again, but it was still not responding. “They must have hit it,” she said. “Whatever they thought it was, they must have launched something small and fast, and knocked it out before the ice storm even started.” She felt numb. So much for the doorway to the stars.
Shelma touched her arm reassuringly. “It might yet reply – but even if it’s gone, we’re not stranded.”
“No?” The microprobes had nowhere near enough power for interstellar transmissions, or even the raw materials to build the kind of hardware they’d need. And the data-ferrying balloon couldn’t send them anywhere; their return path to Mologhat would have involved a gamma-ray mirror on the balloon, modulating and reflecting radiation coming down from the station itself.
Azar slumped into her seat. How had she ever imagined she could do this? Travel fifteen hundred light years as if it was nothing? There was no magic gate leading home, just fourteen quadrillion kilometers of vacuum.
Shelma said, “We have plenty of resources down here.”
Azar rubbed her eyes and tried to concentrate. “That’s true.” Given time, the nanotech could build them almost anything – and they didn’t even have to reach all the way back to Hanuz or Bahar; they just had to connect to the Amalgam’s network. Still, the nearest node was seven hundred light years away; getting a signal that far was a daunting prospect. “Can we do this from the ground?”
“Well ... we could build a radio dish a few hundred kilometers across,” Shelma replied, deadpan. “Factoring in suitable error correction for the signal-to-noise ratio, it might only take two or three centuries to complete the transmission.”
Azar got the message. “OK: better to build a rail gun and launch a transmitter into orbit. But even if we can power a rail gun, how do we power the transmitter? We don’t have any antimatter. There’s virtually no deuterium here; are we going to try to build a hydrogen-boron fusion reactor?” The most efficient way to produce gamma rays was from antimatter – that would certainly make for the lightest power source to loft into orbit – but trying to create even a few micrograms of antihydrogen with nothing but plant carbohydrates as the energy source made Shelma’s giant radio dish sound like a good idea. Whatever was guarding Tallulah might be a tad obtuse, but it was difficult to imagine a particle accelerator powered by industrial-scale deforestation slipping under the radar.
“What’s the point of this transmission anyway?” she said bitterly. “To arrive home empty handed, with no news worth hearing? If it comes to that, I’d rather let my backup wake.”
“So would I,” Shelma said, “but I think you’re missing something.”
“Yeah?”
“The news worth hearing,” she said, “and the energy source we need in order to send it, are the very same thing. Whatever’s keeping this planet warm is just a few kilometers beneath our feet. If we can reach it, study it, understand it and harness it, we’ll have both the means to get home, and the reason.”
4
A few kilometers beneath our feet was an encouraging way of putting it; from where they stood the actual distance was twenty-seven thousand meters. The nanotech built some robot moles, powered by long thermoelectric tails, and sent them on their way. They would reach the heat source in about two hundred days.
The oceanic crust was much thinner in places. Azar did some calculations. It wasn’t clear what kind of food there’d be in the water, but she thought it was worth finding out. Shelma agreed, and they set out for the coast.
The insect made good time, averaging about thirty kilometers an hour, but when they reached the edge of the jungle food became more sparse and the scattered plants less nutritious. Flying across the flat, monotonously glowing savannah, Azar ached for sunrise to come and banish the interminable night. But she fought down the pang of homesickness and tried to find the beauty in this upside-down world.
Other explorer insects were already fanning out from a dozen sites where the spores had landed, building up a picture of the continent’s geochemistry. A tentative analysis of the data suggested that the surface had only been above sea level for about a quarter of a billion years.
“Before that, there might have been no dry land at all,” Shelma suggested. “That would explain why the ecosystem here is so young.”
“So where did all the water go?” Azar wondered. “Unless their antimatter detectors have a lot of false alarms.” Even the modest amount that they’d seen thrown skywards would mostly rain back down again.
“A collision?” Shelma frowned and withdrew the suggestion. “No, the odds would be very low out here, to hit something big enough.” The current estimate of Tallulah’s galactic orbit suggested that it hadn’t even crossed another system’s Oort cloud in the last billion years.
They reached the shoreline. Waves lapped gently on a lifeless beach; the infrared glow of the placid ocean made Azar think of liquid metal, but if she’d been wearing her body this water would have made a luxurious warm bath.
In the waves they found only single-celled creatures, living off a very thin soup of organic debris. They flew out a kilometer and took another sample, sending scout mites a few hundred meters down. The soup was thicker here; with a little tweaking, the insect would be able to make use of it.
There was a trench some six hundred kilometers off the coast, where the mysterious neutrino source was just nine thousand meters below the ocean floor. They set out across the waves, stopping every couple of hours to dive and feed.
Each time they plunged into the water, Azar noticed Shelma tensing. She wondered about the propriety of commenting on this; if she’d been looking at Shelma’s true self-image – a Bahari body with five limbs and five tails, like the rear end of a cat caught in a kaleidoscope – she would not have been able to distinguish tranquility from terror. Still, it was not as if the scape was reading Shelma’s mind; it was merely translating information that she had chosen to make public.
As they approached the trench, Azar finally spoke. “You don’t have to do this if you don’t want to.” The trench was three thousand meters deep; if Shelma had retained a primal fear of drowning, Az
ar had no wish to see her suffer. “We can split the processor, and you can stay up here.”
Shelma shook her head, a little puzzled by this offer. “No, I’ll go with you. But first I want to bring in as much of the library as we can fit.”
“Oh.” Azar understood now; this was nothing to do with how Baharis felt about getting their fur wet. Once they were underwater they’d lose radio contact with everything, including the balloon-borne library.
Shelma began communing with the library, trying to choose a selection of its contents that would fit inside the insect without leaving them ill-equipped in the face of some crucial problem or opportunity. “I don’t want to meet the Ground Heaters and then find we can’t even make sense of their language!”
“If they’re so smart,” Azar replied, “let them make sense of ours.” Then again, if they’d spent a billion years living alone on the ocean floor, it would probably be unwise to expect too much of their communications skills.
Hanuz had sent no travelers out into the galaxy for a hundred thousand years, and that had been long enough. Though Tallulah had been an enticing destination, Azar had left home less for the sake of the orphan’s secrets than for the sake of breaking the curse. As the window for joining Mologhat had approached, she’d thought: If we don’t do this now, it will only grow harder. And she’d finally stopped waiting for someone else to volunteer.
Shelma announced that she’d completed her selection, then she changed her mind and dived back into the interface. Azar thought of her great-great-granddaughter, Shirin, struggling to pack for an overnight trip. Shirin would be ancient when Azar returned; she’d have left all her toy animals behind.
“This is it now,” Shelma declared. “We’re prepared for anything.” Her avatar wasn’t quite hyperventilating, but Azar could imagine her letting a breeze of random skills and factoids play through her mind, soaking her tissues with the oxygen of information before the conduit slammed shut.
“I could always give myself amnesia, if you want a little more room,” Azar suggested. Shelma actually looked tempted for a moment, before smiling thinly at the joke.
Azar took the joystick and the insect dived beneath the waves.
Their infrared vision wasn’t quite useless here; if they tuned to a wavelength a little shorter than the peak thermal emissions around them, they could see the shadows that nearby objects cast against the glow of the warmer water below. Augmented with the strobe flashes of sonar, the vague shadows became fluttering vertical ribbons, drifting in the current but maintaining their orientation. Azar sent in the scout mites, who found the ribbons to be packed with tiny buoyancy chambers that exchanged gases in a complex cycle, eking a few microwatts out of the temperature gradient. The ribbon-weed’s C3 sequence made it a close cousin of the land plants; in fact, it had probably changed very little from the ancestor whose other offspring had invaded the continents.
Five hundred meters down, they saw the first animals: small, segmented worms about a millimeter long, feeding off the ribbon-weed. The scout mites grabbed a few cells from the worms’ skin for analysis. As Azar watched the data coming through, she felt a sense of disorientation to rival anything since she’d stepped on to Mologhat. The worms had no C3 in their cells; they were no more related to the weed they were munching on than she was to Shelma. Their replicator was P2, a polypeptide. What’s more, their genome had been the subject of some blatantly artificial modifications, probably less than a million years ago.
“Introduced?” Shelma suggested.
“They must be,” Azar replied. Colonists from a P2 world must have come to Tallulah, bringing a few species from their home planet and tweaking them in order to survive here. It was a curious strategy; almost all interstellar travelers were digital in transit, not biological, and even if – like the founders of Hanuz – they had a fetish for recreating their original biochemistry upon arrival, such travelers tended to colonize sterile worlds. But then, nobody traveled to an orphan world for the sake of its real estate value. “It looks as if someone else came treasure-hunting long before we did.”
“Apparently,” Shelma concurred. “But if Tallulah is such old news, why isn’t the heating process known halfway around the galaxy by now?”
As they descended further the ribbon-weed grew larger, the soup of microbes thicker, and the P2 creatures more numerous and diverse. There were shrimp-like animals straining microbes from the water, floating gas bladders with poisonous tentacles, and sinuous, muscular fish of every size, feeding on each other, the ribbon-weed and the shrimp.
A vast forest came into sonar range, rising up from the ocean floor. Azar had already been impressed by the size of the free-floating ribbon-weed, but its anchored cousins were giants, fifty or sixty meters high. With convection currents in the water sweeping heat away more efficiently than air, the temperature gradient was far less steep here than on land, but the water also made it easier for taller structures to support themselves. The scout mites counted eighty species of animals in the forest’s upper reaches alone; some were P2, but there were C3 species as well, the first such animals the mites had found. And some were N3, with their genome encoded in nucleic acid.
“This place really has been popular,” Shelma observed dryly. “It’s enough to bring out the biograffitist in anyone. You sprinkle some N2 microbes in the water; I’ll add the C1.” N2 was DNA, Azar’s ancestral replicator.
The N3 species, like the P2, had been engineered, but the best estimates put the date of intervention much earlier, between two and three hundred million years ago. Azar checked their local copy of the library; there was no previous archaeological evidence for an interstellar civilization with N3 ancestry in that epoch – and this was not one of the databases Shelma had trimmed. How could a civilization reach such a difficult target as Tallulah, but leave no trace anywhere else?
As they descended slowly into the forest, the insect announced a discovery that had nothing to do with Tallulah’s polyglot biology. The robot’s mass spectrometer was constantly analyzing samples from the water around them, and it had just stumbled across an extraordinary find. The object in question had a mass of 40.635 atomic units; this far-from-integral figure might have made sense as an average for a sample containing a mixture of calcium-40 and its heavier isotopes – but it wasn’t an average, it was the mass of a single ion. Stranger still, when stripped of all its electrons the thing had a charge, not of twenty or so, but two hundred and ten. This was double the charge of any known stable nucleus, and ten times greater than its atomic weight implied.
“Who ordered that?” Azar quipped. Shelma didn’t even smile; the depleted library wasn’t up to providing the scape with the context for a proper translation.
“It’s femtotech,” Shelma declared.
Azar hesitated, then agreed. It was a staggering notion, but what else could it be? A new fundamental particle ... with a charge of two hundred and ten? Femtotech – the engineering of matter on the scale of atomic nuclei – was still a primitive art within the Amalgam; there had been some ingenious creations but they all had to do their job quickly, before they blew apart in a few trillionths of a second. The insect’s find had endured for at least three hundred seconds, and counting.
“How do you create a femtomachine with a binding energy equal to ninety percent of its mass?” she wondered. The most stable nuclei, nickel and iron, weighed about one percent less than the sum of their parts, thanks to the potential energy associated with the strong nuclear force. But increasing that effect by a factor of ninety was almost unimaginable.
The insect measured the ion’s magnetic moment. The result was orders of magnitude higher than that which a nucleus of atomic number 210 would be expected to possess if it was sitting quietly in its ground state; to generate such a strong magnetic field, it would need to be spinning at relativistic speeds. This only made the overall picture even stranger: the kinetic energy from this rotation should have added substantially to the ion’s total mass, rendering the
actual forty-something value even more bizarre. The one thing that made a warped kind of sense was the failure of the ion to tear itself apart from centrifugal force. How could it explode, when the fragments would need to possess ten times more energy than the whole?
Azar said, “I take it this is ash from the heating process?”
Shelma managed a dazed smile. “If it’s not, it certainly should be. Ninety percent mass-to-energy conversion. No wonder it’s still going strong after a billion years!”
Tallulah’s crust was generating heat at a rate of about two petawatts, stabilizing the planet’s temperature by replacing the energy that leaked out of the greenhouse blanket. At ninety percent mass conversion, that would consume less than eight hundred tonnes of fuel each year, so in principle the process could continue for about ten-to-the-eighteenth years: a billion times longer than it had been running so far. Unlike fission or fusion, even if the starting point for the femtotech process had to be one particular kind of nucleus, it really didn’t matter how rare it was in nature, since the energy required to synthesize it from anything else would be trivial in comparison. If each tonne of the Ground Heaters’ “gold” burned so fiercely that it could power the transmutation of a hundred tonnes of nickel or iron into yet more fuel, then Tallulah’s bonfire would easily outlive the stars.
Technology like this could transform the Amalgam. Antimatter had never been more than a wonderfully compact storage device, costing as much energy to make as it released. The most exquisitely efficient fusion systems extracted about half a percent of their fuel’s mass as usable energy. There were some unwieldy tricks with black holes that could do better, but they weren’t very practical, let alone portable. If everyone could harness the Ground Heaters’ femtotech, it would be like a magic wand that could turn nine parts in ten of anything into energy, leaving nothing behind but this strange spinning ash.