by Greg Egan
“On the surface of the sun,” Cornelio replied. “In our own burning stones.”
“I see.” A burning stone heated its surroundings, adding kinetic energy to them, so true energy would have to be flowing the other way, into the flame. Did that make sense? Cornelio had warned her that energy only flowed from the higher temperature to the lower if both had the same sign.
The mixed-sign case wasn’t hard to understand, though. A system with a positive temperature would gain possibilities if it gained energy. A system with a negative temperature would gain possibilities if it lost energy. Putting the two together, there was no subtle trade-off anymore—this was a win-win situation. Both systems could gain volume in momentum space from the same transaction.
So, any system with a negative temperature would lose true energy to any system with a positive temperature. That was why Cornelio had seen merit in calling ordinary objects “hotter than infinitely hot”; however high the positive temperature of a blazing sunstone, a “hotter than infinitely hot” cool breeze could still heap true energy upon it.
Yalda said, “But how can you know for sure that something has a positive temperature—and not just a large negative one? How do you know when things aren’t just ‘hot’ in the old-fashioned way?”
“Light,” Cornelio replied. “Whenever a system freely creates light—not in the orderly way a flower does it, but in the chaos of a flame—it’s turning true energy into something that didn’t exist before, opening up new possibilities. That’s the very definition of positive temperature.”
“So once an ordinary system with a negative temperature starts creating light,” Yalda ventured, “its temperature must change sign? Crossing infinity along the way?”
“Precisely,” Cornelio said. “Once it’s creating light, it’s lost to the ordinary world.”
Yalda couldn’t help stealing another glance at the workshop’s collection of energetically precarious concoctions. Above the shelves, the ceiling still showed signs of recent repairs.
“In the end,” Cornelio declared, “everything becomes heat and light. It’s not in our power to stop that. All we can do is slow it down a little and try to enjoy the ride.”
Yalda ended up staying in the chemistry department until dusk, then she caught a lift in the department’s truck back to the city campus, along with Cornelio and five of his students. As they drove across the dusty plain, Cornelio explained how the pressure of a gas could remain positive as its temperature changed sign, and finite as its temperature crossed infinity. The old ideal gas law—pressure times volume is proportional to temperature times quantity—was receding into the distance; it wasn’t even true within the flames of an ordinary lamp.
The back of the truck was open to the sky, so Yalda saw the Hurtler’s violet tip rushing toward them from the north, but then the driver panicked and slammed on the brakes, sending the vehicle lurching and skidding. When it came to a halt she could recall nothing but a whirl of color across a spinning bowl of stars.
Everyone clambered out, checking themselves for injuries, but it was soon clear that nobody had been hurt. Yalda was still clutching her light recording supplies; she examined the contents of the box in the starlight, but Cornelio had packed everything carefully and none of the vials appeared to have been damaged. She helped some of the students push the truck back onto the road, wasting no time fretting over the lost opportunity. At this rate, there’d be another Hurtler over Zeugma in a couple of stints.
“What do you think they are?” she asked Cornelio, as the truck lurched into motion again.
“Fragments from a big explosion,” he replied. “Something so distant that even the smallest differences in the speed of the debris could spread out its arrival over many years. My hunch is that successively later fragments will prove to be moving more slowly.”
“That’s an interesting idea.” Yalda tapped the box of goodies appreciatively. “Hopefully I’ll be able to test it soon.” A crisp image of a Hurtler’s light trail, captured at a single moment, might enable her to measure its asymmetry and finally quantify the object’s speed.
It was dark when they reached the city. At the university, Yalda bid farewell to Cornelio, stashed her new supplies in the optics workshop, then braved Ludovico’s wing of the department to see if Tullia was still around. But the place was empty. Tullia might have caught some observations of the Hurtler, then, on her way to her apartment or the Solo Club.
At the Solo, Yalda found Daria and Lidia; they hadn’t seen Tullia, but they persuaded Yalda to join them in a game of six-dice. To everyone’s amazement, Yalda won, so she stayed for a second game. Lidia beat her this time, but it was close.
Yalda was tired now, but she decided it was worth stopping by at Tullia’s apartment; it would be good to share the highlights of her trip to Amputation Alley. Tullia was planning to head up to Mount Peerless in a few stints; she could probably make use of Cornelio’s invention there herself.
When she arrived at the apartment, Yalda found the entrance unbarred but the curtain closed. She called out softly a few times, but received no reply. Tullia didn’t usually sleep so early, but if she’d dozed off after a hard day it would be unfair to wake her.
Yalda turned and started toward the stairs, but then she changed her mind; it wouldn’t hurt to sneak in quietly and check that everything was all right. She walked back, parted the curtain and stepped into the apartment.
Tullia was lying on the floor near the window. Yalda called her name but there was no response. She approached and knelt to examine her; Tullia was limbless, and her skin displayed a strange sheen. For one panicked moment Yalda thought of her grandfather, but then she realized that the patches of light she was seeing were just distorted reflections of the flowers above.
Yalda took her friend by the shoulders and shook her gently; her skin felt strange—tight, almost rigid—and she did not react at all. There was a furrow down the middle of her chest: a deep, narrow fissure, the first line of a symbol that Tullia would never have written by choice.
“No,” Yalda whispered. “That’s not happening.” She scrabbled in her pocket for her vial of holin. If Tullia had taken a weak dose, it might not be too late to augment it. Yalda tipped three of the small green cubes into her palm, then reached for Tullia’s mouth.
But Tullia had no mouth. The darker pigmentation of her lips remained visible, but the skin that bore their color and shape was part of a smooth, seamless expanse. Yalda dropped the holin and moved her fingers over Tullia’s face, gently probing one of her eyes; the eyelids were still discernible, but they’d fused together. Below her mouth, her tympanum was rigid. Her body was becoming as hard and featureless as a seed case.
Yalda was shaking now; she forced herself to be still. Who would know what to do? Daria, surely. Yalda leaned out the window and spotted a boy on the street; she threw him a coin to get his attention, then begged him to run to the restaurant under the Solo and ask the chef to “fetch Daria, urgently, for Yalda”. Two more pieces and the promise of two more on his return did the trick.
As Yalda knelt down again, her shadow fell on Tullia and she saw that there was a faint glow from her body—though it was not on the surface, like her grandfather’s affliction. This light came from deep within, and it flickered and shifted constantly: a frenzy of signaling so intense that it could now be seen through all the intervening flesh.
Yalda stroked Tullia’s forehead. “We’ll fix you up,” she promised. “It will be all right.” If Daria could get hold of some melding resin, they could glue together the sides of the furrow and let Tullia’s own body attack the dividing wall. And if Daria sent for her sunstone lamp, the same kind of flash that had set the captive arborine’s muscles twitching could disrupt the signals that were organizing Tullia’s fission. There were so many things they could do. Tullia wasn’t sick, or old, or frail. She wasn’t enslaved to an impatient co. She was a free woman, in the care of her friends.
Having reached the top of
her torso, the furrow now began to divide Tullia’s tympanum. Yalda took hold of the edges and squeezed them together with all her strength. “No further,” she said. “And when this hand grows tired, there are ten more waiting.” But in fact there was no great force opposing Yalda’s intervention, just the faintest springiness in the underlying flesh.
Tullia would survive; Yalda was sure of that now. Survive, and flourish. She would enlighten her students and delight her friends for a dozen more years. She would find the light of a forest on a distant world.
The furrow itself wasn’t lengthening, but Yalda could see the walls of hard skin stretching beyond the point where she was pinching them together, growing up toward the place where Tullia’s mouth had been. The slight convexity of her blind eyes had vanished now; the organs had been resorbed and their lids subsumed into the featureless skin around them.
Yalda heard footsteps, then the curtain parted. Daria hurried in, followed by Lidia.
“You promised some boy that I’d pay him four pieces?” Daria asked irritably. “This had better be—”
As Yalda moved aside to let Daria apprehend the reason she’d been summoned, she saw that a cross-furrow was forming now, threatening to divide each of Tullia’s lateral halves. “Did you bring some melding resin?” she asked Daria. “Or should we stitch the gaps together? Clamping with my hands doesn’t work.”
Lidia approached. “There’s nothing we can do,” she told Yalda gently. “Any resin, any drug, any surgery—all that would do now is kill the children.”
Yalda looked to Daria. “That can’t be true.”
Daria said, “Once division starts, it’s irreversible.”
Lidia put a hand on Yalda’s shoulder. “Let her be.”
Yalda turned to her. “What—just let her die?”
“It’s not a choice anymore,” Daria explained sadly. “You can hold her body as tightly as you like, but her brain is already in pieces.”
“Her brain’s destroyed?” Yalda stared down at Tullia’s blank face. “It’s her brain that’s making this happen, isn’t it? Sending out the signals. Don’t tell me it’s in pieces.”
Daria walked up and knelt beside her. “Yalda, she’s gone. She would have been gone long before you found her.”
“No, no, no.” No more lives cut short. Claudia and Aurelia had been far away, out of her hands, but not Tullia.
She turned back to Lidia and pleaded, “What do we do? Tell me!”
Lidia said, “All we can do now is remember her.”
Yalda ran her hand over Tullia’s wounds; the furrow now reached the top of her skull. “There must be something.”
Daria spoke firmly now. “Yalda, this is beyond changing. She was our friend, and we loved her, but her mind is gone; she’s as dead to us now as any man in his grave. Tonight we can only grieve for her.”
Yalda felt herself shivering and humming. She didn’t believe Daria’s words, but some traitorous part of her had decided to behave as if they were true.
“And in the morning,” Daria added, “we’ll need to talk about the way we’re going to raise her children.”
10
As Yalda stood in the courtyard waiting for her guest to arrive, she counted two dozen pale streaks of color drifting in mirrored pairs across the afternoon sky. From their leisurely pace she could tell that these Hurtlers were not especially close—perhaps a little farther than the sun itself—but to be visible by day at such a distance they would have to have blazed far more brightly than the specks that only showed at night.
Brighter almost certainly meant larger.
She spotted Eusebio crossing the courtyard and held up a hand in greeting. “Hello, Councilor.”
“It’s good to see you again, Yalda,” he said.
“You too.”
He glanced up at the sky. “It seems almost normal now, doesn’t it? People can get used to the strangest things.”
“Sometimes that’s a useful trait,” Yalda said.
“But not this time,” Eusebio suggested.
“Maybe not.”
They left the courtyard and strolled across the campus; Yalda had offered to organize a meeting room, but Eusebio had wanted to avoid any suggestion that either of them were acting in an official role. Two old friends were getting together to reminisce, that was all.
“I’ve only heard third-hand versions of your theory about the Hurtlers,” he said. “But they were enough to get me worried.”
“The whole idea’s extremely speculative,” Yalda said. “I wouldn’t go jumping off any bridges yet.”
Eusebio was amused. “Yet? Believe me, that’s not where I’m headed, prematurely or otherwise.”
“But you say you’re worried.”
“Of course I’m worried. Wasn’t that the point? Why else would you raise those possibilities?”
Yalda wasn’t sure how to answer that. The truth was, when she’d first begun discussing the idea with a few colleagues, she hadn’t taken it very seriously. It had been nothing more than an audacious stab in the dark—and far too esoteric, she’d thought, to pose any risk of sowing panic.
Eusebio said, “Maybe there’s something you can clear up for me, to start with. Years ago, you gave a talk where you said the cosmos was a flat, four-dimensional torus. In which case… if you followed a bundle of histories through time, wouldn’t they remain pretty much parallel? And when they met, wouldn’t they just form a loop?” He drew a two-dimensional version, first as a square and then curled up to remove the artificial edges.
Yalda said, “The flat torus is just an idealization—the simplest case where the light equation can be solved nicely. The topology of the cosmos might be more complicated, or the geometry might not be flat. Or maybe the histories of the worlds haven’t stayed together in a nice tight bundle, the way you’ve drawn them; it’s true that everything has to join up in the end, but it doesn’t have to do it tidily. If there was a primal world that fragmented in both directions—one we’d think of as the future, and one we’d think of as the past—and the fragments themselves then broke up, and so on… we could end up colliding with the past fragments at almost any angle.”
Yalda sketched a crude illustration of the idea. “I won’t try to draw this wrapped around a torus, but you can imagine the possibilities if those two sets of fragments ever came to overlap.”
“So the primal world gets to explode both backward and forward?” Eusebio gave a chirp of delight tinged with skepticism.
“I know it sounds strange,” Yalda conceded, “but if the primal world is where entropy reaches its minimum value, it’s just as reasonable for it to explode in one direction as the other. Imagine the cosmos filled with a vast tangle of unruly threads—the histories of all the particles of matter—and then demand that somewhere they’re all packed together and perfectly aligned. I’m not sure why that has to occur at all, but unless there’s some extra rule imposed on top of it, the threads are going to break free in the same way on both sides of the constriction—creating two localized arrows of time pointing in opposite directions.”
“But whatever the fine details,” Eusebio said, “so long as the cosmos is finite—and the light equation suggests that it must be—there’s a potential for the Hurtlers to be the harbinger of something worse.”
“A potential. Exactly.” Yalda didn’t want him losing sight of that. “Admitting that there could be places in the cosmos where two sets of worlds cross paths with each other doesn’t mean that has to be what’s happening here and now.”
“Except that the alternative explanations for here and now aren’t working out too well,” Eusebio replied. “If the Hurtlers are just debris from a single monstrous explosion, shouldn’t the fragments be arriving ever more slowly?”
Yalda said, “Yes—but whether we could measure the change in velocity over just a few years is another question. It’s been tough enough quantifying the velocity at all.”
Eusebio was unpersuaded. “Why should the change be
so hard to spot, though? I can imagine an exploding world sending out a blast of high-speed dust, along with a slower-moving barrage of pebbles. But if that really does explain everything, shouldn’t the difference in speed be as striking as the difference in size?”
“Perhaps,” Yalda admitted.
“My third-hand source implied that you’d more or less predicted this”—Eusebio gestured at the color trails crowding the sky—“almost two years ago. A cluster of worlds and stars, much like the one we seem to lie within ourselves, would be surrounded by a halo of fine dust. Then as you penetrated deeper into its environs you could expect to encounter larger objects.”
“It’s hard to know for sure what the structure would be,” Yalda said. “We don’t understand the breakup of worlds, let alone the long-term effects of gravity and collisions between the fragments.”
“But it’s not an unreasonable position,” Eusebio persisted, “to posit rarefied dust at the edges and more substantial material closer in?”
“No.” However much she wished to downplay the conclusions, Yalda couldn’t retreat from the entire argument she’d made.
Eusebio said, “Then if our notion of time corresponded to one of this cluster’s notions of space… we should expect to find ourselves encountering successively larger objects, all moving at similar speeds. Isn’t that right?”
He offered an illustration.
“Couldn’t you at least have us glancing the edge?” Yalda pleaded. “We don’t need to be heading in as deeply as you’ve drawn it.”
Eusebio obliged. “I knew I should have been a physicist,” he said. “If there’s something you don’t like about the world, you merely adjust a free parameter and everything’s perfect.”
“What would you have me do?” she said. “Give up hope for all of our grandchildren?”
“Not at all. I want you to imagine the worst, and then tell me how we can survive it.”
