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.”
Yalda emitted a bitter, truncated buzz. “The worst? The Hurtlers will keep coming, ever larger and in ever-greater numbers, until the odds that we’re struck approach a certainty. If we survive that, we’ll probably collide with an orthogonal clump of gas—turning the world itself into something like a giant Hurtler. Somewhere along the way, there will be gravitational disruption, maybe ripping us free from the sun completely—or maybe tossing us into it. And if none of these things sound sufficiently fearsome, the encounter might scramble our arrow of time completely, leaving us with no past and no future. The world will end as a lifeless mass of thermal fluctuations in a state of maximum entropy.”
Eusebio heard her out without flinching, without disputing anything. Then he said, “So how can we survive that?”
“We can’t,” Yalda said bluntly. She pointed to his chest. “If it’s more than a glancing blow—if you’re going to deny me my choice of impact parameter—then we’re all dead.”
“Are you telling me that it’s physically impossible to protect ourselves?”
“Physically impossible?” Yalda had never heard an engineer use that phrase before. “No, of course not. It’s not physically impossible that we could shield ourselves from all of these collisions, or side-step them, or simply flee from the whole encounter. It would violate no fundamental law of physics if we built some kind of magnificent engine that carried the whole world off to a safer place. But we don’t know how to do that. And we don’t have the time to learn.”
“How long would it take?” Eusebio asked calmly. “To learn what we need to know to make ourselves safe.”
Yalda had to admire his persistence. “I can’t honestly say. An era? An age? We still don’t know the simplest things about matter! What are its basic constituents? How do they rearrange themselves in chemical reactions? What holds them together and keeps them apart? How does matter create light, or absorb it? And you want us to build a shield against collisions at infinite velocity, or an engine that can move an entire world.”
Eusebio looked around at a group of students chatting happily near the food hall, as if they might have overheard this catalog of unsolved problems and decided to rise to the challenge.
“Suppose we’d need an age, then,” he said. “A dozen gross years. How long do we actually have before the danger becomes acute?”
“I can only guess.”
“Then guess,” Eusebio insisted.
“A few dozen years,” Yalda said. “The truth is, we’re blind to whatever’s coming; a whole world, a whole blazing orthogonal star might already be disposed to strike us tomorrow. But from the progression in the size of the Hurtlers that we’ve seen so far, unless we’re especially unlucky…” She trailed off. What difference did it make? Six years, a dozen, a gross? All she could do was go on living day by day, averting her gaze from the unknowable future.
Eusebio said, “We need an age, and we don’t have it.”
“Exactly.”
“That’s what I thought,” he said, “but I had to hear it from you to be sure.”
His tone was solemn but far from despairing. Yalda stopped walking and turned to face him. “I’m sorry I had no good news for you,” she said. “Perhaps I’m wrong about all of this. Perhaps our luck will be far better than—”
Eusebio raised a hand, cutting her off. “We need an age, and we don’t have it,” he repeated. “So we find the time elsewhere.”
He wiped the colliding clusters from his chest. Then he drew two lines, one straight, one meandering, and added a few simple annotations.
“We make a rocket,” he said, “powerful enough to leave the world behind. We send it into the void and accelerate it until it matches the velocity of the Hurtlers. Once it’s done that, there’ll be very little chance that anything from the orthogonal cluster will strike it—but we might need to offset its position at the start, to keep it from colliding with gas and dust in our own cluster.
“The complete journey is as I’ve drawn it. The time that passes for the world will be the time it takes to rotate the rocket’s history by a full turn: one quarter-turn to accelerate, one half-turn to reverse, one final quarter-turn to decelerate. If the rocket accelerates at one gravity—giving the passengers no more than their ordinary weight—the time back home for each quarter-turn will be about a year, making four years in all.
“The time that passes on the rocket for those stages of the journey won’t be much greater: each curved segment is only longer than its height by a factor of pi on two. But when the rocket’s history is orthogonal to the history of the world, no time at all passes back home. So for the travelers, the journey can last as long as it needs to. If they require more time to complete their task, they can prolong the flight for another era, another age; it won’t delay their return by one flicker.”
Yalda was speechless. Their roles really had been reversed: the physics Eusebio was presenting was so gloriously simple that she was ashamed she hadn’t thought of it herself—if only in the same whimsical spirit as that in which she’d first thought of the Hurtlers as past-directed fragments of the primal world.
But when it came to practicalities, where did she begin?
“What kind of rocket can you hope to build,” she said, “in which generations can survive for an age—let alone flourish to the point where they have any chance of fulfilling their purpose? The largest rocket I’ve seen with my own eyes was the size of my arm; the largest I’ve heard of was smaller than my body. If you can send my optics workshop into the void that would be the talk of Zeugma for a generation, but I don’t know where we’d put the wheat fields.”
Eusebio hesitated, considering his reply, but he did not appear the least bit discouraged by her response.
“I believe you’ve been on Mount Peerless,” he said.
“Of course. The university has an observatory there.”
“Then you’ll know that it’s far from any permanent habitation.”
“Certainly.” Yalda thought she knew where he was heading: an isolated, high-altitude site would be the perfect place to test new kinds of rockets.
Eusebio said, “The geologists tell me that the core of Mount Peerless is pure sunstone. I plan to tunnel into it and set it alight, and blast the whole mountain into the sky.”
Yalda collected the children from school and took them back to the optics workshop. Amelia and Amelio were happy playing on the floor with a box of flawed lenses, lining them up to form impromptu telescopes and buzzing hysterically at the sight of each other’s distorted images. Valeria and Valerio were going through a stage of drawing pictures of imaginary animals that they insisted had to be preserved; Yalda gave them some old student assignments that were blank on one side of the paper, and some pots of dye that Lidia had brought home from the factory.
Then she stood at her desk watching over them while she tried to decide what to make of Eusebio’s plan.
Giorgio brought a group of students into the workshop to use the heliostat for an experiment in polarization. The children ran to greet him, and he accepted their embraces without a trace of annoyance or embarrassment before gently shooing them back to their activities. Yalda produced a stack of mechanics assignments and proceeded to mark them, marveling that she could feel guilty for taking a couple of chimes away from her conventional duties to ponder the correct means of averting the planet’s annihilation. She’d shared her ideas about the Hurtlers with Giorgio—and he’d offered his usual perceptive comments and objections—but in the end he’d still treated the whole thing as if it were an exercise in metaphysics.
Yalda arrived home with the children just as Lidia returned from her shift.
“Did you bring some more dye?” Valeria nagged her. She and Valerio had used up the last of their supplies on a series of images of giant worms with six gaping mouths arrayed along the length of their bodies.
Lidia spread her arms and jokingly opened six empty pockets. “Not today. Every batch was perfect.”
Valeria went into a sulk, which meant wrapping six arms around her co and trying to pull his head off. Yalda warned her three times, increasingly sharply, then stepped in and physically disentangled them.
“You always take his side!” Valeria screamed.
Yalda struggled to hold her still. “What side? What did Valerio do?”
At a loss for an answer, Valeria changed tactics. “We were just playing, but you had to spoil it.”
“Are you going to be sensible now?”
“I’m always sensible, you fat freak.”