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.”
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.