Free The Arrows of Time: Orthogonal Book Three by Greg Egan

trajectories are neatly lined up, the curvature is different than when they’re moving around at
random.’
    ‘Different how?’
    Agata said, ‘Ordered matter creates positive curvature along its time axis, so objects that start out at rest are drawn together. But sufficiently disordered matter produces negative
curvature, with parallel histories spreading apart.’ She drew an illustration.
     

    ‘But when would the second case actually apply?’ Gineto wondered. ‘If you’re talking about a hot gas, won’t that always spread out into the void
and become too thin to make any difference? Doesn’t the gravitational pull of a star come mostly from the rock beneath the fire – the solid part that actually stays put?’
    ‘That’s true, of course.’ Agata had underestimated him when she’d thought he might have just been making conversation. ‘In fact, Lila proved that a
positive-temperature gas can’t be gravitationally bound – if the stars weren’t mostly rock, they couldn’t hold together at all! But on a large enough scale there might still
be a disordered state that’s gravitationally significant. Our cluster moves one way, the orthogonal cluster another . . . and if you could step back far enough, you might see clusters moving
in every direction in four-space. So it’s possible that something analogous to a giant hot gas – with clusters of stars playing the role of particles – determines the overall
curvature of the cosmos.’
    Serena said, ‘It’s getting close to the time.’
    All the partygoers were turning to face a screen mounted high on the chamber’s inside wall, showing an animation of an old-fashioned mechanical clock with its dials approaching the sixth
bell. Hanging in the darkness behind the clock was an artist’s rendering of the home world. Medoro caught Agata’s eye, and he didn’t need to say a word for her to read his cynical
mind: this was just the Council playing on their emotions. No doubt that was true, though to give them credit they hadn’t inserted any Hurtlers into the scene, poised to skewer the beloved
planet.
    As the pause-dial on the clock neared twelve the chamber fell silent. To Agata the dial seemed to slow, each click forward taking longer than the last. But then the marks aligned and the room
erupted with exuberant cheers. Medoro’s whole family were emitting deafening chirps – Medoro as enthusiastically as anyone. Agata felt her own tympanum thrumming, so she knew she was
joining in, but the sound of the crowd was so overwhelming that she didn’t have a hope of discerning her own contribution to the din.
    After crossing a dozen vasto-severances of void, the
Peerless
had reached the farthest point in its trajectory, halted for an instant and reversed. They weren’t fleeing any more;
they were on their way home. For the ancestors, a mere two years had elapsed since the launch, and with luck the travellers would return in two more.
    They would not be too late, Agata believed; they would not find a world in flames. The journey would fulfil its purpose – and the generations who’d endured the isolation of the
mountain, who’d suffered through famine and turmoil, who’d struggled and died with no reward, would not have lived in vain.
    Overcome, she sank to the floor on folded legs, her face down-turned, her rear eyes closed. She’d seen the ancestors’ sky, she’d stood motionless beside them. What more could
she have hoped for in her lifetime?
    But these moments of connection would never be repeated. All she had left was the distant promise of the reunion, as remote to her now as the launch.
    Someone touched her shoulder. Agata looked up, expecting Medoro’s hand, but it was his mother’s. The noise was still too great for there to be any point in speaking, but Vala’s
face was eloquent: she shared the same bitter-sweet feeling.
    Agata rose to her feet, hoping that she hadn’t embarrassed her friends too much, but the whole room