The Arrows of Time: Orthogonal Book Three (8 page)

Serena gestured at the food in front of them. ‘We’ve all been stuffing ourselves, so don’t be shy about catching up.’ Agata took a spiced loaf from the table. She
hadn’t been able to eat with Cira in the apartment; every bite she took in her mother’s presence made her feel as if she were betraying her starving dead grandmother – who
hadn’t actually starved for long, and whose co had helped her raise her machine-fathered daughter.

Vala asked Agata how her research was progressing.

‘Still slowly,’ Agata confessed.

Gineto hummed sympathetically. ‘What is it exactly that you’re doing? Medoro’s tried to describe it to us, but I’m not sure he really understands it himself.’

‘I’m just a humble instrument builder,’ Medoro said. ‘You can’t expect me to begin to comprehend Agata’s work.’

Agata ignored his teasing, but Gineto seemed genuinely curious. And if he wasn’t, he was being too polite to be brushed off with ‘it’s complicated’.

She said, ‘Do you know about Lila’s work?’

‘Vaguely,’ Gineto replied. ‘Didn’t she find a way to make gravity compatible with rotational physics?’

‘Exactly. Vittorio’s law of gravity assumed absolute time. Yalda must have known that it wasn’t rotationally invariant, but in those days the discrepancy wasn’t seen as
important. People were busy enough trying to understand light.’

‘So . . . what changes?’ Gineto asked. ‘What does Vittorio’s inverse square law become?’

‘It’s trickier than that,’ Agata warned him. ‘In Lila’s theory, gravity isn’t a force at all, in the traditional sense: it’s a result of four-space
being curved. You know how lines of longitude on a globe come together? Even though they start out parallel at the equator, they don’t remain the same distance apart.’

‘Right,’ Gineto agreed tentatively. There was nothing esoteric in the geometry she’d described, but he couldn’t quite see the connection.

Agata said, ‘In Lila’s theory, gravitational attraction is the same kind of effect. When two massive bodies start out at rest with respect to each other – that is, with their
histories parallel – they don’t stay the same distance apart, they accelerate towards each other. But you don’t need a force for that; all you need is curvature.’

Gineto buzzed: he got it now. ‘That’s an elegant idea. Have the astronomers tested it?’

‘That’s the hardest part,’ Agata admitted. ‘The mathematics is beautiful, but we’re so far from any truly massive bodies that it’s almost impossible to devise
a test.’

‘The ideal thing to study would be a planet orbiting close to its star,’ Medoro interjected. ‘Like the innermost planet in the home system. What was that called? Paolo? Peleo?
I can never quite remember it.’

Agata said, ‘Lila’s theory predicts that a close elliptical orbit would undergo “apsidal precession”: the near and far points of the orbit should move around the star,
instead of staying fixed in space. So careful observations of a system like that could distinguish her theory from Vittorio’s.’ She sketched an example on her chest.

‘If there are other planets in the system it’s more complicated,’ she added. ‘The way they tug on each other will cause precession, too, so you have to
separate out the various contributions. If we had copies of all the ancestors’ astronomical measurements we could hunt for some sign of Lila’s precession, but nobody thought to include
that kind of thing in the library.’

‘And where does your own work fit into all this?’ Gineto pressed her.

‘My work’s about trying to understand entropy in the context of Lila’s theory,’ Agata replied. ‘According to Lila, the curvature of four-space depends on both the
amount of matter present and the way it’s moving. If all the particles’ 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 was full of distraught people, torn between celebration and loss.

Medoro approached and put an arm around her. ‘It’s enough,’ he said. ‘It has to be enough.’

‘Of course.’ Agata willed herself to accept that.

‘I know you don’t want grandchildren,’ he teased her, ‘but you can always tell your stories to my niece’s kids.’

Stories of the spin-down, the exotic gravity, the shrunken stars. All her life, she’d ached to live through these tangible signs that the voyage really would have an end. But now that ache
felt worse than ever. When her apartment’s floor was horizontal again, when the giant stairwells were tunnels and the star trails had stretched out into coloured threads that squeezed into
half the sky, what could she look forward to?

Serena joined them, standing beside her brother. ‘How are you feeling?’ Agata asked her.

‘I couldn’t be happier!’ Serena spread her arms. ‘I know, everyone’s emotional, everyone’s confused . . . but what can I say? Octofurcate me:
we’re
headed home!
’

Agata was ashamed. How many people had kept up the struggle when there’d been no end in sight? She still had her work, she still had her friends, and she’d always have her memories
of this day. What more did she want?

‘We’re headed home,’ she agreed. ‘That’s enough.’

Seated at his console in the main control room, Ramiro watched the image feed from the camera out on the slopes. At his behest, a small tethered engine ran through a series of
moves, tugging on a set of restraining springs and force gauges that allowed its thrust to be measured.

To his astonishment, the rules that the test rig was obeying remained as simple and intuitive as he could have wished: he could point the engine’s outlet any way he liked, and when he
powered up the engine it generated thrust in the opposite direction. No exceptions, no complications – and no dependence at all on the disposition of distant worlds.

‘That’s disturbing,’ he told Tarquinia. An inset showed her in her office near the summit; she’d carried out the tests herself before inviting Ramiro to repeat them.

‘What did you expect?’ she asked. She wasn’t mocking him; it was a serious question.

‘I don’t know,’ Ramiro replied. ‘Maybe part of me always imagined this outcome, but I shouted it down as naïve.’

‘I never knew what to think,’ Tarquinia admitted. ‘My gut feeling – when I was looking at the engine in isolation – was that there’d be thrust in all
directions. But all I had to do to change my mind was picture the consequences of that: all the specks of dust and gas out in the void that would need to conspire to make it happen.’ She sent
Ramiro a sketch via her corset; it appeared in miniature in a second inset. ‘But then all I had to do to change my mind again,’ she added, ‘was to think of the engine magically
“knowing” that it wasn’t meant to work when it was pointed towards the wrong part of the sky. That was just as hard to swallow as the alternative.’

Ramiro said, ‘Well, now you’ve settled it. Either way, something had to offend our intuition – so we should be grateful that the chosen offence happens far away and out of
sight.’ He enlarged Tarquinia’s sketch, which drove home the point: eerie as it would have been to watch the engine selectively fail, if they could have witnessed the actual results in
every detail that would have been at least as unsettling.

Unless the engine’s outlet was aimed at the
Peerless
itself, every photon it pumped out would eventually strike some distant object: usually just a particle of
gas or dust belonging to one of the clusters. Given the present motion of the
Peerless
, it was easy to arrange the geometry so that the light would be arriving from the dust’s future
– which meant that according to its own arrow of time, the dust would be emitting the light, not receiving it. By that account, the engine’s whole exhaust beam was being spontaneously
emitted by countless tiny sources scattered across the void, just as much as it was being emitted by the engine’s own rebounders.