Greg Egan - The Arrows of Time

An amused voice interjected loudly, ‘How?’

‘I can’t answer that,’ Pio admitted. ‘Perhaps a physicist will find a way to transform our positive luxagens into negative ones, letting us walk safely on the Object before we move on to a larger orthogonal world. Perhaps a biologist will find a way for us to sculpt orthogonal matter into a new generation of children, who bear our traits without being shed directly from our own flesh.’ Agata’s neighbours in the crowd were reacting with equal parts hilarity and incredulity now. ‘Did the ancestors know that we’d learn to make an Eternal Flame?’ Pio persisted. ‘Of course not! They merely trusted that, with time and dedication, we’d solve the fuel problem one way or another. We need to respect our descendants’ abilities to deal with a problem of their own.’

The debate timer rang. Pio flipped the lever to silence and restarted it, then moved back along the guide rope, allowing his opponent to take centre stage.

‘Who knows what our descendants will achieve?’ Lila began. ‘I’m not going to try to refute Pio’s speculations. But it does seem clear to me that any attempt to migrate into orthogonal matter would be perilous – and beyond the danger to the actual pioneers aspiring to set foot on the Object, everyone on the Peerless would be hostage to the need to complete the process in a limited time. Over the generations, as their resources dwindled, they’d be forced to keep wondering whether they needed to cut their losses and try to head home after all. But the longer they put it off, the longer that return journey would be, and eventually any misjudgement of the time they had left would be fatal.

‘Why should we subject our descendants to that kind of torture? We can turn the Peerless around right now, confident that it will support us long enough to complete the trip.’

Lila brought an image onto her chest; the room’s camera picked it up and displayed it on the giant screen behind her. ‘This is the plan,’ she said. ‘This was always the plan, from the day Eusebio broached it with Yalda.’ The sight was enough for Agata to feel a latent impression of the same familiar curve, ready to rise up on her own skin. This was the map of her life’s purpose; she’d understood that since the day she’d first seen it.

‘We know we can make the turn,’ Lila said. ‘All the way around that semicircle, the acceleration we need can be produced with the engines sending photons into the future of either the home cluster or the orthogonal cluster. Only the last stage of the journey presents a problem: it’s not clear how we can begin to decelerate in the approach to the home world. But we’ll have six more generations to address that, and I can’t believe it will prove insurmountable.’

Lila glanced at the timer. ‘To describe this plan as “dangerous” is absurd. Dangerous compared to trying to give birth to children made of negative luxagens? I don’t think so!’

The timer rang. Most of the crowd cheered; Agata ignored her mother’s look of lofty amusement and joined in. Lila deserved the encouragement. Pio’s ideas weren’t likely to get much traction, but with the vote less than a stint away they needed to be refuted decisively for the sake of everyone’s morale.

Pio dragged himself forward again. ‘What dangers would the return pose?’ he asked. ‘Let’s start with a wildly optimistic view, and suppose that the entire journey could be completed safely. Once we reach the home world and deal with the Hurtlers, the barbarians are sure to be grateful – for a while. But could we really live among them, after so much time apart? I can’t see them approving of our ideas about governance, let alone our reproductive methods, and my guess is that they’d hold Starvers in almost as much contempt as Shedders. Then again… since we’ve made such a habit of bequeathing tasks to our descendants, maybe the last one could be to devise the kind of weapons they’d need to defend their way of life against the planetary status quo.’

Agata shifted uncomfortably on her rope. She knew he was being sarcastic, but any talk of weapons put her on edge.

Pio said, ‘That’s the optimistic view, but the real problems will arise much sooner. As we decelerate for the turn, we’ll be moving at ever greater speeds with respect to the Hurtlers. For a long time our spin has been enough to fling these specks of dust away, and now we have a fancy system of sensors and coherers guarding the slopes so we can spin-down the mountain with impunity – but even the coherers won’t be able to protect us once the Hurtlers are moving faster than the fastest radiation we can actually detect.’

The audience fidgeted, underwhelmed. Everyone knew that the Peerless was a small target, and though it was true that the mountain’s defences would be useless once the Hurtlers crossed a certain threshold velocity, the period of vulnerability would be brief.

Pio inclined his head slightly, acknowledging the weakness of the point and moving on. ‘Lila assures us that the engines won’t need to violate any thermodynamic laws as we turn the Peerless around. But how certain can we be that they really will keep functioning? And even if the turnaround itself proves uneventful, keep in mind that the entire return journey entails our own arrow of time pointing against the arrow of the orthogonal cluster – a configuration we’ve never experienced before.’

Agata couldn’t contain an exasperated hum. The most dramatic effect she expected from the reversal was for the orthogonal stars to vanish from the sky.

‘Beyond those disturbing uncertainties, no one has the slightest idea how we could commence the final deceleration. Lila herself admits as much!’ Pio paused to let the audience dwell on this – despite his own cheerful confession that he had no idea how a viable migration scheme would work. ‘Imagine what it would mean to be trapped in this mountain, heading back into a region full of ordinary matter but unable to slow down and match speeds with it. Every grain of interstellar dust we encountered would strike us with infinite velocity – rendering it as lethal to us as a Hurtler would be to the ancestors. Astronomers in Yalda’s day searched the sky for years to find the safe corridor we’re moving through now. We should take their gift and make the most of it: we should remain on this trajectory for as long as it’s clear, and use the time to prepare ourselves to step away from all of these colliding worlds and find a home that will be safe for eons to come.’

As Pio reached down to reset the timer there were a few scattered cheers.

Lila took his place. ‘If migrating to the orthogonal worlds would be so much easier than slowing the Peerless for the final approach,’ she said, ‘then let people ponder both questions while we’re travelling back towards the home world. When one problem or the other is actually solved, we’ll be in a position to make an informed choice. What’s more, sticking to the plan and reversing the Peerless would actually make migration easier: all those negative luxagens in the orthogonal worlds will become positive to us! The thermodynamic arrow of the orthogonal stars will be pointing against us, but between coping with that and trying to walk on antimatter, I know which challenge I’d prefer.’

Agata turned to her mother and whispered, ‘The woman just won. It’s over!’ Diehard migrationists might have their reasons to remain committed to the more difficult route, but whatever allure the idea held for wavering voters, Lila had just offered them a vastly less terrifying way to go on thinking about deserting the ancestors, without burning any bridges until their own safety was guaranteed.