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.’