Agata said, ‘What does it matter, if they can’t communicate any of it?’ Lila claimed to have accepted her argument: complex ideas were far more likely to remain unmentioned in the messages than to arise without clear antecedents.

‘What matters is that it seems to have paralysed you.’

Agata struggled to recall some small achievement she could hold up against this claim, but since she and Lila had proved their conjecture on the curvature of four-spheres she’d really done nothing but mundane calculations. ‘Everyone gets stuck,’ she said. ‘I’ll snap out of it soon enough.’

Lila said, ‘I hope so. Because once you learn whether you do or you don’t, it’ll be settled for good, won’t it?’

Agata had set her console to wake her early. She ate quickly, then made her way to the axis and propelled herself down a long weightless shaft, touching the guide ropes lightly to keep herself centred.

She followed the shaft all the way to the bottom, emerging on a level that housed what remained of the feeds for the old sunstone engines. She dragged herself through chambers full of obsolete clockwork, the mirrorstone gears and springs tarnished and clogged with grit, but still offering up a dull sheen in the moss-light. For a year and a half after the launch, mechanical gyroscopes had tracked the mountain’s orientation, and the feeds’ machinery had kept the engines balanced by adjusting the flow of liberator trickling down into the sunstone fuel. Agata doubted that anyone at the time had imagined that a piece of photonics the size of her thumb would take the place of all these rooms.

As she moved further away from the axis, the chambers’ erstwhile floors became walls. Descending the rope ladder that crossed the third room, she passed precariously dangling assemblies of gears and shafts spilling out of their cabinets, the pieces still loosely bound together against the long assault of centrifugal gravity. The eruptions looked almost organic, as if the neglected cogs were sprouting from exuberantly blossoming vines. There must have been people inspecting and maintaining all of this quaint machinery, up until the day when Carla finally proved that none of it would ever be needed again. Now it would only take a couple more rivets snapping for these strange sculptures to come crashing down.

Agata left the decrepit clockwork behind, and dragged herself down a narrow passage to the shabby office of her supervisor, Celia. The office itself looked as old as the Peerless, but Celia’s networked console was a slab of bright modernity among the ruins – its roster of volunteers encrypted, for what that was worth when every strike-breaker could be seen coming and going. Celia handed her an access key and tool belt, and Agata signed for the equipment with a photonic patch, not dye.

‘This must seem pretty menial to you,’ Celia teased her. ‘Your predecessor wasn’t much into cosmology.’