But even their formal, mathematical expression for the vacuum state was a bizarre sleight of hand: they’d imagined taking the simpler vacuum of a more pristine theory – one where all particles stood aloof from each other, refusing to interact – and writing it as a sum of pieces that each corresponded to a different energy level of the true theory. If you followed that sum over a long time, you could pick out the least rapid oscillations that represented the lowest energy level. So in all of Romolo and Assunto’s calculations, they’d pretended that everything happened in an infinitely old cosmos that had started – infinitely long ago – with the simple vacuum, from which a mathematical trick extracted the true vacuum before they set to work adding particles to it in the here and now.

Amazingly, all of this nonsense had worked well enough for their purposes, with the quantities they’d predicted confirmed by experiment again and again. But the real cosmos with its own real history and topology couldn’t be understood by grafting on an infinitely long run-up from a state that had never actually existed.

Someone knocked on the door of the cabin. Agata dragged herself over and opened it.

‘Are you busy?’ Azelio asked.

‘Not really.’

‘Do you want to help me set up the tether?’

Agata felt a surge of excitement, but then she realised that it was premature. ‘You think Tarquinia will let me do it?’

‘Didn’t she give you your void proficiency certificate?’

‘Only because she doubted that anyone else would.’

Azelio frowned. ‘Ramiro doesn’t have much more experience than you. If you’re willing to ask Tarquinia, I’ll support you.’

The two of them approached the pilot in her couch, and Tarquinia heard them out politely.

‘My job is to try to keep you all alive,’ she said. ‘This might not be an especially dangerous task, but Ramiro has the edge on you in confidence.’

Agata said, ‘If the worst comes to the worst, I’m expendable. Ramiro isn’t. If something goes seriously wrong with the automation, no one else will be able to fix it.’

‘I can get us home without automation,’ Tarquinia replied.

‘Of course.’ Agata hoped she hadn’t inadvertently insulted her. ‘But you have to admit that a lot of things would become more difficult if we were forced to do them manually.’

‘Hmm.’

Azelio said, ‘Everyone needs experience to get used to working in the void. The engines are switched off, and we’d both be wearing jetpacks; how much safer could it be? And if Agata does this now, it could make all the difference to the kind of task she can manage in an emergency.’

Tarquinia inclined her head, conceding his point. ‘But she did tell me once that she’d rather not have anyone relying on her.’