Agata struggled to concentrate. ‘It’s the same as the two-dimensional case: there’s only one class of loop.’ She could wind an imaginary thread a dozen times around the four-sphere, weave and tangle it any way she liked, but if she tried removing all those complications and shrinking the loop down to a plain circle, nothing she’d done and nothing about the space itself would obstruct her.

‘And is that true of the cosmos we live in?’ Lila pressed her.

‘How would we know?’

Lila said, ‘If you can find a good reason why it has to be true, that would be the key to the entropy gradient.’

Agata couldn’t argue with the logic of this claim, but she didn’t have high hopes for satisfying its premise. ‘I don’t see how it could ever be forced on us. The solutions to Nereo’s equation are just as well behaved on a torus as they are on a sphere.’

‘Then perhaps we need to look farther afield. You must have some new ideas on this that you want to pursue.’

Lila gazed at her expectantly. Agata felt her skin tingling with shame, but she had no inspired suggestions with which to fill the silence. ‘I’ve been a bit distracted by my new duties,’ she said.

‘I see.’ Lila’s tone was neutral, but the lack of sympathy made her words sound like an accusation.

‘I know that’s no excuse,’ Agata said. ‘Everyone has to help keep things running until the strike’s over. But when my mind’s blocked, what can I do?’

Lila adjusted herself in her harness. ‘That depends on the nature of the obstruction.’

‘It can’t always have been easy for you,’ Agata protested. ‘You must have got stuck yourself, sometimes.’

‘Of course,’ Lila agreed. ‘But I was never forced to contemplate the imminent arrival of messages from a culture that had solved all my problems so long ago that any bright three-year-old would know the answers. I doubt that would have done much for my motivation.’

Agata said, ‘I’m not expecting any help from the future. That would make no sense.’

Lila inclined her head, accepting the last claim. ‘You’ve made a good argument for that. But I’m not sure that you’ve convinced yourself as thoroughly as you’ve persuaded me.’

Agata was unsettled. ‘What do you mean?’

‘Ever since the messaging system was mooted, you’ve been like this.’ Lila waited for her to object, but Agata was silent. ‘Maybe it’s just excitement at the prospect of the thing itself, or the distraction of all the politics. But be honest: can you really block it out of your mind that you might soon be reading the words of people who’ve had six more generations of prior research to call on than you’ve had?’