‘So the final slowdown shouldn’t be a problem,’ Ramiro realised belatedly.

‘If this holds up – no, it shouldn’t,’ Tarquinia agreed.

Ramiro leant back from the console, pondering the political consequences. Even the staunchest reunionists had assumed that they’d be leaving their descendants with the burden of finding a way to start decelerating on the approach to the home world. But the tiny engine Tarquinia had set straining against its springs had had no difficulty achieving thrust in exactly the direction that the Peerless itself would need for that last manoeuvre. The migrationists had lost their most powerful scare story.

But physics had lost a story of its own. From the point of view of the ultimate recipients of the engine’s exhaust, its successful firing was the kind of absurd picture that came from imagining time running in reverse, with the fragments of some shattered object reassembling themselves into the whole.

‘So much for the law of increasing entropy,’ Ramiro said.

Tarquinia was unfazed. ‘That was never going to last.’

‘No.’ If the cosmos really did loop back on itself in all four dimensions, nothing could increase for ever. ‘But what do we put in its place?’

‘Observation.’ Tarquinia nodded towards the image of the test rig.

‘So everything becomes empirical?’ Ramiro was happy to be guided by experiments, so long as some prospect remained that they could yield the same result twice in a row.

‘The cosmos is what it is,’ Tarquinia replied. ‘The laws of optics and mechanics and gravity are simple and elegant and universal . . . but a detailed description of all the things on which those laws play out seems to be nothing but a set of brute facts that need to be discovered individually. I mean, a “typical” cosmos, in statistical terms, would be a gas in thermal equilibrium filling the void, with no solid objects at all. There certainly wouldn’t be steep entropy gradients. We’ve only been treating the existence of one such gradient as a “law” because it was the most prominent fact in our lives: time came with an arrow distinguishing the past from the future.’

Ramiro said, ‘But isn’t there still a question of how brutish the brute facts are? We know that the home cluster’s entropy was much lower in its distant past, and the same was true of the orthogonal cluster. The most economical explanation is that both clusters shared a common past.’

Tarquinia said, ‘So you want to cling to the notion of parsimony? A single region of low entropy is already stupendously unlikely, but even if we have no choice about that, you want to hold the line and refuse to allow two?’

‘You don’t think that’s reasonable?’

Tarquinia thought it over. ‘I don’t know what’s reasonable any more,’ she said.

Ramiro closed his eyes for a moment, raising some crude scrawls on his chest based on Tarquinia’s diagram, but keeping them private. ‘Forget about whether or not the clusters have a common past; forget about the orthogonal cluster entirely. Suppose the only thing we rely on is the fact that the home cluster had vastly lower entropy in the past.’