The Arrows of Time

‘I couldn’t be happier!’ Serena spread her arms. ‘I know, everyone’s emotional, everyone’s confused… but what can I say? Octofurcate me: we’re headed home!’

Agata was ashamed. How many people had kept up the struggle when there’d been no end in sight? She still had her work, she still had her friends, and she’d always have her memories of this day. What more did she want?

‘We’re headed home,’ she agreed. ‘That’s enough.’

5

Seated at his console in the main control room, Ramiro watched the image feed from the camera out on the slopes. At his behest, a small tethered engine ran through a series of moves, tugging on a set of restraining springs and force gauges that allowed its thrust to be measured.

To his astonishment, the rules that the test rig was obeying remained as simple and intuitive as he could have wished: he could point the engine’s outlet any way he liked, and when he powered up the engine it generated thrust in the opposite direction. No exceptions, no complications – and no dependence at all on the disposition of distant worlds.

‘That’s disturbing,’ he told Tarquinia. An inset showed her in her office near the summit; she’d carried out the tests herself before inviting Ramiro to repeat them.

‘What did you expect?’ she asked. She wasn’t mocking him; it was a serious question.

‘I don’t know,’ Ramiro replied. ‘Maybe part of me always imagined this outcome, but I shouted it down as naïve.’

‘I never knew what to think,’ Tarquinia admitted. ‘My gut feeling – when I was looking at the engine in isolation – was that there’d be thrust in all directions. But all I had to do to change my mind was picture the consequences of that: all the specks of dust and gas out in the void that would need to conspire to make it happen.’ She sent Ramiro a sketch via her corset; it appeared in miniature in a second inset. ‘But then all I had to do to change my mind again,’ she added, ‘was to think of the engine magically “knowing” that it wasn’t meant to work when it was pointed towards the wrong part of the sky. That was just as hard to swallow as the alternative.’

Ramiro said, ‘Well, now you’ve settled it. Either way, something had to offend our intuition – so we should be grateful that the chosen offence happens far away and out of sight.’ He enlarged Tarquinia’s sketch, which drove home the point: eerie as it would have been to watch the engine selectively fail, if they could have witnessed the actual results in every detail that would have been at least as unsettling.

Unless the engine’s outlet was aimed at the Peerless itself, every photon it pumped out would eventually strike some distant object: usually just a particle of gas or dust belonging to one of the clusters. Given the present motion of the Peerless, it was easy to arrange the geometry so that the light would be arriving from the dust’s future – which meant that according to its own arrow of time, the dust would be emitting the light, not receiving it. By that account, the engine’s whole exhaust beam was being spontaneously emitted by countless tiny sources scattered across the void, just as much as it was being emitted by the engine’s own rebounders.

‘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.