Greg Egan - The Arrows of Time

Ramiro turned to Agata. ‘It looks as if it’s your vote.’

‘Can we model the air use for different scenarios?’ she wondered. ‘Take a guess about the roughness of the mountainside, and see what the chances are that we can get these things from the dock to the antipode with air still in the tank, for each design?’

Ramiro said, ‘I can try, if you want to help me with the model. “Roughness” isn’t the easiest thing to quantify, but you’re the expert on curvature.’

Agata sat beside him and they spent the next three bells working through the problem. In the end, with some plausible assumptions, there was a chance of about five in a gross that the current version of the occulter would run out of air before it had completed its task. With a new model where the arms could fold together or bend apart – allowing it to keep its grip in bumpier regions – that fell to three in a gross.

Ramiro said, ‘That’s for a single machine. But even if we build half a dozen spares, we can’t afford too many failures.’

Tarquinia had been doing calculations of her own. ‘You need to add in the chance of the modification itself leading to a failure. I get two in a gross for that.’ Ramiro looked sceptical, but when he went through her numbers he couldn’t fault them.

‘Five in a gross… versus five in a gross.’ Agata couldn’t see how to break the tie. None of these numbers were precise, but without more information she couldn’t make the uncertainties any clearer.

She looked across the cabin at the occulter. Their encounter with the Hurtler and their bomb-removal project offered plausible excuses for all sorts of items ending up in the void, or lost in the dust of Esilio – but she was already afraid that their depleted stores might attract suspicion. Ramiro’s proposed changes would require dozens more proximity sensors – spares that ought to have been packed away neatly in a single large box. Why would they have taken that box out of the storeroom for safekeeping, but then never brought it back?

‘I’m voting with Tarquinia,’ she said. ‘What we’ve got now is physically robust – and we’ll already be hard pressed to build and test the whole swarm before we arrive. This isn’t the time to start making things more complicated.’

For a moment Ramiro looked poised to respond with a further plea, but then he was silent.

Tarquinia said, ‘It’s good to have that settled. Everyone should get some sleep now, and tomorrow we’ll go into production.’

‘Do you want some fresh pictures for your wall?’ Azelio asked, offering Agata a sheaf of papers.

She took them from him. The first drawing showed the mountain coming into view through the window of the Surveyor , with Luisa and Lorenzo standing on the summit waving, very much not to scale. In the second, they’d thrown out a docking rope to the craft and were reeling it in by hand. ‘These are great,’ she said. ‘Thank you.’

Azelio lingered in the doorway. ‘Come in for a while,’ she suggested. He followed her into the cabin. There was only one chair, so she sat on the edge of the desk.

‘I’m going mad,’ Azelio said bluntly. ‘I don’t know what to think any more. I don’t know what to do.’

‘And I don’t know what to tell you.’ Agata had talked him through the situation a dozen times, but he was never satisfied with her account.

‘Tell me that the mountain won’t be destroyed,’ he pleaded. ‘Tell me that everyone will be safe.’

‘The occulters aren’t looking too bad,’ she said. ‘There are benign ways that the disruption might happen; I can promise you that.’

Azelio glanced down at the pile of notes on her desk. ‘And doesn’t everything that could happen, happen? Isn’t that what your diagram calculus says?’

‘No.’ Agata nodded at the pile. ‘For a start, you can only add up diagrams that begin and end in exactly the same way: they all take different paths, but their end points have to be identical. Getting to the disruption with benign sabotage leaves the mountain intact; getting there with a meteor strike hardly brings you to the same state. And even when the end points are identical, all the alternatives you draw for a process just help you find the probability that the process takes place. Those alternatives don’t all get to happen, themselves.’

‘Then what makes the choice?’ Azelio pressed her. ‘When a luxagen could end up in either of two places, how does just one get picked?’

‘No one knows,’ Agata confessed. ‘In the years after wave mechanics was developed, there was a big debate about whether it was truly random, or whether there was some hidden structure beneath the randomness where all the results were certain. For a while, one group of physicists claimed to have proved that there couldn’t be a deeper level. Their proof looked quite persuasive – until Leonia showed that it was tacitly assuming that information could never flow back in time.’

‘Ah, the strange things people once believed,’ Azelio observed dryly.

Agata said, ‘No one believed it, even then, but they found it easier than we do to forget that it wasn’t true.’

Azelio lifted a diagram from the stack. ‘So what can this tell us about the disruption?’

‘Nothing.’ Agata wasn’t sure how he’d ended up clutching at the diagram calculus as an answer to their plight, but if she’d been careless in describing her work to him in the past then what she owed him now was as much clarity as she could muster. ‘Just because we don’t know the cause of the disruption, that doesn’t mean that every cause we can imagine will coexist. If you want history to unfold a certain way, forget about wave mechanics. What matters now are the usual things: who we are, what we do, and a certain amount of dumb luck.’

Azelio put the diagram down. ‘So if there’s a meteor coming, how do I stop it? Or avoid it?’

‘You can’t,’ Agata replied. This was the sticking point they always reached. ‘Not if the disruption is the proof that it hits us.’

‘Then what difference does it make “who we are” and “what we do”?’ Azelio asked bitterly. ‘If I go through the motions of enacting something more benign… how will that help? If there’s a murderer trying to kill your family, you don’t protect them by moving your own tympanum to match the threats being shouted through the door. Or do you really believe in safety through reverse ventriloquism?’

Agata wrapped her arms around her head in frustration. ‘We don’t know that there’s a murderer at the door! We don’t know that there’s a meteor on its way!’

‘So we search the sky,’ Azelio pleaded. ‘We make better detectors. We try to peek through a crack in the door.’

‘If we were going to find anything,’ she said, ‘we’d know that already. If we were going to spot a meteor and avoid it, then that’s what the messages would be telling us.’

Azelio said, ‘I can’t accept that.’

Agata dropped her arms. ‘I know.’ There was nothing she could say that would change his mind, and nothing she could do that would bring him any comfort.

‘We should fly over the antipode,’ Tarquinia joked. ‘Do a little reconnaissance.’

‘Fly low enough and you could occult all the channels at once,’ Ramiro suggested. ‘Maybe there’s a disruption earlier than everyone claimed, and all the later messages are just fakes.’

Agata said, ‘I’d rather not test the defences.’

Through the window, the mountain cast a sharp silhouette against the star trails. It had been visible through external cameras for days, but they’d had to wait until they cut the main engines to rotate the Surveyor around for a naked-eye view.