The Arrows of Time

by Greg Egan

  Agata considered this. ‘I think your argument would hold if we knew the cause with certainty. But it’s not so clear-cut when we’re less informed – when we’re not sure that the disruption will be fatal, and we’re not sure that it involves a collision at all.’

  Azelio scowled. ‘So because we can’t know that it’s a collision . . . it’s more likely that it is?’

  ‘Is that really so strange?’ Agata replied. ‘If everyone on the Peerless was confronted with the certain knowledge that the course they were on was suicidal, then there’s no way they’d persist with it – unless some freakishly unlikely set of events undermined the efficacy of their intentions. With three years’ warning to achieve the necessary swerve, what could possibly stop them? The engines would need to drop off, and every last person capable of improvising any kind of substitute would need to die of some convenient affliction. I don’t believe for a moment that the cosmos contains anything so unlikely.

  ‘But taking an unknown risk is different. If we don’t know exactly what would make us safe, there’s no need for an endless barrage of misfortune to keep us from finding the right solution.’

  Azelio abandoned the argument and the cabin fell into a despondent silence. Ramiro almost wished he hadn’t argued against Agata’s first, cheerful verdict. He couldn’t imagine what Azelio was going through, but even his own brief, hallucinatory experiences of fatherhood offered a hint. Nothing could be more harrowing than being forced to contemplate the death of the children you’d promised to protect.

  ‘Maybe the Councillors are going to shut down the system themselves,’ he suggested. ‘Just because Greta denied it doesn’t mean they won’t do it.’

  ‘But why would they?’ Tarquinia asked irritably.

  ‘If it’s a choice between that and the destruction of the Peerless,’ Ramiro replied, ‘then I don’t believe they’d choose the latter. Whatever their flaws, they’re not that deranged.’

  Azelio was taking no comfort from the theory. ‘But are they deranged enough to think that that’s their choice? If you can’t avoid a meteor by choosing your trajectory, how can you avoid it just by switching off the messaging system?’

  Agata had a different objection. ‘If they did shut down the system, wouldn’t that be an unsupported loop? They’d only be doing it because they learnt that it was going to happen.’

  ‘There’s not much complexity to it, though,’ Ramiro argued. ‘It’s hardly the same as learning a whole new theory of the vacuum from your future self; all they have to do is flick a switch.’

  ‘The Council wouldn’t want the mountain destroyed,’ Tarquinia agreed, ‘but they might well share Azelio’s view about their choices. They’ve come into this looking for a vindication of the system – so I don’t see anything inconsistent if they find themselves receiving three years’ worth of reports from the future that all describe them clinging to their original position: that the whole thing’s a boon, and there couldn’t possibly be any reason to shut it down deliberately.’

  Ramiro ran his hands over his face. ‘Forget the Council, then. Let’s assume that there’s no chance of them causing the disruption. What’s the next most benign explanation?’

  ‘We could do it ourselves,’ Tarquinia suggested.

  ‘How?’ Azelio demanded. ‘What could we do that would be harder to see coming than a meteor at infinite speed?’

  ‘I have no idea yet,’ Tarquinia admitted. ‘But at least we’re isolated from the messaging system for a few more stints. We ought to be less vulnerable to the innovation block.’

  Agata said, ‘But in the end, it would only be the shutdown itself that would have driven us to find a way to cause the shutdown.’

  ‘And that’s meant to stop us?’ Tarquinia was undeterred. ‘If that kind of loop really is too unlikely to be true, then we’ll find out eventually. But the only way to know is to try it.’

  Ramiro recalled his own farcical attempt to steal the authorship of the fake inscription from her. It still seemed wisest to keep that to himself, but he didn’t need to confess anything to make the case for a more robust strategy.

  He said, ‘There are plenty of people on the Peerless who could have planned this shutdown long before they heard about it.’

  ‘You mean saboteurs?’ Agata asked coldly. ‘The people who murdered the camera team? You want to replace a meteor strike with a bomb?’

  ‘Of course not.’ Ramiro spoke more carefully. ‘Most of the anti-messagers found those murders abhorrent, but a group of them could still be planning a way to cause the disruption without hurting anyone. And if they’re intent on using explosives at all, we can try to replace that with something better.’

  Tarquinia understood. ‘We have seven stints to work out a plan of our own, and then we can try to sell it to these would-be saboteurs. That way it becomes a hybrid effort: their motives predate the news of the disruption, but if they’ve left the details too late we might be able to offer them a technological edge.’

  Azelio hummed with frustration. ‘What’s all this talk of replacement? If a meteor is going to hit us, it’s going to hit us! You can devise as many ingenious plans as you like to try to sabotage the system at the very same moment, but if there’s a rock on its way, nothing you do is going to make it disappear.’

  ‘If there’s a rock on its way, that’s true,’ Ramiro conceded. ‘But until we know that there is, why should we assume that? The history of the next twelve stints ends with the messaging system failing; we’re about as certain of that as we can be. Some sequence of events has to fill the gap between that certainty and all the other things we know. So which snippets would you rather the cosmos had on hand to complete the story? Just one, where a meteor hits the Peerless? Just two: a meteor, or a bomb? Making our own preferred version possible won’t rule out everything else – but if we don’t even try, we’ll rule out our own best hope entirely.’

  Agata brought a schematic onto her chest. ‘Whatever the details of the final design they used, each channel must have components something like this.’

  Ramiro hadn’t thought about the technical aspects of the system for years, and as he reacquainted himself he was surprised by its apparent fragility. ‘Disrupt the light for a flicker, and the flow of information is cut. There’s no need to damage anything.’ Although the messages were constantly being converted into a less transient form to be boosted and re-sent, that version of the data only endured forwards in time – it couldn’t bridge a gap into the past. He’d often pictured the messages as a storehouse of documents, a kind of future-archeological find, but they were much more vulnerable than anything written on paper, or even in the energy states of a memory chip.

  ‘Could we launch some small objects into the external light paths?’ Tarquinia wondered. ‘If each one starts on the mountain close to one of the channel’s outlets, it could probably occult the target star without being picked up by a surveillance camera first.’

  Azelio said, ‘The outlets will have to be on the base of the mountain, won’t they?’

  ‘Yes,’ Tarquinia replied. ‘Unless they’ve turned everything around while we were gone.’

  ‘We’d need to know exactly which orthogonal stars they’re using,’ Agata pointed out.

  ‘Maybe our collaborators will have that information already,’ Ramiro suggested. ‘So if we can offer them some miniature automated craft to fly up from the mountain and block those stars, why wouldn’t they use them?’

  Azelio said, ‘So who’s going to build these things without being noticed? They’ll need accelerometers and photonics in order to navigate with any precision. If we make them ourselves on the Surveyor, we won’t stand a chance of smuggling them out when we dock. But on the Peerless, all the workshops and stores will be under surveillance.’

  ‘We could release them before we dock,’ Agata suggested. ‘Send them out to hide somewhere. If they’re small enough, and we time the whole thing carefully, they could pass from the Surveyor to the slopes
undetected.’

  ‘And then what?’ Azelio pressed her. ‘They adhere to the slopes somehow, and then crawl towards the base – like insects crawling along a ceiling?’

  ‘Yes.’ Agata wasn’t backing down, but the proposal was growing more ambitious by the moment.

  ‘And then later,’ Azelio said, ‘since we won’t know the coordinates in advance, we have to be able to instruct them, remotely, to crawl to a particular take-off point and then fly along a certain trajectory. Without the signal being detected.’

  Tarquinia disagreed with his last claim. ‘If a brief encrypted signal is picked up by the authorities, what can they do about it? So long as they can’t pin down the exact source or destination, mere detection need not be a problem. Even if they take it as a sign that some form of attempted sabotage is under way . . . they would have had that possibility in mind for the last three years, regardless.’

  Azelio hesitated. ‘So why would they even try to stop us? They know the disruption is going to happen – so unless all this clandestine activity is irrelevant and a meteor is going to be responsible, this is a battle they know they can’t win.’

  ‘They’re not going to give up, any more than we are,’ Agata replied. ‘Do you see any sign in what we’ve heard from the mountain that the Council have resigned themselves to a state of fatalistic powerlessness?’

  ‘No,’ Azelio conceded.

  ‘Think of it as a kind of equilibrium,’ Tarquinia suggested. ‘I’m sure there are limits to how far the Council would go to try to stop the inevitable, but there must be limits, too, on how supine they’ll become: they’re not going to shut down the system themselves, or release all the anti-messagers and let them go on a rampage with mallets. They’ve taken a stance and they’re going to pursue it as far as they can. When this is over they’ll be looking for a political advantage in the details of the fight, as much as in the outcome.’

  Azelio was looking disoriented. ‘I want this to work,’ he said haltingly. ‘But every time I stop and think about it, it feels as if all we’re doing is playing some kind of game. Shouldn’t we be trying to build better meteor detectors? If we really are the only people left with any hope of innovating, why not design a device that could actually save the mountain – instead of one for faking its death?’

  Agata said, ‘If we saved the mountain from a meteor, don’t you think we’d know about it?’

  ‘I have no idea.’ Azelio rose from his seat. ‘But what we’re doing now is pointless.’ He walked out of the cabin.

  In the silence, Ramiro felt his own confidence faltering. ‘I don’t know how to reason about this any more,’ he said. ‘If it’s a meteor that could actually kill us, isn’t that where our efforts should go? Forget what the messages say or don’t say about it: if we do our best to build something useful, how can that fail to make a difference?’

  Agata inclined her head, expressing some sympathy with the impulse. But she wasn’t swayed. ‘I was the one who tried to argue that there’s no such thing as an undetectable meteor – but what do we have on board for tackling that problem? A single time-reversed camera, and no facilities for building new photonic chips or any kind of high-precision optics. Even if we came up with a glorious new design, how are we supposed to manufacture a whole network of surveillance cameras and get them deployed? They can’t just be drifting around the mountain detecting hazards for their own amusement – if they find something, they have to be able to trigger either a coherer powerful enough to deflect the thing, or start up the engines and make the whole mountain swerve. Do you really think we’d be able to do that in secret?’

  ‘Maybe the Council will finally exercise enough discipline to keep it all quiet,’ Ramiro replied.

  ‘If they’re capable of that,’ Agata countered, ‘then they’re capable of doing a vastly better job than we are with the entire project.’

  Ramiro gave up. He desperately wanted everything to work in the old way, when he could wrap his mind around a self-contained problem and take it apart without having to think about the entire history and politics of the mountain. But wishing for those days wasn’t going to bring them back. ‘Then we should go ahead with the star-occulters for our hypothetical saboteurs,’ he said. ‘Find a way to build them, and a way to keep them secret, and then hope that the cosmos takes us up on the offer to explain away the disruption with one simple, harmless conspiracy.’

  26

  Agata took hold of one end of the slab of calmstone, Ramiro the other, then they lifted it onto their shoulders and stood facing each other, some four strides apart.

  ‘Are you ready?’ Tarquinia asked.

  ‘I’m not sure how steady this will be,’ Agata replied.

  ‘That doesn’t matter. I just want you to be able to keep your grip when there’s a force applied from below.’

  Agata put a second hand on the slab. ‘All right. Go ahead.’ The improvised test rig looked alarmingly amateurish, but the ceiling of the cabin was made of the wrong material, and in any case they didn’t want to leave it covered with incriminating marks. They’d hunted through the storeroom for something to serve as a trestle, but there’d been nothing ready-made, so in the end their bodies had seemed like the most expeditious substitute.

  Tarquinia pushed a button on the remote control and the occulter rose from the floor. The core of the tiny craft was a dodecahedron about a span wide, with air nozzles fixed in the centres of eleven of its pentagonal faces. Attached to the top, twelfth face was a linear assembly, a pair of arms three or four spans long, as densely packed with gears and linkages as anything from the age of clockwork.

  Staying low, the occulter steered its way across the cabin until it was hovering in front of Agata’s feet; she could feel the spill of air against her skin. Then it ascended smoothly until it made contact with the calmstone slab – surrogate for the slopes of the Peerless itself. She gripped the slab tightly as four burred tips drilled obliquely into the stone. As Tarquinia had promised, the net force was purely vertical, so the weight of the slab bore most of it, and with the drills counter-rotating in matched pairs Agata felt no torque trying to twist the slab sideways.

  After a few lapses the drills fell silent and the air jets cut off, leaving the device hanging.

  ‘Try to shake it loose,’ Tarquinia suggested. Ramiro ignored the invitation, but Agata slid her end gently from side to side, and when this had no effect she grew bolder and began rocking the slab back and forth. The linkage rattled alarmingly, but the four splayed drill bits remained lodged in place.

  ‘That’s reassuring, isn’t it?’ she said. ‘The mountain is hardly going to sway like that.’

  Ramiro wasn’t so impressed. ‘It doesn’t tell us much about the real hazards. If there’s a hole under the surface, or a powderstone inclusion—’

  ‘If it comes loose that’s not the end of the world,’ Tarquinia stressed. ‘It can always fly back and reattach itself.’

  Agata said, ‘Try the walking mode.’

  Tarquinia tapped the remote. The four bits remained fixed in the stone, but the plate on which the drills were mounted began rotating on the end of its arm – or rather, the arm began rotating around the plate, swinging the entire occulter forward, carrying it from Agata’s end of the slab towards Ramiro’s.

  When this repositioning was complete, the four drills at the end of the second arm pushed up against the stone and began biting into it. The new quartet managed to gain purchase with only the first set bracing them; there was no need to start up the air jets again. Then the first four went into reverse and disengaged from the stone, and the whole process began to repeat itself.

  Agata watched anxiously as the machine whirred and clanked its way down the slope from her shoulder to Ramiro’s. If the slab was unrealistically smooth, at least they’d made sure that it wasn’t gravitationally level.

  When the occulter had come within a span of Ramiro’s body, Tarquinia used the remote again. The craft freed itself from the slab and fl
ew away to alight on the cabin floor. Ramiro looked to Agata, and they carefully put the slab down together.

  ‘Not bad,’ Tarquinia declared.

  Ramiro said, ‘No. But we still need to decide what happens when the surface is uneven.’

  Tarquinia had already reached her position on that. ‘It should detour around the problem if it can, or drop away and fly past it if it can’t. That makes it purely a question of navigation.’

  ‘And a question of air,’ Ramiro corrected her.

  ‘Whatever we do,’ Tarquinia replied, ‘there’ll always be a chance of the air running out. Letting the arms tilt so they can conform to the surface won’t guarantee anything – and it’s one more joint that can jam, two more actuators that can leak, plus six more sensors to make the idea work at all.’

  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.