Suppose we leave a piece of equipment behind on Esilio - say, a small spyglass. Over the eons, from our point of view, we'd expect it to become pitted by dust in the wind, and eventually break up completely and turn to sand. Our spyglass, our rules: that sounds fair, doesn't it? But if that sand stays on Esilio, what origin will it have from Esilio's point of view? Most likely, some ordinary Esilian rock will have broken down to make it - which to us, would look like erosion running backwards. But then, in Esilian time the remnants of that rock will eventually form themselves spontaneously into a spyglass, which lies on the ground until we come along to retrieve it. So if you follow the history of the matter that makes up the spyglass far enough in both directions, it's clear that it's not committed to either side's rdes.' 'Swap the roles of Esilio and the Surveyor,' he replied, 'then tell the same story again. If something from Esilio takes the place of the spyglass, it must be with us already. We must have been carrying it, or the things that will become it, from the very start. Because according to Esilio's arrow of time we've already visited the planet, and it's almost certain that something remained with us when we departed.'

'Tell us one thing that you're sure won't happen,' he challenged her. She said, 'Two objects in thermal contact will not maintain different temperatures over a long period of time.' 'Because ... ?' 'Because there are vastly more possibilities in which they share their thermal energy more equally. If you pick a possibility at random, it's likely to be one of those. Fundamental physics might make the entropy minimum necessary - but we still expect the cosmos to be as random as it can be.'

There was fine red dust covering the grey hardstone walls of the airlock. He hadn't noticed it by the dimmer illumination of the safety light. He ran a gloved finger along the seal of the outer door, trying to find the point where it had been breached, but if there was a hole it wasn't apparent. It hardly mattered now; however the dust had entered, he was about to let in a great deal more. But as he began to turn the crank, the realisation hit him: it hadn't corne from outside. They must have brought it with them all the way from the Peerless, scattered invisibly throughout the craft, with a little more accumulating inside the airlock each time the inner door was opened. Or in Esilio's terms: the Surveyor's visit had just ended, and this residue was something they would soon take away with them

Twice, as she jumped out of some indentation in the sand, it vanished. She and Azelio hadn't actually made all the tracks that he'd attributed to them. Or not yet, they hadn't. 'Come and join us,' Azelio said. 'Some of these must be yours.'

Each time Azelio lifted his feet, scattered sand unscattered itself, grains sliding in around the places where he'd stepped to settle more evenly - though not always smoothing the ground completely. After all, 197 Ramiro reasoned, it was possible to walk in someone else's footprints, or to step several times in your own. It would only be the last footfall on any given spot - prior to the next occasion on which the wind levelled everything - that would unmake the imprint completely.

'What happens if I try to walk on pristine ground?' he asked. 'Try it and see!' Agata taunted him. Ramiro descended to the bottom of the ladder, intending to move quickly and get the ordeal over with, but then his resolve deserted him. When he willed his foot to land on unblemished sand, what exactly would intervene to stop him? A cramp in the muscle, diverting his leg to its proper, predestined target? A puppet-like manipulation of his body by some unseen force too strong to resist, or a trance-like suspension of his whole sense of self? He wasn't sure that he wanted to know the answer. And perhaps that was the simplest resolution: he would lack the courage to walk out across the surface of Esilio for the rest of the mission. He would cower in his room, leaving the work to the others, while he waited to return to the Peerless in disgrace.

He'd scrutinised the ground beforehand, and he was sure there'd been no footprints at all where his feet now stood. He lifted one foot and inspected the sand below. He had created an indentation that had not been there before. That was every bit as strange to Esilio as the erasures he'd witnessed were strange to him.

'How?' he demanded, more confused than relieved. 'You really don't listen to me, do you?' Agata chided him. 'Did I ever tell you that the local arrow was inviolable? 'No.' What she'd stressed most of all was a loss of predictability but the sight of her and Azelio unmaking their footprints had crowded everything else out of his mind. Those disappearing marks 198 in the sand might be unsettling, but if he could ignore them and walk wherever he pleased then they were not the shackles he'd taken them to be.

'What happens if there are footprints that no one gets around to before the next dust storm?' he asked Agata. 'Ones that were there straight after the last storm?' She said, 'There can't be a footprint untouched by any foot. I don't understand the dynamics of wind and sand well enough to swear to you that there won't be hollows in the ground that come and go of their own accord - but if you're talking about a clear imprint, if we could keep our feet away from it, it simply wouldn't be there.'

Esilio was a world where a certain amount of nOiSY, partial - and predominantly trivial - information about the future would be strewn across the landscape. There had always been plenty of trivial things that could be predicted with near-certainty back on the Peerless, and perhaps as many of them would be lost, here, as these eerie new portents would be gained

Emboldened, he strode out across the illuminated ground, pausing every few steps to kick at the sand. Sometimes he simply pushed the dust aside; sometimes the dust applied pressure of its own, as it moved in to occupy the space his foot vacated. But that pressure never came out of nowhere: his feet moved as and when he'd willed them to move, followed by the dust but never forced to retreat. Nor were they thrust without warning into the air

Each time there was a dust storm the record of future movements would be erased, but even in a prolonged period of calm the footprints would overlap, conveying very little information.

'You already dug twelve holes!' he observed. 'And I thought you were messing around with Agata all morning.' Azelio made a noncommittal sound. 'My plan is to dig up all these plants at the end of the trial and take them back to the Peerless for my colleagues to analyse,' Azelio mused. 'So I guess that's when I'll see the transition between cultivated and truly pristine ground. But right now, in Esilio's terms, we've just dug the plants up - so on our terms, we're about to do that. Backwards.' Ramiro said, 'You make it sound as if you've been practising timereversed agronomy all your life.' 'It's not that hard to see what's going on, if you think it through,' Azelio replied lightly. 'But you don't mind following markers like this? Evidence of acts you haven't performed yet?' 'It's a little disconcerting,' Azelio conceded. 'But I can't say that it fills me with claustrophobia to know that I'll carry out the experimental protocols I always planned to carry out.'

He lowered the plant until its roots were in the hole, then he started adding soil from the surrounding mound. Some of the soil was scooped in with pressure from behind, in the ordinary manner. Some appeared to pursue the trowel, the way the dust sometimes pursued Ramiro's feet. What decided between the two? Azelio's own actions had to be consistent with the motion of the soil, but which determined which? Maybe there was no answer to that, short of the impossible act of solving in the finest detail the equations that Agata was yet to discover, revealing exactly which sequences of events were consistent with the laws of physics all the way around the cosmos.

Ramiro's left arm had grown tired from holding the plant in place over the hole. He shifted it slightly to make himself more comfortable, but as he shifted it back he saw soil rising and adhering to the roots. He stared at this bizarre result for a moment, then decided to stop wasting time delaying an outcome he had no wish to oppose. He held the trowel to the side of the mound nearest the hole, then drew it closer. The sand followed the blade - not adhering to it and needing to be brought along, but gently pushing it. He lowered the trowel into the hole then withdrew it; the sand parted from the blade and packed itself between the roots of the plant and the side of the hole.

He hesitated, groping for a clearer sense of his role in the task. But what could he actually do wrong? So long as he was committed to making whatever movements with the trowel were necessary until the plant was securely in place, that state of mind and the strictures of the environment ought to work it out between themselves. He scooped some soil straight into the hole; like the last delivery, it clung to the roots. In Esilio's terms, this soil had spent at least a few stints packed tightly around the plant; if he could have seen the action in reverse, it would have involved nothing stranger than a clump of sand finally coming loose.

But as she moved the broom across the floor, duly concentrating the dust ahead of it, other dust began to appear behind it - some of it falling from the air, some sliding over the stone to pile up against the bristles. Its entropy was decreasing too, as it accumulated from whatever scattered reaches of the Surveyor in which it had been lurking. The net result was that the stretch of floor she'd swept remained as dusty as ever.

But the plants' uptake of nutrients relied on interactions between their roots and the native soil at a microscopic level, and there was no guarantee that the two systems, left to themselves, would simply sort out their differences

'Because the test plots are failing,' Agata explained. 'So we need to take the explosive up into the hills, turn some rock into soil for ourselves - against the Esilian arrow - and see if that imbues the soil with the properties that it needs to support plant growth

'If we do set off this explosive,' he reasoned, 'shouldn't we be able to see some evidence of that already?' Agata said, 'You mean a crater?' 'Yes.' 'If we found a site like that, it would be useless to us. It would imply that after we set off the bomb, the crater would be gone and the sand around it would be rock again.' Ramiro scowled. 'Esilio doesn't care what's useful or useless, or it wouldn't have killed the plants, would it?' 'Esilio doesn't care,' Agata agreed, 'but why would we go ahead and set off the bomb there, knowing that it would do us no good?' 'Because the crater would prove that we did!' Ramiro replied heatedly. 'But as far as we know, there is no such crater.' Agata met his gaze openly, trying to reassure him of her Sincerity: she wasn't playing some verbal game just to annoy him. 'There is no crater, because if we saw it, we wouldn't choose to make it. Esilio can't force our hand; whatever happens has to be consistent with everything, including our motives. Ramiro said, 'It can't force our hand, but there could still be an accident.' 'That's true. But if we saw such a crater, we wouldn't even go near it with the explosive.'

He ran a hand over his face. 'If the plants can't bring their arrow to Esilio, why should a bomb do any better?' 'The roots of a plant aren't entirely passive,' Azelio replied, 'but they do rely on the state of the soil. I don't think the bomb going off will rely on anything like that.' 'But in Esilian time,' Ramiro protested, 'all the soil we're supposedly going to make with this bomb has to mesh perfectly with a backwards explOSion in such a way that it forms a solid rock. How likely is that?' How likely are the alternatives?' Agata countered. 'How likely is it that the explOSive will fail to detonate? How likely is it that we'll allow it to explode in an existing crater instead - just to pander to Esilio's arrow?'

She started swinging her pick into the rock face. Small chips of stone flew out from the point of impact, stinging her forearms, but the rush of power and freedom she felt at the sight of the growing excavation was more than enough to compensate. In Esilian time, the chips were rising from the ground, propelled into the air by conspiracies of time-reversed thermal diffusion, just to aid her as she rebuilt the rock. What stronger proof could there be that the cosmos had a place for her, with all her plans and choices? One day it would kill her, but until then the contract was clear: hardship and frustration and failure were all pOSSible, but she would never be robbed of her will entirely.

the lines on the rock face formed symbols. The sides of the ridges appeared softened and eroded, as if a generation's worth of future dust storms had left their mark. But she could still make out most of the message. ' ... came here from the home world,' she read. 'To offer thanks and bring you ... courage.' Azelia said, 'Who thanks whom for what?' 'It's from the ancestors,' she said. 'They're going to come here and write this. They're going to come here to tell us that everything we've done and everything we've been through was worth it in the end.'

But ever since he'd seen the writing for himself, he'd been unable to stop wondering if the message suited him too well. As far as he could recall, he'd never consciously planned to commit any kind of hoax. What he didn't know was exactly what his lack of preparation meant. The words were there, nothing could change that now. But with every moment that passed it seemed more likely to him that the ancestors had nothing to do with it, and that he would find a way to write the message himself.

To feel alive, he needed to feel himself struggling moment by moment to shape his own history. It was not enough to look down on events from above like a biologist watching a worm in a maze, content to note that this creature's actions had never actually gone against its wishes.

nothing helped a plan run more smoothly than having a law of physics on its side

How could he carve anything into the rock face, if the idea of doing it had only come to him after he'd seen the result? Even the choice of words hadn't sounded like his own. If he'd only selected them because he'd read them, who would have made the choice? No one. Agata had told him endlessly: a loop could never contain complexity with no antecedent but itself, because the probability would be far too low. There could be no words appearing on rocks for no other reason than the fact that they'd done so.

'We've all hit a dead patch,' Lila said sadly. 'Chemists, biologists, astronomers, engineers. Since they switched on the messaging system, there hasn't been a single new idea across the mountain.' 'You mean no one's been sending back new ideas?' Agata had predicted as much - but surely that self-censorship hadn't surprised anyone. 'Oh, the messages have contained no innovations,' Lila confirmed. 'But neither has the work itself.' 'I don't understand,' Agata admitted. Lila said, 'If people did innovate, the results would leak back to them one way or another. I know you believed that they'd be able to keep qUiet, so everything would go on as usual. But everything has not gone on as usual. We've had no new ideas since the system was turned on - because if we'd had them, we would have heard of them before we'd had a chance to think of them ourselves. The barriers to information flow are so porous now that the knowledge gradient has been flattened: the past contains everything the future contains ... which means the future contains nothing more than the past.'

What would her own generation be famous for? Rendering the creation of new knowledge impossible.

'I'm where I need to be.' 'In the administrative sense, or the teleological?'

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.'

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.

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. 'Por 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?

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!'

'This is your pass,' the woman explained, handing her a red disc. 'Please don't lose it.' 'Do I lose it?' Agata asked. 'Of course you don't,' the guard replied. 'Because I asked you not to.' 'Right.' Agata suppressed a shiver.

'I didn't make the inscription,' Tarquinia declared. 'I went out there to try, but nothing happened: no shards of stone rose from the ground to meet the chisel. I tried different tools, different movements ... but I couldn't unwrite those symbols. If anything, when I left they were sharper than I'd found them - as if all I'd done was make the message less clear for Agata and Azelio than if I'd stayed away completely. I wasn't the author of those words. Someone else must be responsible for them.'

I wish I could talk to them,' she said. 'I wish I could thank them. I wish I could tell them that it wasn't for nothing, that it ended well.' Clara said, 'If that's what you want, then I believe you'll find a way.'