The Arrows of Time

by Greg Egan

  ‘How was it ever cool, up here?’ he asked. ‘Before we launched it? Or in Esilio’s terms: what cooled it after it emerged from the atmosphere?’

  ‘Any answer to that will sound strange from either perspective,’ Agata replied. ‘I suppose it must have happened through interactions with the cooling air – but then, from Esilio’s point of view that air was rushing in from the void and striking the probe in just the right way needed to cool it, while on our terms the probe was releasing cooling air but heating up in the process.’

  Ramiro clutched his skull. ‘Why, though?’

  ‘What’s the alternative?’ Agata replied. ‘Retaining all the heat from this ascent for the next six years, while it was sitting in its bay in contact with the Surveyor?’

  ‘That would have been absurd,’ Ramiro conceded. ‘But the fact that it heated up at all before it hit the atmosphere is absurd, too.’

  ‘Less so,’ Agata insisted. ‘And “absurd” is the wrong description. If I handed you two identical-looking slabs of stone at room temperature – one of which had been heated for a while in a fire the day before – would you expect to be able to tell me which was which?’

  ‘Of course not.’

  ‘Now look at the same situation in reverse. Your failure to guess the stones’ history becomes a failure to predict their future – but the one that would become unexpectedly hot well before it was actually in the fire would not have been doing anything absurd.’

  Ramiro couldn’t argue with that. ‘So I should be grateful on those rare occasions when things make perfect sense from a single perspective, whether it’s ours or Esilio’s. But when that doesn’t work . . . what are we left with?’

  Agata said, ‘Why should we expect a system as complex as a slab of stone to be predictable, when we don’t know the detailed motion of all its constituent particles? We’re used to making predictions based on nothing but a single number, like temperature or pressure, but the ability to do that depends entirely on our relationship with the entropy minimum.’

  ‘So we’ll be helpless down there,’ Ramiro concluded glumly. ‘Anything could happen.’

  ‘No! Not anything.’

  ‘What can we rely on, then?’ Azelio asked.

  ‘Nothing should happen that’s unreasonably improbable,’ Agata declared.

  Azelio buzzed. ‘What makes something reasonably improbable?’

  ‘Cosmology.’

  ‘I might need a little more guidance than that,’ Azelio pleaded.

  Agata thought for a while. ‘If you took a cubic stride of air at a certain temperature and pressure,’ she said, ‘and chose the direction of all its particles at random, then in the vast majority of cases the entropy of that system would increase if you followed it either forwards or backwards in time.’

  She sketched an example.

  ‘But the air we actually deal with every day might well have been released into that large container from a smaller one, which immediately tells us that it’s in an improbable state: one that would shrink of its own accord into a smaller volume if you followed it backwards in time.

  ‘Most cubic strides of gas – in a time-blind, mathematical sense of “most” – do not have that property! But the entropy minimum in our past makes it entirely reasonable that we encounter air in that state. The cosmos isn’t full of particles moving purely at random, or there wouldn’t be an entropy minimum at all.

  ‘But the entropy minimum is in our future as well as our past – and Esilio connects us to it in a way we’re not accustomed to. So we’re now in a situation where we might encounter a cubic stride of air that not only occupied a smaller volume in the past, but will occupy a smaller volume in the future.’

  ‘As a fraction of all the ways the particles could be moving, that’s even more improbable than before – but given where we are and the facts of cosmology, it’s not unreasonable.’

  Ramiro accepted Agata’s logic, but it was difficult to see what it offered them in practice. ‘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.’

  Ramiro said, ‘Why am I not comforted by that?’

  Agata buzzed. ‘I don’t mean rocks flying into the air and hitting you in the face for no reason. When individual particles are moving randomly, that makes large assemblies of them more predictable, not less. Most of the time, air will just be air, stone will just be stone, acting the way our instincts expect.’

  ‘And the rest of the time?’

  Agata said, ‘We’ll just have to be prepared for the exceptions.’

  Ramiro was on watch, so he stayed in the front cabin monitoring the probe’s data feed long after everyone else had gone to bed. Sitting meekly on the surface of Esilio sending back images of the surrounding landscape, the probe encountered no conspiracies of air, or rock, or heat to impair it. Its temperature remained stable – despite the heat that its photonics would be generating in the normal course of things – which seemed to imply that it was exchanging thermal energy with its surroundings in the usual way. Agata appeared to have been right about that much: the earlier, unanticipated heating had taken place for a perfectly good reason, and there was no risk of it happening again while the probe was motionless on the ground.

  Tarquinia had put the Surveyor into a new orbit, so high that it matched Esilio’s rotational period, keeping the probe permanently in their line of sight and allowing the link to remain open. Through the window, the planet itself had shrunk to an enigmatic grey disc, but as Ramiro swept the distant cameras back and forth across the starlit plain, the new world appeared as innocent and tranquil as he could have hoped it to be.

  ‘I’m happy with the site,’ Azelio announced. ‘The probe can’t verify every detail, but nothing it’s shown us makes me think we were wrong about the geology of the area.’

  Tarquinia turned to Agata. ‘Any problems?’

  ‘No,’ she replied. ‘If we’re careful, I think we can do this safely.’

  ‘Ramiro?’

  Ramiro had no objection to the site, but they could at least try to deal with the one unsettling phenomenon they’d already witnessed. ‘What if we lower ourselves through the atmosphere more slowly than the probe?’ he proposed. ‘That should keep frictional heating to a minimum, whether you look at it as an ascent or a descent.’

  ‘It would mean more heat from the engines,’ Tarquinia pointed out.

  ‘We’ve had no problem with that for a year at a time,’ he replied. ‘I know: venting cooling air into Esilio’s atmosphere might not be the same as doing it in the void. But wouldn’t it be the most cautious approach: moving slowly, trying to keep our temperature constant?’

  Tarquinia looked to Agata.

  Agata said, ‘I think Ramiro’s instincts are sound. The closer we can stay to thermal equilibrium, the more predictable things should be.’

  ‘All right then. A slow descent it is.’

  Tarquinia turned to her console and began plotting their course down from orbit.

  In the sunlit view through the time-reversed camera, Ramiro could see the broken ring of hills directly beneath the Surveyor, their eroded peaks casting long shadows to the east. Azelio had been ecstatic when he’d found this site, with the strange confluence of ancient dust flows that its peculiar topography had allowed. Ramiro didn’t pretend to understand the details, but over time the central valley appeared to have trapped wind-borne detritus from at least four different sources. From on high, the variety in the soils was impossible to miss, with great splotches of competing hues laid over each other like a m
ess of dyes spilt from a child’s paintbox. But though the colours were layered they remained distinct, which suggested that the whole arrangement was stable. The Surveyor was a great deal heavier than the probe, but if these deposits were prone to subsidence they ought to have shown more mixing under their own weight.

  The temperature in the cabin had barely changed since they’d entered the planet’s atmosphere. Ramiro didn’t want to grow complacent; no one would forget the near-fatal surprise that the Object had held for its first visitors. But if a mismatch in Nereo’s arrow was a guarantee of mutual annihilation, the arrows of time were more pliable. On this world of lifeless dust with its almost timeless landscape, it did not seem too much to hope for that two opposing directions could coexist.

  ‘There’s the probe!’ Agata announced excitedly, pointing to a dark elliptical splotch. It was hard to distinguish the thing itself from its shadow.

  The Surveyor was descending at a constant rate, leaving the cabin subject to Esilio’s full gravity – about a third higher than the home world’s. That standard was usually taken as the limit for prolonged acceleration, on the assumption that the ancestors’ physiology had adapted to it over the eons. But the travellers had coped easily enough with far lower gravity than the ancestral norm, and Ramiro did not believe that the settlers would be troubled by this minor increase.

  Azelio said, ‘I can hear the wind.’

  Ramiro strained his tympanum. It was hard to distinguish it from the sound of the cooling system, but the gusts were sharper, rising and falling less predictably.

  The altitude displayed on the navigation console dropped below one saunter. As Ramiro watched the wind whipping dust across the ground, he began to discern an almost perfect dark circle with a wide penumbral ring, straight below the camera. He would have sworn it was the Surveyor’s shadow, but that made no sense: the sun wasn’t overhead.

  A warning appeared below the image: the ultraviolet glare scattered back from the ground was approaching unacceptable levels. Even though the engines’ beams were splayed out to the side – and the camera was counting photons emitted, not received – too much irradiation could damage the sensor. Tarquinia closed the protective shutter and the image turned black.

  The altimeter kept working, timing slow infrared pulses that were making it down and back through the dust. Half a dozen strides above the ground, Tarquinia cut the main engines and switched on the air jets to cushion their fall. Ramiro had barely registered the plummeting sensation before the impact drove him hard into his couch. The jolt left him shaken, but when he moved slightly in his harness he felt no pain.

  Tarquinia swept her rear gaze across the cabin. ‘Anyone hurt?’

  ‘I’m fine,’ Agata replied, and Ramiro and Azelio echoed her.

  The view through the window was so dark that the pane might as well have been a mirror, reflecting back the lights of the cabin. Tarquinia redirected the time-reversed camera to a side-mounted lens; the image showed red dust swirling over the ground, darkening the sky and blotting out the horizon. The sight dragged Ramiro’s attention back to the sound of the wind on the hull; he could hear the difference as the visible signs of each gust rose and fell.

  ‘Is this just . . . weather?’ he wondered. ‘Like the home world?’ He’d read about dust storms in the sagas, but it was hard to know which parts of those stories were real.

  Agata said, ‘I read a memoir by one of the first travellers, a woman named Fatima. She described the dust blowing around at the site of a rocket test, jamming all the clockwork.’

  ‘But it shouldn’t be harmful to people, should it?’ Ramiro had no idea how fast the wind would need to be blowing before the dust would start abrading skin.

  Tarquinia swivelled her couch around. ‘I don’t expect so, but no one is going out for at least two bells. I want to be certain that the ground isn’t hot – and I don’t care whether or not Esilio thinks we’re yet to use the engines.’

  Ramiro turned beseechingly to Agata, but she said, ‘Good policy. In Esilio’s terms, the engines’ exhaust came from the environment and entered the rebounders, so that’s violating the local arrow already. We should treat all these non-equilibrium situations as uncertain, and only assume that temperatures will be uniform when everything is settled.’

  They passed the time checking the Surveyor for damage, but even the weakest points on the hull – their repairs along the Hurtler gash – seemed to have survived the landing intact. When Tarquinia ran out of things they could inspect – short of taking the engines apart and putting the rebounders under a microscope – they brought out some loaves for a celebratory meal, and Azelio took photographs to show his niece and nephew.

  Two bells after they’d touched down, the instruments showed the hull’s external temperature to be no different from the cabin’s. No one challenged Ramiro when he finally moved towards the airlock.

  He closed the inner door, then hesitated, gathering his courage. There was no need to use the pump – Tarquinia had already raised the cabin pressure to match the external atmosphere. He was wearing his helmet and cooling bag for protection from the dust; he switched on the coherer in his helmet, dazzling himself for a moment until he adjusted the brightness.

  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 come 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.

  Ramiro shivered, disoriented for a moment, but whether or not this account was correct there was nothing to be done about it. A small, stubborn part of him longed to leave the door closed and . . . what? Never open it at all, just to see if he could spite this unsurprising message from the future which claimed that, actually, he would? Every time they’d used the time-reversed camera, subtler but even stranger things had happened, as thermal fluctuations in the sensor conspired to create the orderly pattern of photons that the device needed to emit. Every image the camera had shown them had been encoded all along in the not-quite-random vibrations of various objects throughout the Surveyor, waiting to come together at just the right moment.

  He leant against the crank and broke the door’s seal; a gust of wind forced its way through the narrow gap. Dust flew into the airlock, dust flew out, erasing all distinctions between that which they’d brought and the rest. He slid the door fully open, letting the light from his helmet carve a tunnel through the storm. Amidst the chaos, sheets of darkness fluttered, where the dust piled together in mid-air for a moment before scattering again. Cautiously, Ramiro poked his head and shoulders through the portal. He felt the warm wind insinuate itself beneath the fabric of his cooling bag, but it seemed that nothing it carried was small enough, or sharp enough, to reach his skin.

  He swept the beam of the coherer across the ground; the wind was raising so much dust that it was impossible to make out the surface below, but given that the Surveyor wasn’t sinking its immediate surroundings were unlikely to prove treacherous. He slid the short boarding ladder out from the airlock; its feet vanished before it touched the ground, but a scant or two further down it encountered firm resistance.

  He clambered down the ladder and stood on the surface. Even with the cooling bag encasing his feet there was an unpleasant grittiness against his soles; he took a few steps to see if he’d grow accustomed to the texture, but it remained distracting so he hardened and desensitised the skin. The wind wasn’t strong enough to knock
him down, but he couldn’t move confidently without pausing each time it rose up, to recalibrate his efforts to compensate for the force.

  ‘Ramiro?’ Tarquinia’s voice came through the link in his helmet.

  ‘I’m fine!’ he replied, shouting to be heard above the dust scraping across the surface of his helmet. He closed the outer door of the airlock, then walked around to the window and raised a hand; his crew-mates raised theirs to shield their eyes from his coherer. ‘Sorry.’ He swivelled the beam upwards, out of their lines of sight. ‘The wind’s annoying, and I can’t see much. But I don’t think I’ve started speaking backwards or ageing in reverse.’

  Agata said, ‘I’m coming out.’

  Ramiro made a quick circuit around the Surveyor; he couldn’t see any damage on the outside of the hull. Agata emerged, stepping gingerly across the swirling sand.

  ‘So this is what a planet’s like,’ she said numbly.

  ‘It’s not exactly welcoming,’ Ramiro conceded. ‘But it should be more appealing once the weather improves.’ He glanced up at the stars; he could just make out the arc of the rim, its usual dazzle reduced to a pale broken line. Though the wind and the dust were the most intrusive novelties, even the more familiar elements of their surroundings were juxtaposed so bizarrely that they lost their usual meaning: on the slopes of the Peerless strong gravity and an open sky always lay in the same direction. He wondered if he’d ever be able to sleep out here, or if he’d panic and imagine that he was falling into the stars.

  ‘Whenever I pictured the reunion, I always thought of people meeting in a corridor,’ Agata confessed. ‘But it will probably be outdoors – in the countryside, where the vehicles can land safely. It might even look like this.’

  ‘We’ll recreate the centre of Zeugma for you later,’ Ramiro teased her. ‘To give you some better imagery for the ceremonies in the town square.’