The Arrows of Time

by Greg Egan

  Giacomo said, ‘The angle of approach and the orientation are crucial. We’ve made sure that the hooks are compatible with the dimensions of the arms, but if your machine comes in too steeply or the arms are turned the wrong way, it won’t engage the hooks at all.’

  ‘I understand.’

  ‘And the retreat’s just as important,’ Giacomo stressed. ‘If you pull away vertically, the resin won’t give. The rope will snap, or something else will break.’

  ‘We’ll follow the whole flight plan as closely as we can.’ Ramiro reviewed the list, bringing the figures back onto his skin as he checked them. ‘What are those last sets of numbers?’

  ‘The coordinates of the light collectors.’

  Ramiro hadn’t expected to be given the targets themselves until he’d reported back on the first stage of the process. ‘So that’s it? We just fly the occulters there . . . and then what?’

  ‘The bombs are all controlled by timers,’ Giacomo explained. ‘All you need to do is get them to the right place.’

  ‘What do we do if something goes wrong? How can we contact you?’ Ramiro was prepared to accept responsibility for the occulters, but if anything else malfunctioned he’d have no idea what the options might be.

  ‘Nothing goes wrong,’ Giacomo assured him.

  ‘You can’t know that,’ Ramiro protested. ‘Not after the private messages are squeezed out—’

  ‘That late?’ Giacomo paused, struggling to frame an answer, as if he’d lost the habit of imagining anything beyond the reach of his foresight. ‘The disruption is ours,’ he said finally. ‘We’ve been planning it for longer than the system’s been in existence. We know that it happens – and we know that we’re trying harder to make it happen than anyone else. So how can we possibly fail?’

  Ramiro moved away from the console and let Tarquinia check the alignment of the link against her own calculations. They’d set the beam to be as narrow as they dared, to minimise the chance of anyone detecting it on its way out to the slopes. But if they failed to aim it at the precise location where they’d left the first occulter clinging to the rock they’d be risking discovery for nothing.

  ‘This looks right to me,’ Tarquinia said.

  ‘Are you sure?’

  ‘In the end it’s just arithmetic and geometry,’ she replied. ‘If I do it a dozen more times I’ll still get the same answer.’

  Ramiro had already entered the coordinates of the nearest cache into the software. He tapped a key on the console and a tight burst of UV erupted from the link. The confirmation came back immediately: the occulter had received the message and was proceeding to act on it.

  ‘Perfect!’ Tarquinia declared.

  ‘So far.’ It would take the occulter three days to crawl across the mountain to its first rendezvous. Ramiro pictured the prototype clanking down the plank towards him, back on the Surveyor. They’d made allowances for the machines losing their footing and needing to recover, but the complex manoeuvres required to pick up the cargo would cut into the air supply, and the extra mass being lugged around would shrink the margin for error even further.

  Tarquinia said, ‘Next target.’

  Their run of luck continued for a while, but the fifth occulter failed to reply. Ramiro rechecked the direction of the link, then broadened the transmission, but it made no difference.

  When they’d released the occulters from the Surveyor each one had been given preassigned coordinates, but if the composition of the rock proved unsuitable they were to try again at a number of adjacent sites. A pseudo-random algorithm varied the coordinates; knowing the seed for it they could match the sequence exactly.

  After a dozen steps, Ramiro gave up. If the occulter hadn’t found a secure purchase by then, it would not have had enough air left to be of any use to them even if they could locate it.

  ‘One in five,’ Tarquinia said. ‘We can live with that.’

  By the end of the day they’d set a dozen and three occulters in motion and given up on three.

  ‘If Giacomo had stayed in touch with us,’ Tarquinia mused, ‘he could have spared his people the trouble of planting three of those caches.’

  Ramiro said, ‘Maybe. Or maybe we’ll fumble the pick-ups on three of the others and have to go back and use the ones that seemed superfluous.’

  ‘That’s true.’ Tarquinia reached across and squeezed his shoulder. ‘We’re doing well.’

  Ramiro was exhausted. He stared across the room and tried not to think of the machines scuttling along the slopes; the more he visualised them, the harder it became to avoid picturing a cog jamming or a drill bit coming loose. ‘At the turnaround, all our biggest problems had been solved,’ he said. ‘Every traveller before us had put up with far more hardship and uncertainty than we were facing then. So how did it come to this? Why are we the idiots who could lose it all?’

  ‘Stop thinking about it.’ Tarquinia took him by the arm and led him through into the bedroom.

  When they’d finished, Ramiro clung to her body angrily. He’d wasted half his life on this imitation of fatherhood. If he hadn’t wanted the real thing, why did he keep chasing this shadow? He was as much a slave now as if he’d meekly followed his uncle’s commands.

  Tarquinia eased herself out of his embrace.

  ‘What happens afterwards?’ he asked her. ‘After the disruption.’

  ‘After the disruption,’ she said, ‘life goes back to normal.’

  28

  Agata ascended the stairs slowly, her gaze cast down at the moss-lit rock, hoping that if anyone was watching her she’d appear suitably distracted: a moody theorist wandering the mountain, oblivious to her surroundings. Though every ordinary resident of the Peerless surely knew the size of the excluded zone around the axis, she hadn’t been able to bring herself to ask Serena or Gineto to tell her. There was no way to phrase the question innocently: whoever she consulted, however obliquely, would be instantly burdened with the knowledge that she was contemplating sabotage. Which might have led nowhere, or might have taken her rapidly to a place she didn’t want to be: finding a way to reassure an alarmed friend that she hadn’t gone over to the side of Medoro’s killers, but was actually striving to undermine them.

  To make any progress on that task, she needed a rough idea of the dimensions of the messaging system. It was safe to assume that the designers had made every channel as long as possible, running close to the full height of the mountain, so once she knew how close to the axis the public were permitted to travel she’d have some sense of the mirrors’ width and the volume of each enclosed light path.

  As Agata’s weight diminished, she continued upwards, using the guide rope beside her. The ancestors couldn’t tell her how to halt the system, but they must have chosen her for a reason – and the only hint they could give her had to be encoded in that choice itself. She had measured the bending of light by Esilio’s sun, hadn’t she? There was no prospect of using gravity to distort the light paths in the messaging system, but gravity wasn’t the only way to modify light’s passage.

  A woman passed her, descending, murmuring a casual greeting. Agata had chosen the stairwell at random; she had no reason to believe that she was heading for an entrance to the facility itself. The usual tiers of apartments here should simply come to an end a little sooner than they had before the system was built.

  Above her, less than a saunter away, the twelve long tubes would run from mirror to distant mirror, carrying messages from the future in beams of densely modulated time-reversed starlight. The tubes would be sealed against contamination – against dust or smoke that might scatter the light – and perhaps the Council had made an effort to render them vacuum-safe, in case the ends were breached and they were opened to the void. But that would be a matter of structural reinforcement to limit the damage from a sudden pressure difference, not a matter of impermeability. There was no such thing as a hermetic seal on a container of that size. At the very least, particles of air would be constantly diffusin
g in and out of the tubes.

  For most purposes, air was air. So long as it was chemically inert and dense enough to serve the crucial role of carrying heat away from bodies and machinery, any finer details were of secondary importance. When the cooling system had switched from using the old engines’ exhaust to the gas produced by cold decomposition of sunstone, no one would have much cared that the range of particle sizes was different. But there were countless variations on the basic theme of a stable ball of luxagens, and different mixtures had different properties. The speed of each frequency of light was slightly different in air than in a vacuum, and the precise value depended on the precise composition of the air.

  Agata reached the top of the stairs. A sign right in front of her spelt it out: LAST EXIT. She left the stairwell and dragged herself along the corridor, past the doors of the apartments. The Council would have left a large enough buffer above this unrestricted area so that a bomb planted here could not have breached the nearest of the tubes. But she had a number now, good enough to feed into an order-of-magnitude estimate: how fast could she expect a change in the air to diffuse through a resin seal into the tubes that contained the light paths?

  Air was air, no one would feel a thing. But if she could make it happen, there’d be no need to damage a single component of the messaging system. The exquisitely calibrated timing of the data fed into each tube would include allowances for ordinary variations in the ‘delay’ created by the light bouncing from mirror to mirror, but once it drifted beyond that range and the signal was scrambled beyond recovery, there’d be nothing that the system could do about it – least of all send a message into the past to warn the operators of the nature of the problem.

  Back in her apartment, Agata sat at her desk and worked through the calculations. If she could add a component to the air that was significantly lighter than the smallest particles in the present mixture, it would naturally rise towards the axis and diffuse into the imperfectly sealed tubes. Though a particle of air in isolation had almost no external field, each light wave that passed over it distorted its shape sufficiently to spoil the usual cancellation between the luxagens, and the secondary wave generated by that process combined with the first to slow it down.

  Generations of scientists had uncovered the details she needed to quantify the effect: she was mining the entire intellectual legacy of the Peerless. Agata hadn’t used half of these results since she was a student, but though her memory of some of the formulas was hazy – and she was afraid to consult the photonic library lest she alert someone in authority to her sudden change of interests – she discovered that she still had all her old paper textbooks at the bottom of a cupboard, not yet so insect-damaged as to be indecipherable.

  Without access to the tubes themselves she couldn’t hope for a precise answer, but she could sketch the limits of what was physically possible. In the worst case, time had already run out: if the tubes were large enough and the seals sufficiently tight, it could take half a year to infuse enough modified air into them to corrupt the signals.

  In the best case, it would take close to three stints. So she had, at most, two stints to alter the composition of the mountain’s atmosphere sufficiently to get the process started.

  Agata rechecked the numbers, but they did not improve. She sat at her desk with her tattered books around her, bewildered but refusing to be cowed. The ancestors had spoken to her; she was joined to them across the disruption, across the generations yet to be born. The cosmos had no choice but to find a sequence of events that filled the gap and completed that connection, and it could not come out of nowhere. The right plan had to lie within her, just waiting to unfurl.

  29

  Ramiro was beginning to wish that they’d put cameras on the occulters. The extra transmissions needed to send back the images might have increased the chance of detection, but it would have been worth it just to have an objective version of the rendezvous with the cache in front of him, to take the place of the pictures in his head.

  First, the occulter had to release itself from the rock, unwinding the drills and letting itself fall into the void. Then the air jets had to catch it and send it swooping back towards the slopes, approaching the cache with just the right speed at just the right angle. Two hooks on strings hung down from the cache, each one an open half-circle crossed by a vertical trigger about a third of the way in; the arms of the occulter needed to enter those half-circles and strike the triggers to send the second, spring-loaded halves sliding around to enclose them. Then the occulter had to move away, dragging the cache almost horizontally across the rock, unrolling the adhesive resin that was holding it in place against the vertical tug of its centrifugal weight.

  Tarquinia interrupted his brooding. ‘Relax,’ she said. ‘Or add up the navigational tolerances again, if you want reassurance. We can hit the hooks, I’m sure of it.’

  Ramiro checked the clock on his console. ‘Maybe we can, if the occulter turns up. It’s already three lapses late.’

  ‘Three days crossing the slopes, and you want it to be punctual to the flicker?’

  ‘These things move like clockwork, literally. If not to the flicker, they ought to be punctual to the lapse.’

  Tarquinia said, ‘If this turns out badly, I’ll drop my anti-messaging principles and let you know . . .’ She glanced at the clock. ‘One lapse from now.’

  Ramiro buzzed dismissively. ‘How would that help?’

  ‘It wouldn’t,’ she admitted. ‘But if you can convince yourself that I’m telling the truth, you can relax and assume that silence means success.’

  A row of numbers appeared on the console – a transmission from the occulter, not from Tarquinia’s future self. Ramiro waited, refusing to interpret the numbers in isolation. Then the second brief report followed.

  The occulter was stable, well clear now of the cache site . . . and measurably more massive than before, as revealed by its response to the thrust of the air jets. It had picked up its cargo and held on to it, and as the bomb swung down from above the arms, the occulter had successfully compensated for the spin that would otherwise have been imparted.

  A moment later a third report announced that the occulter had managed to drill itself into the rock again.

  ‘One more,’ Ramiro pleaded. To catch the hooks and stay balanced was miracle enough, but the occulter needed to be able to keep moving down the slopes towards the base. If the strings had become tangled around the arms, they’d either end up breaking and freeing the cargo, or the whole mechanism would grind to a halt.

  ‘And there it is.’ Tarquinia read all the numbers aloud, and worked through the meaning of the torques. The occulter was moving in the normal way, and it was still carrying the bomb. Nothing had jammed, nothing had broken.

  ‘There it is.’ Ramiro bent forward, willing the tension out of his shoulders, but only a fraction of the pain departed. A dozen and two equally finicky and precarious encounters remained.

  Tarquinia said, ‘The mass is less than I was expecting.’

  ‘The mass of the cargo? You think we lost something? Dropped some component—?’

  ‘No!’ Tarquinia hesitated. ‘I suppose I’m just admitting that Giacomo seems to have been honest with us. I was afraid he might have downplayed the size of the bombs.’

  ‘But he didn’t.’ Ramiro was pleased. ‘We’ll need to get every one of them exactly on target, though. A few strides away and we might not even shatter the collector.’

  Tarquinia was amused. ‘We just threaded a needle on the slopes, and you’re talking about missing by strides?’

  ‘We had no time window with the cache,’ Ramiro pointed out. ‘There’s no comparison with the base. In fact, if I was working for the Council I would have told them to build decoys: dozens of structures mimicking the light collectors, with exactly the same optics protruding from the surface. Who’s to know which ones really feed into the tubes?’

  Tarquinia said, ‘Giacomo’s group has had three years to think
about all that. If they’d had any doubts about the coordinates they could have gone for a different strategy. If we start trying to second-guess them now, we’ll go out of our minds.’

  ‘Yeah.’ Ramiro turned back to the console and read through the last report again, until he’d convinced himself that the numbers could not mean anything but success.

  The next two encounters went as flawlessly as the first, but as the time for the fourth pickup came and went there was silence from the link. It stretched on for more than half a bell, until the fifth occulter began reporting.

  Three bells later, the same thing happened again. They’d lost two machines.

  At the second last scheduled rendezvous, the occulter missed the hooks and flew right past the cargo, its mass unchanged. Tarquinia stepped in, sending it looping back to try again – not at the same coordinates, but a progression of slightly shifted locations. Ramiro stood aside and watched her work, wishing more than ever for a camera as she swept the occulter over the slopes, trying to engage with a cache that had either slipped a little out of place or simply fallen away into the void.

  After the fifth attempt she stopped the occulter and had it drill back into the rock.

  ‘Can we send it to another cache?’ she wondered.

  Ramiro checked the positions of the three caches for which they’d had no occulters. This one would have to double back to reach any of the three – depleting its air tank to the point where it would not be able to make it to the base.

  ‘It’s as good as lost,’ he said. ‘We now have no spares.’

  The last pickup was still almost a bell away. Tarquinia said, ‘Do you want to get some loaves? I’ll stay here in case there are any surprises.’

  As Ramiro stood in the queue in the food hall, he noticed a group of diners stealing glances in his direction then turning away with pained expressions, as if his presence were mildly embarrassing. Perhaps he’d become a figure of pity for wasting half his life on the expedition, to so little avail. But if all the real action had been back on the mountain, what exactly had anyone here done to earn the right to look down on him this way?