The Arrows of Time

by Greg Egan

  ‘That was exhausting,’ she said. ‘Can you give me a hand up?’

  Agata jumped down into the shaft and helped her out through the hatch. The flesh of Tarquinia’s torso had become corrugated as she’d forced it between the pipes, giving her the appearance of a decoratively shaped novelty loaf on legs.

  ‘Any luck?’

  Tarquinia said, ‘There’s nothing hidden beside the beam.’

  ‘Oh.’

  ‘But the beam itself is hollow.’

  ‘Really? How can you tell?’

  ‘You can hear it when you tap,’ Tarquinia explained.

  ‘Couldn’t that just be to save mass?’

  ‘In principle it could be. But when I got to the far end I found something peculiar: it looks as if the cooling air is actually routed through the beam. Why do that, except to make life harder on anyone tampering with it?’

  ‘So if there’s a bomb,’ Agata said, ‘it might be anywhere inside a hardstone beam that spans the diameter of the Surveyor. And the only way we’ll ever know for sure is if we cut the whole thing open – in a place where there’s barely room to move, let alone the space to wield tools safely.’

  Tarquinia inclined her head admiringly. ‘Trust Verano to find a civilised solution.’

  Agata hummed with distaste. ‘Is there such a thing as a civilised bomb?’

  ‘Well, no,’ Tarquinia conceded. ‘But the Council would have asked him to fit a booby trap, and at least he made that idea redundant. There’s no way that Ramiro alone – or even the four of us – could have taken that hiding place apart and left the Surveyor functioning. A booby-trapped bomb would probably have been triggered by accident, long ago. We can thank Verano for finding a way to make the thing as good as tamper-proof, without turning it into a death sentence.’

  Agata said, ‘I’ll send him flowers when I get back. But if we can’t get the bomb out and leave the Surveyor functioning—’

  ‘We couldn’t have done it in the void,’ Tarquinia interjected. ‘But with an external atmosphere, there’s no comparison. I think even the most paranoid Councillor would have reasoned that if Ramiro had proposed the mission merely as a cover for an attack on the Peerless, he would hardly have been willing to spend twelve years actually detouring all the way to Esilio just to remove this thing.’

  ‘You really think you can go back down there and slice the beam open?’ Agata gestured at the curves still imprinted into Tarquinia’s body.

  Tarquinia said, ‘Not just like that. First we take out most of the cooling pipes. Then we drill inspection holes in the beam, to see what we can see. The whole exercise could take a while, but it’s not impossible.’

  ‘Assuming there are no other problems. Assuming there really is no booby trap.’

  Tarquinia said, ‘Yes.’

  Agata slumped against the side of the shaft. Before she’d approached Tarquinia, she’d been picturing the bomb hidden behind a false wall at the back of the pantry, requiring nothing more to disarm it than the snip of a cable.

  Tarquinia began smoothing out the kinks in her flesh. ‘I’m not going to try something like this without unanimous assent. And just because you raised the idea yourself doesn’t mean that you can’t change your mind.’

  As Agata described her plan to blast their own arrow into the Esilian soil, she could see an expression of delight growing on Azelio’s face – as if she’d slipped a drawing of a flourishing garden sprouting from a bomb-shattered hillside into the stack the children had left him. There was scepticism too, but she was sure now that he would understand that it was at least worth trying.

  Ramiro, though, remained as dispirited as ever. ‘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.’ Agata would have liked to have taken comfort from the fact that there were no signs at the landing site of any future accident, but if the blast was capable of imposing its own arrow that meant nothing.

  Ramiro’s hostility wavered. ‘I don’t know how to think about any of this,’ he admitted. 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?’

  ‘Don’t ask me,’ Ramiro replied bitterly. ‘I only live here.’ Tarquinia reached over and squeezed his shoulder.

  Agata said, ‘I can’t predict anything with certainty either, but surely it’s worth doing the experiment.’

  Azelio turned to Tarquinia. ‘You think you can extract the explosive safely?’

  Tarquinia phrased her reply carefully. ‘I’m as sure as I can be that Verano wouldn’t have allowed anything on the Surveyor that could kill us from a bump or a broken connection. Whether I manage to set it off anyway is another question.’

  They spent three more chimes talking over the details, then Tarquinia called for a vote.

  Ramiro’s gloom had given Agata pause. Even if the plan succeeded, he might well end up back on the Peerless warning his fellow anti-messagers that the crops counted for nothing when Esilio itself would rot their minds. Why should she risk her life if it would make no difference to the fate of the mountain?

  Azelio said, ‘I’m for it.’

  Tarquinia followed him quickly. ‘I am too.’

  Ramiro was silent. Agata willed him to mutter a surly veto, sparing her the need to make a decision, but having advertised his confusion already he kept his resolve much longer than she could.

  ‘I’m for it,’ she said, unsure now if she had any better reason than her wish to see Azelio hopeful again.

  Ramiro stared at the floor. Agata felt a twinge of sympathy for him: he’d come here with nothing but good intentions, hoping to grant both of the warring parties a chance to live exactly as they wished. It was not his fault that Esilio wasn’t so accommodating.

  ‘I’m for it,’ he said finally. ‘If we baulk at the risk we could still get killed by a Hurtler on the way back – but we can’t go back without trying everything. If people can survive here, they need to know.’

  Tarquinia said, ‘Right.’

  As she rose from her couch Ramiro added, ‘To be honest, though, there’s a better reason to do this than anything it can tell us about the crops.’

  Agata was confused.
‘What’s that?’

  Ramiro said, ‘The look on Greta’s face when we tell her exactly what we did with her beautiful bomb.’

  Agata sat in the tent, wearing her helmet so she could hear the audio link clearly over the noise of the wind. Every chime or so the footfalls and gentle clanking echoing in the empty engine cavity gave way to the bone-shaking whine of hardstone being drilled. Tarquinia was making holes in the beam, hunting for the bomb.

  Agata pictured the scene as she’d left it, with mirrors angled into the cavity to bring in as much Esilian sunlight as possible. But even the safety lights in the cabin above would be off now, leaving Tarquinia to work with nothing but the view through the time-reversed camera. Exposing the bomb to ordinary light might trigger a tamper-prevention device, but it wouldn’t have made much sense to include the means to detect time-reversed light, when any act of sabotage had been expected to take place close to the Peerless, where the only source would have been distant starlight.

  The camera could amplify the faint image obtained by a periscope inserted in each inspection hole, with sunlight introduced by a second mirrored tube. But so far, all Tarquinia had been able to report was that there were dozens of baffles inside the hollow beam, blocking the view along its length, leaving her with no way to proceed but trial and error.

  Ramiro lay on the floor of the tent, one arm covering his faceplate; Azelio was crouched beside him, his head bowed in thought. They had spent eight days stripping as much as they could out of the Surveyor, preparing themselves for the grim contingency that an explosion might leave the hull damaged but not entirely beyond repair. Tools and medical supplies filled the tent; its three neighbours held the rebounder panels, parts of the cooling and navigation systems, and their entire stock of food. Agata understood now why there’d been so much dust around in the preceding days.

  ‘I’ve found it,’ Tarquinia announced calmly. ‘Six strides from the rim of the hull.’

  Ramiro sat up. ‘What is there, exactly?’

  ‘About what you’d expect,’ Tarquinia replied. ‘A UV receiver on a board with a photonic processor. And a cable leading from the processor into the explosive.’ Agata felt sick. She could see the blue dust that had filled Medoro’s workshop; she could picture it spilling from the broken hull to mix with the Esilian soil.

  ‘No other components on the board?’ Ramiro pressed her.

  Tarquinia said, ‘Remember when we shot up into a high orbit, to maintain contact with the probe? If that didn’t set this thing off, nothing will. There’s no accelerometer here.’

  ‘Is the beam warm?’ Ramiro asked.

  Tarquinia buzzed curtly. ‘Yes! I just drilled a hole in it.’

  ‘You should leave it for a chime and see if it cools down completely,’ Ramiro pleaded. Agata understood his logic: a passive system that needed an external signal to wake it would not be generating heat, but the kind of photonics required to detect an incision in the cable would have to be constantly active.

  ‘If it gave out a heat signature, that would defeat the whole point of trying to hide it,’ Tarquinia replied.

  Ramiro said, ‘I think they would have imagined the cooling system still running while we were doing this.’

  ‘All right,’ Tarquinia agreed reluctantly. ‘I’ll wait.’

  Agata caught Azelio’s eye and they exchanged grimaces of relief. Tarquinia’s unwavering conviction that Verano would have gone out of his way to make the bomb ‘safe’ was probably justified – but impugning the man’s honour was quite low on everyone else’s list of calamities to avoid.

  Ramiro took off his helmet and rubbed his eyes. ‘I should be doing this,’ he muttered. Agata offered no opinion; in the end it had been Tarquinia’s decision.

  ‘Is anyone hungry?’ she asked. ‘I could go and bring some loaves.’ She hadn’t seen Ramiro eat all day.

  Azelio said, ‘I’ll go with you.’

  As they unzipped the entrance to the tent a gust of wind entered, sending the walls ballooning out and loosening the stake holding down one corner; it was only the collection of heavy tools arrayed across the floor that kept it from peeling up from the ground. Ramiro went and put a foot on the wayward corner, and Agata dashed out to fix the stake. With the wind pelting her with dust the food run seemed like more trouble than it was worth; she returned to the tent.

  Tarquinia’s voice came over the link. ‘The beam’s down to ambient temperature,’ she announced. ‘There’s no heat coming from the bomb.’

  ‘How’s your visibility?’ Ramiro asked anxiously. They could hear the wind rising; the dust had to be obscuring the sunlight entering the Surveyor’s window.

  ‘Good enough,’ Tarquinia assured him. ‘I’m going to cut the cable.’

  Ramiro said, ‘You’re tired now, and there’s not much light. Why don’t you wait for the storm to pass?’

  Agata heard the drill start up again; Tarquinia would need a third hole to insert the shears.

  Ramiro paced the tent. Azelio crouched in a corner, staring at the floor. The whining of the drill came to an end, replaced by a gentle scraping noise as the folded instrument was manoeuvred through the hole.

  ‘I’ve got the shears around the cable,’ Tarquinia announced. Agata saw Ramiro’s faced contorted with fear. There was a soft click of the blades meeting.

  The wind rose up, pelting the wall of the tent with dust. But one word came clearly through the link.

  ‘Done.’

  As Agata trudged up the rocky incline, the patch of bright ground beside the Surveyor remained visible in her rear gaze. But it looked so out of place against the dark valley floor that a part of her mind began to discount it, treating it as nothing but a flaw in her vision. The first few times she felt it vanish from her mental map of her surroundings she panicked, scanning the view for the comforting beacon until it snapped back into focus, acknowledged again as real. But after a while she stopped worrying and let it fade into the landscape. Tarquinia and Ramiro were not going to turn out the lights and hide from her. When the time came she’d have no trouble finding her way back.

  Ahead, above the grey hills, the sky could not have marked the way more clearly. The direction along Esilio’s axis that they’d chosen to call ‘south’ pierced the bowl of stars about a twelfth of a revolution below its bright rim, and from this valley in the southern mid-latitudes that celestial pole remained perpetually in view, with the rim twirling around it like a burning hoop and the stars in between never setting.

  Azelio walked beside her, carrying two of his potted seedlings from the final dozen he’d held in reserve. He wasn’t complaining, but she could see him struggling with the weight as the slope increased.

  ‘I’d be happy to take one,’ she offered.

  ‘Thanks, but I’d rather you had nothing to distract you from your own load,’ he replied.

  Agata raised the bomb effortlessly above her head. ‘It hardly weighs anything. And even if I drop it, it’s not going to go off.’ Tarquinia had assured her that the explosive could only be triggered by a bright pulse of light at a specific wavelength, and the only means of delivering that pulse was strapped securely to her tool belt.

  Azelio said, ‘I’m more worried that you might damage the detonator and we won’t be able to set it off at all.’

  ‘Fair enough.’

  Azelio had identified a promising outcrop in the images they’d taken from orbit – a body of rock whose spectral signature suggested that it could give rise to fertile soil. No one had objected when Agata had volunteered to accompany him to the site, but she still felt slightly guilty at having wormed her way out of the tedious business of moving everything back into the Surveyor. Blowing up a hillside would be vastly more enjoyable than reassembling cooling pipes and restocking the pantry.

  ‘Can we rest for a bit?’ Azelio suggested.

  ‘Of course.’ Agata placed the bomb gently on the ground, then sat beside it, positioning her body so she’d be blocking its way if it began to slide.
Azelio did the same with his plants.

  ‘Do you think they already know how this ends, back on the Peerless?’ he asked her.

  ‘I expect so.’ Unless there’d been an ongoing campaign of sabotage, it was hard to believe that the messaging system would not have been completed by now.

  ‘In some ways that takes the sting off the separation,’ Azelio mused. ‘If the children are already in contact with me, that’s almost like being there.’

  ‘This from a man who voted against the system,’ Agata teased him.

  Azelio said, ‘If the vote had gone against the system then we wouldn’t have needed to be here at all.’

  ‘Hmm.’ Agata didn’t want to start arguing with him over the attribution of blame.

  ‘So long as there’s peace, I don’t care about the system,’ Azelio admitted wearily. ‘People can use it or ignore it as they wish. We managed not to go to war over shedding; we ought to be able to live with anything after that.’

  ‘We ought to, and we will,’ Agata declared. ‘The fanatics who can’t accept that will be free to leave.’

  Azelio buzzed wryly. ‘Fanatics carrying the necessary stocks of explosive?’

  ‘Maybe we can send all the bombs they’ll need in a separate craft,’ Agata suggested. ‘We could bundle off a whole lot of freight to Esilio in an automated vessel at high acceleration, then let the settlers follow. It’s not an intractable problem; we’ll think of some way to do it safely.’

  ‘Assuming this works at all.’ Azelio nodded towards their own bomb.

  ‘It has to work.’ Agata searched the dark valley for the speck of light that marked the landing site. ‘If the soil is right and the arrow is right, the plants will grow. Nothing else would make sense.’

  The rim of the star bowl was almost vertical as they came over the rise. Agata wished they could have chosen a landscape with more rock than dust from the start; it would have spared them the worst of the storms, and they could have passed the time just sitting outside, gazing at this glorious celestial clock.

  ‘There it is,’ Azelio announced, pointing ahead. Agata could barely distinguish the hue of the outcrop from that of its surroundings, but she trusted Azelio. He’d studied the image of the hills for half a day as he’d plotted their route, and he had too much at stake to be careless.