The Arrows of Time

by Greg Egan

  ‘Oh, I’m getting all nostalgic now,’ Greta replied sardonically. ‘Let’s reminisce about the time you lied to my face and betrayed me.’

  ‘You never used to take things personally,’ Ramiro complained. ‘All the time we worked together on the turnaround, did I ever make a fuss when you took all the credit with the Council? We both treated each other pretty shabbily, but we still managed to solve every problem that was thrown at us.’

  Greta was unmoved. ‘Try to be objective. You’re asking me to give a gnat to an automator whose greatest claim to fame will remind anyone who might have forgotten that automating a gnat is just what you need to turn it into a weapon.’

  ‘That’s a very negative way of looking at it.’ Ramiro thought for a moment. ‘The biggest problem with the rogue gnat was that it took us by surprise. We can arrange things so that this craft has no way of doing that. And you can always send an observer from the messagers’ side to keep me honest – if you can find any volunteers for the job.’

  Greta said, ‘Right now I’m having trouble even thinking of a pilot.’

  Ramiro didn’t reply. For all the help Tarquinia had given him with the debate, after the vote she’d refused to get involved with the dissenters. A dozen years away from the mountain would be too painful a sacrifice to ask from anyone with a clear conscience.

  ‘You’d also need an agronomist,’ Greta added. ‘I doubt that even the diehard migrationists would take your word about the prospects for growing a crop.’

  ‘That’s fine with me.’ That she’d bothered to make the suggestion at all was a sign that this might not be hopeless – that he might have snagged her mind on the rough edges of his plan.

  ‘Do you really have no idea who the bombers are?’ Greta asked.

  ‘None at all.’

  ‘I believe you,’ she said, ‘but I don’t know how to prove it to the Council.’

  ‘Whatever happened to the need to prove people guilty? Half the Peerless voted the same way as I did, but I doubt you’ve even locked up all the strikers.’

  Greta pretended that she hadn’t heard his last remark; he wasn’t allowed to know who else had or hadn’t been imprisoned.

  ‘If I put this to the Council,’ she said, ‘they’ll only agree to it if it comes from them. They have to be the peacemakers, reaching out to their enemies for the sake of the greater good.’

  ‘Well, naturally.’

  ‘And they might not even want you on the mission,’ she warned him. ‘What if they go with the idea, but then pick a crew without you?’

  Ramiro buzzed. ‘At worst, I might have to stay in prison for the whole four years that they’re away. Compared with spending twelve in something not much bigger than my cell, I don’t think the disappointment would crush me.’

  Greta was puzzled. ‘And yet you’re willing to do it, if you’re asked.’

  Ramiro said, ‘Who else could make this work politically? If you send Pio, half the mountain will riot. You trashed his reputation as soon as you locked him up over the rogue.’

  ‘And we haven’t trashed yours?’

  ‘Not yet, I hope.’

  Greta drew herself out of her harness. ‘I’ll give this some thought. In the end, all I can do is take it to the Council.’ She dragged herself towards the door and tapped for the guard.

  ‘And put it to them the right way,’ Ramiro pleaded.

  Greta turned to face him. ‘And what’s the right way?’

  Ramiro said, ‘Forget it. It’s not for me to tell you how to do your job.’

  When she’d gone, he closed his eyes and pictured the scene in the Council chamber. What would you say, Greta the fixer would begin, if we could find a way to inspire every troublemaker in the mountain to march, willingly, into the void, out of our lives for ever?

  14

  Agata reached over and took Arianna from Gineto. The child ran her hands over Agata’s face, then frowned, disappointed. A moment later she started humming in distress. Agata handed her back to her great-uncle.

  Serena said, ‘Don’t worry, she’s moody with everyone. I’m sure she’ll get used to you.’

  ‘I’m happy to look after her,’ Agata declared. ‘Any time you want me to.’

  ‘That’s very kind,’ Gineto said, in about the same tone as Agata might have used if he’d offered to help her prove a theorem in topology.

  Gineto had moved into Medoro’s old apartment, so at least Arianna would still have the same surroundings. Agata didn’t know what she could contribute beyond the occasional period of babysitting, but she needed to do something to assuage the ache she felt from Medoro’s absence. If she couldn’t even help with his niece, what was left to her?

  Medoro’s books still lined the walls of the living room: mostly specialist works on the theory of solids. He’d always teased Agata about her esoteric research, but improving the design of photonic arrays had required far more physics than she’d ever mastered. She would have needed to study for years to have any chance of taking his place in the camera team.

  ‘What do you think of this new mission they’re proposing?’ Serena asked her. ‘The Surveyor?’

  ‘I don’t know.’ When she’d first heard the news, Agata had been disgusted; it had sounded like a gesture meant to appease Medoro’s killers. Since then she’d grown less adamant, but she’d still been too angry to try to think through all the ramifications.

  Gineto said, ‘If someone’s fighting to impose their will over the mountain, they’re not going to give that up and walk away.’

  ‘Not even for a planet all their own?’ Serena replied.

  ‘You think they want freedom? They just want power.’ Gineto was talking in his hyper-happy baby voice, beaming down at Arianna, pulling faces to make her chirp. ‘The Peerless is all they know. If they’d wanted to take their chances on their own, they could have asked for that any time since the turnaround.’

  ‘Then what’s the solution?’ Serena demanded. ‘If it were up to me I’d abandon the messaging system, but the Council’s already talking about the bombing as proof of how much we need the warnings.’ Her voice faltered; Agata reached over and squeezed her shoulder.

  ‘The Surveyor’s not a bad idea,’ Agata conceded. ‘If they can grow crops on an orthogonal world, it would be one of the safest places to start a colony. There’d be a limit to the kind of collisions with home-cluster matter it could suffer in the migrants’ future – since anything really catastrophic in the world’s own past would have destroyed it, or lit it up like Gemma. On a cosmic timescale the entropy gradient would be a problem, but compared with the Peerless or the home world, a planet guaranteed to stay intact for eons would be a haven.’

  ‘Then it’s worth trying, isn’t it?’ Serena said.

  Gineto wasn’t swayed. ‘You think people who don’t want to know the future will migrate to a world where they can step out of their houses and see what a dozen generations of their descendants will have carved into the rocks?’

  Serena said wearily, ‘I just want a plan that both sides can live with. If not this, let them split the mountain in two.’

  Gineto drew Arianna against his chest to hide his face from her as his expression grew grimmer. ‘The war to decide the size of the pieces would kill us all. If that’s the alternative, the Surveyor can fly with my blessing.’

  ‘I’m very sorry about your friend,’ Lila said gently.

  ‘Thank you.’ Agata shifted in her harness. ‘His uncle’s looking after the baby, but it’s hard for the whole family.’

  ‘Of course.’ Lila offered a moment of sympathetic silence, then delicately broached a different subject. ‘Have you had a chance to think about the gradient problem, since our last meeting?’

  ‘Not really,’ Agata confessed. She pictured Medoro standing in the corner of the office, the expression on his face enough to convey exactly what he thought of her laziness. It had been three stints since the bombing; she really had no excuse not to get back to work.

&nb
sp; Lila said, ‘I’ve had one idea myself, if you’re interested.’

  ‘Of course.’ Agata leant forward attentively and tried to concentrate.

  ‘It’s possible that what we’re lacking is a proper understanding of vacuum energy,’ Lila suggested. ‘You know the naïve version: if you look at the free light field, and assume the right kind of relationship between the mass of a photon and the dimensions of the cosmos, each mode of the field that wraps around the cosmos a whole number of times is like a simple oscillator. Wave mechanics tells us that an oscillator like that can’t have an energy of zero: the lowest energy level has some non-zero value. So even if the cosmos were empty, the vacuum would have as much energy as you’d get by adding up the lowest levels of all the possible modes of the light field.’

  ‘Plus the same kind of contributions from all the modes of the luxagen field,’ Agata added.

  ‘Yes. Which are actually negative, if you take the mathematics seriously.’ Lila buzzed softly. ‘When I was developing the gravitational theory, I was never sure if I should claim that this kind of energy would need to be included as a source of curvature – and then go on to insist that the very mild curvature that empty space seems to possess is proof that the two kinds of vacuum energy more or less cancel each other out.’

  ‘Hmm.’ Agata wasn’t sure whether Lila was mocking this idea or trying to resurrect it, so it seemed wiser not to offer an opinion of her own.

  ‘The thing is, though,’ Lila continued, ‘the naïve version is just that. We don’t know if the cosmos really has the right dimensions to allow any free modes of either field, and we don’t know how to calculate the vacuum energy under more realistic assumptions: taking account of the interaction between luxagens and photons, and taking account of the curvature of four-space.’

  Agata said, ‘But if the vacuum energy is a source of curvature, and the curvature itself can influence the vacuum energy . . .’

  ‘Then it’s much less clear what combinations of the two are actually possible,’ Lila replied. ‘That’s what I’m hoping would shed some light on the gradient problem.’

  ‘Ah.’ Agata finally caught a glimmer of that illumination. ‘If we take account of the way the geometry of the cosmos determines what kind of waves can exist – which governs the vacuum energy, which contributes to the curvature – we might end up showing that a uniform cosmos would be self-contradictory.’

  ‘It’s conceivable,’ Lila said cautiously. ‘Of course, that wouldn’t help much if a tiny wrinkle in the curvature was enough to make things work out. What we’d need is for there to be no solutions without a significant entropy gradient.’

  Agata understood the proposal now – and it was terrifying. To make any progress they would need to combine field theory, wave mechanics and cosmology in a manner that no one had ever achieved before.

  ‘Can I think about this?’ She didn’t want to agree to the project only to find that she couldn’t summon up the kind of focus and stamina that an undertaking of this scale required.

  Lila said, ‘Of course.’

  Agata hadn’t set out with any intention of going near the demolished workshop, but when her path took her to the boarded-up entrance she wasn’t surprised.

  The floor of the tunnel outside was still covered in a layer of fine dust; she knew it was bluish by artificial light, but in the red glow of the moss it looked almost black. The last time she’d stood here the entrance had only been covered by a curtain, and she’d peered in through a gap. A string of coherers hung across the ceiling had illuminated a team of investigators at work in the rubble. They’d been photographing everything as they sifted patiently through the debris, hoping they might find fragments of the bomb.

  As far as she knew, they never had. The Council had locked up all the anti-messagers who’d argued their case most vigorously in public, but as likely as not Medoro’s killers were still free. When the new team of instrument builders was assembled they’d have bodyguards around the clock, and no one would get within a stroll of the workshop without being searched. But if the bombers’ first choice of target became impossible to reach, they would find another one. Even if all of the system’s components could be built without another incident, when the whole thing was finally assembled it would be vulnerable to other kinds of sabotage.

  The Surveyor mission wasn’t an act of appeasement; everyone would be better off with the two factions living apart. Agata didn’t know how many people would be prepared to abandon the familiar surroundings of the Peerless, but it was brave of Ramiro to be willing to make the journey to discover if migration was possible at all.

  She leant against the wall, humming and shivering. Sometimes she missed Medoro so badly that she wanted to die, but everyone expected her life to go on as if nothing had happened. And now Lila was inviting her to spend the next few years struggling with some beautiful ideas that for all she knew might have no bearing on reality at all.

  Agata stilled herself and stared down into the black dust at her feet. She felt as if she’d been waiting all her life for just one message from the future, telling her that everything would be worth it in the end – but the hungrier she grew for that scrap of comfort the further it receded, and the greater the cost. She would have given up all hope of it to get Medoro back, but no one was offering her that choice.

  She couldn’t spend another day sitting in her office juggling equations, with no idea if they were true or false. And she couldn’t bear to be around Medoro’s family if she had nothing useful to contribute to their lives. Everything on the Peerless was ash to her, now. She needed to find another reason to live, or she was finished.

  The Surveyor team were still looking for volunteers. At the cost of spending twelve years cooped up in a glorified gnat, the crew would be the first travellers in six generations to set foot on anything like a planet. If they could bring peace to the mountain, all the better, but just making that trip would be extraordinary.

  Agata turned away from the entrance and began retracing her steps. She’d very nearly talked herself into it – but if all she had to offer the rest of the crew was her desperation, they’d be better off leaving her behind to go insane on her own time. She could go through the motions with the vacuum energy calculations, then if the messaging system survived the saboteurs she might at least get a verdict from the future as to whether or not Lila’s theory of gravity applied in the real world. There’d be nothing inconsistent with the laws of physics in being told that she’d wasted her life.

  She stopped dead, her skin tingling, ashamed of her self-pity but grateful for one detail of her maudlin fantasy. A verdict on Lila’s theory, how? She’d always imagined that such a thing would never come until after the reunion itself, once there’d been a chance for a future generation of astronomers to make observations from the home world. But the home world was no longer the only planet worth imagining.

  15

  Ramiro scratched the skin around his fetter; it had been itching horribly for the last three days. Despite his pleas, Greta had insisted that he remain in chains even when they were interviewing candidates. He was beginning to wonder if he’d be kept in restraints even once he was on the Surveyor itself.

  ‘The planet Esilio is orbiting a massive star,’ Agata enthused. ‘But we’ll be blind to the light of that star; it will appear to us as nothing but a pale grey disc. That combination offers the perfect conditions for the observations I want to carry out.’

  Ramiro had no idea what she was talking about, but they hadn’t seen anyone else with pro-messager credentials half as eager to make the journey. ‘Go on,’ he said.

  ‘If gravity is really just the curvature of four-space,’ Agata continued, ‘then light that passes close to this star will be bent less than it would be under Vittorio’s theory. I know that sounds strange, but in Lila’s theory the curved space around the star makes centrifugal force stronger than it would be in flat space, so it’s harder to bend the light’s trajectory. With no glare to
hamper the observations, we could measure the apparent positions of home-cluster star trails as they approach the edge of this star’s disc, and see which predictions turn out to be correct.’

  She summoned an illustration of the phenomenon onto her chest.

  ‘I’ve exaggerated the scale of the effect here,’ Agata admitted, ‘but it would certainly be measurable with a small telescope.’

  Ramiro thought this sounded harmless enough. The last applicant who’d claimed to be drawn to the mission by the chance to carry out a scientific project had wanted to experiment with exotic methods of pulverising Esilio’s surface from orbit, in order to impose their own entropic arrow as firmly as possible. Listening to the woman’s wish-list of weapons that she hoped to load onto the Surveyor had been entertaining, but a theodolite or two would probably be easier to sell to the Council than a flying armoury.

  Greta, though, was as suspicious as ever. ‘You’d be willing to give up twelve years of your life, just to observe this minor optical effect?’

  ‘I would,’ Agata replied. ‘The precise angle by which a beam of light is deflected by a star might not sound important, but until we know for sure whether matter and energy really do curve four-space, the answers that we’re struggling to find to much bigger questions – the geometry of the cosmos, the reasons for the entropy gradient – will just be guesswork.’ She paused, then added, ‘I also think the mission’s worthwhile for its own sake. If there are people who can’t live with the majority’s decisions on the Peerless any more, we should let them leave.’

  ‘I understand that you were close to one of the instrument builders?’ Greta pressed her. ‘To Medoro?’