The Arrows of Time

by Greg Egan

  Agata was about to retort that she had every intention of being that ‘someone’, but she caught herself; he was just goading her. ‘That’s enough cosmology,’ she said. ‘How’s the camera business?’

  ‘Cosmological,’ Medoro replied. ‘Actually, that’s why I came looking for you. I’m starting a new project, and I wanted to hear your thoughts on it.’

  Agata was intrigued. Medoro made cameras for the astronomers from time to time, but he’d never felt the need to consult with her before. ‘What are you building?’ she asked.

  ‘A new imaging chip,’ he said. ‘One that can visualise the orthogonal cluster.’

  ‘Visualise it?’ Agata scrutinised his face, half suspecting that she was being set up for a joke, but either way she couldn’t resist the bait. ‘How?’

  Medoro said, ‘Instead of polling the array of pixels on the chip and counting how many photons have struck each of them, it will count how many photons each pixel has emitted. Point the camera at the sky . . . and when it emits light towards the orthogonal stars, you can read off the details.’

  Before the turnaround Agata would have been sceptical, but now she could see that the possibility of a camera like this had been implicit in the results of the very first engine tests after the reversal. Just as the engines had happily given off light that the ultimate recipients would consider to be arriving from their future, the orthogonal stars were – presumably – still shining down on the Peerless, despite being rendered invisible by the very same property. People’s eyes had not evolved to know when they were the joint authors of a beam of light, as responsible for creating it as the distant star at the other end. But a camera could be made to catch its own strange radiance in the act.

  ‘Who commissioned this?’ she asked.

  ‘Do you know Greta?’

  ‘No.’ Agata knew all the astronomers, and there was no Greta among them.

  ‘She’s a technical adviser to the Council,’ Medoro explained. ‘She supervised the turnaround, but now that it’s over she’s been given this new thing.’

  ‘Which is . . . ?’

  Medoro leant forward as if to share some delicate confidence. ‘I was told that the camera would be part of a general upgrade of the navigation systems. The rationale being that the old maps are fine for most purposes, but if we can find a way to keep getting real-time images of the orthogonal stars, so much the better.’

  ‘Except that this is better than real-time,’ Agata joked. ‘Instead of seeing where the star was, we’ll know where it will be.’

  Medoro said, ‘That, and a great deal more.’

  ‘I’m sorry?’

  He buzzed impatiently. ‘Come on, you’re the physicist! Do I have to spell it out?’

  Agata stared at him, bemused. Knowing the future positions of the orthogonal stars would not be a momentous revelation: their trajectories were already predictable over a time-span of eons. And in fact, these stars’ ‘future’ positions would be positions in which they’d already been observed, earlier in the Peerless’s own twisted history. Telescopes had improved since then, but there were unlikely to be any spectacular, collision-avoiding surprises.

  ‘You’ve lost me,’ she confessed.

  ‘Suppose something occults an orthogonal star that I’ve been watching with this camera,’ Medoro said. ‘What happens then?’

  ‘The occulting object will take the place of the camera as the second source of the light.’

  ‘So we’ll know about the occultation?’ he pressed her.

  Agata said, ‘Of course! If there’s no light passing between camera and star, the “image” of the star will disappear, just as an ordinary image would.’

  ‘And when will we know about it?’

  ‘When? The exact time will depend on the geometry: the location of the object that blocks the light, and the speed of light for the part of the star trail that’s obscured.’

  Medoro said, ‘Now suppose that we arrange a sequence of occultations – of the slowest detectable light, with the blocking taking place as far from the telescope as possible.’

  Agata thought she knew where he was headed. ‘Then the image of the star will blink out before the blocking object is actually in place. But you know, even the slowest detectable infrared is quite hard to outpace. So unless you build some massive engines, these flying shutters of yours would need to be launched long before you see their effect on the star.’

  But Medoro wasn’t finished with his thought experiment. ‘Now add a pair of mirrors and fold up the light path, so we can achieve the same effect while manipulating an object that’s much closer.’

  Agata raised a quick sketch of the proposal.

  ‘Depending on the dimensions of the system and the number of bounces before the loss to the mirrors is too great,’ she said, ‘you’ll be able to make observations that reveal the shutter’s position some time into the future. I’m no expert on practical optics, but I’d guess that a realistic time-span would be measured in flickers at most.’

  Medoro said, ‘Maybe. But suppose it’s more than twice the response time of an automated signal booster. You might only be able to receive the message from a short way into the future, but so long as you can resend it to a time when the booster will be free to handle it “again” – without any overlap with the later boost – the process can go on indefinitely.’

  Agata gazed at the picture on his chest. If there was a flaw in his plan, she couldn’t see it.

  ‘Greta didn’t mention anything like this?’ she asked.

  Medoro scowled. ‘No – but do you really think anyone could commission a camera that detects light from the future, without this sort of thing crossing their mind?’

  Agata was ashamed that she’d failed to see the possibilities herself long ago. This was the most beautiful idea she’d ever encountered.

  ‘You’re right,’ she said. ‘The Council must be working on a messaging system. And if they can boost the signal like this . . .’ She reached over to Medoro, almost touching the diagram. ‘Then I don’t see why we couldn’t use it to learn about the journey still to come, all the way up to the reunion.’

  7

  ‘Have you lost your mind?’ Ramiro had come to the interview with high hopes, but within a few lapses his mood had been transformed from anticipation to bemusement to horror. ‘That’s the most deranged thing you’ve ever asked me to do – which is not an easy contest to win.’

  Greta motioned with her hand on her tympanum, imploring him to keep his voice down.

  Ramiro said, ‘If you’re going to keep raising this subject with people, you might want to think about soundproofing your office.’

  He began drawing himself out of the harness facing her desk. ‘Where are you going?’ Greta asked anxiously.

  ‘Relax,’ he said. ‘I’m not going to tell anyone. Though next time you ask me to keep something between us, I’ll take that as a sign that I should turn and run. And when you put this to a referendum, I promise you I’ll be campaigning very noisily—’ Ramiro caught a flicker of discomfort on her face. ‘There is going to be a vote, before you actually build this?’

  ‘That’s up to the Council,’ Greta replied. ‘But there’s not much point voting on a system whose feasibility is entirely hypothetical.’

  Ramiro slipped back into the harness. ‘So you were thinking of building it first? And then what? Hope you can learn something in advance that will guarantee the outcome of the vote? But how would that work? What if the message from next year’s Council is simply that whatever they tried, failed?’

  Greta said stiffly, ‘It’s not about vote rigging. It’s about security. You of all people should appreciate that.’

  ‘Me of all people?’ Ramiro stared back at her in disgust. ‘If we’re being frank, I blame you for the farce with the gnat just as much as I blame the migrationists.’ As the words emerged he wondered if he was letting his anger get the better of him. But Greta showed no sign that she was wounded, let alone contr
ite.

  ‘I don’t understand why you’re so vehemently against this,’ she said. ‘You should take a few days to think it over.’

  Ramiro buzzed. ‘What’s funny is that you’ve been planning this for a year – but you still can’t see that your last remark should answer the preceding question.’

  Greta spent a pause or two struggling to parse that, but it seemed to be beyond her. Ramiro said, ‘You’re inviting me to take my time to ponder all the pros and cons before reaching a decision – but the answer you want from me would eradicate my ability ever to go through the same process again.’

  ‘That’s nonsense,’ Greta said amiably. ‘No one’s asking you to surrender your free will.’

  ‘And that’s not how I’d put it, myself,’ Ramiro replied. ‘But I’m not going to start debating terminology. The simple fact is that anyone who knows their own actions in advance will be living a different kind of life than someone who doesn’t.’

  ‘What makes you think that you’d be forced to know anything about your own actions? The Council will use this facility for planning and security purposes. Any other applications will be carefully controlled – and exposure to information is hardly going to be compulsory.’

  Ramiro said, ‘That’s naïve: information would spread through third parties. You could never come close to promising me that I wouldn’t end up hearing things I didn’t want to hear.’

  ‘What if we’d known the rogue gnat’s trajectory in advance?’ Greta demanded. ‘Are you honestly telling me that it wouldn’t have been worth it?’

  Ramiro wasn’t going to let her use the rogue to bludgeon him into submission. Perfect knowledge of the future might have spared him the dangerous encounter, but the whole mountain shouldn’t have to pay the price for the way the threat had been mishandled from the start. ‘I’m sorry I blamed you for that débâcle,’ he said sarcastically. ‘The fault was mine: I should have just let the gnat hit the Object.’

  He struggled out of the harness.

  ‘Are you going to keep your word?’ Greta asked. ‘I knew I was taking a risk, but I thought I could trust you.’

  Ramiro freed himself and clung to the guide rope leading to the doorway. If he said the wrong thing, could she have him imprisoned until the messaging system was complete? They’d set a precedent with the migrationists, and if he vanished from sight he wasn’t sure that anyone would come looking for him.

  ‘I’ll keep my word,’ he said. ‘I don’t break promises.’ He contemplated adding that he trusted the Council to ensure that the matter was put to a vote, but even Greta was likely to pick up the sarcasm.

  Outside the office, Ramiro still felt rattled by the confrontation, but as he set off down the corridor he began to regain his composure. It was never exactly prudent to hurl abuse at potential employers, but Greta had a thick skin and he doubted that he’d be thrown in prison for refusing a job. So long as he kept quiet about the offer he’d be left alone.

  He reached an intersection and turned into a busy corridor. People strode by, purposeful, intent on their various plans, shaping the minutiae of the unfolding morning. But every child knew that, to the ancestors, the sequence of events that a traveller perceived as evolving over time was no different from the fixed pattern in a tapestry. From the right perspective, each life was a completed picture from birth to death, there to be taken in at a glance.

  Every child was also taught that this incontestable fact did nothing to rob them of their freedom. The laws of physics bound people’s choices to their actions, as firmly as they bound a tumbling rock’s positions from moment to moment into a single, coherent history. Though no one ruled unchallenged over their own flesh – no one could be immune to coercion or injury, no woman to spontaneous division – the exceptions only made it clearer that most acts were acts of will. An omniscient observer who could read the fine details of the tapestry would see that woven into the pattern: deliberation beside resolve, resolve beside deed. Each choice would have its own complex antecedents, inside the body and beyond it – but who would wish to sit in isolation, churning out decisions that came from nowhere?

  Ramiro had long ago reconciled himself to this picture of time and choice, and though he couldn’t claim to perceive his own life in these terms from day to day, he felt no disquiet at all at the prospect of the timeless point of view growing more compelling.

  But Greta’s system would do far more than confront the travellers with a stark confirmation of abstract principles that most of them already acknowledged. The one thing a message from the future couldn’t tell a person was what they would have decided in the absence of that message – it would not be as if the ordinary deliberation really had taken place elsewhere, and was now being delivered to them as a kind of executive summary to spare them from needlessly repeating the effort. The old process wouldn’t merely be rendered more efficient, so it reached the same endpoint with less uncertainty or stress. The endpoint itself could be completely different.

  And even if it wasn’t, was that all that mattered? Ramiro stopped walking and moved to the side of the corridor so he wasn’t blocking the guide rope. If he heard from the future that he’d raised Rosita’s child, then in the end he would choose to make that happen. If he heard that he hadn’t, he would choose differently. He couldn’t claim that this would turn him into some kind of hollow puppet, when both outcomes were already possible in the ordinary course of events.

  But the nature of the decision would still be utterly different if he reached it with foreknowledge. All that the need for consistency could impose was the requirement that he actually went along with the choice – however reluctantly, resignedly or apathetically he closed the loop. The revelation wouldn’t need to ring true, or fill him with joy, or cast any light on the dilemma it resolved. He merely had to be capable of acceding to it – of muttering ‘Yeah, that’ll do.’

  He couldn’t live like that – and he couldn’t stand by and let the Council force it on everyone else for the next six generations. Greta’s promise that the information would be contained was just wishful thinking; that would certainly make the technology more useful to its owners, but Ramiro had no doubt that the content of the messages would still leak out.

  And the sooner he broke his own promise, the safer he’d be. He called out to a woman approaching on his left, ‘Excuse me!’

  She stopped. ‘Yes?’

  ‘My name’s Ramiro, I’m an automation engineer.’

  The woman looked puzzled, but she introduced herself. ‘I’m Livia, I’m a shedding technician. Didn’t you—?’

  ‘Lose an exploding leg near the Station? Yes, I’m that idiot.’

  Livia paused expectantly. Famous or not, his claim on her time was strictly limited.

  Ramiro said, ‘I’ve just heard that the Council is planning a new messaging system; they invited me to work on it, but I declined. If they build it, it will affect all of us, so if you can spare a couple of lapses I’d like to tell you about it.’

  By the time he was halfway through his account there were six more people listening. Ramiro confined his message to the science itself; people could ponder the implications at their leisure, and if he started philosophising that would only encourage disputatious onlookers.

  When he’d finished, Livia thanked him and walked away, but some of the others gathered nearby in heated discussion. Ramiro left them to it; it was more important to keep spreading the word than to try to influence one small debate. He raised a schematic on his chest and spread his arms. ‘New messaging system! Hear all about it!’

  A dozen people walked past him, bemused or embarrassed, but then a woman stopped. ‘What new system?’

  ‘The one that uses light from the orthogonal cluster to bring information from the future.’

  ‘You’re joking?’

  Ramiro said, ‘Hear me out, then decide for yourself.’

  As he started speaking, more people gathered, while the remnants of his last audience dispersed. In a
chime or two there’d be no hope at all of tracking down everyone who knew about the scheme, let alone locking them up.

  8

  ‘You should ask Pio for some debating tips,’ Medoro suggested. ‘He’s the expert.’

  Agata hummed frantically. ‘Are you going to help me or not? I only have three more days to get this right.’ The last time she’d sought Medoro’s advice he’d fobbed her off with an excuse about the camera creating a conflict of interest. The Councillors themselves were required by law to stay out of the debates, but if every last person who had some stake in the outcome kept themselves at arm’s length from the process there’d be no one left with a reason to argue the case on either side.

  Medoro relented and invited her into his apartment. ‘I’ll do what I can. If your enemies portray you as part of my self-serving cabal, so be it – but when we’re in public you’ll have to call me “puppet master” and answer to “stooge”.’

  Agata rewarded this suggestion with silence.

  He said, ‘Seriously, wouldn’t Lila be more use to you?’

  ‘Lila’s staying out of this. I’m not sure if she’s even made up her mind how she’ll be voting.’ Agata hadn’t pushed her on the matter, and she respected Lila’s right to take a different view, but without even her long-time mentor backing her she was beginning to feel desperately isolated.

  ‘What’s your brother’s position?’ Medoro asked.

  ‘Against, of course!’

  ‘Why “of course”? What happened to protecting our descendants from unforeseen risks?’

  Agata said, ‘Don’t ask me for his detailed rationalisation, but from what I’ve heard he’s claiming that the messaging system is the unforeseen risk – unforeseen by everyone but him, since he warned us that there’d be dire consequences from the clash of arrows.’

  ‘Fair enough,’ Medoro judged. ‘His message from the future told him that the rest of us would soon want messages from the future – but he couldn’t tell us that until now, because otherwise we might have wanted our messages sooner.’