Greg Egan - The Arrows of Time

‘I was going to start with a few practical benefits,’ Agata said. ‘Suppose we learn that a new crop disease shows up three years from now. We can’t prevent it arising altogether – or we’d never hear about it – but we can still take early quarantine measures and ensure that the outbreak is limited.’

‘Boring but sensible,’ Medoro declared approvingly. ‘Exactly the kind of thing people want the Council to be doing. Throw in a reference to the Great Holin Shortage, and you’ll have won over half the women in the room. What about averted collisions?’

Agata said, ‘No – in most realistic cases we would have had plenty of warning by conventional means.’

Medoro was disappointed. ‘You don’t want to go for the frisson of danger? Message: “Thanks for starting a sideways swerve when you read this, it just paid off and saved the whole mountain”?’

‘I could raise it briefly,’ Agata decided. ‘Almost as a joke, so Ramiro can’t go too hard on the implausibility.’

‘What else?’ Medoro pressed her.

‘Next, reassurances about privacy, validation and containment. Everything can be encrypted and signed, in exactly the same way as with the ordinary network. So there’ll be no chance of anyone else reading a message intended for you personally, and no chance of the sender convincing you that they’re someone they’re not.’

Medoro said, ‘I can check a message that purports to be from you because you’ve published a validation key – and you’re here to complain loudly if someone else tries to distribute a different key and claim that it’s yours. But if I get a message from someone who claims to be my grand-niece… how do I authenticate that ?’

‘You just need a chain of trust,’ Agata explained. ‘You get a message from your niece, signed with a key that you’ll give her personally when she’s old enough. Her message gives you a key for authenticating her daughter’s message.’

Medoro grimaced. ‘So I need to have one key ready right now, and be sure that it won’t get lost or stolen before I have a chance to give it to a child who hasn’t been born yet?’

‘Yes. But is that so much harder than keeping your own key secure?’

‘I think I deleted my key,’ Medoro confessed. ‘That’s why I never sign messages any more; I’m too embarrassed to apply for a replacement.’

Agata said, ‘If Ramiro raises that kind of problem, what can I say? Nothing’s perfect, we can’t expect it to be, but we cope with the flaws in every other technology.’

‘You mentioned containment,’ Medoro reminded her. ‘Suppose your neighbour hears from her son about your death; how can you stop her telling you the details, if you don’t want to be told?’

‘Punishment,’ Agata said. ‘If you violate someone’s right to be future-blind, all messages addressed to you will be deleted henceforth.’

‘Not that anyone would bother sending them any more,’ Medoro reasoned. ‘But can’t we strengthen that further, and make it impossible to do the damage in the first place?’

Agata was amused. ‘I doubt it. I mean, the Council could declare that if someone receives a message that leads to an offence, the sender will never be allowed to send it in the first place. And if their ability to follow through on that were infallible, then it would never even need to be acted on. But realistically… ?’

Medoro said, ‘I might have been overreaching there. What’s next?’

‘I won’t have time for much more, but I should end with something about the reunion.’ Agata waited for Medoro to mock her, but he listened in silence. ‘We all have stories of the launch, passed down from generation to generation. Why shouldn’t it be the same with the reunion? Why shouldn’t we all have that sense of completion?’

‘That’s reasonable,’ he said. ‘Don’t overdo it, though; just say enough for the people who’d be swayed by it to complete the picture for themselves – without starting the cynics groaning about ancestor worship.’

‘No talk of receiving photographs of our descendants strolling through Zeugma beside Eusebio?’

‘I think not.’

‘That’s all I have,’ Agata said. ‘Practical benefits, rigorous safeguards, future-nostalgic finish. The rest of the time I’ll need to be dealing with Ramiro’s arguments. So if you can think of any downsides I haven’t addressed, this would be a good time to hear them.’

Medoro took a few moments to consider the request. ‘What about the idea that the system could be demoralising for innovators? You’ve been struggling for years to understand why the entropy gradients exist; how would you feel if you read a message from the future that handed you all of the answers, robbing you of the chance to discover them for yourself?’

Lila had expressed similar misgivings, but when Agata had thought the matter through she had not been swayed. ‘Complex ideas don’t come out of nowhere,’ she said. ‘Not because that would violate some law of physics or logic, but because it’s stupendously improbable. The most probable routes to complexity involve some kind of backstory. There were no people around at the entropy minimum – it took eons for the first simple organisms to arise, and eons more for life to evolve to include a species with a complex culture.’

Medoro was confused. ‘What has that got to do with some future rival stealing your glory?’

‘Complexity grows in a sequence of steps,’ Agata stressed. ‘So far, we’ve never seen it appear fully formed, and we have no reason to think that the messaging system could change that. If some future researcher did send me a theorem that made all my own efforts obsolete, it would enter the scientific culture and be passed down the generations – so whoever wrote the message would merely have heard the result during their own education: they wouldn’t be its discoverer.’

‘In which case… who would?’ Medoro struggled.

Agata buzzed. ‘Nobody! And there’d be no contradiction in that, but it’s as unlikely for the cosmos to contain such an isolated loop of unexplained complexity as it would be for the same idea to have popped into my mind all by itself this morning, with no prompting from any future informant.’

Medoro thought it over. ‘Suppose I take your word for all that. You’ve still only told me what you think won’t happen – you’ve given me an unlikely scenario. So what’s the alternative? What’s the likely story?’

Agata said, ‘There’ll have to be some self-censorship of the messages. People won’t pass back ideas that would entail the creation of complexity out of nothing.’

Medoro hummed with frustration. ‘Now you’ve introduced some magic cosmic censor?’

‘It’s not magic,’ Agata insisted. ‘It’s not some extra premise or some new constraint. If the messaging system can be built at all and the cosmos is self-consistent, the less improbable scenario – by far – is the one that contains some limits on the content of the messages, not the one that allows whole new sciences to come into existence without a day’s toil by anyone.’

Medoro plucked at the rope, dissatisfied. ‘What if someone in the future decides to break this rule?’

‘They can’t, they don’t,’ Agata said flatly. ‘Or to be precise: it’s prodigiously unlikely. The fact that ordinarily such an act would be unexceptional is beside the point: the messaging system will put them in a situation where the prerequisite for such a disclosure is that they have something massively improbable to disclose in the first place.’

Medoro wasn’t placated. ‘What happened to the freedom the engine tests gave us?’ he demanded.

Agata said, ‘That’s just the freedom to send messages in general, not some open-ended guarantee that the usual range of actions will always be possible. You didn’t complain about our lack of freedom to ignore an outbreak of crop disease, if we get a message spelling out everything we will have done to contain it.’