The Arrows of Time

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.