‘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.’