‘That’s not impossible,’ Agata agreed. ‘But we can’t expect to be able to partition everything as neatly as that. While we’re still far apart we can talk about the two sides and their rules . . . but deep down, matter is just matter, it doesn’t come with allegiances. The real laws of physics treat all directions in time and space identically, and they’re the laws that every photon and luxagen obeys — without knowing or caring about anything called entropy, let alone what side it’s meant to be on in some clash of thermodynamic arrows.

‘Suppose we leave a piece of equipment behind on Esilio – say, a small spyglass. Over the eons, from our point of view, we’d expect it to become pitted by dust in the wind, and eventually break up completely and turn to sand. Our spyglass, our rules: that sounds fair, doesn’t it? But if that sand stays on Esilio, what origin will it have from Esilio’s point of view? Most likely, some ordinary Esilian rock will have broken down to make it — which to us, would look like erosion running backwards. But then, in Esilian time the remnants of that rock will eventually form themselves spontaneously into a spyglass, which lies on the ground until we come along to retrieve it. So if you follow the history of the matter that makes up the spyglass far enough in both directions, it’s clear that it’s not committed to either side’s rules.’

Ramiro said, ‘That’s all very fascinating, but you still haven’t told me whether or not I’d burn myself by touching a cold rock.’

Tarquinia broke in. ‘No one will be touching anything until we’ve done enough experiments to know what’s safe and what isn’t.’

Ramiro gave up and dragged himself away, muttering about the uselessness of theoreticians.

Azelio caught Agata’s eye. ‘Your story about the spyglass was unsettling,’ he said, ‘but I’ll tell you what disturbs me more.’

‘What?’

‘Swap the roles of Esilio and the Surveyor,’ he replied, ‘then tell the same story again. If something from Esilio takes the place of the spyglass, it must be with us already. We must have been carrying it, or the things that will become it, from the very start. Because according to Esilio’s arrow of time we’ve already visited the planet, and it’s almost certain that something remained with us when we departed.’

‘The black sun awaits your pleasure,’ Tarquinia announced from the doorway.

Agata looked up, startled. ‘Already?’

‘It’s now, or wait until we’re on our way back.’

‘Of course.’ Agata hesitated. ‘The telescope’s mine, until we switch orbits?’

‘Absolutely,’ Tarquinia replied. ‘But if you break it, you can grind a new lens.’

‘From what?’

‘The other part of your punishment will be hunting down suitable materials on Esilio.’