‘Something to do with us?’ Stef asked.

Penny said, ‘And what’s it to do with you, Earthshine? What do you want?’

‘Well, that’s rather nebulous at the moment. Suffice to say—’ He paused, as if choosing his words. ‘I want to stop being afraid.’

Stef stared at him, startled by that peculiar non sequitur. He’d said this calmly, his expression still, faintly artificial. Yet that, somehow, made it all the more convincing.

Penny seemed more aggressive. ‘You, afraid? Afraid of what? You’re an artificial mind stored underground in massively paralleled and distributed processor and memory banks, with your own dedicated manufacturing units and energy supply. You and your partners rode out the climate Jolts like they were bumps in the road, while millions of us died. What could you possibly be afraid of?’

‘I will explain, in time.’ He held out his hands. ‘I know this is difficult for you. But here you are, together. Would you like to talk?’

Penny and Stef looked into each other’s eyes, just as they had in that first moment of revelation in the Hatch on Mercury. Then they looked away.

At length Stef said, ‘I’ve done some research. On us, on our past.’

‘I know. You’ve been doing it for years. My firewall traced you. I let you go ahead.’

‘I saw the records,’ Stef said. ‘As they exist now. We are twins, genetically identical. I am the older by a few minutes. We seem to have been close companions when we were small.’

‘I remember,’ Penny said more softly. ‘I wouldn’t need to research it. We played all the time.’

‘We were put through the same schools by our father. We showed the same kind of aptitude, basically mathematical, logical, verbal. We both joined the ISF for the sake of scholarships that put us through grad school and sponsored our early researches, and enabled us to get access to the kernel labs on the moon.’

‘The ISF split us up,’ Penny said. ‘Their psychs thought it would make each of us more self-reliant. Still we did the same training and development, more or less, just in a different order.’

‘But our careers converged again, when we started working on kernel physics.’

Penny said, ‘It all came from that day we were on Mercury with Dad, we were eleven years old, when the first hulk ship was launched. That was what inspired us to go into kernel research in the first place.’

Stef closed her eyes, just for a moment. No. I was there alone. With Dad. You weren’t there, not even as some unwelcome ghost. That was my day, not yours . . .

‘And then it was all fine until we went into the Hatch on Mercury,’ Penny said sadly. ‘I went through first, Stef. You followed me in. And when I went into that second chamber, and I turned around and you saw me, I could see you didn’t recognise me. We’d only been out of each other’s sight for a minute—’

‘Less than that,’ Earthshine said. ‘I have studied the record. Thirty-eight seconds.’

‘And my memory is different,’ Stef said. ‘I went alone into the Hatch. I opened the second hatch. There was Penny, already in the next chamber.’

‘Before that time, you, Stef, clearly knew your sister. Afterwards you were baffled by her very existence, though you did your best to conceal it when you realised that something was very wrong.’

Stef felt resentment flare. ‘You’re not allowed access to any material on kernel physics. That’s a UN law.’

‘Of course,’ Earthshine said smoothly. ‘But any such law needs a defined boundary. And I, or my legal advisers, push assiduously at that boundary. Wouldn’t you? I am entitled to explore the implications of kernel science, even if I must turn my head away from the physics itself. A visual record of events at the Hatch tells me little about the underlying physics, and much of it is in the public domain anyhow.’ He leaned forward. ‘Major Kalinski – I mean, Stef. Only you remember how it was before. Your life as an only child. Yes? Most people therefore assume your memory is faulty.’

Stef said, ‘Or that exposure to the Hatch messed with my mind and sent me mildly crazy.’

He shook his head. ‘But that’s not what you believe, is it, Major? Now consider the alternative. If your mind hasn’t been tampered with – if your memories are authentic—’

‘This makes no sense,’ Penny said, growing hostile.

Earthshine urged, ‘Just run with this for a moment. Stef, what’s your alternative hypothesis?’

She took a deep breath. ‘History changed. What else? The minute I opened that Hatch.’

Earthshine nodded. ‘Before, there was a different history.’

‘Where I was an only child. Where I had a different name, for God’s sake. I was Stephanie Penelope Kalinski, not Stephanie Karen, and Penelope Dianne never existed. And when I opened that Hatch and stepped inside, there you were, Penny – real, live, impossible. With a set of memories of a different past. Memories that were in everybody else’s head too.’

‘All except yours,’ Earthshine said. ‘Just suppose you’re right, Stef. Just suppose reality was changed, that the Hatch, on accepting you, immediately tinkered with the past – at least with your own past. Giving you a sister you never had. And presumably causing subsequent small changes that rippled away from that big central adjustment.’

Penny was clearly uncomfortable, and Stef was sure she knew why. They were talking about a world where she’d never even been born, and that must be existentially terrifying. Penny said now, ‘Occam’s razor. Basic principle of science. The idea that Stef somehow got a kind of amnesia is a lot simpler than the idea that the whole universe has been changed to generate a new reality.’

‘Well, Occam has been dead a long time,’ Earthshine said mildly. ‘And is the alternative really so preposterous? We know that the Hatch technology involves some kind of manipulation of space-time. You both clambered down into a hole beneath the Hatch that could not exist, according to the geophysics measurements. What is a history change but another such manipulation? In time, rather than space. Stef, I suspect you may not have gone much further with this line of thinking yourself, even in the privacy of your own head.’

‘What are you getting at?’

‘I mean that if there has been some kind of history change, effected by the Hatch, or whoever built the Hatch—’

Penny snorted. ‘Oh, this is—’

‘Then it’s been kind of a messy change, hasn’t it? I mean, it hasn’t been clean. We know that it’s left at least one trace of what went before, in your own memory, Stef.’

‘That’s hardly evidence,’ Penny snapped.

‘It is to Stef. Maybe it had something to do with you being inside the Hatch itself, at the moment the change was effected—’

‘And what would be the point?’ Penny demanded now. ‘You’re talking about changing history. If you can do that, why not, hell, wipe out a climate Jolt or two? Or even wipe out the warming altogether – why not go back and shoot Henry Ford?’

Stef said, her mind racing, ‘Maybe it – or they, the Hatch-makers, whoever is behind this – couldn’t manage anything on that scale. Maybe they didn’t know enough about us, about humanity, to make more than the smallest change. Maybe this was all they could manage. For now, anyhow.’ She looked at Earthshine. ‘But why us? I mean, why me? What’s significant about me, or my life?’

‘Everything, ‘ Earthshine said. ‘Or nothing. Maybe it was just the fact that you were first into the Hatch. This was a kind of – test run. An exploration. But if so, as I said—’

‘The execution was sloppy,’ Stef said. ‘With one loose end left, in my memory. Trant and King remembered Penny opening the second hatchway and going through ahead of me. I remember opening the second hatch myself, then seeing Penny for the first time . . . Sloppy.’

‘At least one loose end.’

Stef looked at him sharply.