Stef shrugged.

‘You see, Julius Caesar had already set foot in our island, and had planted the dream of conquest in the Romans’ empty heads. Fifty years later Kartimandia, queen of a realm in the north, was informed that the time had come, that the legions were massing in Portus Itius on the coast of Gaul for the invasion. It was she who travelled in person to Rome, she who managed to persuade Emperor Claudius that there was much greater glory to be gained if he turned his legions north, into Germania transrhenus, which even his glorious predecessor Augustus had failed to conquer. Continental provinces would be easier to consolidate for the Romans, and besides she pledged to become an ally of Rome, so that the invasion was unnecessary. She made a good case, it was said, much to the surprise of many Romans. But, despite the Romans’ prejudice at the time – and despite what Caesar said about us – we were no hairy savages, and Kartimandia was sophisticated and wily.

‘Well, it was Outer Germania that felt the tramp of the legionary’s boot and not the fields of Pritanike. Kartimandia, with some Roman help, went on to consolidate her hold on the whole of southern Pritanike, and her successors made themselves valuable allies of Rome by becoming a secure exporter of grain, wool and leather to supply the Empire’s continental armies. The Brikanti have never forgotten the achievements of Kartimandia. And forever since Brikanti women have won positions of power.’

Stef and Yuri had quietly talked over some of this with the ColU, as they speculated how this history had diverged from their own. In the account lodged in the ColU’s memory, at the time of the invasion of Britain a woman Roman historians knew as Cartimandua had indeed ruled a kingdom in the north of Britain, called by the Romans ‘Brigantia’. And northern Germany, meanwhile, had never been conquered by Rome after the disastrous loss of three legions in the Teutoberg forest a generation earlier. Not so here. Stef supposed that even if they could figure out how history had diverged to deliver this strange new outcome, there was a deeper question of why. Why this history – why the change now? And how had she and her companions survived the transformation of human destiny?

Eilidh, evidently sharply intelligent, was watching her. ‘Much of this is unfamiliar to you, isn’t it? Some day we must explore our differences fully. Yet, whoever you are, wherever you come from, I see your soul. Watching you at the Hatch, I saw the wonder in your eyes.’

Stef shrugged. ‘Guilty as charged. In my – home – I was a philosopher, as the Romans would say. I studied the kernels, and later Hatches, because I wanted to understand how it all worked.’ That had been her goal since she was eleven years old and she’d stood with her father on Mercury, and watched a kernel-driven manned spacecraft drive like a spear of light into the heavens. ‘Where do the kernels get their energy from? How do the Hatches work? What are they for? Why are they here? How was it I and my companions came walking out of that thing ourselves? And, frankly, I’m fascinated by what you’ve done here. On this world you’ve gone beyond anything my people ever achieved. You’ve built a Hatch …’

Eilidh grinned. ‘We have, haven’t we?’

Eilidh had the cetus pause over the Hatch construction site: the dull sheen of the Hatch installation itself at the centre, the land shattered and melted for a wide area around that central point, and a loose cordon of bored-looking legionaries playing knucklebones with fragments of broken rock.

Eilidh and Stef sipped Xin tea. There was no coffee to be had, one miracle of globalisation that evidently hadn’t translated to this timeline. Yuri had joked about going into business cultivating the stuff once they got back to Earth. But Yuri’s health was worsening; he’d been in a continual decline since they’d emerged from the Hatch …

Stef tried to concentrate on what Eilidh was telling her.

‘To create a Hatch is like mating wild boar: a simple act to understand but dangerous in practice, especially if you get in the way  … You take kernels. You arrange them in a spherical array, with all their mouths directed inward, to a single point in space. And at that centre you place one more kernel, its mouth tight closed. You understand that kernels can be handled with etheric fields?’

By which, Stef had learned, she meant electromagnetic fields. ‘Of course. We too first found kernels on Mercury. You can position them, even close or open their mouths to control their energy output.’

Eilidh frowned. ‘Some of your terms are unfamiliar, but clearly we agree on the essence. Well then, with sufficient kernels, held with sufficient precision, there is an inward blast of energy. You can only watch this from a distance, and many lives were spent in determining that distance precisely.

‘The configuration holds for only a splinter of time before the arrangement is blown apart. The land, the air all around is shattered, melted, by an outpouring of heat and shock waves – well, you see the result here. But if you get it right, when the glowing gases and the rain of liquid rock and the shocked air have all passed, and you can go back in to see – when all that is done, what is left is a brand new Hatch in its neat installation, just as you see here.’

Stef frowned. ‘I’m not sure I understand. You don’t have to construct the Hatch?’

‘No more than we have to “construct” a chicken emerging from the egg. Our druidh speculate that there is a Hatch implicit in the form of every kernel. It is merely a question of breaking the egg to release the chick, to use the kernels’ own energy to shock one of their brood to adopt this new form. You never discovered this?’

‘My culture was more cautious than yours. More timid, perhaps. We would never have won approval for such an experiment.’ For better or worse, she thought, we cared more about the lives of our technicians than to spend them on such stunts. Even if it had occurred to us to try. ‘How did you get the idea? I can hardly believe you found such a specific arrangement by trial and error.’

Eilidh smiled. ‘We did not. Somebody else found it for us.’ Now the cetus was rising, turning its prow to the jagged row of mountains on the misty horizon. ‘We first found the kernels on Mercury – as did you, yes? We were already travelling beyond Terra – well, obviously. We had big ships driven by Xin fire-of-life, and by potent liquid elixirs  … I fear our common vocabulary is not yet rich enough.’

Gunpowder and chemical propellants. ‘I get the idea.’

‘Such substances had been discovered and developed during centuries of war. We had already flown to Luna, to Mars, though many died in those days, and our first attempts to plant colonia on those bodies were often catastrophic …’

Stef’s head swam. Without the fall of Rome in the west, without the Dark Ages, could technological development have been that much faster? She imagined a medieval world with crude rocketships lumbering into space, with lessons slowly being learned about the vacuum of space, about radiation, about weightlessness, by cultures utterly unsophisticated in the relevant science – lessons learned the hard way, at the expense of many deaths. She was thrilled at the idea. Thrilled and appalled.

‘Then came Mercury,’ Eilidh said. ‘There was a war of acquisition, more intense than most. We all wanted Mercury and its resources to capture the energy of the sun, you see. It was seen as a strategic position in terms of advantage for the future. And just how strategic only became clear when a Xin party stumbled across a field of kernels.’

‘Ah.’

‘After the usual blood toll the kernels were tamed, their energies used to drive our ships, and they were unleashed as weapons of war.’

That simple phrase managed to shock Stef, despite all she’d witnessed in her own home timeline. ‘Surely not on Earth itself.’