In a short, low-res holographic message – a cube showing his well-groomed head, his smiling middle-aged-politician-type face – the Core AI requested that she come visit him on Earth, at what he called his ‘node’ in Paris. He said he had a matter to discuss of global importance, but specifically of interest to ‘you and your sister’. There was also an avowal, in legal wording, that the AI would make no attempt to access the growing knowledge base on kernel physics during his meeting with the sisters. Without that avowal Stef supposed the message would never have been allowed through the various layers of security that surrounded her, here at Verne.

A similar message, an attachment noted, had been sent to Penny on Mercury.

Stef shut down the hologram with a curt acknowledgement of receipt, and spent a full dome-day thinking it over. That was her way when faced with dilemmas she found difficult or personally unpleasant, a way she’d developed of managing her own instincts over nearly thirty-six years of life. Let the news work its way through her conscious and subconscious mind, before formulating a decision. She even slept on it.

For one thing there was the sheer time she would need to take out of her own programme. Right now Stef was in a work jag that she was reluctant to climb out of. Well, she was always in a work jag. Seven years on from the Hatch’s first opening and the Penny incident – as she thought of it – explorations of the Hatch and investigations of its physical properties were shedding some light on the complementary studies of the kernels that had been going on for decades now. It was a slow, painstaking process, and it was full of gaps. Stef had the feeling she had been handed the two ends of a long chain of discovery, and she had a way to go before she worked her way from either end in towards the centre. But it was absorbing – there was more than a lifetime’s work here for her and her colleagues, she was sure. And that was a pleasing thought, since it pushed the need to make any drastic decisions about her own future off beyond the horizon.

Decisions such as about her relationship with her sister.

There was another reason for her to be wary about Earthshine’s note. She was actually working now with Penny. Her sister, who was on Mercury, was running direct experimentation on the Hatch emplacement, trying to detect emissions of various exotic high-energy particles. Unlike some siblings, indeed some twins, the sisters worked well together, as a long string of academic publications to their individual and joint credit from the beginning of their careers proved. In this particular project at this particular time Penny was the experimentalist, Stef the theoretician, but on other projects in the past, the record showed, it had often been the other way around. They were flexible that way, with close but complementary skill sets.

It was all fine and dandy, a family relationship to be admired and envied, and something that both their father and mother would have been proud of. It was just that Stef had no memory of any of this before the damn Hatch on Mercury had opened.

The news of the discovery had quickly leaked, and the Hatch had been a sensation for about twenty-four hours. It was after all evidence of alien intelligence in the solar system. But a Hatch leading nowhere had since been largely forgotten, or dismissed as a hoax, though it still trailed conspiracy theories like a comet tail.

But Stef was left with a massive rewiring of her own past.

Before the Hatch, she had been an only child. After it, suddenly she had a twin. Not only that, she suddenly had a whole different lifetime behind her, intertwined with that of her twin. Papers that had been to her sole accreditation, for instance, were now under joint authorship with her sister. She’d read some of them; they were much as she remembered writing them, but not quite – not significantly better or worse academically – and there were others, reflecting bits of work she couldn’t remember, that she’d never generated herself at all.

Only Stef remembered her solitary past before. Nobody else. Everybody in her life, including colleagues she’d known since her graduate-student days, thought of her now as half of a pair, not Stef alone. Not even King and Trant, who had been there at the moment of transition, remembered the old timeline.

Not even Penny remembered it. As far as Penny was concerned, their joint careers had just carried on, after a hiccup as Stef had tried to absorb what had happened at the Hatch. To Penny, Stef was a sister who had suddenly developed a kind of selective amnesia.

And maybe that was what it was. Some kind of mild craziness, perhaps triggered by some bizarro radiation field leaking from the alien artefact into which she’d climbed. That was the simplest explanation, after all, that her own perception, her memory, was somehow faulty. Though she’d looked hard, Stef hadn’t found a single shred of evidence to contradict the reality of it. The alternative, that history had somehow been changed around her, that the fault lay in the external universe rather than in her own small head, seemed an absurdly over-elaborate explanation by comparison.

She didn’t believe that, however. She knew herself, she knew her past, her life. And this past wasn’t hers.

She’d learned not to talk about this, not to anybody – not after the first few minutes of utter bafflement, up there on Mercury, in her pressure suit, in the Hatch, facing a sister she’d never known existed, and everybody had stared at her in dismay as she babbled out her confusion. After all she’d rather be working on kernel super-physics than spend the rest of her life on medication and therapy intended to rid her of ‘delusions’. She wouldn’t even talk to Penny about it, despite her sister’s tentative attempts to break through the barrier. Stef had been very happy to see Penny posted to a different planet, happy to just get on with her work; at least the work had stayed a constant comfort.

But now here was this summons from Earthshine, evidently intended to bring the two sisters together.

It seemed to Stef that despite much study and commentary, even while everybody acknowledged their power, most experts were unsure what the real agenda of the Core AIs might be. The three antique minds, a legacy of a difficult past, had no formal role in human society, no legal status – no rights, in a sense. But everybody knew that human agencies, from the UN and nation states on downwards, had to deal with them. Their power was recognised the way you would acknowledge the power of a natural phenomenon, a hurricane; you couldn’t ignore them, but they were outside the human world. And unlike hurricanes, the Core AIs could think and communicate.

Now Earthshine had chosen to communicate with Stef and her sister. Why? That depended, Stef supposed, on Earthshine’s own agenda. Maybe Earthshine had some kind of insight to share. But did she want her personal tangle of a life to be unpicked by such a monster?

On a personal level she was repelled. But on an intellectual level she was intrigued.

She acknowledged the note, logged the trip in her personal schedule, and with relief went back to work.

CHAPTER 42

Stef Kalinski dropped from the moon’s orbit to Earth, following the usual leisurely three-day unpowered trajectory. At Earth orbit she had to wait a day before she was transferred to an orbit-to-ground shuttle, like a snub-nosed plane with black heat tiles and white insulation, its cabin crowded with passengers and luggage.

The little craft glided down through the air with looping, sweeping curves.

On its final track the shuttle crossed the eastern coast of South America, coming down towards a strip of flat coastal savannah. The land glimmered with standing water, flooded by the rising ocean despite crumbling levees that still lined the coast. This was Kourou, Guiana, the old European space agency launch centre, now converted to a surface-to-orbit transit station. Further inland Stef saw bare ground, scrub, some of it plastered with solar-collector arrays like a coat of silver paint. This site was only a few hundred kilometres north of the mouth of the Amazon. Now there was no forest, and the river was reduced to a trickle through a semi-desert.