Hideaway

Merlin absorbed that. 'Why haven't we encountered anything like shadow matter before?'

'There must be strong segregation between the two types across the plane of the galaxy. For one reason or another, that segregation has broken down around Bright Boy. There seems to be about half a solar mass of shadow matter gravitationally bound to this system - most of it sitting in Bright Boy's core.'

Merlin tightened his grip on the balustrade. 'Tell me this answers all our riddles, Sayaca.'

Sayaca told them the rest, reminding Merlin how they had probed Cinder's interior via sound waves, each sonic pulse generated by the impact of an in-falling meteorite; the sound waves tracked as they swept through Cinder, gathered by a network of listening posts sprinkled across the surface. It was these seismic images that had first elucidated the fine structure of the Digger tunnels. But - unwittingly - Sayaca had learned much more than that.

'We measured Cinder's mass twice. The first time was when we put our own mapping satellites into orbit. That gave us one figure. The seismic data should have given us a second estimate that agreed to within a few per cent. But the seismic data said there was only two-thirds as much mass as there should have been, compared with the gravitational mass estimate.' Sayaca's semblance paused, perhaps giving the two of them time to make the connection themselves. When neither spoke, she permitted herself to continue. 'If there's a large chunk of shadow matter inside Cinder, it explains everything. The seismic waves only travel through normal matter, so they don't see one-third of Cinder's composition at all. But the gravitational signature of normal and shadow matter is identical. Our satellites felt the pull of the normal and shadow matter, just as we did when we were walking around inside Cinder.'

'All right,' he said. 'Tell me about Bright Boy too.'

'It makes just as much sense. Most of the shadow matter in this system must be inside the star. Half a solar mass would be enough for Bright Boy's shadow counterpart to become a star in its own right - burning its own shadow hydrogen to shadow helium, giving off shadow photons and shadow neutrinos, none of which we can see. Except just like Bright Boy it would be an astrophysical anomaly - too bright and small to make any kind of sense, because its structure is being affected by the presence of an equal amount of normal matter from our universe. Both stars end up with hotter cores, since the nuclear reactions have to work harder to hold up the weight of overlying stellar atmosphere.'

Sayaca thought that the two halves of Bright Boy - the normal and shadow-matter suns - had once been spatially separated, so that they formed the two stars of a close binary system. That, she said, would have been something so strange that no passing culture could have missed it, for the visible counterpart of Bright Boy would have appeared locked in orbital embrace with an invisible partner, signalling its oddity across half the galaxy. Over the ensuing billions of years, the two stars had whirled closer and closer together, their orbital motions damped by tidal dissipation, until they had merged and settled into the same spatial volume. Whoever comes after us, Merlin thought, we won't be the last to study this cosmic mystery.

'Then tell me about Pauraque's storm,' he said, flinching at the memory of her crushed survival egg.

Gallinule nodded. 'Go on. I want to know what killed her.'

Sayaca spoke now with less ease. 'It must be another chunk of shadow matter - about the mass of a large moon, squashed into a volume no more than a few tens of kilometres across. Of course, it wasn't the shadow matter itself that killed her. Just the storm it caused by its passage through the atmosphere.'

And not even that, Merlin thought. It was his decision that killed her; his conviction that it was more vital to save the first egg, the one falling into the storm's eye. Afterwards, discovering that there was no gamma-ray point there, he had realised that he could have saved both of them if he had saved Pauraque first.

'Something that massive, and that small . . .' Gallinule paused. 'It can't be a moon, can it?'

Sayaca turned away from the sunset. 'No. It's no moon. Whatever it is, it was made by someone. Not the Huskers, I think, but someone else. And I think we have to work out what they had in mind.'

Nervously, Merlin watched seniors populate the auditorium - walking in or simply popping into holographic existence, like card figures dropped into a toy theatre. Sayaca had bided her time before announcing her discovery to the rest of the expedition, but eventually the three of them had gathered enough data to refute any argument. When it became clear that her news would be momentous, seniors had flown in from across the system, leaving the putative hideaways they were investigating. A few of them even sent their semblances, for the simulacra were now sophisticated enough to make many physical journeys unnecessary.

The announcement would take place in the auditorium of the largest orbiting station, poised above Ghost's cloud-tops. An auroral storm was lashing Ghost's northern pole, appropriately dramatic for the event. He wondered if Sayaca had scheduled the meeting with that display in mind.

'Go easy on the superstring physics,' Gallinule whispered in Sayaca's ear, as she sat between the two men. 'You don't want to lose them before you've begun. Some of these relics don't even know what a quark is, let alone a baryon-to-entropy ratio.'

Gallinule was right to warn Sayaca. It would be like her to begin her announcement by projecting a forest of equations on the display wall.

'Don't worry,' Sayaca said. 'I'll keep it nice and simple; throw in a few jokes to wake them up.'

Gallinule kept his voice low. 'They won't need waking up once they realise what the implications are. Straightforward hiding's no longer an option, not with something as strange as the Ghost anomaly sitting in our neighbourhood. When the Huskers arrive they're bound to start investigating. They're also bound to find any hideaway we construct, no matter how well camouflaged.'

'Not if we dig deep enough,' Merlin said.

'Forget it. There's no way we can hide now. Not the way it was planned, anyway. Unless--'

'Don't tell me: we'd be perfectly safe if we could store ourselves as patterns in some machine memory?'

'Don't sound so nauseated. You can't argue with the logic. We'd be nearly invulnerable. The storage media could be physically tiny, distributed in many locations. Impossible for the Huskers to find them all.'

'The Council can decide,' Sayaca said, raising a hand to shut the two of them up. 'Let's see how they take my discovery, first.'

'It was Pauraque's discovery,' Merlin said quietly.

'Whatever.'

She was already walking away from them, crossing the auditorium's floor towards the podium where she would address the congregation. Sayaca walked on air, striding across the clouds. It was a trick, of course: the real view outside the station was constantly changing because of the structure's rotation, but the illusion was flawless.

'It may have been Pauraque who discovered the storm,' Gallinule said, 'but it was Sayaca who interpreted it.'

'I wasn't trying to take anything away from her.'

'Good.'

Now she stepped up to the podium, the hem of her electric-blue gown floating above the clouds. She stood pridefully, surveying the people who had gathered here to hear her speak. Her expression was one of complete calm and self-assurance, but Merlin saw how tightly she grasped the edges of the podium. He sensed that beneath that shell of control she was acutely nervous, knowing that this was the most important moment in her life, the one that would make her reputation amongst the seniors and perhaps shape all of their destinies.