Greg Egan - The Arrows of Time

‘I don’t know.’ When she’d first heard the news, Agata had been disgusted; it had sounded like a gesture meant to appease Medoro’s killers. Since then she’d grown less adamant, but she’d still been too angry to try to think through all the ramifications.

Gineto said, ‘If someone’s fighting to impose their will over the mountain, they’re not going to give that up and walk away.’

‘Not even for a planet all their own?’ Serena replied.

‘You think they want freedom? They just want power.’ Gineto was talking in his hyper-happy baby voice, beaming down at Arianna, pulling faces to make her chirp. ‘The Peerless is all they know. If they’d wanted to take their chances on their own, they could have asked for that any time since the turnaround.’

‘Then what’s the solution?’ Serena demanded. ‘If it were up to me I’d abandon the messaging system, but the Council’s already talking about the bombing as proof of how much we need the warnings.’ Her voice faltered; Agata reached over and squeezed her shoulder.

‘The Surveyor ’s not a bad idea,’ Agata conceded. ‘If they can grow crops on an orthogonal world, it would be one of the safest places to start a colony. There’d be a limit to the kind of collisions with home-cluster matter it could suffer in the migrants’ future – since anything really catastrophic in the world’s own past would have destroyed it, or lit it up like Gemma. On a cosmic timescale the entropy gradient would be a problem, but compared with the Peerless or the home world, a planet guaranteed to stay intact for eons would be a haven.’

‘Then it’s worth trying, isn’t it?’ Serena said.

Gineto wasn’t swayed. ‘You think people who don’t want to know the future will migrate to a world where they can step out of their houses and see what a dozen generations of their descendants will have carved into the rocks?’

Serena said wearily, ‘I just want a plan that both sides can live with. If not this, let them split the mountain in two.’

Gineto drew Arianna against his chest to hide his face from her as his expression grew grimmer. ‘The war to decide the size of the pieces would kill us all. If that’s the alternative, the Surveyor can fly with my blessing.’

‘I’m very sorry about your friend,’ Lila said gently.

‘Thank you.’ Agata shifted in her harness. ‘His uncle’s looking after the baby, but it’s hard for the whole family.’

‘Of course.’ Lila offered a moment of sympathetic silence, then delicately broached a different subject. ‘Have you had a chance to think about the gradient problem, since our last meeting?’

‘Not really,’ Agata confessed. She pictured Medoro standing in the corner of the office, the expression on his face enough to convey exactly what he thought of her laziness. It had been three stints since the bombing; she really had no excuse not to get back to work.

Lila said, ‘I’ve had one idea myself, if you’re interested.’

‘Of course.’ Agata leant forward attentively and tried to concentrate.

‘It’s possible that what we’re lacking is a proper understanding of vacuum energy,’ Lila suggested. ‘You know the naïve version: if you look at the free light field, and assume the right kind of relationship between the mass of a photon and the dimensions of the cosmos, each mode of the field that wraps around the cosmos a whole number of times is like a simple oscillator. Wave mechanics tells us that an oscillator like that can’t have an energy of zero: the lowest energy level has some non-zero value. So even if the cosmos were empty, the vacuum would have as much energy as you’d get by adding up the lowest levels of all the possible modes of the light field.’

‘Plus the same kind of contributions from all the modes of the luxagen field,’ Agata added.

‘Yes. Which are actually negative, if you take the mathematics seriously.’ Lila buzzed softly. ‘When I was developing the gravitational theory, I was never sure if I should claim that this kind of energy would need to be included as a source of curvature – and then go on to insist that the very mild curvature that empty space seems to possess is proof that the two kinds of vacuum energy more or less cancel each other out.’

‘Hmm.’ Agata wasn’t sure whether Lila was mocking this idea or trying to resurrect it, so it seemed wiser not to offer an opinion of her own.

‘The thing is, though,’ Lila continued, ‘the naïve version is just that. We don’t know if the cosmos really has the right dimensions to allow any free modes of either field, and we don’t know how to calculate the vacuum energy under more realistic assumptions: taking account of the interaction between luxagens and photons, and taking account of the curvature of four-space.’

Agata said, ‘But if the vacuum energy is a source of curvature, and the curvature itself can influence the vacuum energy…’

‘Then it’s much less clear what combinations of the two are actually possible,’ Lila replied. ‘That’s what I’m hoping would shed some light on the gradient problem.’

‘Ah.’ Agata finally caught a glimmer of that illumination. ‘If we take account of the way the geometry of the cosmos determines what kind of waves can exist – which governs the vacuum energy, which contributes to the curvature – we might end up showing that a uniform cosmos would be self-contradictory.’

‘It’s conceivable,’ Lila said cautiously. ‘Of course, that wouldn’t help much if a tiny wrinkle in the curvature was enough to make things work out. What we’d need is for there to be no solutions without a significant entropy gradient.’

Agata understood the proposal now – and it was terrifying. To make any progress they would need to combine field theory, wave mechanics and cosmology in a manner that no one had ever achieved before.

‘Can I think about this?’ She didn’t want to agree to the project only to find that she couldn’t summon up the kind of focus and stamina that an undertaking of this scale required.

Lila said, ‘Of course.’

Agata hadn’t set out with any intention of going near the demolished workshop, but when her path took her to the boarded-up entrance she wasn’t surprised.

The floor of the tunnel outside was still covered in a layer of fine dust; she knew it was bluish by artificial light, but in the red glow of the moss it looked almost black. The last time she’d stood here the entrance had only been covered by a curtain, and she’d peered in through a gap. A string of coherers hung across the ceiling had illuminated a team of investigators at work in the rubble. They’d been photographing everything as they sifted patiently through the debris, hoping they might find fragments of the bomb.

As far as she knew, they never had. The Council had locked up all the anti-messagers who’d argued their case most vigorously in public, but as likely as not Medoro’s killers were still free. When the new team of instrument builders was assembled they’d have bodyguards around the clock, and no one would get within a stroll of the workshop without being searched. But if the bombers’ first choice of target became impossible to reach, they would find another one. Even if all of the system’s components could be built without another incident, when the whole thing was finally assembled it would be vulnerable to other kinds of sabotage.

The Surveyor mission wasn’t an act of appeasement; everyone would be better off with the two factions living apart. Agata didn’t know how many people would be prepared to abandon the familiar surroundings of the Peerless , but it was brave of Ramiro to be willing to make the journey to discover if migration was possible at all.