Greg Egan - The Arrows of Time

As Agata’s weight diminished, she continued upwards, using the guide rope beside her. The ancestors couldn’t tell her how to halt the system, but they must have chosen her for a reason – and the only hint they could give her had to be encoded in that choice itself. She had measured the bending of light by Esilio’s sun, hadn’t she? There was no prospect of using gravity to distort the light paths in the messaging system, but gravity wasn’t the only way to modify light’s passage.

A woman passed her, descending, murmuring a casual greeting. Agata had chosen the stairwell at random; she had no reason to believe that she was heading for an entrance to the facility itself. The usual tiers of apartments here should simply come to an end a little sooner than they had before the system was built.

Above her, less than a saunter away, the twelve long tubes would run from mirror to distant mirror, carrying messages from the future in beams of densely modulated time-reversed starlight. The tubes would be sealed against contamination – against dust or smoke that might scatter the light – and perhaps the Council had made an effort to render them vacuum-safe, in case the ends were breached and they were opened to the void. But that would be a matter of structural reinforcement to limit the damage from a sudden pressure difference, not a matter of impermeability. There was no such thing as a hermetic seal on a container of that size. At the very least, particles of air would be constantly diffusing in and out of the tubes.

For most purposes, air was air. So long as it was chemically inert and dense enough to serve the crucial role of carrying heat away from bodies and machinery, any finer details were of secondary importance. When the cooling system had switched from using the old engines’ exhaust to the gas produced by cold decomposition of sunstone, no one would have much cared that the range of particle sizes was different. But there were countless variations on the basic theme of a stable ball of luxagens, and different mixtures had different properties. The speed of each frequency of light was slightly different in air than in a vacuum, and the precise value depended on the precise composition of the air.

Agata reached the top of the stairs. A sign right in front of her spelt it out: LAST EXIT. She left the stairwell and dragged herself along the corridor, past the doors of the apartments. The Council would have left a large enough buffer above this unrestricted area so that a bomb planted here could not have breached the nearest of the tubes. But she had a number now, good enough to feed into an order-of-magnitude estimate: how fast could she expect a change in the air to diffuse through a resin seal into the tubes that contained the light paths?

Air was air, no one would feel a thing. But if she could make it happen, there’d be no need to damage a single component of the messaging system. The exquisitely calibrated timing of the data fed into each tube would include allowances for ordinary variations in the ‘delay’ created by the light bouncing from mirror to mirror, but once it drifted beyond that range and the signal was scrambled beyond recovery, there’d be nothing that the system could do about it – least of all send a message into the past to warn the operators of the nature of the problem.

Back in her apartment, Agata sat at her desk and worked through the calculations. If she could add a component to the air that was significantly lighter than the smallest particles in the present mixture, it would naturally rise towards the axis and diffuse into the imperfectly sealed tubes. Though a particle of air in isolation had almost no external field, each light wave that passed over it distorted its shape sufficiently to spoil the usual cancellation between the luxagens, and the secondary wave generated by that process combined with the first to slow it down.

Generations of scientists had uncovered the details she needed to quantify the effect: she was mining the entire intellectual legacy of the Peerless . Agata hadn’t used half of these results since she was a student, but though her memory of some of the formulas was hazy – and she was afraid to consult the photonic library lest she alert someone in authority to her sudden change of interests – she discovered that she still had all her old paper textbooks at the bottom of a cupboard, not yet so insect-damaged as to be indecipherable.

Without access to the tubes themselves she couldn’t hope for a precise answer, but she could sketch the limits of what was physically possible. In the worst case, time had already run out: if the tubes were large enough and the seals sufficiently tight, it could take half a year to infuse enough modified air into them to corrupt the signals.

In the best case, it would take close to three stints. So she had, at most, two stints to alter the composition of the mountain’s atmosphere sufficiently to get the process started.

Agata rechecked the numbers, but they did not improve. She sat at her desk with her tattered books around her, bewildered but refusing to be cowed. The ancestors had spoken to her; she was joined to them across the disruption, across the generations yet to be born. The cosmos had no choice but to find a sequence of events that filled the gap and completed that connection, and it could not come out of nowhere. The right plan had to lie within her, just waiting to unfurl.

Ramiro was beginning to wish that they’d put cameras on the occulters. The extra transmissions needed to send back the images might have increased the chance of detection, but it would have been worth it just to have an objective version of the rendezvous with the cache in front of him, to take the place of the pictures in his head.

First, the occulter had to release itself from the rock, unwinding the drills and letting itself fall into the void. Then the air jets had to catch it and send it swooping back towards the slopes, approaching the cache with just the right speed at just the right angle. Two hooks on strings hung down from the cache, each one an open half-circle crossed by a vertical trigger about a third of the way in; the arms of the occulter needed to enter those half-circles and strike the triggers to send the second, spring-loaded halves sliding around to enclose them. Then the occulter had to move away, dragging the cache almost horizontally across the rock, unrolling the adhesive resin that was holding it in place against the vertical tug of its centrifugal weight.

Tarquinia interrupted his brooding. ‘Relax,’ she said. ‘Or add up the navigational tolerances again, if you want reassurance. We can hit the hooks, I’m sure of it.’

Ramiro checked the clock on his console. ‘Maybe we can, if the occulter turns up. It’s already three lapses late.’

‘Three days crossing the slopes, and you want it to be punctual to the flicker?’

‘These things move like clockwork, literally. If not to the flicker, they ought to be punctual to the lapse.’

Tarquinia said, ‘If this turns out badly, I’ll drop my anti-messaging principles and let you know…’ She glanced at the clock. ‘One lapse from now.’

Ramiro buzzed dismissively. ‘How would that help?’

‘It wouldn’t,’ she admitted. ‘But if you can convince yourself that I’m telling the truth, you can relax and assume that silence means success.’

A row of numbers appeared on the console – a transmission from the occulter, not from Tarquinia’s future self. Ramiro waited, refusing to interpret the numbers in isolation. Then the second brief report followed.

The occulter was stable, well clear now of the cache site… and measurably more massive than before, as revealed by its response to the thrust of the air jets. It had picked up its cargo and held on to it, and as the bomb swung down from above the arms, the occulter had successfully compensated for the spin that would otherwise have been imparted.