Agata ascended the stairs slowly, her gaze cast down at the moss-lit rock, hoping that if anyone was watching her she’d appear suitably distracted: a moody theorist wandering the mountain, oblivious to her surroundings. Though every ordinary resident of the Peerless surely knew the size of the excluded zone around the axis, she hadn’t been able to bring herself to ask Serena or Gineto to tell her. There was no way to phrase the question innocently: whoever she consulted, however obliquely, would be instantly burdened with the knowledge that she was contemplating sabotage. Which might have led nowhere, or might have taken her rapidly to a place she didn’t want to be: finding a way to reassure an alarmed friend that she hadn’t gone over to the side of Medoro’s killers, but was actually striving to undermine them.

To make any progress on that task, she needed a rough idea of the dimensions of the messaging system. It was safe to assume that the designers had made every channel as long as possible, running close to the full height of the mountain, so once she knew how close to the axis the public were permitted to travel she’d have some sense of the mirrors’ width and the volume of each enclosed light path.

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.