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.