Otherlight, also known as ‘Deeplight’ or ‘Underlight’, is an alternative dimension that exists ‘below’ real space, in which electromagnetic radiation travels faster than ‘on the surface’. The separation from reality is not binary; Otherlight has ‘depth’, corresponding to far it is from real space, with the speed of electromagnetic radiation increasing according to distance from the ‘surface’. EM radiation travels constantly through Otherlight, but under normal circumstances it cannot alter its depth and therefore remains totally imperceptible and irrelevant to reality, forming a kind of invisible ‘background radiation’ constant throughout known space.
Accessing Otherlight is extremely difficult; it takes enormous amounts of energy to force a physical object or regular radio transmission beneath reality. The amount of energy required increases exponentially with depth.
In general, the ‘burying’ process is conducted by an Otherlight Shift Module. These devices are usually dependent on a fusion reactor to generate the requisite quantities of energy, and work by raw power, forcing an object or wave beneath reality and holding it there against nature’s howling rejection. Shift modules thereby enable faster-than-light travel and communication, since the speed of light increases correspondingly with the speed of local Otherlight. This therefore increases the potential top speed of objects and waves, and reduces the energy requirements to accelerate to a given speed (since the energy needed to accelerate increases the closer an object gets to C).
Otherlight can also be accessed using an Otherlight Interdictor. This is a device that monitors nearby Otherlight to detect changes in the passing stream. Any such changes in this constant background denote the passage of either an object or artificial wave, which can then be ripped out of Otherlight using a shift module built in to the interdictor. Objects are usually fairly easy to detect in this way, but picking out the very minor variations produced by artificial waves is very difficult, especially if they are deeply buried. For this reason, intercepting communications is very difficult.
Otherlight Communication typically works on pre-specified frequencies, for obvious reasons given the above-mentioned difficulties with comms interception. Even so, it is a complex process, and most civilisations will develop faster-than-light communications somewhat later than FTL travel. This is due entirely to difficulties in receiving Otherlight transmissions (sending them presents no difficulties that would not be overcome by physical transfers).