Tachyon

Tachyon particles were particles which moved faster than light.

According to CS-NA's internal encyclopedia, they were created as a byproduct of the linear flow of time. “Bursts” of tachyon particles floated out of the reality that produced them and into other layers of time. Thus, by catching their momentum through their tachyonic sails, tachyonic sailing ships could propel themselves out of reality and onto the layer of existence called the Chronon Sea, where they could be steered towards the desired time stream. This was an unconventional method of time travel, more cumbersome and unpredictable than ordinary Void Ships or shifting, but did have its advantages. (PROSE: The Winter Quests)

When a mailbag mysteriously disappeared from the Cupid Post Office, the Blue Feather's investigation included looking for tachyon particles, “and/or” inter-dimensional residue. (PROSE: Acquaintanceship-982 and the Missing Mail Mystery)