Diagram to help explain how FTL works in Okanverse without creating time paradoxes, as far as I understand. I've been researching this at surface level for like several years now lol
The most important things to take away here are (very, very roughly) as follows:
1. The purpose of FTL is to achieve "instantaneous" travel, but defining "instantaneous" becomes different with space travel because motion in space and motion in time are linked. Gaining speed in space slows you down in time, and vice versa. A ship moving at FTL travels back in time relative to the world at large.
2. For this reason, every FTL journey requires a slower-than-light (STL) half as well as an FTL half to cancel out the travel time.
3. Additionally, time dilation is everything. The amount of subjective time that passes aboard our FTL ship determines the minimum amount of time that has passed at home by the time it returns. If our ship has experienced 2 weeks of travel time, it can arrive home no earlier than 2 weeks after it left. This makes time dilation a desirable feature.
A Krasnikov tube is a kind of spacetime tunnel. It is somewhat like a wormhole, except that whereas a wormhole provides a shortened tunnel between two points in spacetime, a Krasnikov tube spans the entire normal-space distance between the two points (as far as I can tell) -- except that once it's built, using it to travel back the way you came sends you back in time.
Warp bubbles in this scenario are simply a fast drive for slower-than-light propulsion - they do not go FTL by themselves.
For a more detailed explanation, see the relevant article on my WorldAnvil