The controversial scientist Bob Lazar, the guy who made Area 51 a household name, put out a video when he first went public that, regardless of whether you think he's full of shit or not, is pretty interesting and better than most sci-fi in terms of explaining how an advanced vehicle might travel without propulsion and not be limited by the speed of light [0].
His premise is that the strong nuclear force is a purely attractive sub-force of gravity that affects spacetime in the same way, and that there are heavier elements that were created in star systems with much more energy than our sun and its predecessors. These stable, high atomic weight elements (specifically element 115, which was synthesized and added to the periodic table much later) have large nuclei past a certain size threshold, in which the strong (gravitational) force reaches nonlinearly beyond the nucleus and can be amplified and focused (somehow?) into a gravitational wave that distorts spacetime. He claims that antimatter annihilation, also fueled by the same element, is the source of power for that amplification.