In the limit, if you're describing something with no grounding in physics, you need magic. But you can make this magic very much constrained. There's a fundamental difference between "fusion reactors that are absurdly efficient for magical reasons" and "FTL drives that work by magic". A difference in the level of abstraction, if you like. The more lower-level your magic, the bigger and more interesting the network of potential consequences. And also the easier to keep universe self-consistent - because you have small-scale axiom, and large-scale rules are extrapolations using real-world physics and reasoning.