> Expanse authors have paid careful attention to the physics of space-flight, which they then use create their vision of "realistic" space battles

Yes and no. I'm told that if you do the math on the "Epstein Drive" fusion rockets that are in use throughout The Expanse, they are actually _impossibly_ effective regarding reaction mass.

Still, unlike the star gates, they don't take you across light-years in less than actual years, so they're "harder" than the later books. There is indeed no hard boundary.

When asked on Twitter how the Epstein Drive works, the authors responded: "Efficiency"

Alternative answer: blackmail

Reminds me of the time a Star Trek writer was asked how the Heisenberg compensator works.

The response was that "It works very well, thank you!"

Sure the engine tech is fantastical, but the "realism" is in the attention paid to g-loading, momentum, the vast expanse (heh) of space, etc. The authors have chosen strategically which aspects of their story to be fantastical and which parts to keep hard. And of course, every good sci-fi has to do this. Which is why I personally like the metric of "does part of the enjoyment of this story come from a (somewhat) realistic portrayal of real science". Conveniently categorized by my non-sciencey reading buddy. The more it does this, the "harder" it is. However that means the location of the fuzzy boundary is subjective, but we can probably rank different stories relative to each other. The Martian is harder than The Expanse is hard than Star Wars. Pretty clear I'd say. The Expanse may have unrealistic engine tech, but at least the spaceships aren't dogfighting Second World War style.