As you mentioned, there is disagreement. Many folks would characterize as Hard SciFi some stories that make use of FTL travel but which give a "rigorous" expla

As you mentioned, there is disagreement. Many folks would characterize as Hard SciFi some stories that make use of FTL travel but which give a "rigorous" explanation of why it initially appeared to be impossible (in present day) but is actually possible according to physics discoveries made after the present day. In some sense, the important criteria is that the laws of physics are "taken seriously", and that the author constructs a hypothetical scientific framework that (1) is internally self-consistent and (2) is compatible with current observations. https://en.wikipedia.org/wiki/Hard_science_fiction#Scientific_rigor https://en.wikipedia.org/wiki/Hard_science_fiction#Scientifi...