There is another view of the hard/soft sci-fi scale that has gained popularity among younger (i.e. under-30) commentators and reviewers where the scale is from:
soft == any and all science things, discussions, or principles are merely set dressings to tell a story;
hard == the science is the basis for the work and the consequences, interactions, etc of dealing with or addressing the ‘science things’ is the purpose of the work
It’s kind of like saying the scale goes from just telling a story set on a space ship to writing a dissertation on long-haul space flight’s effects on human behaviors, which might have a narrative through line.
I am not really sure I even like the premise of this usage of hard/soft sci-fi, I am much more prone to use the possible/impossible technology type of scale when discussing ‘hardness’ of sci-fi, but I don’t get to dictate the usage of language.
The two are correlated. To write proper hard sci-fi in your sense, you have to a) understand the relevant science and engineering (and I mean understand - not necessarily to a PhD level, but enough that you can reason about the phenomena and principles on your own), and b) let it permeate and constrain the plot. This naturally makes it more likely that a hard sci-fi story will be about "science things", because science and engineering not only constrain your plot, but you also have to explain their basics to the audience, which takes space.
Conversely, the soft end of sci-fi has a lot more authors with little to no familiarity with the relevant sciences, and even less care for them.
There are obviously exceptions to this "new hardness scale", but I think overall the two scales give almost identical readings in practice. And personally, I'm not against the new scale either, because it aligns with what I personally care about. That is, I want to read sci-fi that's hard on this new scale. I like my sci-fi to be about science, technology, social dynamics, and everything other than individuals and their emotional journeys and petty conflicts. There's enough of that in every other genre, not to mention, in real life itself.
It’s kind of like saying the scale goes from just telling a story set on a space ship to writing a dissertation on long-haul space flight’s effects on human behaviors, which might have a narrative through line.
I am not really sure I even like the premise of this usage of hard/soft sci-fi, I am much more prone to use the possible/impossible technology type of scale when discussing ‘hardness’ of sci-fi, but I don’t get to dictate the usage of language.