Some of you may be familiar with TV Tropes' "Mohs Scale of Science Fiction Hardness," and while it's certainly workable, it's also very linear.
My system is also linear, but it extends in two different directions, instead of just being "soft" vs. "hard." I think this is important because some science fiction settings combine realistic physics with implausible technology, or realistic technology with fictional physics, and I wanted to distinguish them.
So, because I should put it somewhere, here it is:
The Official(ish) Hardspaceships pH Scale of Science Fiction Hardness!
Acidic SF violates the laws of physics in some significant, unpardonable capacity. The nature of the violation becomes more precise as pH increases.
0-1: Technology is whatever looks cool on screen. Technobabble abounds. Spaceships bank in space and blasters go "pew". This is where you find Star Wars, Doctor Who, and Hitchhiker's Guide to the Galaxy.
2-3: Certain laws of physics are acknowledged, but liberally-applied handwavium technology helpfully ignores anything that gets in the way of the story. Space is probably silent, at least. Star Trek goes here, as does the Lancer RPG.
4-5: The laws of physics are now assumed to reign supreme, but we use handwavium to tweak our setting into the form we desire. Battlestar Galactica, Babylon 5, and Firefly all go here.
6: We take the real world, change it, and justify our changes with precisely-engineered handwavium, with clearly-articulated effects. Hard magic systems, basically. Mass Effect and Mobile Suit Gundam go here.
7: Apart from our "free pass", we aren't breaking the laws of physics at all. We aren't taking pains to implement realistic technology, either. The Expanse is my go-to example here.
Alkaline SF incorporates realistic technology. The overall realism of the technology increases with pH.
8-9: This is where you see technology that is purely speculative. Quantum radiators, black-hole drives, Dyson spheres/lasers, etc. We can't even begin to know how to pull these off, but we can consistently model their effects using real-world physics. High-concept and distant-future SF resides here.
10-11: This is where you see technologies that have had papers written on them, and may have fields of study dedicated to making it work. Fusion reactors/drives, Nuclear Salt-Water Rockets, nuke-pumped lasers, etc. Most "hard" SF is probably going to wind up here, where things are realistic-yet-still-speculative.
12-13: This is where you find technology that has actually been tested in real life. The Orion Drive, Nuclear Thermal Rocket engines, ion drives, etc. Typically the realm of near-future SF, like The Martian and 2001: A Space Odyssey.
14: Essentially reality, with no speculative elements. Space Brothers and Gravity go here.
This is mostly just vibes, and it would probably work better as a four-quadrant chart, but I do think it's worthwhile to distinguish between physics and technology when analyzing "realism" in science fiction.
I've always held that "hard" isn't a binary, and what it even means can be different for different people (I've heard people call Starfield "hard sci-fi" when it's a 3 at best). I might write about that in the future.
Hopefully this provides some insight into how I think about science fiction.