Often the line between plausible and implausible in science fiction is blurry. It’s the same way in real science.
Science fiction stories tend to lumped into two categories: hard and soft.
In hard science fiction the science in the story is a plausible extrapolation from real science. The characters are struggling to deal with problems arising directly or indirectly from real-world scientific phenomena. One of my favorite examples is Andy Weir’s The Martian, where astronaut Mark Watney is left for dead on the surface of Mars and must use his scientific skills and ingenuity to survive on a planet where nothing lives. It takes years to rescue him because spaceships can only travel so fast.
On the other hand you have soft science fiction. Here the science is a little more of a mystery, where the author dares to ask, what would happen if…? Spaceships travel faster than the speed of light and accelerate with minimal g-force effects on humans, robots are sentient, and aliens speak English. The Star Wars movies are excellent examples of this.
Science fiction nerds love to argue about the threshold between hard and soft. Where, for example, would the Star Trek franchise go? You have space ships that travel faster than light, which would make is soft. And yet, some recent physics papers have shown that warp drives may in fact be possible, at least under certain circumstances.
Personally I think of hard and soft SF not as buckets, but as a spectrum of scientific plausibility. No matter where you try to draw the line, there will always be someone who argues some concept in the story is implausible and someone else arguing it is.
This happens in real science too.
When I was a medical physics resident, I recall having dinner with some very highly-esteemed medical physicists in our field. I won’t name them, but to me, this was like a computer nerd sitting with Steve Jobs and Bill Gates. At the time I was working with a team attempting to merge an MRI unit with a linear accelerator so we could take detailed images of cancer patients while treating them with radiation. Unfortunately I spent most of that dinner on the receiving end of a lecture about why that wouldn’t work… the linear accelerator would would interfere with the radio frequency signals the MRI used.
Two years later my group took the world’s first MRI image while the object in question was being irradiated by a linear accelerator. And if you fast forward to today, linac-MRI hybrid machines are a reality in radiation therapy clinics around the world.
The point is that it’s not always easy to tell what’s plausible and what isn’t, even for experts in the field. There is a quote attributed to A. A. Michelson (of the Michelson-Morley ether experiment) at the end of the 19th century in reference to physics that “the grand underlying principles have been firmly established… the future truths of physics are to be looked for in the sixth place of decimals.” For reference, shortly afterward, in 1905 Einstein published the special theory of relativity, one of several advances that reframed the field of physics in the early 20th century.
For a scientist who writes science fiction, there can be a lot of pressure to write on the harder side of the spectrum. Writers like Issac Asimov cast long shadows.
With First Command, I certainly break a few rules that make it lean toward the softer side of science fiction like the faster than light travel. But ultimately that’s a plot device to tell the kind of story that I want to tell. I also try to include some harder stuff… like a sequence where the cadets are trying to shoot down an incoming drone loaded with explosives, but can’t because the planet’s gravity has altered the ballistic trajectory of their weapons.
In the hard or soft debate, a lot comes down to the kind of story you want to read, or if you’re a writer, to tell. For me, the first goal is always the same: tell a good story.