Everyone has heard Reward Kipling’s saying: I keep six honest serving-men, (They taught me all I knew); Their names are What and Why and When , And How and Where and Who. These are the six questions every journalist should ask when pursuing a source of news.
These are the question s every author should ask when they are putting together their story. But I would go further and add a seventh question: Measurement? Annoyingly there isn’t a single question word that describes, which is why I have had to resort to a noun with a question mark.
The science and technology development is based on the ability to measure things like distance, time, quantity of an object or fluid, temperature, any property you can think of that has a variation of some sort. And yet, this is not what J. Bloggs naturally thinks of as a query when investigating something.
But it is what a science fiction writer has to think of when they are pulling together their story, because they have science somewhere in that story. This is the only genre that is forced to use measurement. Other genres like fantasy and horror can have it there, but they don’t need science or technology of whatever. (Crime thrillers strictly speaking don’t need measurement. Yes, it makes their plot easier to have a specific measurement, but it is not necessary. If the crime is committed in a modern setting, it is far more natural to use a handy measuring stick of some sort, which is why there is so much technology in crime fiction.)
So a good litmus test of whether a speculative fiction novel is true science fiction is whether the story is dependent on measurement of some sort, and I mean dependent in the sense there is no way that story could be written without that measurement. If the answer is no, then it is not science fiction in the true sense. It is probably horror or fantasy veering towards science fiction.
As you will surmise, a lot of faster that light space opera does not meet the measurement litmus test. Star Wars for instance has long since been recognised as fantasy, enjoyable though it is.
Measurement as far as I can tell does not appear in any definition of science fiction. That is because the essential measurement property to the plot may in what I call the hidden layer of assumptions in the world building. Which of course can lead to endless arguments among the fans.
For now it is simpler to say the science fiction must have some element of science that is essential to the story. This by default includes the measurement criterion. But if you do identify the measurement criterion of a story, then you know you have science fiction, or the real kind.