http://tvtropes.org/pmwiki/pmwiki.php/Main/MohsScaleOfScienceFictionHardness

Hal Clement, Whirligig World , Whirligig World "The fun, and the material for this article, lies in treating the whole thing as a game. I've been playing the game since I was a child, so the rules must be quite simple. They are: for the reader of a science-fiction story, they consist of finding as many as possible of the author's statements or implications which conflict with the facts as science currently understands them. For the author, the rule is to make as few such slips as he possibly can."

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Speculative Fiction fanatics are always raving about how "hard" the science is in various stories — but it's not like you can rub a story with a piece of quartz and see if it leaves a scratch on the plot. So what is "hardness" in SF? Why do some people want it? And how do we put a number to it?

Beginning with the first question: "Hard" Science Fiction is firmly grounded in reality, with only a few fantastic flights of fancy not justified by science, or with the technology being nonexistent in today's world but probably scientifically possible at some point. "Soft" Sci-Fi is more flexible on the rules. Even the fantastical aspects of the story will show a divide — in hard SF, they operate through strict, preferably physical, laws, where in soft SF they work in whatever way suits the story best. What this leads to for hard SF is a raised bar for the amount of scientific research the writer must put into the story, and usually this is shown quite clearly.

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Example: a character is shown a time machine and asks, "How does it work?"

In hard SF: "A good question with an interesting answer. Please have a seat while I bring you up to speed on the latest ideas in quantum theory, after which I will spend a chapter detailing an elaborate, yet plausible-sounding connection between quantum states, the unified field theory, and the means by which the brain stores memory, all tied into theories from both Albert Einstein and Stephen Hawking."

"A good question with an interesting answer. Please have a seat while I bring you up to speed on the latest ideas in quantum theory, after which I will spend a chapter detailing an elaborate, yet plausible-sounding connection between quantum states, the unified field theory, and the means by which the brain stores memory, all tied into theories from both Albert Einstein and Stephen Hawking." In soft SF: "You sit in this seat, set the date you want, and pull that lever."

"You sit in this seat, set the date you want, and pull that lever." In really hard SF: "It doesn't. Time travel to the past is impossible."

Unfortunately for analytical purposes, this pattern is not universal - hard SF stories can skip over the details as long as the basic explanation is correct given what's been established so far. Therefore, regardless of the typical stylistic flourishes ("If all stories were written like science fiction stories" by Mark Rosenfelder, a conlanger) of hard SF, the only way to define it is self-consistency and scientific accuracy.

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Which leads us to the Scale.

Notes (please read!)