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.
Example: a character is shown a time machine and asks, "How does it work?"
- In soft SF: "You sit in this seat, set the date you want, and pull that lever."
- 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."
- 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.
Which leads us to the Scale.
Note 2: Contrary to what one might expect, there is no apostrophe in "Mohs" — the name is a reference to the Mohs scale of mineral hardness, named for Friedrich Mohs. (Grammatically speaking, an apostrophe after the 's' would be permissible; its addition would produce a possessive, i.e., "Mohs' scale", denoting the scale created or promulgated by Mohs. However, it's apparently not used in the standard name for the scale, so its use here would approach a degree of informality utterly unacceptable on The Other Wiki.)
Note 3: While the term "soft science fiction" is used above as the antonym of "hard science fiction", another common use of the term is to describe soft-science fiction: sociological and psychological science fiction. This can, in some cases, make it appropriate to talk about "hard soft science fiction", but doing so is likely to confuse people. By and large, though, science-fiction "hardness" doesn't correlate well with realism in areas such as characterization, views of human nature, or views of human societies. In such areas, some works that barely qualify as SF might be unsparingly realistic and some of the hardest SF imaginable might deal in out-and-out fantasy.
Note 4: Sometimes a study hits the news that, if confirmed, would reassign many works on the scale. For example, the September 2011 OPERA experiment which measured faster-than-light travel by neutrinos might have moved works whose One Big Lie was Faster-Than-Light Travel into the Speculative Science category. There are three reasons to be cautious about doing so: first, because mass media reporting of scientific results is often inaccurate due to the difficulty of presenting technical results to a non-technical audience; second, because revolutionary new results (and results in the news are generally new) are far more likely to be overturned than they appear (indeed, the OPERA anomaly was caused by faulty equipment); and third, for purposes of the Scale, the yardstick of scientific plausibility is what the science said at the time the work was written, not what scientists discovered later. If the story in question was based on a scientific model that, while now discredited, was widely accepted in its day, it still qualifies as "hard" science fiction because the author did his best with the information available at the time.
Note 5: As far as this wiki is concerned, Tropes Are Tools. "Hard" and "soft" may be considered as denotations of the quality of the story by those who prefer one over the other, but we don't want to hear about it that way.
Note 6: In science fiction fandom, classifying something as hard science fiction generally relies on more than just the plausibility of the technology used. "Hardness", in that sense, also depends to the level of scientific explanation used in the story. This scale, however, is based mainly on closeness to real world science and the consistency of the science fiction elements. For this reason you may find examples of works on the higher end of the scale that are not generally described as hard science fiction.
Note 7: When adding this trope to a work page, don't simply put down the number and leave it at that. This would require a troper to visit this page to learn more about it. That's fine if the troper is interested, but if they're already working down the work's page (and only at the M's), they probably don't want to wander off on a Wiki Walk. You can say the number, but please go on a bit explaining what the number is. For instance:
- Mohs Scale Of Science Fiction Hardness: 5. This work leans heavily into Speculative Science — the science of the tale is (or was) genuine speculative science or engineering, and the goal of the author to make as few errors with respect to known fact as possible.
- Science in Genre Only: The work is unambiguously set in the literary genre of Science Fiction, but scientific it is not. Applied Phlebotinum is the rule of the day, often of the Nonsensoleum kind, Green Rocks gain New Powers as the Plot Demands, and both Bellisario's Maxim and the MST3K Mantra apply. Works like Futurama, Star Wars, Tengen Toppa Gurren Lagann, The DC and Marvel universes,note and The Hitchhiker's Guide to the Galaxy fall in this class.
- World of Phlebotinum: The universe is full of Applied Phlebotinum with more to be found behind every star, but the Phlebotinum is dealt with in a fairly consistent fashion despite its lack of correspondence with reality and, in-world, is considered to lie within the realm of scientific inquiry. Works like E. E. Doc Smith's Lensman series, Neon Genesis Evangelion, Star Trek: The Original Series, and StarCraft fall in this category.
A subclass of this class (arguably 2.5 on the scale) contains stories that are generally sound, except the physics aren't our own. Plot aside, they are often a philosophical exploration of a concept no longer considered true (such as Aristotelian physics), or never considered true in the first place (e.g. two spatial dimensions instead of three, like Flatland). Some of Arthur C. Clarke's stories fall here. However, given the overlap with fantasy, it can prove tricky to even classify a story as SF.
- Physics Plus: Stories in this class once again have multiple forms of Applied Phlebotinum, but in contrast to the prior class, the author aims to justify these creations with real and invented natural laws — and these creations and others from the same laws will turn up again and again in new contexts. Works like Schlock Mercenary, David Weber's Honor Harrington series, David Brin's Uplift series, and Battlestar Galactica (2003) fall in this class. Most Real Robot shows fall somewhere between Classes 2 and 3.
- One Big Lie: Authors of works in this class invent one (or, at most, a very few) counterfactual physical laws and writes a story that explores the implications of these principles. James Blish's Cities in Flight stories fall squarely into this category, courtesy of the "Dirac Equations" leading to the "spindizzy motor" and instantaneous communication. Most works in Alan Dean Foster's Humanx Commonwealth series, the Ad Astra board games and Robert A. Heinlein's Farnham's Freehold fall in this category, as do many of Vernor Vinge's books.
This class also includes a subclass (4.5 on the scale) we call One Small Fib, containing stories that include only a single counterfactual device (often Faster-Than-Light Travel), but for which the device is not a major element of the plot. Many Hal Clement novels (e.g. Mission of Gravity, Close to Critical) and Freefall fall within the subclass.
- Speculative Science: Stories in which there is no "big lie" — the science of the tale is (or was) genuine speculative science or engineering, and the goal of the author to make as few errors with respect to known fact as possible. The first two books in Robert L. Forward's Rocheworld series and Robert A. Heinlein's The Moon Is a Harsh Mistress fall in this class.
A subclass of this (5.5 on the scale) is Futurology: stories which function almost like a prediction of the future, extrapolating from current technology rather than inventing major new technologies or discoveries. (Naturally, Zeerust is common in older entries.) Gattaca, Planetes, Transhuman Space and the more speculative works of Jules Verne fall in this subclass.
- Real Life (aka Fiction in Genre Only): A Shared Universe which spawned its own genre, known as "Non-Fiction". Despite the various problems noted at Reality Is Unrealistic, it is almost universally agreed that there is no other universe known so thoroughly worked out from established scientific principles. The Apollo Program, World War II, and Woodstock fall in this class.