Mohs Scale of Science Fiction Hardness

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"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 fact as science currently understands them. For the author, the rule is to make as few such slips as he possibly can."
Hal Clement, Whirligig World

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 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 few fantastic flights of fancy not justified by Science. "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 mathematical, laws, where in soft SF they just work however the author feels like. What this leads to for hard SF—and this is part of the attraction for many people—is a raised bar for the amount of work the writer must put into the story, and usually this is shown quite clearly.

Example: a character is shown a machine for traveling into the past and asks, "How does it work?"

  • In really soft SF: "With science."
  • 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's a ride. Obviously, time travel to the past is impossible, but this multi-axis motion ride will make you think you're really there."

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 of hard SF, the only way to define it is self-consistency and scientific accuracy.

Paradoxically, hard SF often does include technology that looks impossible. Many works of hard SF embrace the maxim, "A sufficiently advanced technology is indistinguishable from magic." This was coined by Arthur C. Clarke (one of the definitive hard SF writers) and is embraced at the end of his novel (and movie) 2001: A Space Odyssey (a definitive hard SF work), where the many fantastic abilities of the monoliths are simply presented as "sufficiently advanced" and inexplicable. Essentially, a deep understanding of how scientific advancement has worked in the past leads to the knowledge that we have no real idea how it will go in the future.

Which leads us to the Scale.

Notes (please read!)[edit | hide | hide all]

Note: The works mentioned below are purely for illustrative purposes—please add new examples to the subpages.

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.

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.

Note 4: There are sometimes in the news reports of studies which would reassign many works on the scale—for example, tropers might claim the September 2011 OPERA experiment which measured faster-than-light travel by neutrinos can raise works whose One Big Lie is FTL Travel to the Speculative Science category. There are three reasons to be cautious about making such alterations: 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 seem to have been 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.

Note 5: As far as this wiki is concerned, Tropes Are Not Good and Tropes Are Not Bad. "Hard" and "soft" may be considered as denotations of the quality of the story by those who prefer one over the other. We don't hold to that here, as each category has readers who find it "just right".

Note 6: Examples Need Context. 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 are already working their way 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.




  1. 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, Tengen Toppa Gurren Lagann, The DC Comics and Marvel universes, and The Hitchhiker's Guide to the Galaxy fall in this class.
  2. 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 Skylark of Space and Lensman series, Star Trek: The Original Series, and StarCraft fall in this category.
  3. 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 Brin's Uplift series, and the 2003-2009 Battlestar Galactica Reimagined fall in this class.
  4. 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. David Weber's Honor Harrington series, most works in Alan Dean Foster's Humanx Commonwealth series, and Robert A. Heinlein's Farnham's Freehold fall in this category. 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 FTL 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), Freefall, and the Alien series fall within the subclass.
  5. 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. Early works in Larry Niven's Known Space series, the first two books in Robert L. Forward's Rocheworld series, Eternal Sunshine of the Spotless Mind, 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, The Machine Stops by E. M. Forster, and the more Speculative Fiction works of Jules Verne fall in this subclass.
  6. Real Life: A Shared Universe which spawned its own genre, known as "Nonfiction". 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.