Sliding Scale / Mohs Scale of Science Fiction Hardness

In using the Scale, please keep the following in mind:

• Note 1: All the works mentioned below are solely for illustrative purposes. Please don't add any new examples to this page; instead, add them 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. Grammatically speaking, you could put an apostrophe in the title in the sense that it's someone's scale, but if you did that, it would read "Mohs' Scale". And that's apparently not the way the original "Mohs scale" is named, so we're not going to do it here either. This also means that in dialogue, it should be referred to as "the Mohs scale".
• Note 3: Remember that Tropes Are Tools. A story being on any given end of the scale doesn't make it any "better" or "worse", especially because different people prefer different ends of the scale, and we don't want any fights like that here.
• Note 4: Also remember that Type Labels Are Not Examples. Don't just put down the number and force the reader to visit this page to decipher what it means—you're free to list the number on the scale, but please add an explanation.
• Note 5: 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 6: 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 7: 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.

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, Star Wars, Tengen Toppa Gurren Lagann, The DC and Marvel universes,^note although some heroes' individual comics might fit elsewhere occasionally Doctor Who, 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-universe, it's considered to lie within the realm of scientific inquiry. Works like Neon Genesis Evangelion, the various Star Trek series, and StarCraft fall in this category.
   
   A subclass of this class (let's say 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 the Luminiferous Ether) 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 such a story as SF.

3. Physics Plus: Still multiple forms of Applied Phlebotinum, but here the author aims to justify these creations with natural laws both real and invented—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.

4. One Big Lie: The author invents one (or, at most, a very few) counterfactual physical laws and writes a story that explores the implications of these principles. Consider, for instance, Cities in Flight's "Dirac Equations" and "spindizzy motor" leading to instantaneous communication, or Mass Effect's "Element Zero" being the basis for all of the series' futuristic technology. Other works in this class include most works in Alan Dean Foster's Humanx Commonwealth series, the Ad Astra board games, Robert A. Heinlein's Farnham's Freehold, and many of Vernor Vinge's books.
   
   This class also includes a subclass (4.5 on the scale) one might call One Small Fib, containing stories that include only a single counterfactual device (often Faster-Than-Light Travel) which 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.

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. 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 here. The Martian is famously about as hard as science fiction can go, falling at the hard end of this subclass. The Mundane Science Fiction (MSF) genre/movement, which was developed in 2004, calls for plausible science fiction using existing science and technology. MSF is typically set in our Solar System and it has no aliens, FTL spaceships, or telekinesis.

6. Real Life (a.k.a. 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.

There exists a classification known as "Hard Fantasy", which is an attempt to describe fantasy settings that try to play by Magic A Is Magic A so hard that a clever reader can predict how a mysterious technique was done well before the author reveals it. We mention it mainly to say that such a setting has a position on the above scale; a particularly effective version of a "Hard Fantasy" setting is usually between a 4 (One Big Lie) and a 4.5 on the above scale.