->''"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."''
-->-- '''Creator/HalClement''', ''Whirligig World''

SpeculativeFiction 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 [[SortingAlgorithmOfTropes how do we put a number to it]]?

Beginning with the first question: "Hard" ScienceFiction 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" SciFi 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 [[ShownTheirWork this is shown quite clearly]].

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

* '''In soft SF:''' "You sit in this seat, set the date you want, and pull that lever."

* '''In medium SF:''' "You sit in this seat, set the date you want, and [[Film/BackToTheFuture drive to 88 mph]]."

* '''In hard SF:''' "A good question with an interesting answer. [[{{Infodump}} 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 UsefulNotes/AlbertEinstein [[SmallReferencePools and]] StephenHawking."

* '''In really hard SF:''' "It doesn't. Time travel to the past is impossible." [[note]]General Relativity allows for solutions for Kerr black holes where closed timelike curves, and therefore time travel, are possible. It is expected that a proper theory of Quantum Gravity will remove this possibility.[[/note]]

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 [[MagicAIsMagicA given what's been established so far]]. Therefore, regardless of the [[http://archive.is/9HERI typical stylistic flourishes]] ("If all stories were written like science fiction stories" by Mark Rosenfelder, a [[ConLang conlanger]]) of hard SF, the only way to define it is self-consistency and scientific accuracy.

Which leads us to the Scale.

[[folder:Notes (please read!)]]
''Note:'' The works mentioned below are sheerly 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 [[http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness 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: [[AsimovsThreeKindsOfScienceFiction 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, [[http://www.telegraph.co.uk/news/science/8782895/CERN-scientists-break-the-speed-of-light.html the September 2011 OPERA experiment which measured faster-than-light travel by neutrinos]] might have moved works whose Mohs/OneBigLie was FTLTravel into the Mohs/SpeculativeScience 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 [[http://www.sciencemag.org/news/2012/02/breaking-news-error-undoes-faster-light-neutrino-results 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 [[ScienceMarchesOn scientists discovered later]].

''Note 5:'' As far as this wiki is concerned, TropesAreNotGood and TropesAreNotBad. "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.

''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:'' Keep in mind that ScienceMarchesOn when categorizing older works. 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 [[FairForItsDay did his best]] ''[[FairForItsDay with the information available at the time]]''.

''Note 8:'' When adding this trope to a work page, [[Administrivia/TypeLabelsAreNotExamples 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 who is already working down the work's page (and only at the M's) who probably doesn't want to wander off on a WikiWalk. You can say the number, but please go on a bit explaining what the number is. For instance:
* MohsScaleOfScienceFictionHardness: 5. This work leans heavily into Mohs/SpeculativeScience -- 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.


# '''Mohs/ScienceInGenreOnly:''' The work is unambiguously set in the ''literary genre'' of ScienceFiction, but ''scientific'' it is not. AppliedPhlebotinum is the rule of the day, often of the [[ItRunsOnNonsensoleum Nonsensoleum]] kind, GreenRocks gain NewPowersAsThePlotDemands, and both BellisariosMaxim and the MST3KMantra apply. Works like ''WesternAnimation/{{Futurama}}'', ''Anime/TengenToppaGurrenLagann'', The {{DC|Comics}} and {{Marvel}} universes,[[note]]although the individual comics of some heroes might fit elsewhere occasionally[[/note]] and ''Franchise/TheHitchhikersGuideToTheGalaxy'' fall in this class.

# '''Mohs/WorldOfPhlebotinum:''' The universe is full of AppliedPhlebotinum with more to be found behind every star, but the Phlebotinum is dealt with in a [[MagicAIsMagicA 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 Creator/EEDocSmith's ''Literature/{{Lensman}}'' series, ''Anime/NeonGenesisEvangelion'', ''Series/StarTrekTheOriginalSeries'', and ''Franchise/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 [[ScienceMarchesOn no longer considered true]] (such as [[{{Creator/Aristotle}} Aristotelian physics]]), or never considered true in the first place (e.g. two spatial dimensions instead of three). Some of Creator/ArthurCClarke's stories fall here. However, given [[ScienceFantasy the overlap with fantasy]], it can [[GenreBusting prove tricky]] to even classify a story as SF.

# '''Mohs/PhysicsPlus:''' Stories in this class once again have multiple forms of AppliedPhlebotinum, but in contrast to the prior class, the author aims to justify these creations with [[ShownTheirWork real]] and [[MinovskyParticle invented]] natural laws -- and these creations and others from the same laws will [[ChekhovsBoomerang turn up again and again in new contexts]]. Works like ''Webcomic/SchlockMercenary'', DavidWeber's ''Literature/HonorHarrington'' series, David Brin's ''Literature/{{Uplift}}'' series, and ''Series/BattlestarGalactica2003'' fall in this class. Most RealRobot shows fall somewhere between Classes 2 and 3.

# '''Mohs/OneBigLie:''' 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. Most works in Creator/AlanDeanFoster's ''HumanxCommonwealth'' series, the Ad Astra board games and Creator/RobertAHeinlein's ''Literature/FarnhamsFreehold'' 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 FTLTravel), but for which the device is not a major element of the plot. Many Creator/HalClement novels (e.g. ''Literature/MissionOfGravity'', ''Close to Critical'') and ''Webcomic/{{Freefall}}'' fall within the subclass.

# '''Mohs/SpeculativeScience:''' Stories in which there is no "big lie" -- the science of the tale is (or [[ScienceMarchesOn 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 Creator/RobertAHeinlein's ''Literature/TheMoonIsAHarshMistress'' 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.) ''Film/{{Gattaca}}'', Manga/{{Planetes}}, ''[[http://en.wikipedia.org/wiki/The_Machine_Stops The Machine Stops]]'' by E. M. Forster, and the more SpeculativeFiction works of Creator/JulesVerne fall in this subclass.

# '''RealLife''' (aka Fiction in Genre Only): A SharedUniverse which spawned its own genre, known as "NonFiction". Despite the various problems noted at RealityIsUnrealistic, it is almost universally agreed that there is no other universe known so thoroughly worked out from established scientific principles. [[UsefulNotes/{{NASA}} The Apollo Program]], WorldWarII, {{a|rsonMurderAndJaywalking}}nd {{Woodstock}} fall in this class.