(Update (April 2017): I’ve published a new version of this scale on Omni)
Just as there’s a Mohs Scale of Hardness in mineralogy, so there’s a metaphorical Scale of hardness in Science Fiction, where “hardness” refers to Hard Science Fiction (Hard SF for short), the genre that has the greatest technological plausibility and scientific realism. (Hard SF on its own also tends to dryness and socio-cultural and character shallowness, but that’s another story)
For more on Hard SF, here’s the wikipedia page, here’s a website devoted solely to the topic, here’s a list of books considered Hard SF (as always, there’s a lot of subjective assessment, unlike minerology or chemistry where there’s easy empirical tests). Little in the way of Hard SF movies, one could list 2001, Gravity, and The Martian.
The following scale of realism combines and updates my earlier scale (written years ago and now out of date) with the excellent TV Tropes Mohs Scale of Science Fiction Hardness (which I have quoted from at length). Update: See also other comments and versions by Mike Brotherton, and Howard Miller, Juliette Harrisson’s Discworld Mohs Scale of Fantasy Hardness Parts One and Two, and J.P. Flarity’s Mohs Scale of Fiction Weirdness.
Regarding the following, four things should be pointed out.
First, the opposite of Hard SF is not Soft SF. Soft Science Fiction refers to science fiction that explores themes and possibilities based on the social or “soft” sciences such as psychology, anthropology and sociology. This is a perfectly respectable field of science fiction, and indeed Hard SF stores may be decidedly lacking in the social realism stakes, just as social science fiction may be lacking in the astronomy and rocket science stakes.
Second, even when science marches on, a work still remains hard science. To quote Wikipedia: “Later discoveries do not necessarily invalidate the label of hard SF… Arthur C. Clarke’s 1961 novel A Fall of Moondust (is still) hard SF,…even though a crucial plot element, the existence of deep pockets of ‘moondust’ in lunar craters, is now known to be incorrect.” HG Wells War of the Worlds for example was hard science by the standards of its day. Nevertheless, the future never turns out as we imagine or predict it, so “Future Rust” is common in older books, and even more recent technothrillers.
And third, no story or worldbuilding is uniform in its realism or lack thereof. A scenario may include ultra hard SF (9 out of 10) ships for interplanetary travel but give them hyperjump capability for FTL (6 out of 10).
And finally, all science fiction is still fiction. The science in science fiction enhances the fiction, but generally less so the reverse, although there are exceptions, e.g. many scientists and astronauts who work at NASA grew up watching Star Trek. But for a writer it is less what is absolutely, objectively, real, and more what serves the story.
Here then is my current provisional list, from most to least realistic, where 10 is the real world, and 0 is total cartoon nonsense, with especially emphasis on spaceships and space opera:
(Update: have included decimal-tenths just for the heck of it. So n.6 is more speculative and n.5. more realistic)
10. The real world. Science is explained, no handwavium. Also, Non-Fiction. As TV Tropes puts it: “The Apollo Program, World War II, and Woodstock fall in this class.”
9.6. to 9.9 Technothrillers and Futurology. Technothrillers take place only a few years in the future, with only a few plausible near future tech projections. This category overlaps with Futurology which includes, as explained in TV Tropes “stories which function almost like a prediction of the future, extrapolating from current technology but do not assume or invent any important new technologies or discoveries.” Quite likely some of the more rigorously and scientifically realistic of the ultra hard space exploration categories of the following category actually go here.
8.6.to 9.5 Ultra Hard Science – only extrapolation from known laws of physics. In TV Tropes this is called Speculative Science. In contrast to the previous categories, the science is “genuine speculative science or engineering, and the goal of the author to make as few errors with respect to known fact as possible.” Interplanetary vessels have milligee thrust. Rockets have to choose between high thrust low isp, or high isp low thrust, you can’t have both. Adherence to thermodynamics, real science. No torchships. Preferably no aliens, if there are, they are limited to the same physics we are, and also must justify in terms of Great Silence (Fermi Paradox). Greg Egan, Kim Stanly Robinson Mars Trilogy, Arthur C Clarke 2001, Asimov The Gods Themselves, GURPS Transhuman Space, Andy Weir’s The Martian, 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. 9.5 is the most realistic, with only plausible near future tech projections. For some excellent examples on Ultra Hard SF ships in fact and fiction, with authentic rocket science, see Winchell Chung’s Atomic Rockets website, a resource that I simply cannot recommend highly enough. For tabletop spaceship combat game, see Ken Burnside’s Attack Vector: Tactical (although this also has less realistic FTL as a background explanation for why the battles are not in our solar system). John Lumpkin’s Human Reach series features similarily authentic spaceships, although the setting includes wormholes which pertain to one or two realism grades down.
7.6.to 8.5. Very Hard Science – extrapolation from laws of physics, with minimum handwavium for story purposes. Basic torchships (high thrust high isp), interplanetary vessels average around 0.3 gee thrust or less. Adherence as much as possible to thermodynamics and real science. Interstellar, non-relativistic travel. In TV Tropes this is called “One Small Fib”, “stories that include only a single counterfactual device (often FTL Travel), but for which the device is not a major element of the plot.” Some alien or posthuman handwavium / unobtanium allowed. Big Dumb Object without handwavium. Preferably have an explanation for Great Silence (Fermi Paradox). Arthur C Clarke Rama novels, Gregory Benford, Bruce Stirling Schizmatrix, Stephen Baxter. Many Hal Clement novels (e.g. Mission Of Gravity, Close to Critical) and Freefall belong here.
6.6 to 7.5. Hard Science – extrapolition from laws of physics, plus some handwavium for storytelling purposes. In TV Tropes this is called “One Big Lie”. “Authors invent one, or, at most, a very few, counterfactual physical laws and writes a story that explores the implications of these principles.” Tech otherwise consistent and explained. Interplanetary vessels average around 1 gee. Relativistic travel. May have reactionless drive but must be realistic, say exotic matter or singularity or other unusual tech. FTL only if plausibly explained, otherwise relativistic travel. Includes thermodynamics, rocket science, etc. Aliens with realistic-seeming biology, non-humanoid only. Token explanation of Great Silence (Fermi Paradox). 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. Also Haldeman Forever War, Niven Mote in God’s Eye, Greg Bear various stories, Alastair Reynolds Revelation Space/Galactic North Universe, Posthuman Studios Eclipse Phase (everything else very hard, say 9 out of 10, but “sleeving” consciousness with its associated cartesian ghost in the machine implications, also alien wormholes that are pure unexplainedium). John C. Wright’s The Golden Age probably would go here as well (say 6.7)
5.6.to 6.5. Firm Science – with handwavium and unobtanium, but these reasonably explained in the context of the setting. Torchships rather than reactionless drive, but absurd acceleration given energy allowance (fudging the figures). Token thermodynamics only. FTL with reasonably plausible explanation. Ignores Fermi Paradox. Niven’s Known Universe, Peter Hamilton, James Corey’s The Expanse, a lot of space opera in general.
4.6.to 5.5 Physics Plus (to use the TV Tropes moniker). “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.” Some Science. Reactionless drive ships easily pull dozens or hundreds of gees. Doesn’t explain why relativistic reactionless drive ships aren’t used as doomsday devices to annihilate whole planets, or as perpetual motion free energy devices. Differs from Token Science in that there is still some rocket science. Heinlein Citizen of the Galaxy, Starship Troopers, Brin Uplift universe, Iain M Banks Cultureverse, Schlock Mercenary, David Weber’s Honor Harrington series, David Brin’s Uplift series, and Battlestar Galactica (2003) fall in this class.
3.6.to 4.5 Token Science. Some real science, but only because this is “science fiction”; the science is there only for storytelling purposes rather than realism. For the most part handwavium and unobtanium with little or no explanation. Ignores thermodynamics (no radiator fins). Reactionless drive, agrav, forcefields, tractor beams, stealth, some aliens look like humans, others very different. FTL with arbitrary explanation. Consistency in worldbuilding, in contrast to the Pure Technobabble category. Frank Herbert’s Dune, Dan Simmons Hyperion Cantos, Firefly/Serenity, maybe Eve on line. Babylon 5 may belong here, although humanoid aliens belong to the next category down, and conversely fighters use vector thrust and larger ships generate artificial gravity by centrifugal spin, which put them in at least the 6 or 7 out of 10 category.
2.6 to 3.5. Technobabble and Handwavium. Differs from Science Fiction In Name Only in that at least there is some attempt, no matter how poor, at a rational explanation. Or in otherwords, to quote TV Tropes: “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.” So for example the Enterprise cannot land on a planet, showing that large ships are outer space only. Even so, this is still full of sillytech and inconsistent worldbuilding. Because of handwavium and phlebotinum, anything can be explained. You can have ST style matter transportation because it is assumed all the technical problems were solved, most aliens are humanoid, many identical in appearance to H. sapiens, human and aliens can interbreed, and humans live in similar environments can eat same food etc, because it is assumed there were humanoid progenitors. At the same time, there is serious worldbuilding incompatabilities e.g. if you have ST matter teleportation otherwise you could easily drop a nuke in an enemies lap, have immortality, limitless clones, etc. But it is never explained why this never happens. Star Trek is the classic example, but also included here is E. E. “Doc” Smith’s Lensman series, Neon Genesis Evangelion, Warhammer 40k, the Star Wars Expanded Universe novels (now called “legends” and considered non-canon) which at least make some attempt at explanation over the totally unrealistic canonical movies, Cowboy Bebop, and StarCraft.
1.6 to 2.5 Artistic License. Although unambiguously considered to be “Science Fiction”, there is no actual science to be found. Rather, the story follows what looks good artistically from our contemporary early 21st century perspective. Ships wheel and bank in a vacuum, blaster bolts move slower than tracer rounds. Battles directly mapped from WW II naval engagements. Lantern-jawed heroes and busty maidens, or scary monsters, or both. More realistic than comic book superheros only in that it’s assumed you need a spaceship to travel through space, and there is an attempt at a consistent plot. Star Wars, Guardians of the Galaxy, Dr Who, Red Dwarf, Farscape, Space Battleship Yamoto. Some Star Trek movies, e.g. Star Trek out of Darkness has worldbuilding inconsistencies that are if anything even worse, e.g. Transwarp allows teleporting across interstellar distances, but it is never explained why people still use spaceships
0.1. to 1.5 Comic books. Only token attempts at explanations – Marvel and DC superheros, e.g. superpowers because from a planet with a red sun, Genetic mutation, or Peter Parker bitten by a radioactive spider. Sometimes not only that, so that, to quote TV Tropes: “Green Rocks gain New Powers as the Plot Demands”. The DC and Marvel superhero universes belong here, along with Futurama, manga and anime like Dragonball Z and Tengen Toppa Gurren Lagann, and humorous aspects of The Hitchhiker’s Guide to the Galaxy.
0. Cartoons that are not meant to make sense, e.g. Marvin the Martian