Update (3rd April 2017): I wrote this some fifteen years ago. Every so often I'll revise this whimsical scale. The current version is at Omni magazine, see The Scale of Hardness in Science Fiction.
I now consider that the simple linear scale used in that thesis is manifestly inadequate to the complex possibilities of the hard and soft sciences and the imagination, involved in envisaging future or alternative worlds. Ideally,a number of parameters should be used. Specifically, there is the distinction between what is actually known, and what is speculation and hence can go either way.
What is actually known tells us what SF and Sci Fi themes and tropes are absolutely impossible, no matter what future developments in science and technology may bring. e.g. we know that lifeless planets cannot have a breathable atmosphere, that atomic radiation won't give rise to "postapocalyptic mutants", that technobabble is not the way that engineering works, that insects can't grow as big as elephants, that 20 gigaton ray guns require an energy source and have to deal with waste heat, that aliens wont look like H. sapiens sapiens and won't innately speak English either, and so on.
This is not to criticise any of these things as fun escapist entertainment, and/or as mytholologic metaphors, only to say that they cannot occur in the "real world".
What is speculation and hence can go either way refers to matters regarding which we currently simply do no know. e.g. aliens may or may not exist and if they do may or may not be roughly humanoid, wormholes may or may not be allowed given current or future insights regarding the laws of physics, FTL and hence acausality may or may not apply, the current understanding of chemistry at nano scales may (wet nano only) or may not (drexlerian nanotech) apply, ditto the Heisenberg Uncertainty Principle (hence femtotech etc), ditto non-biological intelligence (hence AI and infomorph intelligence). Regarding those critics who state that certain things are absolutely impossible, Arthur C. Clarke had some choice words about pronouncements by elderly but distinguished professors. This does not mean that these things are absolutely possible. It means we don't know. And SF should be allowed to explore either or both options.
There is a third parameter to be considered, and that is Science Fiction verses Fantasy. We can call this variable "Magic". This constitutes a gradation from "hard science" in which everything has a physical technical explanation, to some psionic or mystical elements, to full fantasy. There can also be an overlap, in which the same universe may have elements of fantasy and hard science. For this, reason, this variable is considered distinct from "what is known"
However the current page is retained as it would be too difficult to revise everything, and a lot of what is written here is still, I feel, valid, and may be of interest.
On the basis of rating of common sci fi tropes from ultra-realistic to pure soft sci fi unscientificness, one can posit a grading of the genre from Ultra Hard to Very Soft. This is not intended as, and should not be taken as, a list of absolutes. It is simply one interpretation of what is more and what is less likely, given the current understanding of the universe and how it works. Future discoveries in physics, AI, etc may very well change the placement of some of these stories - some things that are here listed as implausible may become very plausible, and vice versa. However even if, say, warp drive turns out to be viable and wormholes not, there is no way that the Star Trek "bumpy-headed humanoid of the week" galactography could in any way be possible. Hence some things are patently absurd, no matter what future advances in technology or exploration of space reveal.
However, this should not be taken as a value judgment, because no grade is "better" than any other. So soft science SF as a genre is just as valid as hard science SF. Moreover, you may have a completely different, but equally valid, approach to these things. Other standards might equally well be used to define various levels of "hardness", and we in no way wish to claim that our interpretation is the only or the best one! In addition, to reiterate, we are not trying to suggest that "hard" science fiction is in any way "better" than the soft sci fi, or pop sci fi franchises. All that is simply a matter of personal taste, opinion, and preference. None of what is written here should be taken as any sort of value judgement.
|Major Categories||Rating used here||Common Tropes||A few examples|
|Hard Sci Fi||"Present Day Tech"||Cutting edge Present Day Tech, some developments and speculation, but nothing major that has not been attained today (so no AI). Basic space exploration, very near future||Technothrillers, Allen Steele's Orbital Decay|
|Ultra Hard (Diamond Hard)||Plausible developments of contemporary technologies - AI, Constrained Nanotech, DNI, Interplanetary colonisation, Genetically engineered lifeforms. Nothing that conflicts with the laws of physics, chemistry, biology etc as currently understood||William Gibson, Neil Stephenson, Kim Stanley Robinson's "Mars" Trilogy, Robert Forward|
|Very Hard||Plausible developments of provocative contemporary ideas, bot nothing that conflicts with the known laws of physics, information theory, etc - Assembler Nanotech, Nano-Goo, Uploads, Interstellar colonisation, Relativistic ships, vacuum-adapted life||Greg Egan, Linda Nagata, Greg Benford's Galactic Center series, Stephen Baxter's Manifold Series, GURPS Transhuman Space|
|Plausibly Hard||The above but with the addition of some very speculative themes, some of which may well turn out to be impossible, others may be possible. Requires some modification of current understanding, but nothing that is logically impossible, or has been conclusively proved to be impossible (so no FTL without time travel) - Wormholes, Reactionless Drive, Sub-nanotech (Femto-, Plank, etc), Domain Walls, exotic matter, FTL drive with time travel, etc||Stephen Baxter's Xeelee universe, Greg Bear's Forge of God series, Orion's Arm|
|Firm||As realistic as the above categories were it not for unrealistic/impossible plot devices (e.g. FTL without time travel paradoxes), although these are kept to a minimum as much as possible||Asimov's "Foundation" Series, "Giants" series by Hogan, Vinge's A Fire Upon the Deep and A Deepness in the Sky|
|Medium||Similar to the above but with a larger number of unrealistic plot devices; e.g. FTL without real explanation (ore with pseudo-explanation), alien biota in some instances very similar to terragen life, psionics, a great many alien civilizations. However still preserves plot and worldbuilding consistency, and the science is good and consistent.||Niven's "Known Space" series, Robert Heinlein's Starship Troopers, Banks' "Culture" novels, David Brin's "Uplift" series, Frank Herbert's Dune, Traveller RPG|
|Soft Sci Fi||Soft||A number of unscientific themes - e.g. aliens as anthropomorphic "furries", handwavium disintegrator guns, Alien Cultures and psychology all extremely uniform, and so on. However, still retains story consistency.||Various TV series: Babylon 5, Farscape, Andromeda, Matrix, StarGate for the most part|
|Very Soft||As above but either even more unscientific elements (humanoid of the week, lifeless planets with beathable atmosphere, etc), and story with less consistency||Various TV and movie series; for the most part the Star Trek Canon and Star Wars Canon|
|Mushy Soft||As above but even more unscientific (alien races never before encountered speak perfect English without a translator, animals too large to stand in Earth gravity (Godzilla), weapons that make energy beams without putting energy in, interstellar travel without FTL or centuries long voyage, mutants with super energy powers, etc)||Godzilla, Comic Book Superheros, badly written TV sci fi, elements of some franchises|
Note that the above scale does not include Science Fantasy or proper fantasy. Also one person's fantasy might be another person's sci fi (e.g. Star Wars is considered "Space Fantasy" by it's creator, but sci fi by others)
Hard Sci Fi is Science Fiction in which the science and tech remains plausible, and the universe is explained in a consistent rationally. It may not always be realistic, and indeed it can sometimes be very speculative and even include unrealistic or impossible plot devices, but the overall approach is one of careful research, scientific, technological, and sociological consistency, and real science rather than meaningless technobabble.
Hard Sci Fi ranges from the most realistic stories limited only to current knowledge and set in the present day or very near future, to science fiction that is only "medium" in realism, but, being more speculative, can be set much further in the future or explore more themes.
Note that some would claim that only the nearest future categories included here can be considered "Hard", the others being too speculative. However I am following the "John W Campbell" definition of what constitutes "Hard SF" or Hard Sci Fi, and that is that while some speculative ideas are allowed, the story as a whole must be based on scientific research, avoid technobabble or cliches like Bug Eyed Monsters stealing Earth women.
PRESENT DAY TECH sci fi deals only with known technologies and science, and only the most conservative extrapolation therefrom. This may include such things as flying cars or fusion reactors. Does not incorporate radical or controversial concept like wormholes, any kind of aliens etc. The term is given in inverted commas because the story and even technical details may still turn out to be implausible or impractical in real life, much as it reads well in fictional form. Generally "Present Day Tech" SF would generally take place in either the "present day" or the very near future, as the further ahead the harder it becomes to make decent predictions and the more likely you are to be wrong. While this makes this form of sci fi much more realistic, it can also limit the imagination.
The following are common themes that occur in "Present Day Tech" settings. All are absolutely certain through extrapolation of current technology.
ULTRA HARD can also be called Diamond Hard; this is so-called because it represents the most extreme (realistic) side of the Hard SF spectrum. The term DIAMOND HARD is here used as something of a pun - diamond refers to nanotech building material (diamondoid) but also in the hardness scale to very realistic sci fi. Does not incorporate radical or controversial concepts like wormholes or femtotech. Generally Ultra Hard SF would generally take place in the near future, as the further ahead the harder it becomes to make decent predictions and the more likely you are to be wrong.
In Ultra Hard Sci Fi, handwavium and anything that might be dubious is completely absent.
The following are common themes that occur in Ultra Hard sci fi settings. From our present understanding, there is nothing in the laws of physics, chemistry, biology, or information theory that says these things are actually impossible.
VERY HARD: Deals with known technologies and expands on existing scientific theories in a speculative, but still rigorous and plausible, fashion. The story doesn't break any of the known laws of physics, information theory, and so on. Includes some controversial concepts, but nothing that conflicts with the laws of physics as they currently stand. A bit more speculative than Diamond Hard, and hence Very Hard SF can take place future in the future, or on a larger canvas. Handwavium is kept at an absolute minimum.
The following are common themes that occur in Very Hard sci fi settings. According to the current understanding of how the universe works, some of these things can be considered either almost or absolutely certain, others are very probable, but may well be proved wrong in the future. However, from our present understanding, there is nothing in the laws of physics, or information theory as currently understood that says these things are actually impossible. And were they to be proved possible our understanding of how the universe works would only need to be revised in a minor way.
PLAUSIBLY HARD: Deals with known technologies and expands on existing scientific theories in a speculative, but still rigorous and plausible, fashion. The story doesn't break any of the known laws of physics, although it may make reasonable, explained extrapolations of physical laws well beyond current leading-edge concepts, including ideas that may be controversial, but have not yet been shown to be impossible. Inevitably there is always some degree of handwavium, but it is always within the context of the story, and no unrealistic plot device sillytech or unobtanium is ever allowed. Technology has to follow a particular logical sequence, and while the advanced elements of the sequence may seem highly speculative by today's physics, they still follow a logical chain, e.g. you can't just jump from today's tech to FTL or wormholes or blasters or whatever. Nothing that can be shown to be logically impossible is allowed.
The following are common themes that occur in Hard sci fi settings. As well as everything found in Ultra Hard / Very Hard SF, the somewhat less rigorous Plausibly Hard Sci Fi aspect of the genre includes things that may or may not be possible, but can still be considered plausible or reasonable, at least until proved wrong by future discoveries. Although some of these points are currently considered unlikely or even impossible by conservative physicists (but not, mind you, by all physicists), that in itself does not make them impossible (Arthur C Clarke's comments on pronouncements by elderly and distinguished professors come to mind here too). Even so, were any of these things to be proved possible, our understanding of how certain aspects of the universe works would need some pretty radical revision, but would still be accomodatible with what we know and understand at present.
FIRM : Deals with known technologies, sciences and theories, but often incorporates new theories or ideas with plausible explanations. Breaks some physical laws, but provides a solid rationale for it. Differs from Hard only in the inclusion of some form of FTL Plott-DeVice Drive * :-) or equivalent. Apart from these non-hard elements, everything is described in technical terms, using real and authentic science and engineering, and apart from the aforementioned plot devices, and the story is never allowed to make the science look silly.
The following are some themes and technologies that might occur in Firm (but not in Hard) sci fi settings.
MEDIUM : Breaks physical laws but attempts a rationale which sounds reasonable in context with the work, regardless whether or not it makes sense within the current scientific paradigm. Also describes things in a scientific manner. So while concepts like FTL appear, which are unproven and indeed contradictory to the laws of physics, the story is still arranged in a logical manner. This material is generally considered "hard science SF", but is not as rigorous as the above category, and often (unlike Firm science fiction) the writer will deliberately fudge or even ignore the science for the sake of a more entertaining story.
The following are some themes and technologies that might occur in Medium (but not in Firm) sci fi settings. These tropes can still be used (if not overdone) in a Campbellian hard science manner (which is a less rigorous grading than we use - e.g. an FTL drive is allowed if the rest of the story hangs together, the repercussions on society as a whole are acknowledged or explored, etc). Most traditional SF uses at least one of these tropes, as plot devices. All of these ideas have been excluded from the Orion's Arm setting.
Soft Sci Fi as defined here is Science Fiction, Science Fantasy, or (to use George Lucas' term) Space Fantasy, in which there is little or no attempt to keep the science and tech plausible, or to have social trends follow logically from technological development. Frequent absurd elements are common (e.g. it would be impossible for Godzilla, or for giant ants or spiders, to stand or move in Earth gravity), and the universe is explained in terms of storyline entertainment rather than plausibility. In some cases technobabble is used instead of proper science.
As with Hard Sci Fi, one can suggest various grades of Soft Sci Fi.
SOFT: Breaks physical laws but still attempts a rationale which may sound reasonable in context with the work, although in all other respects it is very implausible. Moreover it shows pretty much complete ignorance of how real science and the real universe works. These tropes are found in almost all television and much cinema-based SF. Nevertheless, occasionally a classic SF tale will involve one or more of these tropes.
Note that the term Soft SF is often also used to refer to SF that explores sociological and psychological themes; here we are using it instead specifically in the context of works that do not attempt to be scientifically rigorous, even if they are still logically consistent.
The following are some themes and technologies that might occur in Soft (but not Medium Hard) sci fi settings. These things are highly improbable to impossible and cannot by today's understanding of physics, astronomy, etc, be considered in any way realistic. Often they are the result of poor background research, or of (in the case of movies and TV shows) the limitations of working with human actors, make-up , and so on (now with improved CGI this may change). Hence they are excluded from the harder versions of Hard SF (Hard or Ultra Hard). But who knows, they may be proved correct in the future, should the laws of physics be completely and drastically turned around. This would however require a revision as great as the difference between, say, Aristotlean and Contemporary science.
VERY SOFT : Breaks physical laws and while the explanations, if any, may sound reasonable in the context of the story, the result has so many inconsistencies and implausibilities one is hard pressed to feel in any way comfortable about this material as "Science Fiction", enjoyable as it may be as pure entertainment. Knowledge of real science, real engineering, and so on is basically non-existent. Unlike Soft Sci Fi it does not even provide any consistency within the context of the story. e.g. in Star Trek matter transports allow perfect molecular replication, but this is never used to heal illness or attain bodily immortality. Incidentally, the author of one website (don't have the url) argued (dubiously) that much of this material is still "hard SF" because it seeks and provides a rational explanation of things. e.g. in Star Trek you know that you need some form of FTL to travel between the stars. But this is really stretching the term "hard sci fi".
This relates to the inconsistency of these fictional universes in failing to follow through technological development into other areas of science and of society. Also some concepts that are clearly totally absurd and against the laws of physics. In addition, technobabble might be used instead of real science.
MUSHY SOFT : Differs from standard Soft Sci Fi in giving up any pretense at all of trying to be plausible. This category is often disparagingly referred to as "science fantasy" by people who are serious about Sci Fi (however this is not to be confused with Science Fantasy in the sense of sci fi that incorporates fantasy elements).
At its worst (regarding scientific plausibility or lack thereof) Mushy Soft Sci Fi does not even attempt any explanation, or else those it does are patently absurd, just complete scientific nonsense. However, it cannot be called Science Fantasy, Magical Realism, or whatever, because it is still supposedly set in the "real world". Differs from Very Soft in that it does not even give the pretense of realism (e.g. in munchkin sci fi you can still have a zillion erg blaster beam as long as there is some technology like a big ray gun cannon to generate and "explain" it; in this category you don't even need that). Note that, as always, this is not a value judgment, only a science and realism comparison. This material (e.g. superhero comics) can still be very entertaining and enjoyable to read.
The following are some themes and technologies that might occur in Mushy Soft (but not in reasonably thought out Soft ) sci fi settings. These things are impossible and often outright fantasy. We can safely say though that all these things are the result of sci fi clichés, bad science, or both. They are included in sci fi for entertainment purposes, rather than being attempts to describe an authentic reality. They show not the slightest attempt at even the most basic understanding of how the universe works.
For more on the sort of absurdities one frequently finds in pop soft sci fi, I again recommend the brilliant and hilarious Overused Science Fiction Clichés
Science Fantasy is a sort of overlap with sci fi, but contains specifically fantasy elements, and hence does not describe the universe in a rational way, although it still has rational elements. Pure Fantasy, and Horror, are distinct genres to Science Fiction, although some journals publish all of them together. Often in Sci Fi Entertainment forums and media (magazines, websites, etc), "Sci Fi" is used as a generic term that also includes fantasy, horror, supernatural stories, and so on.
SCIENCE FANTASY: claims to be SF, and indeed has or is based on self-consistent SF elements that would otherwise include it under SF, but also includes one or more supernaturalist elements that remove it from the realm of pure SF. George Lucas uses the term "Space Fantasy" to describe his work, although it seems to me that - except for its fairy tale prologue ("a long time ago in a galaxy far away") - Star Wars can more properly be included with other Soft Sci Fi popular universes. The only "fantasy" element in SW is The Force, and even this is explained in technobabble terms (midichlorians") in the prequel trilogy.