The Mohs Scale of SF Hardness

In the ongoing discussion this corner of fandom has been having about genre, a sub-argument broke out about what counts as “hard” SF. Some people think “hardness” is a yes or no property and are indignant if some work is excluded. So it’s time to go over the Mohs Scale of SF Hardness.


(For non-geologists, the Mohs scale is a way of rating the hardness of rocks. It ranges from talc (anything can scratch it) to diamond (no lesser rock can scratch it). For genre discussions this is, of course, a metaphor.)

Disclaimers up front: “Hardness” is a separate property from whether a story is entertaining, actually science fiction, part of a particular sub-genre, or possessing the Campbellian “sense of wonder.” Yes, there are stories that are diamond-hard without stirring any sense of wonder. Us Hard SF fans call them “boring.”

Eight: Starting at the hardest level of hardness, there’s Real Life. Everything in the story exists today. SF fans consider this the least interesting level. Pulp stories with hard-boiled detectives are here.

Seven: The next level is A Simple Matter of Engineering. The gadgets in the story are compliant with known science and could be built if we put in the effort. The settings are as realistic as current knowledge allows. The Martian is at this level with the exception of its initial dust storm. The ion drive of the Hermes and automated Martian fuel manufacturing landers are just awaiting funding. The recent discovery of permafrost in the Martian soil means the hero could have dug for his water instead of messing with hydrazine (shudder) but this doesn’t make the story less hard, it just dates it. James Cambias’ Corsair is here, and hasn’t been ruined by a new discovery yet.

One problem with this level of hardness is that it only makes sense a short distance into the future. If your story is set a thousand years from now it’s ludicrous to think there will be no new rules of physics discovered in that time. If the setting isn’t as different from today’s as our lives are different from the world of 1000 AD that’s a failure of imagination.

Six: The third level is One New Thing. Invent a gadget, scientific law, or strange place, and examine the implications as it interacts with known reality. Heinlein’s The Moon Is a Harsh Mistress is level two with a spontaneously-created artificial intelligence. Niven’s “Inconstant Moon” is Real Life with an exploding sun.

Five: New Physics is where writers can invent lots of stuff. The trick to keeping it on this level is picking a few new inventions and dealing with their consequences rigorously. The Mote In God’s Eye and the related stories of Pournelle’s CoDominion series were here–Alderson drive, Langston Field shield, and weird yet plausible aliens.

Four: Artificial Gravity and Other Toys. Spaceships no longer need seat belts. This can still involve running numbers: Weber’s Honor Harrington series includes careful calculations of how much acceleration ships of each size can get from their gravitic impellers. This can still be “sense of wonder” hard SF. Ringworld had all sorts of impossible tech (teleporters, hyperdrive, invulnerable hulls, hereditary luck, and the scrith the Ringworld was made out of) but expanded our imaginations with what could be done with it.

Three: Who Cares How it Works? Spaceships fly, blasters zap, Death Stars blow things up. The gadgets enable the heroes to do their stuff. Numbers are a distraction. Firefly and Star Trek fall here. Most space opera is here as well, such as Lee and Miller’s Liaden series.

Two: Add A Dash of Magic. It looks like a cyberpunk or space opera setting, but some of what’s going on is just beyond physics. Star Wars Jedi can control minds, hurl objects, and create lightning with their minds.

One: Never Mind Science. Sometimes the author wants to do something and doesn’t care if it’s proven impossible. Mammals interbreed with egg-layers, rocks hang in the air, and Rule of Cool is all.

Too complicated? Oh, it gets worse. Many stories are rigorous in some areas of science and hand-wave others. An argument broke out over Dune in the CH comments. Where does it rank on this scale? The treatment of ecology is at One New Thing–a desert dominated by sandworms, with the implications for human society and future terraforming worked out in detail. Meanwhile the Bene Gesserit and Navigators Guild were effectively working by magic.

Another example of that mixed level is Bujold’s Vorkosigan series. Most tech in the stories is generic space opera, but the biological tech for reproduction, genetic engineering, and terraforming approach A Simple Matter of Engineering in the detail and accuracy provided. Whether this counts as “hard” SF depends on which aspect the viewer cares most about. If forced to assign a label I’d go with “partially hard” or “biologically hard,” which I’m sure amuses the twelve year olds in our midst.

So where is the line for defining something as “hard” SF? With most of the other things we argue about in the genre war it’s a matter of taste. I’ve noticed the most common line is artificial gravity. If the author make the crew strap in for acceleration and float the rest of the time the book will be called hard SF even if he’s resorting to blatant handwaves such as passing through a “loop of cosmic string” to travel between star systems.


This also points out the weaknesses in the definitions people are tossing around for “Campbellian SF.” Plenty of non-hard SF books such as Ringworld and Hogfather (a pure fantasy) provide the sense of wonder or “conceptual breakthrough” Campbellian fans desire.

The “hardness” of a story is just one way to describe it, separate from whether it includes any entertainment or other qualities. A hard SF story doesn’t guarantee it will provide sense of wonder. What the term “hard SF” does is give readers an idea of what to expect from the story, so they have a hint of whether it’s what they’re looking for. Which is all any genre label is really useful for.

FOOTNOTE: TVTropes has a version of this scale but I disagree with part of their analysis so I wrote this.

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About Karl Gallagher

Karl K. Gallagher is a systems engineer, currently performing data analysis for a major aerospace company. In the past he calculated trajectories for a commercial launch rocket start-up, operated satellites as a US Air Force officer, and selected orbits for government and commercial satellites. Karl lives in Saginaw, TX with his family. His books Torchship and Torchship Pilot are available on Amazon and Audible.
  • Nathan Housley

    Mundane Science Fiction sought a hardness at about 8 on this scale, but as Karl said, “SF fans consider this the least interesting level.” Even the pinkest of the Pink Sf are glad that Mundane is no more.

    • ‘If You Were a Dinosaur’ requires American rednecks to sit around swilling gin, which, as we know, violates the rules of biology. Gin corrodes a redneck and turns him green – an ugly sight. The proper lubricating fluid for a redneck is whiskey, preferably untaxed.

      • Richard Ranft

        And home-made.

  • Bellomy

    Pulp stories with hard-boiled detectives can be pretty fun.

  • MishaBurnett

    I have the most difficulty suspending disbelief on the level that you are calling “A Simple Matter Of Engineering.” I’ve spent my professional life rebuilding, retrofitting, and replacing things that looked good on paper. A great many technological innovations that are theoretically possible end up being unworkable in practice.

    A lot of authors who write what they consider to be HardSF are engaging in covert handwavium–often without seeming to be aware that they are doing it–by assuming unreasonable efficiency and durability in their new inventions, ignoring maintenance issues, failing to consider the costs of retrofitting to new technology, and so on.

    The recent “Solar Roadway” debacle is a perfect example of this–sadly one that was not limited to a fictional setting.

    • Richard Ranft

      “A Simple Matter of Engineering” doesn’t mean “can be completed quickly and cheaply within one to three years.” It means “it is achievable through a logical step in technological progress.” Improvements in manufacturing, materials technology, mining, etc. may need to be made (and may need to be very significant) in order to bring them to widespread installation. It could be fifty or a hundred years before we see all aspects of technology align to bring them about – but the fact is that the tech is a logical step from something available and understood today. If we’re saying that assuming the additional factors is hand-waving then I’d agree with your position, but do we really want to read about that part anyway?

      • MishaBurnett

        Do I want to read about the advances in materials science that make, say, an orbital solar power collection array possible? Not necessarily, but I do want to see how that technology has changed other areas. Don’t just tell me how much energy the sun puts out and claim that makes fossil fuels unnecessary.

    • Karl Gallagher

      Clearly a communication failure on my part. I’ve worked enough engineering projects that failed to meet their original goals that the phrase “A simple matter of X” has an inherently sarcastic tone to it, much as saying something will be delivered “Real Soon Now” does in other circles.

      No, building the stuff in the stories would not be easy or practical. The definition of that level is that it’s possible, in that no technology or event violates the known laws of physics. Actually delivering on that could be as unlikely as the heroics of an adventure story.

      Let’s face it, the least plausible part of The Martian was that a nation over twenty trillion dollars in debt would spring for a set of five flags and footprints missions to Mars.

  • Geoarrge

    A kind of converse to the principle that you can’t realistically go too far into the future and stick to high-hardness standards, is that it’s probably a good compromise to allow one or a few decisive handwaves if they allow you to keep your future timeline from extending too far out.

  • Brony Reviewer

    Well I wouldn’t say Barsoom or Amtor (John Carter and Carson Napier respectively) were made up whole cloth without scientific input. Percival Lowell’s theory of the canals of mars were accepted scientific theory for many astronomers at the time and indeed it does play into Barsoom’s makup, the canals being a feature in the stories.

    The idea of a hot Venus wasn’t a new one either, so his sword and planet novels for the time they were not scientically out of the realm of possibility….. For the early nineteen hundreds.

    So maybe his work would be considered around a four on your scale for the time period?

    https://en.m.wikipedia.org/wiki/Martian_canal

  • Overgrown Hobbit

    Heh. I read this a while ago and ran into a wall with this bit: “One: Never Mind Science. Sometimes the author wants to do something and doesn’t care if it’s proven impossible. Mammals interbreed with egg-layers”, but couldn’t quite figure out why.

    Then I remembered. In A Princess of Mars, it’s the Martian advanced gengineering technology (ala Bujold’s Haut’s) that allow the cross-breeding of humanoid platypuses (Thoris) & humanoid apes (Carter :-). In fact, nearly all of ERBs Martian stuff is at A Simple Level of Engineering for 1917 science. It was just Level 7 applied by an author who’d mastered the Rule of Cool as well.

    So… does it still count as “hard” SF when scientific knowledge has advanced to the point that it appears to be softness level 1 to an inattentive modern reader?

    Also, where does SF in which ordinary everyday scientific knowledge such as “cats don’t go “baaaaaa” or “you can’t put goldfish in 7-up” aren’t honored? Just really, really, khrepptastic writing? Or does the scale go to negative numbers?

    Hmmm. How does a graph with “hardness” along one axis and “rule of cool” along another sound?

    • Karl Gallagher

      I can imagine four or five dimensional measures for stories. Even more if you rate them on their aspects of different subgenres or mystery/romance subplots.

      As for APoM . . . sorry, I’ve read it too recently to buy that. I’ll certainly agree the goldfish in 7-Up story was khrepp.

  • Lela E. Buis

    What I consider the best “hard” SF is stories that present ideas. Jules Verne promoted ideas that became reality because of the interest he generated. We need that to continue.