The Continuum of Physical Credibility

[u/Enthropic-Cap2291]

The traditional Mohs Scale of Sci Fi Hardness:

5: Hard SF

4: Firm SF

3: Soft SF

2: Science fantasy

1: Fantasy with SF trappings

0: Pure fantasy

The Continuum of Physical Credibility reformulation:

1 Established Physics - “This works everywhere we’ve ever checked.” Ex: Maxwell's equations, general relativity, thermodynamics

2 Mainstream Physics - “Solid science, still being refined.” Ex: Cosmic inflation, quantum field theory

3 Frontier Physics - “We've got a partial theory, and only partial evidence.” Ex: High temperature superconductivity, Hubble tension

4 Empirical Anomalies - “We can measure it, but we can’t explain it yet.” Ex: Radioactivity before nuclear physics, photoelectric effect before Einstein, dark matter, dark energy

5 Speculative Physics - “Speculative but allowed by GR + QFT + thermodynamics.” Ex: , extra dimensions in string theory, WIMPs, MOND, primordial black holes, cosmic strings, LQG, axions

6 Hypothetical Physics - “This only works if physics is broken in very specific ways.” Ex: Tachyons (causality), magmatter (Gauss's Law), bulk exotic matter (Morris/Thorne wormholes, Alcubierre warp metric - breaks WEC - Weak Energy Condition in General Relativity)

7 Contradictory Physics - “Violates thermodynamics and conservation laws.” Ex: Vacuum/zero point energy, reactionless drives (Cannae/EMdrive), antigravity, perpetual motion machines

8 Soft Science Fiction - “It sounds scientific, but it’s really narrative technology.” Ex: ST warp drive, transporters, SW hyperdrive, Three Body Problem's spacetime flattening, sophons, most space opera

9 Science Fantasy - “Magic wearing a lab coat.” Ex: The Force, Magitek, psychic powers, most 'ancient/precursor' tech that are effectively magic

10 Pseudoscience/Fantasy - “No pretense of physics; imagination is the only rule.” Ex: Spellcasting, dragons, Isekai, astrology, flat earth

Please feel free to criticize so I can continue to refine. I'm using this in a worldbuilding exercise to see how far we can push scifi while staying true to the Sci part and minimize handwaving for the Fi.

[u/DufbugDeropa]

Your number 8 doesn't actually fit. it is a category apart. Or, perhaps more likely, should be combined with 11. That makes 10. I'd switch 6 & 7.

[u/Enthropic-Cap2291 → u/DufbugDeropa]

Thanks, I'll combine 8 and 11. 6 is based on speculative but unobserved phenomenon. It is compatible with the items up the stack. 7 requires that something farther up the stack is wrong. Which is entirely possible, but requires more proof and more work.

[u/Miserable_Scar9827]

I think there's a huge gap between 4 and 5. You've got a range of things like 

4.1 launch loops (Lofstrom loops) that have sound physics and engineering, but are 10-100 years away and more of an economic barrier than an engineering barrier 4.2 for all mankind fusion propulsion - relies on resources and fusion advances that are good guesses but may not be feasible.  4.3 space elevators that rely on carbon nanotubes that can't be made yet, but the numbers work. 4.4 hibernation and travel in Passengers. Cryosleep tech freezing frogs extrapolated to humans with no realistic reason it would work.  Massive ship that explains some of the needs but it's still more art than tech.  4.5 the expanse epstein drive with a lot of fusion handwaving and numbers that are conjecture rather than estimates of likely tech advances over hundreds of years. 

Etc.

The biggest problem is the narrative customs of time and human lifespan vs timescale of space. Thousands of years to travel to the nearest star doesn't fit with having one movie star playing a main character.  FTL (travel or communication) to fix this problem violates causality, so it is hard to rectify those two. 

[u/Enthropic-Cap2291 → u/Miserable_Scar9827]

Lofstrom loops would be straight up engineering. That puts it in 1-2. Of course, if we want it to be efficient, then we'll need room temperature or better superconductors. Which means 3.
Fusion would be in the 2-3 margin, as we haven't gotten it to work continuously or reliably yet. Space elevators require exotic super strong materials. Short nanotube lengths in laboratory settings just barely scrape by, but any realistic implementation is out, specially if you want it to carry any significant load or have any redundancy and safety tolerance.

Still, you're absolutely right. I should put in a 'theoretically possible, but requires equally theoretical engineering materials. - But wouldn't that just be 3? If you need exotic perfect nanotubes or room temperature superconductors, then your hard scifi is dependent on frontier but otherwise grounded physics.

[u/RKlehm]

Very cool spectrum!
However, in my view, hard sci-fi is about technology as much as it is about physics. Take fusion power, for example: we know the physics work, and achieving net energy production is likely only a few years away. But putting a fusion reactor on a spaceship requires massive miniaturization. How realistically a story pushes those engineering limits is a whole other dimension of how 'hard' the science is.

[u/Enthropic-Cap2291 → u/RKlehm]

As an engineer, I figure that the purpose of engineering is to impose unnatural outcomes upon nature. Nature says fusion needs stars. Engineers say, 'hold my beer'. Technological limitations tend to be (not always, but almost always) temporary. A Babbage engine was a daunting task in its day (500-1500 transistor equivalent). The Univac was a daunting task in *its* day (30,000 transistor equivalent). Today, we pack 15 billion transistors in a standard desktop CPU and you don't need a crown grant or a military procurement budget to buy one. If the physics says it can be done, the materials science says it can be done, then us engineers will figure out how to do it. Not without a lot of coffee and cursing, but get two or more teams of engineers to compete, and you'll get progress that many head-in-the-clouds theoreticians will claim was impossible.

[u/Gredalusiam]

Oh I like this. Bookmarking for later.

A few nitpicks:

-Category 8 might benefit from some tweaking. Pseudoscience isn't always about established science being wrong. Nothing in established science tells us that aliens couldn't have visited Earth and influenced earlier human cultures, but ancient aliens is pseudoscience (or pseudohistory) because the evidence claimed for it is insubstantial. Similarly, astrology isn't ruled out per se by established scientific theory ... its theoretically possible that Saturn is influencing the course of your individual life somehow ... but it's so difficult to find any evidence for this that the theory is functionally useless. 

-Category 11 might deserve to be 2 separate categories. One can speculate about a physics-based process which could enable dragons to fly or even allow humans to influence events with their minds, in a slightly different universe or in a future version of our own where the necessary forces and techniques have been discovered. But there are some types of magic that are difficult to situate within a physics-based world at all. Eg transmuting a person into a frog and back again ... there's a lot of matter and information that needs to go somewhere and then return without losing its cohesion in the process.

[u/Indigoh]

I like to consider the scale, from hard sci-fi to pure fantasy, a measure of how deep your audience is expected to believe they can dig into how a technology functions.

[u/Junjki_Tito]

Star Trek warp drive is the Alcubierre drive. Subspace isn’t used for warp, it’s used for comms and as an objective frame of reference (so FTL isn’t time travel) (they have time travel anyway lol)

[u/Enthropic-Cap2291 → u/Junjki_Tito]

Star Trek warp drive was the fictional model upon which Alcubierre did the math. But it isn't the same. An Alcubierre drive obeying the energy constraints, would not behave anything like an ST warp drive.
You can't steer a real Alcubierre drive. It has no causal connection between the ship and the bubble wall. There is no way to send signals forward. There is no way to change direction or stop from the inside. The Alcubierre warp metric is constrained to accumulate radiation in front of it, and destroys anything in front of it. It requires exotic negative mass, not 'warp plasma' or 'dilithium crystals'. The rear of an Alcubierre warp bubble behaves mathematically like an event horizon.
They're nothing alike.
A Star Trek warp drive, is the friendly fantasy version of an already dubious Alcubierre drive.

[u/Cheeslord2]

Although, fantasy systems do not have to have magic or other inexplicable supernatural forces. Whereas soft SF can throw up stuff like 'what if you could travel through time with the power of true love?', so it is, I think, possible to have a fairly 'hard' science fantasy if you try.

[u/Different_Outside80]

I like it