It feels like a natural term that might arise. It's not literally gravity but it feels like gravity so people call it gravity. In the same way we casually say "zero gravity" for situations where there is gravity but you can't feel it.
this post was submitted on 25 Aug 2026
-6 points (20.0% liked)
The Expanse
945 readers
4 users here now
founded 3 years ago
MODERATORS
It'd make more sense of it was just in-universe jargon used in dialogue, but it's also used in narration, which doesn't feel like it fits with the rest of the scientific focus, at least not to me.
For accelerating reference frames, you need to move past newtons laws and use general relativity. In that framework, an experimenter enclosed in a box cannot distinguish between acceleration and gravity except in detecting very minute lack of parralelism due to the radial direction of gravity.
https://www.einstein-online.info/en/spotlight/equivalence_principle/
I personally would not get hung up on that term.
Sure, it feels similar, but it still doesn't seem appropriate to conflate the two. If someone said airplanes use "anti gravity" to fly, that'd blatantly incorrect, no? It doesn't help that the series focuses on actual gravity and it's consequences constantly, which just shines a spotlight in this particular incongruence (for me, at least).
Not just feels similar; any experiment conducted would yield identical results assuming the source of thrust is perfectly constant without variations.
But rather than continuing to discuss the ways in which you dislike "thrust gravity," is there a term you would find preferable instead?
Moderately misleading terms is pretty common in real science and technology though. For example, particle spin has nothing to do with actual rotation and instead just has properties that is similar to objects with rotation. So getting obsessively hung up on semantics would be a very frustrating activity even in actual scientific literature.
That said, it keeps bugging me when the term “thrust gravity” is always used to describe something that’s explicitly Newton’s 3rd law - ie, not gravity.
Does it bug you when someone says "almond milk"?
Because it's literally the same thing.
"Almonds imitating milk" and "thrust operating gravity".
I have a bad habit of back rationalizing science fiction, but if they just say "thrust gravity" conversationally, then it is 100% realistic even outside of science to be a shorthand and not literal. I never did read the books, so I might be wrong.
The series talks about three different types of "gravity", being down the well, which is being on a planet and experiencing mass attracting mass, spin gravity, which is created using the Coriolis effect of being stuck to the inside surface of a spinning project, and thrust gravity, being under acceleration of a ship.
For all practice purposes, thrust and well gravity are identical, aside from hearing the engines, you would not be able to tell the difference between 1g on earth and a ship under 1g of acceleration in a straight line. The whole point of the Epstein drive in the show is "We have such a powerful engine we can just keep accelerating", get places very fast, and live with a defined up and down.
Spin gravity is distinctly different as the rotation is detectable, and depending on the radius of the ship / station / rock, can cause a dizzying effect. Belters like to distinguish themselves because the are generally immune to effects of spin gravity.
All of these effects are quite real, and in my opinion the series does a great job of being scientifically accurate.
'gravity-like acceleration due to thrust' 'thrust gravity'
Works for me, especially when you actually have a need to distinguish the source of the acceleration
need to distinguish the source of the acceleration
Sure, and that's definitely helpful, but I'd be more satisfied by "thrust [other word]".
What other word would you use for a gravity like effect, that works to describe how it affect the people on the ship?
The ship experiences accele forward, and the occupants inside experience that as a gravity like effect.
The ship experiences accele forward, and the occupants inside experience that as a gravity like effect.
Right - Newton's 3rd law. As I mentioned in my side note, I don't know if a singular word exists for that. "Action-reaction" is a term I've seen, but that's pretty clunky. Afaik, a new word would have to be coined to express that interaction. If I had to suggest something, my off-the-cuff, "probably wrong in even more ways" idea would be "pronertia".
not a bad word, problem is i hear that, and for some reason i think potential energy, not a gravity like effect! How about gravnertia? Inertia which causes a gravity like effect
Yeah, but the whole point is that the effect felt is the 3rd law in action - one thing pushes against another thing, and that thing pushes back. While gravity can cause that, it's not gravity itself, right? If I catch a ball that's in motion, it's the force I apply to it that stops it, not gravity. Same thing for ships in thrust, or spinning stations. It's all 3rd law in action, not just different versions of gravity.
Yea, but language is about communication, not absolutely precision.
This isn't a scientific paper, this characters and naritive trying to express ideas, and there clarity of intent and idea is more important than scientific rigour.
The term I'm aiming for is for the concept of Newton's 3rd law, not the book.
It's likely for the reader's sake more than anything. A person in The Expanse can experience two types of pulling downward. Actual gravity on a planet/moon, or during a burn. I would agree that they probably wouldn't say "thrust gravity" usually because of context (they know they're on a ship), but technically "gravity" alone would be incorrect, even though in its use it's realized it's referring to the force from the thrust and not a gravity field. A new word might even be created to talk about ship "gravity".
Most likely, though that does feel pretty patronizing if true. A new word would've felt better, imo. Or even something like "thrust g's", reminiscent of modern jet planes of today.
You know what the "g" stands for right? XD
Gravity is the effect. You can use thrust, spin, or a gravity well to create it.
You are wrong to assume gravity can only mean one of those.
Force is the effect, gravity is the cause. Gravity, thrust, and spin accelerate us or other objects and it's our interaction other objects that we feel. So "thrust gravity" is taking two concepts that work similarly and mashing them together. Like, if someone called an electric stove "electric gas", that is just the two things used to create heat, but nothing about the heat itself. It's a misuse of the terms.