In The Expanse ships use linear acceleration for gravity with the decks placed perpendicular to the thrust vector, thus creating a "floor."
When it comes time to decelerate a ship would need to turn 180° and begin burning in the opposite direction. Is the ship's "floor" now a "ceiling?"
I have tried to think through this and can see it both ways. The "floor" is still the side with the thrust pushing against it, but the motion is now in reverse. Is that right?
In case a ship were to simply stop accelerating, the content of the ship would not experience any force anymore and become weightless, i.e. float around.
Should a ship need to do an emergency brake and have negative acceleration, as you asked, the ceiling would force wise become the floor and people would fall there.
However, the standard maneuver is to turn the ship around and then decelerating the same way you accelerate, i.e. in direction of the main thruster. This can be seen here:
The same way, you can see it at the end, when the fleet of Marco Inaros slows down to enter the ring:
The ships main thruster are directed to the ring in order to slow down enough before entering.
So, yes, the floor stays the floor and is simply defined as where you stand on and has no specific direction in the X-Y-Z coordinate system.
Think about suddenly accelerating in a car or an airplane. You're pressed back against the seat, right? Now imagine you put the car in reverse and accelerate. You're still being pushed "backwards" (against the direction of acceleration), but because the vehicle is facing the other way, instead of being pushed into your seat, you'd be pushed away from it.
Now imagine you accelerate while driving forward, spin the car 180, and then accelerate in the opposite direction from before. Both times, you'd be pressed against the seat. You wouldn't be thrown against the windshield, like you would if you'd put the car in reverse instead of spinning it.
The way ships work in The Expanse is to have a big engine at one end of a ship. Because of :handwavy future technology:, they're able to pretty much indefinitely accelerate. That means that to get where they're going, they can accelerate (relative to their destination) to the halfway point, which gives them artificial gravity in the opposite direction of the thrust. (I.e., the ship feels like a tower, with the engine in the basement and the top floor in the nose.) At the halfway point, they stop accelerating and rotate the ship 180; they're still moving in the same direction because they have (a lot of) inertia, but because they're not thrusting, there's no perceived gravity. Then, with the engine pointed at their destination, they start accelerating again, which means they're decelerating relative to the destination while accelerating away from it. This again gives them artificial gravity in the opposite direction of their actual acceleration.
It doesn't matter which way they're traveling, just which way they're accelerating. So by having engines only on one end of the ship, and building the ship like a tower with that engine at the "bottom," they have "gravity" anytime they're under thrust, and the floor will always be the floor and the ceiling will always be the ceiling, because that thrust will always be in the same direction, regardless of which way the ship is traveling. So they build up a bunch of speed going towards their destination, and then slow down in the second half of the trip by accelerating in the opposite direction, which gradually sheds the speed they'd built up in the first half.
The feel of gravity on these ships is all about the push of acceleration from the engine. The speed and direction of travel of the ship is never felt on the ship.
The only thing felt is the push of this engine and that direction of that engine always feels like “down”.
This is known as the Equivalence Principle from Einstein's Theories.
