SPOILER ALERT: This question will refer to, and may have spoilers, regarding Arthur C Clarke's Rendezvous with Rama. Read below at your own peril. BTW the book is totally worth reading. In the book, the astronauts basically encounter an immensely large cylinder, rotating along its long axis at high speeds. Once inside, they describe how along the long axis ("center of the cylinder") there is no sense of gravitational pull, but along the inside shell of the cylinder, a pull is felt. I believe I understand the concept here. Any force that will "pull" someone "down" (or into the ground) can be compared to a gravitational force. In this particular case, Rama does not distort spacetime and does not create gravitational forces, but the passenger's inertia against the rotation shell simulates gravitation (centrifugal force, though some object to this term). Now, Rama has an atmosphere. In another part of the book, one of the astronauts "bikes" with a flying bike from one axis to another, and experiences gravitational force of different intensity, depending on the height of the flying trajectory (perpendicular distance from the rotating shell, alternatively the perpendicular distance from the axis). If traveling along the axis, no pull whatsoever is felt. What is exerting the force on this biking astronaut? Is it Rama's atmosphere? The astronaut, when flying/biking, does not have any contact with the spinning shell. In other terms, if Rama's atmosphere was evacuated, and one decided to push themselves from one end-axis to the other, would they fell a pull as well if they deviated from the main central axis?