The sun is far, far away. The earth is far, far away. No stars. Nothing but unbounded spacetime. And three rockets. Rocket #1: Velocity (according to its odometer): 700 mph Towards Earth (constant speed) Rocket #2: Accelerating (according to its odometer) to 800mph towards some galaxy far away from Milky Way Rocket #3: Engines are off. It's "at rest." Since space has no boundaries and thus direction has no (absolute) meaning, I cannot see why being at rest is any different from being accelerated. Rockets #1 and #2 are not really going anywhere, just like rocket #3. This is why proper acceleration is not making any sense. Sure, rocket man in rocket #2 feels the force of the engines, or does he? Can you have acceleration (feel that jerk) without gravity (a gravitational field)? Additionally, if you can feel that jerk in deep, deep space, what is causing you to feel it, while keeping in mind that space(-time) is unbounded and that direction is meaningless? What am I missing? Edit: I guess the one thing I may not be accounting for is that direction here would be one rocketship moving away from the other, but to me, neither is really going anywhere in a significant sense.