Sorry if this is a dupe: An astronaut is in a windowless, sensorless starship that is drifting in an unknown location in space. There is an accelerometer aboard which reads zero. The astronaut turns on his ideal linear propulsion engine for one second, losing a negligible amount of propellant mass, and reads the maximum value on his accelerometer before it returns to zero. The astronaut then goes to sleep for an arbitrary amount of time. Any unknown forces or fields may affect his vehicle's speed and position on any axis during that time, any of which may or may not affect his accelerometer, but he does not experience any of them while asleep. When he wakes up, he sees that his accelerometer reads zero. He repeats the experiment. Disregarding non-ideal effects such as component wear, or physical alterations to his ship, will he always see the same reading on his accelerometer? Thanks!