Final answer:
In a closed system, you cannot distinguish between gravity and acceleration; they are experienced similarly. However, observing external reference points, such as celestial bodies, may help differentiate the two.
Step-by-step explanation:
According to Einstein's principle of equivalence, gravity and acceleration are indistinguishable in a local frame of reference, meaning if you were inside a very large spaceship, without an external reference, you could not easily tell the difference between gravitational pull and acceleration. They have the same effect and are perceived similarly within a closed system. However, if you can observe nearby celestial bodies or other external references, you might be able to discern the spaceship's acceleration through changes in position or velocity relative to those bodies. In contrast, a gravity field would not provide such relative motion. Moreover, Newton's third law entails that for every action, there is an equal and opposite reaction; therefore, the apparent feeling of gravity due to acceleration in the spaceship must be caused by the propulsion system exerting force on the ship. In the case of gravity, this reaction force is due to the curvature of space-time caused by mass.