equivalence principle

The equivalence principle states that locally, the effects of gravity are indistinguishable from acceleration. In other words, being in a sealed, accelerating spaceship feels just like being in a stationary spaceship in a gravitational field. For example, if you're inside an enclosed room, you can't tell if the force you feel is due to gravity or acceleration. This principle is fundamental to Einstein’s theory of general relativity, showing that gravity isn't just a force but related to the curvature of space and time caused by mass and energy.