The Equivalence Principle

The Equivalence Principle states that the effects of gravity are locally indistinguishable from acceleration. In simple terms, if you're inside a sealed space—like a spacecraft—you can't tell whether the force you feel is gravity pulling you down or the ship accelerating upwards. This principle reveals that gravity can be thought of as a curvature of space-time, not just a force, and it forms the foundation of Einstein's theory of general relativity. Essentially, being in a gravitational field is equivalent, locally, to being in an accelerating frame of reference.