Physics - Modern Physics - Relativity - General relativity - gravity as curved spacetime - Elevator or Planet?

Einstein's happiest thought experiment. Scene: You're in a closed box with no windows. You feel a force pulling you to the floor. Question: 'Are you standing on Earth, or being accelerated upward in a rocket at 9.8 m/s²?' Options: A) I can tell the difference, B) I cannot tell - they feel identical.