Take the tour ×  I understand the equivalence principle as "The
physics in a freely-falling small laboratory is that of special
relativity (SR)." But I'm not quite sure why this is equivalent
to "One cannot tell whether a laboratory on Earth is not
actually in a rocket accelerating at 1 g".  share|improve this
question add comment  4 Answers  Say there were two identical
rooms with no windows, one on earth and one on a spaceship
accelerating at 9.8 m/s^2 (acceleration due to gravity on
earth.) You are in one of them, and can conduct any mechanical
experiment you like as long as it does not involve peeking
outside of the room in any way shape or form.  Unfortunately no
matter what you try you will not be able to determine which room
your in because experiments in both rooms will yield identical
results.  However one room is experiencing gravitational
acceleration and the other inertial acceleration. The principle
of equivalence tells us that these two things are equal, or as
Einstein said "The outcome of any l