Equivalence principle is one of the logical experiments of Einstein. It helps to foundation of General Covariance Principle which is very important in physics. According to equivalence principle, the physics in an elevator falling freely on Earth and being stable in space are not distinguishable. When the observers in elevators drop a ball, ball would be hanged on the same height in the elevator. So the observer in the elevator can not understand where he is and he would not feel any weight.
The second version of this experiment is that a stable elevator on Earth and an upward moving elevator having g acceleration in space. When the ball is being dropped as before experiment, the ball would fall down with g in both elevators.
Consequently, gravitational fields and massless fields are LOCALLY equivalent. Note that this equivalence is broken in the case of big distances.
The next post I will discuss this locality issue.
