•Einstein (1907):“hypothesis
of complete physical equivalence”
In a freely
falling system all masses fall equally fast, hence gravitational
acceleration has no localdynamical effects
Weak Equivalence Principle (WEP)
An observer
inside Einstein elevator will not be able to tell, before
hitting the ground, whether he is moving with an acceleration g in empty space, far away from all masses, or else
he is falling in the vicinity(height of fall<<Earth radius)
of a body (the Earth) whose local gravitational acceleration
is also g(and in the
same direction).