The Equivalence Principle - Implications
In a sufficiently small laboratory, there is no experiment done entirely inside the laboratory which
can tell whether the laboratory is in free fall in a gravitational field or in uniform motion in empty space.
Similarly, there is no experiment which can tell whether the laboratory is at rest in a gravitational field or
being uniformly accelerated in empty space.
To see how light would travel in a gravitational field, imagine it moving through an
Weak Equivalence Principle or Universality of Free Fall --- all objects fall with the same
acceleration regardless of their mass or composition. Easy to test by dropping objects as Galileo did or by watching the motion of pendulums.
Strong Equivalence Principal --- All of the laws of physics are the same in a gravitational field as in an acclerated reference frame. Direct tests of this version of the principle are difficult. However,
physical theories which violate it usually lead to violation of the Weak Equivalence Principal.