Optical definition of gravity under static conditions

The experiment of Pound & Repka shows that light undergoes a frequency shif t in the gravitational field of the earth in accordance with General Relati vity. Conversely, in the static case, we can use only the observed frequenc y shifts to define the gravitational field, presupposing the (constant) 3-g eometry of the 3-space slices is known. The latter can be probed in princip le by rigid rods, but more elegantly by the light geometry as developed by Abramowicz, shortly reviewed here. Our optical definition is independent of the theory of relativity. However, in the second part, we show that, in th e static case, it coincides with the predictions for the acceleration of te st particles in General Relativity. For the non-static case, our definition of gravity is no substitute for that one given in General Relativity. Howe ver, the static case is sufficient for certain discussions about the validi ty of the Principle of Equivalence.