checking explicitly the physical equivalence between the two frames, one has to specify which physical field, or physical process is considered and the equations describing it. The equivalence has to be shown explicitly, but there is no proof that holds in any situation (e.g. scalar fields, spinors, cosmology, black holes derived from the same theory). While physical equivalence can be proven for various physical aspects, no proof comprehensive of all physical fields and different physical applications exists.

It is important to stress that Dicke seems to mix the concept of dimensional units and the concept of measuring units. For example, spatial distance has the dimensional unit length $L$ but the measuring unit length, as we already know from Special Relativity, has to be defined in a relativistic inertial frame (RIF). After, by some rule, spatial lengths are compared in different RIFs. One of such rules is the Einstein one. It can be defined by means of light signals which are simultaneous in the RIF in which the measurement is taking place. This is the so called [*rest length*]{} (which, by assumption, coincides with the corresponding Euclidian length measured by a Newtonian inertial observer). This type of measurement has nothing to 