point was that physics must be invariant under a rescaling of units and the conformal transformation is a local rescaling: units do not change rigidly over the entire spacetime manifold, but by amounts which are different at different spacetime points. From Dicke’s point of view, the two frames are equivalent provided that mass, length, time, and quantities derived from them scale with appropriate powers of the conformal factor in the Einstein frame [@Dicke62].

From this point of view, it is not difficult to see why many authors consider the issue of which is the physical frame a pseudo-problem. In principle, it is difficult to object to this argument, but there are some difficulties.

Even though the above argument is clear in principle, its application to practical situations gives rise to problems. The assumption that the two conformal frames are just different *representations* of the same theory, similar to different gauges of a gauge theory, has to be checked explicitly by using the field equations of a given system. Physical equivalence is a vague concept because one can consider many different matter (or test) fields in curved spacetime and different types of physics, or different physical aspects of a problem. When 