frame by saying that the “physical equivalence” of the two frames must be precisely defined and this concept cannot be used blindly. In fact the Equivalence Principle holds only in one frame but not in the other. This fact could be used as an argument against the physical equivalence of the frames. However, the fact that the Equivalence Principle holds in a given frame and not in [*all frames*]{} means that it is not a covariant requirement but a kinematic one. In other words, the Equivalence Principle could not be sufficient to discriminate among conformally related frames.

In the scalar-tensor theories of gravity, the energy conditions are easily violated in the Jordan frame, but they are satisfied in the Einstein frame [@MagnanoSokolowski94]. This fact does not eliminate singularities and then the two frames remain equivalent with respect to the singularities and not with respect to the energy conditions. This difficulty arises because part of the matter sector of the theory, in the Einstein frame, comes from the conformal factor; in other words, the conformal transformation mixes matter and geometric degrees of freedom, which is the source of many interpretational problems [@Cap01; @Cap02]. Thus, even if the theory turns out to 