  Does a physical field exist independently of the interaction between the
field source and the test body used to measure it at a given point? What does
propagate between two physical bodies when they interact? These are the
fundamental questions we answer in the present work for electromagnetic and
gravitational interactions. We come to the conclusion that gravitational fields
as well as spacetime curvature cannot depend on the passive-gravitational to
inertial mass ratio of the test bodies, because this would lead to energy
conservation and causality violation except when this ratio is universally
exactly equal to one. For what concerns the hypothetical dependence of
electromagnetic fields on the charge to inertial-mass ratio of test bodies,
this would call for the propagation of an operationally ill defined quantity
(Coulomb . Meter$^2$ / Second$^2$) that would replace energy (Joule = Kilogram
. Meter$^2$ / Second$^2$) transfer in the interaction between two electrically
charged bodies. Hence electromagnetic fields cannot depend on the charge to
inertial-mass ratio of test bodies. A consequence of these results is also the
disqualification of Murat Ozer's unification theory of gravitation and
electromagnetism.
