According to this principle, the relativistic changes occurring to the bodies, after velocity changes, cannot be detected by observers moving with them because bodies and stationary radiations change in identical proportion after identical circumstances, i.e, because bodies and stationary radiations have identical relativistic laws with respect to any fixed observer. Effectively the theoretical properties of particle models made up of stationary radiations agree with special relativity, quantum mechanics and the gravitational (G) tests. They fix lineal properties for all of them: the G fields, the black holes (BHs) and the universe. The BHs, after absorbing radiation, must return to the gas state. An eventual universe expansion cannot change any relative distance because the G expansion of matter occurs in identical proportion. This fixes a new kind of universe. In it matter evolves in closed cycles, between gas and BH states and vice versa, indefinitely. Galaxies and clusters must evolve rather cyclically between luminous and black states. Most of the G potential energy of a matter cycle must be released around neutron star and black hole boundaries. Nuclear stripping reactions would transform G energy into nuclear and kinetic energies. This accounts for many non well explained phenomena in astrophysics. This work has been published, in more detail, in a book.