The trouble with it is the word 'equivalence'. Einstein used the term because inertial and gravitational masses could be different properties. It looks very much like they are not equivalent but the same.
This is not a problem for GR as its built in as a core principle. Gravitational Mass ===...
Ah! so as light travels through the 'vacuum' of space it will be impinge upon these virtual particles and loose energy to them. Any two photons will loose almost exactly the same amount of energy over large distances. This will appear as red shift. As the photons wavelengths grow the volume of...