I note there are many question about gravitational redshift and I realize that my question might be buried in one of those but I can’t see it.
I am thinking about two different processes in relation to gravitational redshift:
The photon on its way out from the Sun’s gravity field is experience gravitational redshift
The atom is subject to gravity induced effects which slows down the atom’s processes implying that the photon is red-shifted already at emission.
Are both correct and valid ?
I read in http://www.einstein-online.info/spotlights/redshift_white_dwarfs that: A combination of Newtonian gravity, a particle theory of light, and the weak equivalence principle (gravitating mass equals inertial mass) suffices. It is, therefore, perhaps best regarded as a test of that principle rather than as a test of general relativity.
My follow-up question is: Are both examples of the principle of equivalence or is the second version different from the first (and perhaps in need of the full general theory of relativity for its explanation)?