  A physical process of the gravitational redshift was described in an earlier
paper (Wilhelm & Dwivedi 2014) that did not require any information for the
emitting atom neither on the local gravitational potential U nor on the speed
of light c. Although it could be shown that the correct energy shift of the
emitted photon resulted from energy and momentum conservation principles and
the speed of light at the emission site, it was not obvious how this speed is
controlled by the gravitational potential. The aim of this paper is to describe
a physical process that can accomplish this control. We determine the local
speed of light c by deducing a gravitational index of refraction nG as a
function of the potential U assuming a specific aether model, in which photons
propagate as solitons. Even though an atom cannot locally sense the
gravitational potential U (cf. Muller et al. 2010), the gravitational redshift
will nevertheless be determined by U (cf. Wolf et al. 2010)- mediated by the
local speed of light c.
