"Gravitational redshift" of light
     In 1960, R. Pound and G. Rebka, Jr. at Harvard University conducted experiments in which photons (gamma rays) emitted at the top of a 22.57 m high apparatus were absorbed at the bottom, and photons emitted at the bottom of the apparatus were absorbed at the top. The experiment showed that photons which had been emitted at the top had a higher frequency upon reaching the bottom than the photons which were emitted at the bottom. And photons which were emitted at the bottom had a lower frequency upon reaching the top than the photons emitted at the top. These results are an important part of the experimental evidence supporting general relativity theory which predicts the observed "redshifts" and "blueshifts."
     In the quantum medium view, the frequencies of the photons do not change as they travel between the top and bottom of the apparatus. The photons emitted at the top of the apparatus have a higher frequency than the photons emitted at the bottom. The difference in emission frequencies of the photons is due to the differences in rg and energy exchange rate between the top and bottom of the apparatus, just as in the laboratory of Fig 7. According to Eq. (33), a 22.57 ma change in elevation at the surface of Earth is equivalent to a change in rg of 2.45·10^-15. This agrees with the Pound-Rebka experiment where the measured frequency differences between the top and bottom of the apparatus were 2.45 parts in 10¹⁵.
     Photons moving along the z axis in Fig. 10 and photons moving up or down in the lab of Fig. 7 have small changes in velocity due to the small changes in rg in the qm. A change in cag does not change a photon's frequency and energy because the change in cag is due to a change in wavelength. A photon moving along a path of increasing rg in the qm has an increase in wavelength, but its frequency is constant at every point on the path. (Imagine a photon as a wave, ~, of length, λ, being propagated with velocity cag=.25 ca along a path where rg suddenly changes from .5 to 1. During the λ/.25 second it takes the photon to pass the point of change in rg the front of the wave has a velocity of 1 ca while the rear of the wave has a velocity of .25 ca. This increases the length of the wave by a factor of 4, but the frequency, cag/λ, is not changed.)