On the interpretation of the red shift in a static gravitational field, L.B. Okun, K.G. Selivanov and V.L. Telegdi, Am. J. Phys. 68(2), February 2000
The classical phenomena of the redshift of light in a static gravitational potential, usually called the gravitational redshift, is described in the literature essentially in two ways: On the one hand, the phenomena is explained through the behavior of clocks which run faster the higher they are located in the potential, whereas the energy and frequency of the propagating photon do not change with height. The light thus appears to be redshifted relative to the frequency of the clock. On the other hand, the phenomena is alternately discussed (even in some authoritative texts) in terms of an energy loss of a photon as it overcomes the gravitational attraction of the massive body. This second approach operates with notions such as the "gravitational mass" or the "potential energy" of a photon and we assert that it is misleading. We do claim to present any original ideas or to give a comprehensive review of the subject, our goal being essentially a pedagogical one.