On the gravitational redshift

• Gravitational redshift is still an important subject in modern physics.
• There is no consensus on the physical process(es) causing the shift.
• Solution is formulated in analogy with Fermi’s treatment of the Doppler effect.
• Physical processes with conservation of energy and momentum result in observed shift.
• Gravitational field affects the release of photon and not atomic transition.

The study of the gravitational redshift—a relative wavelength increase of ≈2×10-6≈2×10-6 was predicted for solar radiation by Einstein in 1908—is still an important subject in modern physics. In a dispute whether or not atom interferometry experiments can be employed for gravitational redshift measurements, two research teams have recently disagreed on the physical cause of the shift. Regardless of any discussion on the interferometer aspect—we find that both groups of authors miss the important point that the ratio of gravitational to the electrostatic forces is generally very small. For instance, the ratio of the gravitational force acting on an electron in a hydrogen atom situated in the Sun’s photosphere to the electrostatic force between the proton and the electron in such an atom is approximately 3×10-213×10-21. A comparison of this ratio with the predicted and observed solar redshift indicates a discrepancy of many orders of magnitude. With Einstein’s early assumption that the frequencies of spectral lines depend only on the generating ions themselves as starting point, we show that a solution can be formulated based on a two-step process in analogy with Fermi’s treatment of the Doppler effect. It provides a sequence of physical processes in line with the conservation of energy and momentum resulting in the observed shift and does not employ a geometric description. The gravitational field affects the release of the photon and not the atomic transition. The control parameter is the speed of light. The atomic emission is then contrasted with the gravitational redshift of matter–antimatter annihilation events.