Weighing photons

  title={Weighing photons},
  author={Robert V Pound},
  journal={Classical and Quantum Gravity},
  pages={2303 - 2311}
Albert Einstein pointed out, in 1907 and 1911, that his principle of equivalence would lead to the shift of stellar spectral lines to the red when produced in more massive sources than the Earth. This became one of the `three crucial tests' of his 1917 general theory of relativity. Astronomers searched for it for several decades with rather inconclusive results. A new approach was created by the discovery of recoil-free emission and the absorption of gamma-rays, with widths basically determined… 

Gravitational redshift and the vacuum index of refraction

A physical process of the gravitational redshift was described in an earlier paper (Wilhelm and Dwivedi, New Astron. 31:8, 2014). This process did not require any information for the emitting atom