Weighing Photons, II

After completion of the initial experiment confirming the gravitational red shift by an application of the gamma-ray resonance of ⁵⁷Fe as described in "Weighing Photons, I," Physics in Perspective 2 (2000), 224-268. I proceeded to design and carry out a new version with improved components that would allow better control over sources of systematic error. This second article describes that effort, which was carried out between 1960 and 1964 with the collaboration, briefly, of R. W. P. Drever and then of Joseph L. Snider. It led to a result further confirming the anticipated fractional frequency shift of 2.45᎒^⪣, with an uncertainty of less than 1%, for the height of our "tower." Studies of external reflection of X rays and of possibly better isotopes, aiming toward a possible third-round experiment, were carried out with the collaboration of William T. Vetterling.