Einsteinians compelled to stick to Newton's emission theory of light:

A light source on top of a tower of height h emits light with
frequency f and speed c (relative to the source). The light reaches an
observer on the ground with frequency f' and speed c' (relative to the
observer).

Equivalently, a light source at the front end of an accelerating
rocket of length h and accelaration g emits light with frequency f and
speed c (relative to the source). The light reaches an observer at the
back end with frequency f' and speed c' (relative to the observer).

Consider equations (13.2) on p. 3 in David Morin's text:

http://student.fizika.org/~jsisko/Kn...Morin/CH13.PDF
f' = f(1 + v/c) = f(1 + gh/c^2) (13.2)

where v is the relative speed of the light source (at the moment of
emission) and the observer (at the moment of reception) in the rocket
scenario. By combining these equations with:

(frequency) = (speed of light)/(wavelength)

one obtains the fundamental equations of Newton's emission theory of
light:

c' = c + v = c(1 + gh/c^2)

In the absence of ad hoc auxiliary hypotheses the Pound-Rebka
experiment, just like the Michelson-Morley experiment, unequivocally
confirms the fundamental equations of Newton's emission theory of
light and refutes the principle of constancy of the speed of light:

http://student.fizika.org/~jsisko/Kn...Morin/CH13.PDF
David Morin (p. 4): "This GR time-dilation effect was first measured
at Harvard by Pound and Rebka in 1960. They sent gamma rays up a 20m
tower and measured the redshift (that is, the decrease in frequency)
at the top. This was a notable feat indeed, considering that they were
able to measure a frequency shift of gh/c^2 (which is only a few parts
in 10^15) to within 1% accuracy."

David Morin's text referred to above reappears as Chapter 14 in:

http://www.people.fas.harvard.edu/~djmorin/book.html
Introduction to Classical Mechanics With Problems and Solutions, David
Morin, Cambridge University Press

Pentcho Valev