Einstein's relativity and Newton's emission theory of light give different predictions for the variation of the speed of light in a gravitational field. Which prediction is compatible with the Pound-Rebka experiment? Let us see:
The top of a tower of height h shoots a bullet downwards with initial speed u. As the bullet reaches the ground, its speed (relative to the ground) is:
u' = u(1 + gh/u^2)
According to Newton's emission theory of light, light falls with the same acceleration as ordinary falling bodies. Therefore, if the top of the tower emits a light pulse downwards, this pulse will reach the ground with speed (relative to the ground):
c' = c(1 + gh/c^2)
The frequency an observer on the ground will measure is:
f' = c'/λ = f(1 + gh/c^2)
where λ is the wavelength and f=c/λ is the initial frequency (as measured at the top of the tower). This frequency shift (predicted by Newton's emission theory of light) has been confirmed by the Pound-Rebka experiment:
http://www.einstein-online.info/spotlights/redshift_white_dwarfs
Albert Einstein Institute: "One of the three classical tests for general relativity is the gravitational redshift of light or other forms of electromagnetic radiation. However, in contrast to the other two tests - the gravitational deflection of light and the relativistic perihelion shift -, you do not need general relativity to derive the correct prediction for the gravitational redshift. A combination of Newtonian gravity, a particle theory of light, and the weak equivalence principle (gravitating mass equals inertial mass) suffices. (...) The gravitational redshift was first measured on earth in 1960-65 by Pound, Rebka, and Snider at Harvard University..."
http://courses.physics.illinois.edu/phys419/sp2013/Lectures/l13.pdf
University of Illinois at Urbana-Champaign: "Consider a falling object. ITS SPEED INCREASES AS IT IS FALLING. Hence, if we were to associate a frequency with that object the frequency should increase accordingly as it falls to earth. Because of the equivalence between gravitational and inertial mass, WE SHOULD OBSERVE THE SAME EFFECT FOR LIGHT. So lets shine a light beam from the top of a very tall building. If we can measure the frequency shift as the light beam descends the building, we should be able to discern how gravity affects a falling light beam. This was done by Pound and Rebka in 1960. They shone a light from the top of the Jefferson tower at Harvard and measured the frequency shift. The frequency shift was tiny but in agreement with the theoretical prediction. Consider a light beam that is travelling away from a gravitational field. Its frequency should shift to lower values. This is known as the gravitational red shift of light."
Now Einsteinians are to show that the variation of the speed of falling light predicted by Einstein's relativity is compatible with the Pound-Rebka experiment. Needless to say, the first thing to do is to give an explicit and quantitative description of this variation. How does the speed of falling light vary, Einsteinians? This question always paralyses Einsteinians' thinking:
http://ebooks.adelaide.edu.au/o/orwell/george/o79n/chapter2.9.html
"Crimestop means the faculty of stopping short, as though by instinct, at the threshold of any dangerous thought. It includes the power of not grasping analogies, of failing to perceive logical errors, of misunderstanding the simplest arguments if they are inimical to Ingsoc, and of being bored or repelled by any train of thought which is capable of leading in a heretical direction. Crimestop, in short, means protective stupidity."
Pentcho Valev