"Much of modern physics is founded on the principle that the speed of light is constant. Yet when Einstein proposed this in 1905, he recognised that no experiment could be performed to distinguish it from Lorentz's earlier theory, in which measuring instruments become distorted when they move through the light medium so that merely the measured speed of light is constant. Since no experiment can distinguish the two approaches, it follows that the theory of special relativity is “not even wrong”." https://warwick.ac.uk/fac/sci/physic...6ddf09a11b0628

No need to compare the two theories - the speed of light is OBVIOUSLY variable:

Stationary light source, moving observer (receiver): http://www.einstein-online.info/imag...ector_blue.gif

The speed of the light pulses as measured by the source is

c = df

where d is the distance between the pulses and f is the frequency measured by the source. The speed of the pulses as measured by the observer is

c'= df' c

where f' f is the frequency measured by the observer.

Insofar as their speed is concerned, photons are Newtonian particles. The speed of light varies, both in the presence and in the absence of gravity, just as does the speed of ordinary projectiles.

Actually this is a well-established truth but no one cares (post-truth science):

"Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment of 1887. [...] The name most often associated with emission theory is Isaac Newton. In his corpuscular theory Newton visualized light "corpuscles" being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v)." https://en.wikipedia.org/wiki/Emission_theory

"To see why a deflection of light would be expected, consider Figure 2-17, which shows a beam of light entering an accelerating compartment. Successive positions of the compartment are shown at equal time intervals. Because the compartment is accelerating, the distance it moves in each time interval increases with time. The path of the beam of light, as observed from inside the compartment, is therefore a parabola. But according to the equivalence principle, there is no way to distinguish between an accelerating compartment and one with uniform velocity in a uniform gravitational field. We conclude, therefore, that A BEAM OF LIGHT WILL ACCELERATE IN A GRAVITATIONAL FIELD AS DO OBJECTS WITH REST MASS. For example, near the surface of Earth light will fall with acceleration 9.8 m/s^2." http://web.pdx.edu/~pmoeck/books/Tipler_Llewellyn.pdf

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