Two valid (the premises do entail the conclusion) arguments showing why Einstein had to introduce the idiotic DECREASE in the speed of falling light:

Argument I:

Premise 1: In a gravitational field the speed of falling light INCREASES - photons fall with the same acceleration as ordinary falling bodies (g = 9..8 m/s^2 near Earth's surface), as predicted by Newton's theory.

Premise 2: The formula (frequency)=(speed of light)/(wavelength) is correct.

Conclusion: Gravitational time dilation does not exist - Einstein's general relativity is nonsense.

Argument II:

Premise 1: In a gravitational field the speed of falling light DECREASES - the acceleration of falling photons is NEGATIVE, -2g near Earth's surface.

Premise 2: The formula (frequency)=(speed of light)/(wavelength) is correct.

Conclusion: Gravitational time dilation does exist.

Premise 1 in Argument II is an idiotic fudge factor reconciling the miraculous gravitational time dilation Einstein had fabricated in 1911 and the gravitational redshift predicted by Newton's theory:

"Contrary to intuition, the speed of light (properly defined) decreases as the black hole is approached. [...] If the photon, the 'particle' of light, is thought of as behaving like a massive object, it would indeed be accelerated to higher speeds as it falls toward a black hole. However, the photon has no mass and so behaves in a manner that is not intuitively obvious."x http://www.physlink.com/Education/AskExperts/ae13.cfm

"...you will find in section 3 of that paper Einstein's derivation of the variable speed of light in a gravitational potential, eqn (3). The result is: c'=c0(1+φ/c^2) where φ is the gravitational potential relative to the point where the speed of light c0 is measured. Simply put: Light appears to travel slower in stronger gravitational fields (near bigger mass).. [...] You can find a more sophisticated derivation later by Einstein (1955) from the full theory of general relativity in the weak field approximation. [...] Namely the 1955 approximation shows a variation in km/sec twice as much as first predicted in 1911."x http://www.speed-light.info/speed_of_light_variable.htm

"Thus, as φ becomes increasingly negative (i.e., as the magnitude of the potential increases), the radial "speed of light" c_r defined in terms of the Schwarzschild parameters t and r is reduced to less than the nominal value of c."x https://www.mathpages.com/rr/s6-01/6-01.htm

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