"However, it was two years after special relativity, in 1907, that Einstein had what he'd later refer to as his happiest thought: the equivalence principle."
https://bigthink.com/starts-with-a-bang/einstein-shift/
The equivalence principle proves Newton's variable speed of light and, accordingly, disproves gravitational time dilation:
James Hartle, Gravity: An Introduction to Einstein's General Relativity, p. 113: "If we accept the equivalence principle, we must also accept that light falls in a gravitational field with the same acceleration as material bodies."
https://www.amazon.com/Gravity-Introduction-Einsteins-General-Relativity/dp/0805386629
Paul A. Tipler, Ralph A. Llewellyn, Modern Physics: "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
Max Planck Institute for Gravitational Physics : "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..."
http://www.einstein-online.info/spotlights/redshift_white_dwarfs.html
See more here:
https://twitter.com/pentcho_valev
Pentcho Valev