I have been thinking about Einstein's equivalence principle, wherein acceleration is equated with gravity. The point is: that this is just sleight of hand. Newton's whole theory of gravity is based on the fact of inertial mass. Without this concept it would be impossible to calculate the orbits of planets or the trajectory of comets. There would literally be no way to calculate gravitational effects. It is central to Newton's theory of gravity. The question is this: where did Einstein get off rephrasing this idea and what did he hope to gain from it if not to discredit Newton's theory of gravity? Surely, a simple ploy such as equating gravity and acceleration, creates enough doubt to let the thin edge of general relativity in?
The irony is that the equivalence principle DISPROVES general relativity. According to this principle, as light falls towards a source of gravity, its speed INCREASES as per Newton:
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."
https://courses.physics.illinois.edu/phys419/sp2011/lectures/Lecture13/L13r.html
James Hartle, "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
"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
Einstein's general relativity predicts that, as light falls towards a source of gravity, its speed DECREASES:
"The change in speed of light with change in height is dc/dh=g/c."
View: https://www.youtube.com/watch?v=FJ2SVPahBzg
"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."
http://www.physlink.com/Education/AskExperts/ae13.cfm
"Simply put: Light appears to travel slower near bigger mass (in stronger gravitational fields)."
https://speed-of-light.com/speed_of_light_gravity.html
"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."
https://www.mathpages.com/rr/s6-01/6-01.htm