Einstein's relativity has a preposterous prediction: The speed of light decreases as light falls towards the source of gravity, and increases as light ascends. Theoretical physicists find this prediction too dangerous and never discuss it. Still they know that absolute secrecy is impossible, and accordingly have devised a paralyzing tool against those who may bump into the preposterous prediction and start investigating it:
"Suppose that you have a clock and a ruler (which is not rotating with respect to stars) and that you are not accelerating (inertial). Locally (where you are) you will always measure the speed of light at 299792.458 km/sec. However in the presence of gravity if I am at a different location than yours then I could measure the speed of light at your location to be any value smaller than or greater than 299792.458 km/sec. It depends on where I am and where you are (it depends on locations). So in the presence of gravity the speed of light becomes relative (variable depending on the reference frame of the observer). This does not mean that photons accelerate or decelerate; this is just gravity causing clocks to run slower and rulers to shrink." https://www.speed-of-light.com/speed_of_light_gravity.html
How can an observer measure the speed of light at another observer's location? Nonsense, but the reader does not know that. He/she finds the topic "variable speed of light in gravity" too complicated and loses any critical power. Now the preposterous prediction is not dangerous:
"Simply put: Light appears to travel slower near bigger mass (in stronger gravitational fields)." https://speed-of-light.com/speed_of_light_gravity.html
"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
"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
The truth is so simple and obvious that some theoretical physicists temporarily lose allegiance to the Einstein Cult and become Newtonians. According to the equivalence principle, as light falls towards a source of gravity, its speed increases as per Newton:
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
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
Paul A. Tipler, Ralph A. Llewellyn, Modern Physics: "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
"Suppose that you have a clock and a ruler (which is not rotating with respect to stars) and that you are not accelerating (inertial). Locally (where you are) you will always measure the speed of light at 299792.458 km/sec. However in the presence of gravity if I am at a different location than yours then I could measure the speed of light at your location to be any value smaller than or greater than 299792.458 km/sec. It depends on where I am and where you are (it depends on locations). So in the presence of gravity the speed of light becomes relative (variable depending on the reference frame of the observer). This does not mean that photons accelerate or decelerate; this is just gravity causing clocks to run slower and rulers to shrink." https://www.speed-of-light.com/speed_of_light_gravity.html
How can an observer measure the speed of light at another observer's location? Nonsense, but the reader does not know that. He/she finds the topic "variable speed of light in gravity" too complicated and loses any critical power. Now the preposterous prediction is not dangerous:
"Simply put: Light appears to travel slower near bigger mass (in stronger gravitational fields)." https://speed-of-light.com/speed_of_light_gravity.html
"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
"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
The truth is so simple and obvious that some theoretical physicists temporarily lose allegiance to the Einstein Cult and become Newtonians. According to the equivalence principle, as light falls towards a source of gravity, its speed increases as per Newton:
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
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
Paul A. Tipler, Ralph A. Llewellyn, Modern Physics: "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