Richard Feynman: "I want to emphasize that light comes in this form - particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you probably learned something about light behaving like waves. I'm telling you the way it does behave - like particles. You might say that it's just the photomultiplier that detects light as particles, but no, every instrument that has been designed to be sensitive enough to detect weak light has always ended up discovering the same thing: light is made of particles." QED: The Strange Theory of Light and Matter p. 15 https://www.amazon.com/QED-Strange-T.../dp/0691024170

If taken at face value, this text suggests that light cannot stretch or shrink - the idea of a variable wavelength is unreasonable. Then one can define the wavelength as a CONSTANT (for a given emitter) coefficient in the formula

(speed of light) = (wavelength)(frequency)

Hence the fundamental axiom of future, Einstein-free physics:

"For a given emitter, the wavelength of light is constant"

An important corollary:

"ANY frequency shift entails (is caused by) a proportional speed-of-light shift"

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