Two crucial biconditionals (if and only if propositions):

Biconditional 1: The speed of light is constant as per Einstein IF AND ONLY IF the wavelength of light is variable.

Biconditional 2: The speed of light is variable as per Newton IF AND ONLY IF the wavelength of light is constant for a given emitter.

Both biconditionals are immediate logical consequences of the formula

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

Both the antecedent and the consequent in Biconditional 2 are true. The consequent,

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

will become the fundamental axiom of future, Einstein-free physics (the fundamental axiom of present, Einstein's physics is "the speed of light is constant", the antecedent in Biconditional 1).

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