There is absolute truth in science. Of the following two statements one is absolutely true, the other is false:
(A) The speed of light, as measured by the observer (receiver), varies with the speed of the emitter.
(B) The speed of light, as measured by the observer (receiver), does not vary with the speed of the emitter.
Einstein hesitated between (A), a tenet of Newton's theory, and (B), a tenet of the ether theory, and finally chose (B) as his 1905 second postulate:
Einstein: "I introduced the principle of the constancy of the velocity of light, which I borrowed from H. A. Lorentz's theory of the stationary luminiferous ether..."
https://en.wikipedia.org/wiki/Lorentz_ether_theory
The true statement is (A). The speed of light does vary with the speed of the emitter, as posited by Newton's theory and proved by the Michelson-Morley experiment in 1887 (prior to the introduction of the length-contraction fudge factor):
https://qph.cf2.quoracdn.net/main-qimg-f10f1c25528a4e5edc9bae200640f31c-pjlq
"Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment of 1887...The name most often associated with emission theory is Isaac Newton. In his corpuscular theory Newton visualized light "corpuscles" being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v)."
https://en.wikipedia.org/wiki/Emission_theory
Banesh Hoffmann, Einstein's co-author, admits that, originally ("without recourse to contracting lengths, local time, or Lorentz transformations"), the Michelson-Morley experiment was compatible with Newton's variable speed of light, c'=c±v, and incompatible with the constant speed of light, c'=c:
"Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether." Banesh Hoffmann, Relativity and Its Roots, p.92
https://www.amazon.com/Relativity-Its-Roots-Banesh-Hoffmann/dp/0486406768
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