Reference: Einstein’s 1920 Book
Section VII (Part 1)
The Apparent Incompatibility of the Law of Propagation of Light with the Principle of Relativity
Please see Section VII at the link above.
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Summary
From all observation, the velocity of light depends neither on its color, nor on its direction, nor on the velocity of its source. We may consider it to be constant and absolute in nature.
But, if we apply the theorem of addition employed in classical mechanics, the velocity of light must change when observed from a moving system of reference.
The view based on classical mechanics contradicts the observed fact that the velocity of light is constant. If the speed of light is a universal constant and a natural law, then, according to the principle of relativity, it should be the same for all Galilean coordinate systems irrespective of their motion.
The theoretical investigations of H. A. Lorentz on the electrodynamical and optical phenomena connected with moving bodies show that the law of the constancy of the velocity of light in vacuo is a necessary consequence.
When we consider both the law of propagation of light, and the principle of relativity to be correct, we arrive at the special theory of relativity.
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Comments
The order of magnitude for the velocity of light is so different from the velocity of material bodies that the sum of the two velocities would not be appreciably different from the velocity of light.
One may also argue that the changes in the velocity of light due to differences in colors, directions and the velocity of its source could similarly be imperceptible.
The theoretical research in electrodynamics and optics simply relates the velocity of light to electrical constants related to space, and we do not understand the nature of space.
It is true that the special theory of relativity improves upon the law of addition of the classical theory, because it takes the finite velocity of light into account. But, it is not totally satisfactory because its assumptions are not completely explained.
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