Stephen Hawking teaches Einstein's idiotic syllogism (the laws of physics are the same in every inertial frame; the speed of light is a law of physics; therefore, the speed of light is the same in every inertial frame):

Stephen Hawking, A Brief History of Time, Chapter 2: "The fundamental postulate of the theory of relativity, as it was called, was that the laws of science should be the same for all freely moving observers, no matter what their speed. This was true for Newton’s laws of motion, but now the idea was extended to include Maxwell's theory and the speed of light: all observers should measure the same speed of light, no matter how fast they are moving." http://index-of.co.uk/Science/Stephe...0Of%20Time.pdf

Richard Feynman is more prudent and teaches an implication (variable speed of light contradicts the principle of relativity):

Richard Feynman: "Suppose we are riding in a car that is going at a speed u, and light from the rear is going past the car with speed c. Differentiating the first equation in (15.2) gives dx'/dt=dx/dt-u, which means that according to the Galilean transformation the apparent speed of the passing light, as we measure it in the car, should not be c but should be c-u. For instance, if the car is going 100,000 mi/sec, and the light is going 186,000 mi/sec, then apparently the light going past the car should go 86,000 mi/sec. In any case, by measuring the speed of the light going past the car (if the Galilean transformation is correct for light), one could determine the speed of the car. A number of experiments based on this general idea were performed to determine the velocity of the earth, but they all failed - they gave no velocity at all." http://www.feynmanlectures.caltech.edu/I_15.html

The original "argument":

Albert Einstein, On the Principle of Relativity: "After all, when a beam of light travels with a stated velocity relative to one observer, then - so it seems - a second observer who is himself traveling in the direction of the propagation of the light beam should find the light beam propagating at a lesser velocity than the first observer does. If this were really true, then the law of light propagation in vacuum would not be the same for two observers who are in relative, uniform motion to each other - in contradiction to the principle of relativity stated above." https://einsteinpapers.press..prince.../vol6-trans/16

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