2017-06-18 09:57:29 UTC
Premise 1: The laws of physics are the same in every reference frame (Einstein's 1905 first postulate, the principle of relativity).
Premise 2: Einstein's 1905 second postulate, the principle of constancy of the speed of light, is a law of physics.
Conclusion: Einstein's 1905 second postulate, the principle of constancy of the speed of light, is a consequence of the first, the principle of relativity.
Needless to say, Einstein did not believe that his second postulate is a consequence of the first - it takes imbecility to believe such nonsense. The argument was meant to provide his silliest sycophants with some clue about the fundamentals of relativity - otherwise the creatures may feel insecure and frustrated. Here are examples of such creatures (some of them don't look so silly and may be just dishonest, not imbeciles - like Einstein):
Professor Raymond Flood (5:05): "A consequence of Einstein's principle of relativity is that the speed of light in a vacuum has the same value in two uniformly moving frames of reference."
Lubos Motl: "The second postulate of special relativity morally follows from the first one..."
Dave Slaven: "Einstein's first postulate seems perfectly reasonable. And his second postulate follows very reasonably from his first. How strange that the consequences will seem so unreasonable."
Leonard Susskind (10:26) : "The principle of relativity is that the laws of physics are the same in every reference frame. That principle existed before Einstein. Einstein added one law of physics - the law of physics is that the speed of light is the speed of light, c. If you combine the two things together - that the laws of physics are the same in every reference frame, and that it's a law of physics that light moves with certain velocity, you come to the conclusion that light must move with the same velocity in every reference frame. Why? Because the principle of relativity says that the laws of physics are the same in every reference frame, and Einstein announced that it is a law of physics that light moves with a certain velocity."
Chad Orzel: "The core idea of Einstein's theory of relativity can fit on a bumper sticker: The Laws Of Physics Do Not Depend On How You're Moving. Absolutely everything else follows from the simple realization that physics must appear exactly the same to person in motion as to a person at rest - the constant speed of light, the slowing of time for moving observers, E=mc2, black holes, even the expanding universe (I've written a whole book about this, explained through imaginary conversations with my dog)."
Michael Fowler: "Therefore, demanding that the laws of physics are the same in all inertial frames implies that the speed of any light wave, measured in any inertial frame, must be 186,300 miles per second. This then is the entire content of the Theory of Special Relativity: the Laws of Physics are the same in any inertial frame, and, in particular, any measurement of the speed of light in any inertial frame will always give 186,300 miles per second."
Vesselin Petkov: "One of the fundamental facts of modern physics is the constancy of the speed of light. Einstein regarded it as one of the two postulates on which special relativity is based. So far, however, little attention has been paid to the status of this postulate when teaching special relativity. It turns out that the constancy of the speed of light is a direct consequence of the relativity principle, not an independent postulate. To see this let us consider the two postulates of special relativity as formulated by Einstein in his 1905 paper "On the electrodynamics of moving bodies": "the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good. We will raise this conjecture (the purport of which will hereafter be called the "Principle of Relativity") to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of the motion of the emitting body". As the principle of relativity states that "the laws of physics are the same in all inertial reference frames" and the constancy of the speed of light means that "the speed of light is the same in all inertial reference frames (regardless of the motion of the source or the observer)" it follow that the second postulate is indeed a consequence of the first - the law describing the propagation of light is the same for all inertial observers."