Gary,
"The measured Light is not the emitted light. That is the fallacy of SR."
Beg pardon? Your question as to how we can know the velocity of light from the velocity of the emitter was spot on. We can know the velocity of light that we measure, which always turns out to be 'c'. It doesn't matter if we could know a priori what the velocity of the emitter is.
The ad hominem attacks against a reputedly mischievous, long-haired, leaping gnome, typically argue that the questions he raised are 'proof' of SR being a fallacy. What Einstein despaired of was that in conclusion of SR, there is the outstanding question that given the axiom of all motion is relative, how can it physically be that light velocity taken as a universal absolute value results in that universal relative motion displaying an absolute vector? That's the neat part, not the 'flaw'. Why is that? what can it tell us? And why say, 'see! it's wrong!'
We can know the velocity of a detector from the difference in frequency, we can know the velocity of the emitter from the Doppler shift of emission or absorption lines. Because independent of either, the velocity of the physical form of energy we call 'light' is a universal constant absolute velocity.
Let's take the persistent Hydrogen line, the emission line of incandescing hydrogen which is that beautiful 'electric' blue in a methane gas flame cone. Countless observations have been made of interstellar gas clouds emitting a wide range of EMR, and the rotational aspects and the nebulous direction and velocity is calculable from spectroscopic analysis of 'Doppler' effect. Where a gas cloud is generally receding from our point of observation, that line becomes more greenish, the wavelength gets 'stretched out' (Wow!). Where a cloud is generally approaching us that line becomes more indigo-ish, the wavelength gets 'scrunched up'. How do the hydrogen atoms 'know' which way they are going and how fast, for the emitted light to do that! (That's a great question, Al.) If we accept all motion as relative, it suggests that motion is registered by the volume of the energy of the atomic mass in relation to its own physical shape, not as a first order effect in relation to others.
A vector is simply a direction and magnitude. We can determine an emitter vector and a detector vector from taking light velocity as the absolute value in protracting measure. Many people just don't like 'the idea' of length contraction or time dilation. jrc
