When you hear for the first time the idea that the speed of light is the same in any direction for any observer, regardless his state of motion wrt the source, it sounds as if you were being told that light magically adjusts itself to tease you. Today I pass by a guy who emits a beam of light. I am riding on a monkey: I measure c. Tomorrow I pass by riding on the fastest vehicle in the universe: I still measure exactly c. You think: obviously, the measurement conditions are different, why should I obtain the same result? If I did change the measurement frame, does it mean that light also reacted to change its behaviour so as to fool me? As obviously that cannot be, I’ve given myself, based on my current understanding of SR, this explanation: what I measure is not necessarily what light “does” or what “happens”. What I measure is what the instruments of my frame (held by myself and my lattice of clocks and rulers at rest with me) do, while light passes by. Due to a complex compensation of effects, I always measure c for light speed, but that does not mean that the same thing has “happened” on each occasion: the proof is that each day the frequency of the light beam is different, so my efforts have not been totally in vain. Actually, the reasoning is similar to the classical one, it’s just that the actors have changed: - In classical physics it was thought that a clock would give off the same reading in any frame; certainly, the “ticker” of my clock was not doing the same thing as the ticker of your clock (there is relative motion between the two of us), but since the “counter” of each clock did a different thing as well in the same proportion, a fortunate compensation of effects arose, we all got the same readings and this enabled different frames to transform their measurements leaning on such common element. (The same could be said of rulers.) However, the fact that our respective tickers do different things would still be apparent: if your ticker escapes your clock and hits my nose or your nose, we will feel different impacts (energy, isn’t it?). - SR stems from the observation that nature works differently. Clocks do not work the same way in any frame (just as rulers). But a similar compensation of effects still takes place in favour of light speed, which thus takes the role formerly held by homogeneous time (and length). However, the fact is that light is not really behaving wrt me as wrt you and that is why, if light escapes out of your clock, I will measure the same speed but different frequency (energy, isn’t it?). Well, is this more or less on track? Many mistakes…?