Tim Maudlin: "To begin with, we will focus on just a single physical phenomenon that can be checked experimentally. The phenomenon is that the trajectory of light in a vacuum is independent of the physical state of its source. In particular, suppose there are two flashbulbs in very rapid relative motion, moving past each other. As the flashbulbs pass each other, they both go off. It is an empirically verifiable fact that the light from the two flashbulbs will reach an observer anywhere in the universe exactly together. So the trajectories of the light, the paths the light follows, do not depend on what the source was doing when the light was emitted. There are other aspects of the light from the two flashbulbs that can be different. For example, observers will typically record the light as having a different color or wavelength or frequency, depending on which bulb it came from. But nonetheless the two rays of light will arrive at the observer together. This is clearly a necessary condition if we are to maintain that "the speed of light is constant": if one light ray could outrun or overtake another, then their speeds could not be considered to be the same."
Tim Maudlin,
You could replace the lightbulbs with sound sources - the conclusion would be the same: the sound waves would reach the observer together but the wavelength and frequency would be different depending on which source the waves come from. The physical mechanism is explained in textbooks hence a crucial question:
Is the physical mechanism that makes the wavelength and frequency vary with the speed of the wave source the same for sound and light waves?
If your answer is no, you will have to explain, in physical terms, how the wavelength and frequency of LIGHT vary with the speed of the light source.
If your answer is yes, you will have to continue the analogy and face the awful fact that, for both sound and light, the speed of the waves relative to the observer varies with the speed of the observer.
Pentcho Valev
