One of the main principals of Special Relativity is that the speed of light is constant for all observers. This concept has been the focus of endless explanations. The primary problem is one of understanding how the speed of light can be constant for different observers moving with respect to each other and still maintaining the speed of light constant in the space between them. The simple answer is that it cannot.
The S-S concept gives a very simple answer to the question as to why the speed of light is constant for all observers. Since space is sinked and sourced by protons and electrons, space is intimately connected to each particle and the space immediately surrounding each particle must move with the particle. Any conglomerate of basic particles bound together will also be firmly attached to the space surrounding it since this is so for each particle making up the conglomerate.
Several basic configurations present themselves for analysis:
The first configuration is one where a single electron-proton pair is travelling alone through space. Being alone and with the space immediately surrounding the pair firmly attached to them, the pair has no indication that it is moving since its only frame of reference is itself. Even uniform rotation is meaningless. An accellerated translational or rotational motion could be detected since there would be a time delay between the application of accelleration and the motion of the spatial field loosely surrounding the pair, but there would be no way to effect the accelleration without the intervention of other bodies or fields.
The second configuration is one in which two electron-proton pairs (or aggregate of pairs) are moving with respect to each other but separated by a great distance. Here, each body represents an independant frame of reference with the two frames of reference moving with respect to each other. Without a light signal (or other electromagnetic radiation) passing from one body to the other, each body will act the same as in configuration 1. However,with a signal being transmitted from one toward the other, the receiving body would be able to determine the existance of the transmitting body as well as infer relative motion between them. The transmitting body could not. Uniform rotational motion of the receiving body could be interpreted several ways. It could interpret the relative motion as one of self rotation with the source fixed, as one of motion of the source with respect to the receiver which is fixed or as a combination of these motions. There would be no way of knowing which was correct without additional information. In fact, any choice would be correct.
The speed of light in the space between the two pairs is actually meaningless since there is no instrumentation to measure it. If there were, the measuring equipment would establish its own frame of reference and measure the speed of light constant. The actual speed of light in empty space should be constant with respect to the average motion of local space.
The third configuration to be considered is the case where a small body is moving within the influence of a much larger body. Here, the basic frame of reference is established by the larger body. The smaller body would be moving with respect to the velocity of propegation of the spatial field of the larger body and, as far as the larger body were concerned, the measured mass of the smaller body would change as the relative velocity changes.