It is an excepted fact that speed of light is constant in vacuum. It's it correct? I find following factors are ignored. In physics the word speed is rarely mentioned accept in special relativity. The usual word is velocity. Velocity is speed with direction. Speed of a car is mentioned when the car travels in an irregular route, dividing the distance traveled from the time taken. Speed is a loose concept, most physics equations have velocity.
It is an accepted fact that speed of light varies with the medium. It is slower in air than in vacuum, then much slower in water. According to quantum theory vacuum is not wholly void. Dark matter and dark energy can also be present. To use in an universal theory like special relativity, it should be more constant.
So far all accurate measurement of speed of light was done by measuring light from a fixed source and a fixed observer, with no displacement between the source and observer. Michelson interferometer is used to prove nonexistence of a medium to transport electromagnetic waves. It also proves that light had a fixed velocity when the source and observer are fixed, but both moves in unison.
It is a fundamental mistake to jump into the conclusion that speed of light is constant everywhere. For instance light coming from the sun when it moves towards us may be faster than when it moves out. These need to be checked before coming to a conclusion.
Red shift of stars may be explained by time dilation and length contraction, but a simpler explanation may be variance of velocity of light. (It may not be exotic, it may be mundane, and it may be correct)
In explanations of special relativity observer and source of light are very loosely defined. For instance in the explanation of lightning striking at two ends of the train, the source can be fixed to the platform or may be moving with the train. These two factors are irrelevant to the outcome of the thought experiment.
Further the observer on the platform sees the two lighting strikes are simultaneous, but for the observer on the train the lightning in front occurs before the lightning the rear. This can be explained by light from front travels faster than the light from rear. Special relativity uses time dilation and length contraction to explain this. But is it necessary? There can't be a correlation between the two observers because each observer sees different set of photons in there respective observations.
Velocity of light may be constant with respect of the source, irrespective of it's movement. But in special relativity it is loosely defined, that it is constant both to source and observer irrespective to the movement between the observer and source. The reason that it will give contradictions is, it is possible to have multiple observers that move relative to each other and observing the same source(say different persons who move about each other looking at the sun ). Then how can it be speed of light is constant between each pair?
It may be difficult but not impossible to measure velocity of light that come from moving stars. This measurement should be pure. In this measurement you cannot use mirrors, because the source can shift to the mirror. A photon is emitted when electron changes it's obit. This photon is related to the atom that emits it. We need to see velocity of this photon by a moving observer. If the photon is disturbed in any way the source might shift, therefore we need to be careful in this measurement. I feel most of the measurements done up to now has ignored this factor.
