One of the most risible aspects of the philosophy of Relativity, is its indiscriminate and peculiar use of an ‘Observer’. If you prove that light speed is variant (Sagnac, Michelson-Morley, Dayton Miller etc), the Relativist will simply declare that this is only true in ‘your reference frame’ by ‘your observer’.
If we move the reference frames and ‘observer’ around, then presto, everything is ‘relative’ and light speed is indeed finite. The entire philosophy of Relativity is based on an ‘observer’ viewing something at the absolute and finite speed of light (whatever that might actually be), hitting the retina. Relativity is composed of the following assumed postulates:
Light speed is the only absolute in the universe and its speed is finite (this is wrong and admitted as erroneous by Einstein). Relativity demands that every observer receives light hitting their retina at an absolute speed.
Virtually every idea and formula surrounding Special Relativity is based on ‘what the observer sees’ at this invariant (unchanging) speed of light as it hits the retina.
Each ‘observer’ sits in their own ‘reference frame’ or grid or map. You in your chair is one ‘frame’. Myself crushing my own chair is another frame. We both see the same event. We can both mathematically describe it from completely different observer viewpoints. I saw the cat eat the bird. You saw the bird fly into the cat’s mouth. We can use maths to prove both.
There is no absolute framework, just ‘relative frameworks’ and the only ‘absolute’ in Relativity is the speed of light in vacuo (vacuums don’t exist in space of course).
This observer-related reference frame is referred to as the ‘inertial frame of reference’. Newton’s First Law of Motion is also called the Law of Inertia.
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