There is really no conflict between the constant velocity of light, the principle of relativity and the theorem of the addition of velocities employed in classical mechanics.
The key discovery by Dutch Astronomer De Sitter was that the velocity of propagation of light cannot depend on the velocity of motion of the body emitting the light. That means the relative velocities among masses within the Mass Coordinate System (MCS) have no bearing on the velocity of light in the Field Coordinate System (FCS).
Velocity of light is constant in terms of “wavelength to period” ratio in FCS. So the concept of “frequency”, being a variable, is more useful in terms of relativity in FCS. On the other hand, the concept of “velocity” is more useful in terms of relativity in MCS.
Thus, light seems to form a uniform background in which objects move relative to one other. Light does not move relative to objects. Light takes time to reach an object in space according to the distance between that object and another object, which is the source of light.
Physical awareness of an object at a physical location takes place when light from the object reaches that physical location. This is local awareness which depends on the physical location. This local awareness determines TIME locally by establishing the order in which it perceives various objects or events.
Objective awareness in universal terms comes about by piecing together all local observation logically. This is what we know as scientific awareness. This scientific awareness determines time objectively by establishing order within the parts of the universe.
Explaining time in terms of local awareness, which is limited by finite “velocity of light”, simply offers a subjective view of time. Objective view of time comes from a scientific (logical) view that does not depend on the “velocity of light”.
The theory of relativity deals only with local (subjective) idea of time, and not with the universal (objective) idea of time.
Objective time is established by looking at sequences from a universal viewpoint. In a universal viewpoint one can logically visualize the whole universe at once.
When one looks at two light waves passing each other in opposite directions from a universal viewpoint, one shall find their relative velocity to be 2c.
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