Some physical theories such as special theory of relativity are based on the invariance of the speed of light.
However, is the invariance of the speed of light, in the first place, logically possible?
I'm talking about this version or its equivalence: the speed of light in vacuum is always measured to be the same value from any uniformly moving observer even if at different (but constant) velocities.
Suppose there are two uniformly moving observers at different velocities:
An observer O moving at constant velocity v with respect to some observer
Another observer (light in vacuum) L moving at constant velocity c with respect to the same observer
where c = k * v for some real number k except 1
Also consider the velocity of L with respect to each observers:
The velocity of L with respect to O = c - v
The velocity of L with respect to L itself = c - c
If the invariance of the speed of light is true, those values should be identical for O and L are defined as uniformly moving observers.
Then, we can obtain this equation:
c - v = c - c
But the solution v = c contradicts our definition v and c are different.
Therefore, I believe the invariance of the speed of light is untrue.
Or is there any error in my disproof above?