As many contributors here seem to be maths experts, who therefore find the explanations of Physics a difficult to grasp subject, I thought it was necessary to explain in especially simple terms, one of the many reasons why Einsteins Special Relativity hypothesis is hopelessly incorrect.
Einstein fails to define the speed of light, calling it a constant, but fails to state unequivocally what that velocity is relative to.
If I say my car is going 100, then its taken for granted that the speed is 100 relative to the ground.
A statement of velocity or speed MUST have a reference from which the measurement is related.
Einstein has no reference for the speed of light, therefore its undefined.
These days, scientists, realizing that this is an oversight of Einstein, modify Einstein's original claims to now say that light speed is c "relative to every and any inertial reference frame."
The original 1905 paper stated that light goes at c in empty space, regardless of the motion of the light source.
So we have light, no source and only empty vacuum of space, so the velocity of light is c relative to what? There are no frames, inertial or otherwise.
Light in this case is motionless because there exists nothing to compare it with.
However, Einstein did actually personally, in writing, years later address this problem.
He stated that an "ether" must exist, and is essential if Physicists were going to be able to explain or understand the universe.
He further stated that light is not a constant in a vacuum, thus repudiating his own theory of Special Relativity.
Also, he specifically states that GR is based on a real ether model. With locally ascribed properties related to observable bodies.
So, my nerdy math friends, SR and GR are deceased, and this is the reason why you are wasting your time trying to find math to match up these theories with anything from the real world.
Einstein realized his theories were incorrect from around 1920 onwards, and after admitting that physics requires an ether as a backdrop from which to do real science, he never again produced any new papers or contributed to any new discoveries for the rest of his life.
(maybe because he was no longer with Mileva, so she could not "help" him with the Physics or math.)