Half-truth second postulate of Special Relativity:
Light always goes at c in any inertial frame.
Why is this a half-truth?
Maxwell never suggested that light will be measured at c in any inertial reference frame.
Maxwell would turn over in his grave if he knew what the Einstein quack scientists did to his equations.
Maxwell said that light has a fixed velocity.
He also knew that the velocity of light was different when it was passing through water, or vacuum or air or glass or diamond.
What did Maxwell actually say about light speed then?
He said that light speed was constant, always the same RELATIVE TO THE MEDIUM THAT IT WAS IN.
If light and its source were moving inside a diamond, for instance, and you accelerated the diamond, then the light speed inside the diamond would never be affected by the motion of the diamond. It would still be going at the same speed inside that diamond BECAUSE light speed is relative to the medium it is passing through!
Light speed is NEVER relative to any observer in any inertial frame of reference. It's only relative to the medium it's passing through, and that speed is c in the medium of a vacuum.
How light moves through a vacuum in the first place is not part of Maxwell's equations.
So, despite the false claims that Maxwell's equations needed "fixing" because they were "not invariant" for motion, this is a false claim, as it falsely places the speed of light relative to the observers frame, when Maxwell was clear that light is always only relative to the medium that it's passing through.
When Einstein falsely made light speed relative to the observer, ANY observer, moving in ANY direction, he took us all for fools, as this creates a total impossibility that to this day, no one including Einstein has even offered a solution for.
Its simply IMPOSSIBLE that one can measure light speed at C when moving alongside the light, and also measure that same light as still moving at c as measured by someone moving directly against the direction of the light.
Really, any sane reasonable person should recognize that Einstein was playing a big trick on everyone, and so far he has been successful. We bought into his delusions without stopping to think for ourselves.
Light speed IS constant, that much is true BUT it's only constant, relative to the medium that it's passing through. Never to any observer.
No one knows how light manages to pass through the apparent vacuum. (we don't even know how it passes through glass or water, our speculations are not very rational)
This is still a mystery, but it is not necessary to solve in order to see that EInstein's Special Relativity is more than wrong, its now science fraud.