That's Special relativity silly not General relativity.
The one you cite can be shown by simple math and the following.
It comes from light being measured as exactly the same speed regardless of the inertial frame of reference, which is different than with the speed of matter which is measured as the speed differences.
Learned that in physics a lifetime ago.
That doesn't help the young Earthers though.
But it does help the Biblical literalists though who read Genesis 1 from the perspective of the different "frame of reference" of the author.