if you think about the light why it appears to have a fixed speed depending on the medium it propagates through, it’s because the light has in fact way more energy to move faster than the measured speed, but the medium where it moves in is slowing it down. in the vacuum of space the medium is the quantum field. the true speed of light is actually dependent on the energy it has — the wavelength is in fact a rotation of the electric and magnetic components of the light around the axis of its overall unidirectional movement. this means that the components of the higher energy light are making many more rotations, having to pass through way more distance in the same time period, while the medium is forcing the combined light into a fixed length it passes in time, regardless of available energy. this is the limitation imposed to the movement in one direction by the medium, but not the inherent true speed of light.
of course the speed of light is the same in all frames of reference in the same substance, including in the quantum field, it is squeezed down to the specific limit, not allowing to go on with its full potential. if somehow the limit could be removed then you will find out that a regular addition and subtraction of speeds will apply.
thus the measured speed of light in vacuum of space isn’t the maximum speed the ligth could theoretically have, only the maximum speed in a particular region of space. this region of space with the relatively uniform speed of light could be our entire visible space, while in farther away regions it could be different. i wrote “relatively uniform speed of light” not by accident, the speed of light has been varying in time (obviously because we move through the universe with the earth and are not stationary) and it must be corrected regularly to be exact. this is a little known and well hidden fact from larger public, revealed to specialists in the fields where the extreme precision of light speed is needed to know. you can find out about this inconvenient truth to many scientists by doing a proper research on the subject of light speed.
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