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Only_Philosopher7351 t1_j28hs0j wrote

The speed of light can be calculated using the vacuum permittivity ε0 and vacuum permeability μ0 established by Maxwell's theory: c2 = 1/(ε0μ0).

Einstein used this fact to show that the speed of light in a vacuum must be constant in every inertial frame, thus relative.

Because permittivity and permeability are constant in electromagnetic theory, c must have always been c.

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tranion10 t1_j28l9xt wrote

All this does is reframe the original question, not answer it. OP could ask if vacuum permeability and permittivity have always been constant. Simply saying that they are considered constant in EM theory is not a satisfying answer.

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