## Speed of Light, Correct Interpretation The following was provided as an answer to this question of Quora.

If everything in the universe expands n times (say 10) then will the velocity of light in vacuum change?

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Let’s look at the history of this idea called “velocity of light.” The idea of velocity comes from the concept of observer that is attached to an inertial frame of reference. Einstein used this concept in describing his special theory of relativity. But then there was this discovery of inertia-less field.

Earlier ideas of “temperature field” and “flow field” relayed material conditions at various coördinate points. Scientists assumed that the electromagnetic field relayed material conditions of an invisible substance called aether. Therefore, when light was established to be an electromagnetic phenomenon, it was postulated that light traveled as a disturbance in this invisible material substance known as aether.

Experimental evidence backed by theoretical calculations from Maxwell equations made Einstein conclude that there was no such thing as material aether, and that electromagnetic field itself was a fundamental substance more basic than matter. Compared to the inertia of matter, the electromagnetic field was practically inertia-less.

Apparently, the idea of an observer attached to an inertial frame of reference never got updated for electromagnetic phenomenon. The idea of velocity cannot be applied to an inertia-less frame of reference required for electromagnetic field.

The constant “c” used for the “velocity of light” is, however, a valid constant. It is still required per the principle of relativity. Only it has to be interpreted differently in an inertia-less frame of reference. The constant “c” is now to be interpreted as the “wavelength to period” ratio that does not change throughout the frequency spectrum.

So, the answer to the above question is that “c” as wavelength to period ratio for the electromagnetic disturbance does not change with expansion of the universe because “c” is based on spatial and not material characteristics.

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• vinaire  On January 3, 2016 at 5:30 PM

From https://vinaire.me/2014/01/11/mindful-subject-clearing-physics/#comment-16356

“When more energy is pumped into electromagnetic radiation, its frequency increases, since the speed cannot increase further.

Since the electromagnetic radiation cannot be accelerated further, the additional energy would appear as increased frequency, inertia, and mass.”

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This is a very interesting observation because the idea of velocity exists only in inertial (mass) frame of reference. That frame of reference does not apply to a field. For a field an increase in frequency is more meaningful than “increase in velocity”.

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