Speed Of Light In Vacuum Is Not Actually Constant and the Reason

The paper, by a team of French physicists, finds that the speed of a photon in a vacuum fluctuates by 50 quintillionths of a second per square meter, the Christian Science Monitor reported. In addition, it posits that vacuums may actually have extremely short-lived particles in them that keep appearing and disappearing.These ideas may be testable, the physicists wrote. Ultra-fast lasers should be able to capture the changes in the speed of light they're proposing.

Why the speed of light is not a constant？An article <Why the Speed of Light Is Not a Constant>from Scientific Research Publishing sheds light on this problem:

A variable Speed of Light is supported by the fact that all direct measurements of that speed are basically flawed, because the “meter per second” is proportional to the Speed of Light. Since it is impossible to measure the Speed of Light directly, any variations of it can only be obtained in an indirect way. It will be shown that the recent Supernovae data are in very good agreement with a universe that is slowly expanding exponentially with a Speed of Light that falls over time, inversely proportionally to the expansion of the universe. It will be shown that the definition of the angular and standard impulse momentum has to be modified to get a consistent expansion of the universe. And that all clocks run inversely proportionally to the red-shift z + 1. General Relativity remains valid even with a varying Speed of Light and also Quantum Mechanics is unaffected.

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