And you have proved this, how?
See here:
"The speed of light is a fundamental constant of nature. The word constant, of course, means something that is unchanging. In a sense, the speed of light can be considered to be derived from other constants - most directly from the permittivity and permeability of free space, but these constants, too, are related to other constants such as the charge on an electron, Planck's constant, and the fine structure constant. So if the speed of light were changing, these other constants wouldn’t be constant anymore and this would show up in many kinds of experiments and observations in physics, astronomy, and chemistry - indeed, everything, for atoms and molecules would not be as we know them today. If the speed of light was faster in the past, then we should see this in the ratios of spectral lines of distant quasars coming from different types of atomic transitions. The resulting frequencies have different dependencies on the electron charge and mass, the speed of light, and Planck's constant, and we can compare these to their present values on earth. Needless to say, the spectral signature of elements found in the quasars agrees perfectly with those here on the earth today."