Although most people can recognize the familiar E=mc^2 equation. c is itself the result of the permeability (mu) and permittivity (epsilon) "constants" of free space where c=1/sqrt(mu*epsilon). In other words E=mc^2 is not just the equation of rest mass to energy but the m is the actual mass of a given particle. Since m=mo/sqrt(1-(v^2/c^2)), an equation which relates the relativistic mass of a particle to its rest mass, it follows that E=mc^2=(mo*c^2)/sqrt(1-(v^2/c^2))=mo/(mu*epsilon*sqrt(1-(v^2*mu*epsilon))). c is itself subject to this background, and not a constant in and of itself (as seen in its absence in the latter equation). This gives rise to the question...what are the permittivity and permeability fields all about?
Good article....
All comes from observation. We would not have a clue how reality works if we didn't take a look. Not that we can know the ultimate reality anyway.
That would have been prettier in MathML