Speed of light the fundamental excitations (or quanta) of the electromagnetic field, called photons. In QED, photons are massless particles and thus, according to special relativity, they travel at the speed of light in vacuum. Extensions of QED in which the photon has a mass have been considered. In such a theory, its speed would depend on its frequency, and the invariant speed "c" of special relativity would then be the upper limit of the speed of light in vacuum. No variation of the speed of light with frequency has been observed in rigorous testing, putting stringent limits on the mass of the
Speed of light permeability or magnetic constant, "ε" for the vacuum permittivity or electric constant, and "Z" for the impedance of free space. This article uses exclusively for the speed of light in vacuum. Since 1983, the metre has been defined in the International System of Units (SI) as the distance light travels in vacuum in of a second. This definition fixes the speed of light in vacuum at exactly . As a dimensional physical constant, the numerical value of is different for different unit systems. In branches of physics in which appears often, such as in relativity, it is common to use