Speed of light the "local" speed of light is constant and equal to "c", but the speed of light along a trajectory of finite length can differ from "c", depending on how distances and times are defined. It is generally assumed that fundamental constants such as "c" have the same value throughout spacetime, meaning that they do not depend on location and do not vary with time. However, it has been suggested in various theories that the speed of light may have changed over time. No conclusive evidence for such changes has been found, but they remain the subject of ongoing research. It
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