Speed of light a laser beam is swept quickly across a distant object, the spot of light can move faster than "c", although the initial movement of the spot is delayed because of the time it takes light to get to the distant object at the speed "c". However, the only physical entities that are moving are the laser and its emitted light, which travels at the speed "c" from the laser to the various positions of the spot. Similarly, a shadow projected onto a distant object can be made to move faster than "c", after a delay in time. In neither case
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