Speed of light

The speed at which light travels through vacuum, exactly 299,792,458 meters per second. It is the maximum speed at which information or causal influence can propagate.

Special relativity: A theory of space, time, and motion in which physical laws are the same in all inertial frames. Its postulates make the vacuum speed of light identical for all inertial observers.
Ole Rømer: A Danish astronomer who used delays in eclipses of Io to infer that light travels at finite speed. His 1676 observations supplied the first quantitative evidence for light’s finite travel time.
Vacuum permittivity: The electric-field constant that appears in Maxwell’s equations for electromagnetic phenomena in vacuum. Together with vacuum permeability, it determines the predicted electromagnetic wave speed.
Time of flight: A measurement method that infers distance or properties from the travel time of a signal. Knowing light speed converts measured travel times into distances.
Phase velocity: The speed at which a wave’s phase advances through space. In some settings it can exceed light speed without carrying information faster than light.
Electromagnetic waves: Waves of coupled electric and magnetic fields that propagate through space and matter. Maxwell’s equations predict their vacuum propagation speed from electric and magnetic constants.
James Bradley: An English astronomer who discovered stellar aberration while seeking evidence of stellar parallax. Aberration provided an independent way to estimate light’s speed.
Vacuum permeability: A physical constant describing magnetic fields in vacuum and their relation to electric currents. It enters the classical expression connecting electromagnetic constants to light speed.
Laser ranging: A distance-measurement technique that times laser pulses reflected from a target. It uses light’s known speed to measure distances, including the Earth–Moon distance.
Group velocity: The speed at which the envelope of a wave packet moves. In dispersive media it can differ from both phase speed and information speed.

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