The speed of light in vacuum is exactly 299792458 metres per second, because the metre has been defined using this value of the speed of light. It is commonly approximated as 300000 kilometres per second or 3 times 10 to the eighth metres per second. The symbol c is conventionally used for the speed of light in vacuum. This speed is not merely the speed at which visible light travels.
It is the invariant propagation speed that appears throughout special relativity and is associated with electromagnetic radiation in vacuum. Radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X rays and gamma rays all propagate at c in vacuum. The frequency and wavelength of the radiation can differ enormously while the vacuum propagation speed remains the same. The relation c equals lambda multiplied by f connects wavelength and frequency for electromagnetic radiation in vacuum.
The exact numerical value of c became fixed as part of the modern definition of the metre. Before this definition, scientists measured the speed of light experimentally with increasing precision. Ole Rømer provided an early astronomical indication that light travels at a finite speed by analysing timing variations in observations of Jupiter's moon Io. Later terrestrial experiments by researchers including Hippolyte Fizeau and Léon Foucault measured light propagation using rapidly rotating mechanical systems.
Albert Michelson conducted highly precise measurements using optical interferometry and rotating mirrors. As measurement technology improved, the value became known with sufficient precision that metrology could reverse the relationship and define distance in terms of time and the fixed speed of light. Since 1983, the metre has been defined as the distance travelled by light in vacuum during a specified fraction of a second, making c an exact constant rather than an experimentally uncertain measured value in the definition of the SI unit.
Light travels more slowly through transparent materials than it does through vacuum in the usual macroscopic description. The phase velocity in a material is related to the refractive index by v equals c divided by n, where n is the refractive index for the relevant conditions and wavelength. Glass, water and many other materials therefore reduce the propagation speed associated with the optical wave. The refractive index can depend on wavelength, producing dispersion.
It is important to distinguish the phase velocity of a wave from group velocity, signal velocity and the microscopic processes occurring during propagation. A pulse travelling through a material can be reshaped by dispersion and absorption, and the group velocity can differ substantially from c. None of these effects permit ordinary usable information to propagate through vacuum faster than c. The statement that light slows down in matter therefore requires careful interpretation rather than being treated as though photons simply lose a permanent intrinsic speed.
The speed of light is central to special relativity because c is an invariant speed shared by all inertial observers. According to Einstein's postulates, the laws of physics have the same form in inertial reference frames and the speed of light in vacuum is measured as c by every inertial observer, regardless of the observer's uniform motion relative to the light source. This leads to consequences including time dilation, length contraction and relativity of simultaneity.
It also means that the classical Galilean addition of velocities cannot be applied to light. If one observer travels toward a light beam at a high velocity, that observer does not measure the light as travelling at c plus the observer's speed. Instead, relativistic transformations preserve c. The Lorentz transformations mathematically encode this structure and replace the older Newtonian transformation rules at relativistic speeds.
The finite speed of light also determines causal relationships between events. Information cannot ordinarily be transmitted through vacuum faster than c, so events separated by sufficiently large spatial and temporal intervals may be unable to influence one another. The spacetime interval and light cone structure of special relativity describe which events can be causally connected. Light from astronomical objects also provides a view into the past because the radiation requires time to travel to Earth.
When astronomers observe a galaxy millions or billions of light years away, they detect radiation that left that region millions or billions of years earlier. A light year is therefore a unit of distance equal to the distance light travels in vacuum in one Julian year, not a unit of time. The finite propagation speed is fundamental to astronomy, telecommunications, radar, navigation and any system in which timing and distance are linked.
Although c is frequently described as the speed of light, its deeper significance extends beyond electromagnetic radiation. In relativistic physics, c functions as a fundamental conversion factor linking space and time and appears in the structure of spacetime. It also appears in the famous mass energy relation E equals mc squared, which expresses the equivalence of mass and energy in special relativity.
Electromagnetic fields propagate according to relativistic field equations, and gravitational disturbances in general relativity also propagate at c in vacuum. Modern theories therefore treat c as a fundamental invariant speed rather than merely an optical property. Massless particles such as photons travel at c in vacuum, while objects with nonzero rest mass cannot be accelerated to c because the required relativistic energy would increase without bound as their speed approached the invariant limit.
Measurements of the speed of light are now deeply connected to precision timekeeping and metrology. Atomic clocks provide exceptionally accurate measurements of time, and because the metre is defined through c, high precision distance measurement can be achieved by timing electromagnetic signals or measuring optical frequencies. Technologies such as satellite navigation depend critically on relativistic timing corrections because tiny timing errors correspond to significant positional errors. Radar and lidar systems determine distance by measuring signal travel time.
Fibre optic networks use electromagnetic signals to transfer information over large distances, although the propagation speed in fibre is below c. Astronomy uses light travel times to establish distances and temporal relationships. Particle accelerators also rely on relativistic dynamics, where particles approach but do not exceed c.
The phrase speed of light can create several misconceptions. Light does not always travel at exactly c through every material, and the speed of a wave pattern, pulse or signal in a medium requires context. Refraction does not violate relativity because the relevant propagation occurs through matter and remains consistent with relativistic causal constraints. Quantum entanglement also does not provide a mechanism for sending controllable information faster than light.
Apparent superluminal motion can occur in certain astronomical observations because of projection and relativistic geometry, without any physical object or usable signal locally exceeding c. Some mathematical phase velocities can exceed c without carrying information faster than c. The experimentally and theoretically established invariant speed c therefore remains a central boundary for local causal information transfer in relativity.
