The speed of light in a vacuum is a universal constant exactly defined as 299,792,458 m/s (about 300,000 km/s or 186,000 mi/s). This exact value is fixed because the metre itself is defined by how far light travels in 1/299,792,458 of a second.
This constant speed—denoted by “c”—is the maximum speed at which information, matter, or energy can travel. It remains the same regardless of the relative motion of the source or observer, a foundational postulate of Einstein’s theory of relativity.
Light (and all electromagnetic radiation) travels at this constant speed in a vacuum. In materials like air, water, or glass, it moves slower—dependent on the medium’s refractive index. For example, light travels at about two-thirds the vacuum speed in glass.
The finite speed of light gives rise to time delays over large distances. Sunlight takes about 8 minutes and 20 seconds to reach Earth, while radio signals to distant spacecraft (like those at Jupiter or Mars) can take minutes or even over an hour to arrive and return.
Light’s invariant speed underpins the structure of modern physics and cosmology. It implies that no object with mass can reach or exceed c, and it leads to profound effects like time dilation, length contraction, and mass-energy equivalence (E=mc²) under special relativity.
Accurate measurement of c in vacuum is now a relic: since 1983 the metre is defined by c. Instead, we use precise frequency and wavelength measurements (via lasers and interferometry) to realize the metre standard.
Astrophysical observations—such as pulsar timing, spacecraft tracking, and laser ranging—depend on c. For example, the delay of light or radio signals gives us distances and tests of gravitational theories. These methods are essential for GPS systems, deep-space navigation, and tests of relativity.
The constancy of the speed of light is one of the best-tested principles in science. Experiments like Michelson-Morley and measurements of electromagnetic constants reinforce that c is invariant, a cornerstone of physics and a link among electromagnetism, gravity, and spacetime.