In simple terms
Light in a vacuum travels at exactly 299,792,458 meters per second. Not approximately. Exactly.
That precision does not come from an ever more careful experiment. Since 1983 the number has been fixed by international agreement, and the meter is defined from it.
How it works
In 1983 the General Conference on Weights and Measures resolved that the meter is the length of the path traveled by light in vacuum during 1/299,792,458 of a second.
That reverses the usual logic. Length is no longer an independent standard kept as an object in a vault. It is derived from time, which atomic clocks measure extremely well, and from a constant that is now settled by definition rather than by measurement.
The value was picked to match the speed recommended in 1975, so the switch introduced no appreciable discontinuity in the size of the meter. The definition it replaced, adopted in 1960, used the wavelength of radiation from krypton-86. Before that the meter had begun in the 1790s as one ten-millionth of the distance from the North Pole to the equator, with the Treaty of the Metre standardizing it internationally in 1875.
Why it matters
A fixed constant makes measurement portable. A laboratory in Nairobi and one in Ottawa can realize the same meter without ever comparing physical bars.
It also sits underneath everyday technology. Satellite navigation works by timing radio signals between satellites and a receiver. Laser ranging measures the Earth’s surface and tracks satellites the same way. In each case distance is speed multiplied by time, and the speed is known exactly, so the accuracy of the result depends on the clock.
The same arithmetic gives distances to spacecraft and to objects far out in space.
Where you’ll see it
- The symbol c in physics equations.
- Satellite navigation systems, which convert signal travel time into position.
- Laser ranging used in surveying and satellite tracking.
- Astronomy, where distances are worked out from how long light has been traveling.
Example
Because the value is exact, one nanosecond of travel time corresponds to a light path of 0.299792458 meters, a little under 30 centimeters.
Often confused with
A measured quantity. The figure is not refined by new experiments each year. It is fixed, and modern experiments measure lengths against it rather than measuring it.
Key facts
- The 17th General Conference on Weights and Measures defined the meter in 1983 as the length of the path traveled by light in vacuum during 1/299,792,458 of a second.1
- That definition fixes the speed of light in vacuum at exactly 299,792,458 meters per second.1
- The value was chosen to match the speed recommended in 1975, so the new definition introduced no appreciable discontinuity.1
- The previous definition, adopted in 1960, was based on the wavelength of radiation from krypton-86.2
- The meter originated in the 1790s as one ten-millionth of the distance from the North Pole to the equator, and the Treaty of the Metre standardized measurement internationally in 1875.2
- Because length is now derived from time and a fixed constant, satellite navigation and laser ranging compute distance as speed multiplied by time.2
Related concepts
In the news
Quick checkWhy is the speed of light an exact number rather than a measured one?Show answer
Because the 1983 definition of the meter fixes it by agreement. Length is now derived from the constant, so the constant cannot be measured against length.
Sources
- Bureau International des Poids et Mesures. Resolution 1 of the 17th CGPM (1983). 1983 (accessed 15 September 2026)
- National Institute of Standards and Technology. Meter. Undated (accessed 15 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 16, 2026Researched and drafted with AI assistanceReport an issue