How the Metre Is Defined by the Speed of Light
Source: BIPM. Published: 2025-02-01.
Early Definitions: The Earth as a Ruler
The metre was born from the French Revolution's ambition to replace the chaotic patchwork of local units with a rational, universal system. In 1793, the French National Assembly defined the metre as one ten-millionth of the distance from the North Pole to the equator, measured along the meridian passing through Paris. A massive geodetic survey was commissioned to measure this arc, and the resulting metre was cast into a platinum bar. The idea was elegant: a unit derived from nature itself, not from a king's body part or a local tradition. In practice, measuring the Earth to the required precision was extraordinarily difficult, and the resulting metre was slightly off from the ideal. Nevertheless, the concept — a unit grounded in a physical constant rather than an arbitrary object — proved enormously influential and endured as a philosophical principle for two centuries.
The Platinum-Iridium Metre Bar
In 1889, following the signing of the Metre Convention in 1875, the BIPM produced a new international prototype metre — a bar made from a 90% platinum, 10% iridium alloy, chosen for its hardness, low thermal expansion, and resistance to corrosion. The bar's length was defined by two fine lines engraved near its ends, measured at the melting point of ice. Copies were distributed to member states, and the international metre was defined as the distance between those two lines on the prototype bar under specified conditions. This definition served for over seventy years, but it was not without problems: the bar could be damaged, it required careful temperature control during measurement, and there was no way to reproduce it if destroyed. The scientific community knew that basing a fundamental unit on a fragile physical object was philosophically unsatisfying and practically risky.
Light Wavelengths: The 1960 Redefinition
Advances in spectroscopy during the mid-twentieth century opened a new possibility: defining the metre in terms of the wavelength of light. Light emitted by a specific atomic transition is extraordinarily reproducible — any laboratory with the right equipment can generate it. In 1960, the 11th General Conference on Weights and Measures (CGPM) redefined the metre as 1 650 763.73 wavelengths of the orange-red light emitted by krypton-86 atoms undergoing a specific transition in a vacuum. This was a significant conceptual leap: the metre was now defined by a quantum-mechanical phenomenon rather than a manufactured object. The krypton standard was more reproducible than the platinum bar and could be realised independently by any national metrology institute. But it was not perfect — different laboratories using krypton lamps sometimes measured slightly different wavelengths — and more precise laser techniques were already making the krypton standard look dated.
The 1983 Redefinition: Fixing the Speed of Light
By 1983, laser interferometry had made it possible to measure the speed of light with remarkable precision. The value was converging on 299 792 458 metres per second, with an uncertainty of only a few parts per billion. Rather than continuing to use the speed of light as a measured quantity, the CGPM took a bold step: they fixed its value by definition. The metre was redefined as the distance travelled by light in a vacuum in 1/299 792 458 of a second. This made the speed of light exact — a defined constant rather than a measured one — and it anchored the metre to the second, which was already defined with exceptional precision using atomic clocks. The 1983 definition remains in force today, updated only to align with the 2019 SI revision's framework of fixing fundamental constants.
Consequences: Length Derived from Time
The 1983 definition has a profound implication: length is now derived from time. To realise the metre, you measure how long it takes light to travel a given distance, then multiply by the fixed speed of light. This means that improvements in timekeeping — which have been relentless and dramatic since the invention of atomic clocks — automatically improve the precision with which the metre can be realised. The second, defined through the cesium-133 atom, is currently the most precisely realised SI base unit, with uncertainties at the 10⁻¹⁶ level. As optical lattice clocks push that precision further, the effective precision of the metre improves in lockstep. No other definition of length could be improved simply by improving a clock — yet that is exactly what the 1983 definition achieves.
What This Means for the Length Converter
When you use Unit Master to convert kilometres to miles or metres to feet, the calculation rests on exact conversion factors that are themselves derived from the defined values of physical constants. The international yard, for instance, was fixed at exactly 0.9144 metres by agreement in 1959; the mile follows directly. These exact relationships exist because the metre is no longer a thing that can change — it is a number, fixed by the speed of light and the second. The practical result is that length conversions between metric and imperial units are mathematically exact at the defined values, with no measurement uncertainty introduced by the definitions themselves. Any imprecision in a real-world measurement comes from instruments and conditions, not from the underlying definitions of the units.
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