Meter (m)
Definition, realization and practical length measurement in the International System of Units
Length is a fundamental physical quantity used throughout science, engineering and everyday measurement. How the meter is defined today is not widely known outside metrology. The meter is no longer tied to a physical reference object; it is defined using fundamental constants of nature.
To understand how this definition is put into practice, it is necessary to distinguish clearly between three concepts: definition, realization and practical length measurement.
The definition of the meter in the SI system
Since 1983, the meter has been defined within the International System of Units (SI) as:
the distance travelled by light in vacuum during a time interval of exactly 1/299 792 458 of a second.
The definition rests on two exactly fixed quantities:
- the speed of light in vacuum, fixed at exactly 299 792 458 m·s⁻¹,
- the second, defined via the hyperfine transition frequency of the cesium-133 atom.
In 2019, a comprehensive reform of the SI system was carried out, in which all base units were reformulated so that they are explicitly derived from fixed numerical values of fundamental constants of nature. For the meter this changed nothing about its physical meaning; it was a formal harmonization of how the definition is expressed.
Fact box: Historical definitions of the meter
| Year | Definition of the meter |
| 1799 | The meter is defined as one ten-millionth of the distance from the equator to the North Pole along the Paris meridian. |
| 1889 | The meter is defined by an international platinum-iridium bar (the international prototype meter). |
| 1960 | The meter is defined via the wavelength of radiation from the krypton-86 atom. |
| 1983 | The meter is defined as the distance light travels in vacuum in 1/299 792 458 of a second. |
| 2019 | The SI system is reformed; the definition of the meter is reformulated but remains physically unchanged. |
The meter is thus a unit whose value is universal, independent of time and independent of material references.
Definition, realization and use
The concepts of definition, realization and use refer to distinct levels within length metrology and should not be conflated.
- The definition states what the meter is in principle and is entirely abstract.
- The realization refers to the experimental methods by which the meter is made real in practice at national metrology institutes.
- The use refers to the actual length measurement carried out in industry, research and technical applications.
This distinction is central to a correct understanding of the relationship between the SI definition and practical measurement conditions.
Realizing the meter
In theory, the meter can be realized by directly timing the propagation of light. Such a method is only practical over very large distances, for example in space geodesy. At shorter lengths the time intervals are on the order of nanoseconds, which makes direct timing impractical.
For this reason, optical interferometry is used instead as the primary method for realizing length.
Optical interferometry
Optical interferometry exploits the wave nature of light. When two coherent light waves are superimposed, an interference pattern arises whose phase depends on the difference in their optical paths.
By using laser light with a very well determined frequency, changes in length can be determined by analyzing phase shifts. In practice, length is measured by counting the number of whole and partial wavelengths that correspond to a displacement.
This method enables length measurement with very low measurement uncertainty and forms the basis for the realization of the meter at national metrology institutes.
Vacuum, air and practical measurement conditions
The definition of the meter and its primary realization are strictly tied to vacuum. National metrology institutes realize the meter either in vacuum or in environments where the influence of air can be determined and corrected with high precision.
In practical applications, measuring in vacuum is often impractical or impossible. Length measurement in industry, calibration work and geodetic surveying is therefore carried out in air. For such measurements to be traceable to the SI definition, the influence of air on the propagation of light must be taken into account.
The refractive index of air
The speed of light in air is slightly lower than in vacuum because of the refractive index of air. Under normal laboratory conditions this difference corresponds to a deviation of about 0.27 millimeters per meter if no correction is applied.
The refractive index depends on several environmental parameters, chiefly temperature, air pressure, humidity and carbon dioxide content. In precision measurement these quantities are measured and used to calculate a vacuum-equivalent length value, which ensures traceability to the SI definition.
Temperature and material effects
Length measurement is also affected by thermal expansion of the materials in the measuring system. Most solid materials change length when the temperature varies.
As an example, a steel component one meter long changes its length by about ten micrometers for a temperature change of one degree Celsius. In precision contexts this is a significant effect, which makes temperature control and correct material modelling essential.
Secondary realizations at the nanoscale
At lengths on the nanometer and sub-nanometer scale, optical interferometry reaches its practical limits. For these applications, secondary realizations based on atomic structures are used.
Silicon is particularly well suited for this purpose because its crystal structure is very well characterized. The spacing between certain atomic planes in crystalline silicon is known with very low uncertainty and can be used as a length reference in nanometrology.
The meter as the basis for unit conversions
All length units within the SI system are derived directly from the meter. Unit conversions between, for example, meters, millimeters and nanometers rest on the definition of the meter and its established realizations.
This ensures global comparability and consistency in length measurement across all fields of application.
Summary
The meter is defined by how far light travels in a vacuum in a fixed time. Realization methods turn this definition into practical measurements of very high precision, while everyday length measurement shows how sensitive the quantity is to temperature, air and the materials involved.
A fixed definition together with carefully controlled realization is what keeps the meter consistent from one laboratory to the next, anywhere in the world.