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Thematic toursGuardian of the unitsThe SI > The metre
The metre

Definition

Iodine-stabilized helium-neon laser, PTB’s "workhorse" (wavelength standard) for the realization of the metre.

Iodine-stabilized helium-neon laser, PTB’s "workhorse" (wavelength standard) for the realization of the metre.

The metre is the length of the path travelled by light in vacuum during a time interval of (1/299 792 458) of a second.

The definition of the meter assigns a fixed value to the light velocity c. This fundamental constant can thus no longer be measured, but is now exactly predefined. From this follows that the unit of length depends on the unit of time, the second.

 

Realization

There are different possibilities of realizing the unit of length on the basis of this definition. Two methods are applied for this purpose: measurement of the travelling time and laser radiation. Measurements of the travelling time are well suited to measure distances in astronomy. The distance between the earth and the moon, for example, is measured by directing a short pulse of a high-power laser beam at the moon where it is reflected from a mirror installed there. The distance can be calculated from the time which the laser pulse needs to travel from the earth to the moon and back to the earth. The GPS (Global Positioning System), too, is based on the measurement of the travelling time of electromagnetic radiation.

In the case of the second method, the known wavelengths of lasers are used to carry out precise length measurements at laboratory scale. In so-called laser interferometers, the wavelengths of the laser (~ 0.5 µm) are compared with the length to be measured.


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