The Metre Is Now Defined by the Speed of Light
Length is extent from end to end. The world's standard unit stopped being a physical object in 1983.
Length is the measurement of something from end to end. The standard has moved from object to constant: the metre began as a fraction of the earth's meridian, became a platinum bar in Paris, and since 1983 has been defined as the distance light travels in a vacuum in one 299,792,458th of a second. The speed of light is now exact by definition.
- Extent from end to end. *The length of the field.*
- Duration. *The length of the film.*
- A measure in racing. *Won by two lengths.*
- At length — eventually, or in detail. *Discussed at length.*
Word forms
| Form | Word | Example |
|---|---|---|
| Noun | length | Overall length. |
| Plural | lengths | Twenty lengths. |
| Adjective | long | A long road. |
| Verb | lengthen | Lengthen the hem. |
| Adjective | lengthy | A lengthy delay. |
How it is used in a sentence
- He swam forty lengths.Of the pool.
- She went to great lengths.Made considerable effort.
- The metre is defined by light.Since 1983.
How the metre was defined
- 1793 — one ten-millionth of the distance from pole to equator
- 1889 — the length of a platinum-iridium bar in Paris
- 1960 — a number of wavelengths of krypton-86 radiation
- 1983 — the distance light travels in a fixed fraction of a second
- Consequence — the speed of light is now an exact defined value
From Old English *lengthu*, from *lang*, long, with the noun-forming suffix *-th* — the same pattern as *warmth* and *depth*.
Why define the metre by light?
Because a physical bar can be damaged or drift. A universal constant gives every laboratory the same standard without comparison to an object.
Is the speed of light measured or defined?
Defined. Since 1983 it is fixed by agreement, and the metre is derived from it rather than the other way round.