How We Agreed on the Metre
A metre is not a fact about the universe. It is a decision, made once in the 1790s, revised several times since, and now defined by a constant of nature so that anyone with the right equipment can reproduce it anywhere.
The original idea
French revolutionaries wanted a unit based on the Earth rather than on a king's foot. They defined the metre as one ten-millionth of the distance from the equator to the North Pole through Paris, and sent surveyors to measure a meridian arc to compute it. The survey took years and the resulting metal bar became the standard instead of the calculation.
From object to constant
A physical bar in a vault is only useful if you can visit it. In 1960 the metre was redefined in terms of a wavelength of krypton light, and in 1983 in terms of the speed of light: one metre is the distance light travels in a vacuum in 1/299,792,458 of a second. The number was chosen so the new metre matched the old one closely enough that nobody had to re-measure their property.
Why it is worth caring about
Standards are infrastructure. A shared definition of the second and the metre is what makes GPS, financial timestamps, medical dosing and international manufacturing possible. In 1999 a unit mix-up between metric and imperial contributed to the loss of a Mars orbiter — an expensive demonstration that agreement is not a formality.
Knowledge check
Two questions, no score
How is the metre defined today?
Why does a physical standard object cause problems?
Now prove it
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