The kilogram has a new definition. A physicist explains why and how we redefined a basic unit of measure.

Pedro_P

A kilogram is a kilogram. Unless it isn't.
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▼ How much is a kilogram? 1,000 grams. 2.20462 pounds. Or 0.0685 slugs based on the old Imperial gravitational system. But where does this amount actually come from and how can everyone be sure they are using the same measurement?
Since 1889, countries who are members of the General  Conference on Weights and  Measureshave agreed to use a standard block of metal—kept near Paris—to define the kilogram. But although the modern block is stored in a highly controlled environment, its weight can change by tiny amounts as wear and tear causes it to lose mass and dirt causes it to increase. To address this problem, scientists around the world have spent nearly two decades discussing how the kilogram could instead be defined in relation to constant measurements of nature. And now they have finally reached a decision.
The first kilogram(originally called a grave) was defined in 1793 by a commission of the French Academy of Sciences, who wanted a better standard than the fixed amounts of grain that had traditionally been used. The commission decided that the new measure would be the mass of one cubic decimeter of distilled water at 4℃ (the temperature at which water has its highest density under standard conditions). This had the advantage in that most properly equipped labs would be able to reproduce this standard. Subsequently, a prototype of this mass was cast in brass.
Unfortunately, this definition of mass depended upon another variable measurement, the meter. At this point, the meter was only provisionally defined as part of the distance from the North Pole to the equator. Once the value of the meter and the temperature of water at its densest were more accurately defined, the kilogram also had to be replaced. And a new prototype was cast in platinum to represent this mass.
Eventually, this was replaced with the international prototype kilogram (IKP) used today, cast from a mixture of platinum and iridium to make it very hard and prevent it reacting with oxygen. The IPK and six copies are kept by the International Bureau of Weights and Measures in the Pavillon de Breteuil, Saint-Cloud, near Paris in France to act as a reference to measure against. Copies of the IPK are transported across the world to ensure all participating countries use the same standard.
But even the modern IPK can gradually change in mass. Radically, the answer from the International Bureau of Weights and Measures is to overhaul the definitions of a kilogram, as well as all the other basic units of measurement used in science (known as SI units, from the French for international system).
Instead of measuring the kilogram against a block stored in a vault, we can define it based on precise values of constants of nature. Agreeing on a definition has taken a long time because we needed to be able to measure these constants to exacting standardswith an uncertainty of 30 parts per billion (meaning the measurements are accurate to 0.00000003 of a unit). (▪ ▪ ▪)

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soothsayer

very informative. never knew there was so much to relate about weights and measures