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You might have heard that a few weeks ago on the 20th of May the definition of the kilogram changed. However, despite being the dominant measure of weight around the globe, this change is unlikely to have any impact on you at all. Given this, one has to ask: why did the kilo even need to change?

The short answer is science, but the long answer is much more interesting.

First a little background:

In the mid-13th century Marco Polo, son of wealthy Venetian trading family, left his home in Venice and headed east on a sort of 24 year-long “gap year”, touring much of Asia, particularly China, and returning with techniques and products unknown to Europe at the time. He wasn’t the first European to travel to and through East Asia, but he was the first to return with a highly detailed travel diary recounting his experiences and the locations he visited. Such tales had a significant impact on the European explorers who would follow, most prominently the Dutch arriving in the Indonesian Archipelago (which they dubbed the East Indies) in the late 15th century. This was then followed by a substantial increase in global trade, like spices, silks and exotic plants and animals (to the Europeans at least) were now able to be cheaply, efficiently and quickly transported around the globe.

This new trade route, however, was still long and arduous and the Dutch and English, with relatively small land holdings and heavy investments in navies, had the advantage of the route, setting up supply colonies along the way and eventually forming an alliance. For the Spanish (at this time still two separate nations Castile and Aragon) this couldn’t be allowed to go uncontested, they too wanted a slice of the global trade pie.

Enter Christopher Columbus, a divisive figure throughout the Americas despite him never actually setting foot on the mainland. Columbus, also Italian and inspired by the travels of Marco Polo, convinced Queen Isabella I of Castile, to sponsor an exploratory voyage to find a western passage (knowing the world was a globe) to the East Indies, and in this way gain an advantage over the English and Dutch. Of course, he instead happened upon the Caribbean and the Americas which were to provide Europe with a large number of new fruits and vegetables including potatoes, tomatoes, corn and chocolate as well as providing Spain with an inordinate amount of gold.

Fun fact: Pasta was developed in Italy thanks to Marco Polo bringing back noodle making techniques around 1300, but tomatoes didn’t arrive until the Spanish introduced Italians to them in the 1550s, and even then it took some time for the Italians to warm to the fruit.

Despite the world turning out to be a bit larger than Columbus had expected—Indonesia and the Caribbean are literally on opposite sides of the globe. It was functionally smaller than ever before, and increases in ship designs and technology would only shrink it further.

The Western European powers then set about dividing up the world amongst themselves (Germany didn’t exist until the late 19th century and Austria was far more focussed on managing the Holy Roman Empire than exploring the world). Fighting for power, influence, resources and, most importantly, trade, colonialism and capitalism were cut from the same cloth. This competition culminated in the American War of Independence. The 13 colonies (with substantial assistance from France and Spain) won the war in 1776 and became the United States, then almost immediately resumed trade relations with the British rather than the French and Spanish. In other words, while France had gone into debt to free the US, they didn’t see any benefits in American trade or influence because of it.

This weakening of France’s global standing and internal wealth, combined with the long-term largesse of the Sun King Louis XIV and his heirs, culminated in the French revolution; the death of Louis XVI and formation of the First Republic of France. If you want some spoilers as to how well this republic went, the current President of France, Emmanuel Macron, is President of the Fifth Republic of France.

But what does this have to do with the Kilo?

This new revolutionary France sought to distance itself from traditions, building on the age of enlightenment seen over the preceding decades and following Britain’s lead. France set about properly industrialising, becoming far more meritocratic and investing in educating its population. Given this large uneducated population was soon to be educated and being highly receptive to new ideas, the metric system was conceived and implemented.

By standardising base units and determining them in relation to each other, measurements of distances and volumes (both crucial for trade) could be far more easily determined. Plus, given the widespread use of the base 10 Hindu-Arabic numerals, popularised by Leonardo Fibonacci in the 13th century, a base with prefixes and suffixes being applied for varying factors of 10 e.g. Metre, Centimetre and Kilometre were selected. This is despite most European languages being, at least initially, base 12 (you don’t say one-teen or two-teen), although they generally become base 10 from number 20 onwards.

The proposed system was substantially simpler than others in use at the time, which were mostly derived from the Romans. For example the British Imperial system of 12 inches to a foot, 3 feet to a yard, 220 yards to a furlong and 8 furlongs to a mile. Thanks to Roman influence, different countries also used different values for the same measurement term. Apparently the Dutch and British agreed when working on the Cape Colony that each would build 1 side with a central road running between them, with each side road set 1 mile apart. They were slightly off for the first roads to go in but because their miles were different lengths a few roads later they didn’t overlap at the plus junction at all. The dream of the metric system was so global cooperation could thrive and such mistakes wouldn’t happen again.

So the metric system would be base 10 and relate to itself, but what would it actually be based on? Water was selected a good basis thanks to its widespread availability and importance to people. A kilogram would be 1 Litre of water, water would boil at 100° Celsius and freeze at 0°, seconds and hours (already widely agreed on) would be maintained and so all that really remained was the metre which was defined as one ten-millionth of the distance between the north pole and equator. Due to the impracticalities of having to measure the distance between a pole and the equator and issues with the consistency of the weight of a kilo of water, prototypes against which all other measurements would be defined were determined, with exact replicas of these prototypes to be distributed around the world.

And it worked!

The system was so effective, simple and reliable that today the entire world uses it through the upgraded SI system. Sure the USA likes to claim that they don’t, but that isn’t actually correct. With American pounds and feet actually defined in relation to metric prototype replicas which the United States National Institute of Standards and Technology holds its own copies of.

Now, this is all great and, as I promised, (surprisingly) interesting. However, we still haven’t arrived to the main question.

Why did the Kilo change?

Basically, as scientific equipment and experiments have gotten more and more complex, and smaller and smaller in size and scope (hey there sub-atomic physics), more accurate readings are required. Additionally, physical prototypes have some issues, the length of a metre prototype made from metal would change depending on temperature, this is not enough to worry a tape measure, but enough to throw off a laser surveyor.

What’s more, despite being stored in double vacuum sealed containers, when many of the kilogram prototype replicas were brought back to France to be compared, none of them were the same weight. Basically, it was impossible to know what the true weight of a kilogram was. Lastly, when your units are defined in relation to a physical thing, if the said thing ever goes missing so too does your ability to measure it.

In an effort to avoid these issues the General Conference on Weights and Measures (which has been meeting to define and maintain the SI units since 1875), has, over the years, decided that physical constants would make better and far more exact definitions for these units. This has resulted in the metre now being defined as the distance light travels in a vacuum in 1/299792458th of a second. All the units except the kilogram had been changed up until now.

  • The ampere is defined in terms of the charge held by a single proton. With the coulomb defined in relation to the ampere and the second.
  • The second is defined as the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
  • The Candela (brightness) is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of certain frequency, with a certain radiant intensity in that direction per steradian—a sort of cone leaving the centre of a sphere of light and expanding in radius at the same rate the sphere of light does.
  • The Mole is defined as the amount of substance of exactly 02214076×1023 elementary entities (atoms).
  • Lastly, the Kelvin (expansion of the Celsius scale) is defined in relation to the Boltzmann constant in relation to the kilogram, metre and second.

And now finally the last SI unit, the Kilogram, which on 20/5/2019 CE was redefined by setting the Planck constant h exactly to 6.62607015×10−34 J⋅s (J = kg⋅m2⋅s−2), and given the definitions of the metre and the second defines the kilogram as: