We measure things through the time – how long, how heavy, how hot, and so on – because we need to do things like trade, health and information. But making sure our measurements compare apples with apples has been a challenge. How to know if my kilogram weight or the length of your meter is the same as your weight?
Efforts have been made to define the units of measurement over the years. But on Monday – International Metrology Day – a full review of those standards came into force.
You will not notice anything – you will not be heavier or lighter than you were on a Sunday – because the change was made seamlessly.
Only the definitions of the seven basic units of the OS (Système International d´Unités, or the International System of Units) are now completely different.
How we used to measure
People have always been able to count, but as we developed we quickly moved to measure length, weight and time.
The Egyptian Pharaoh caused pyramids to be built based on the length of the royal arm, the Royal Cubit. This was kept and distributed by priests engineers who maintained the standard under the pain of death.
But the cube was not a stable unit over time – it was about half a meter, as well as or a few tens of millimeters by today's measurement.
The first suggestion of a general set of decimal measures was made by John Wilkins, in 1668, then Secretary of the Royal Society in London.
The impetus came to do something practical with the French Revolution. The first length and mass standards were defined by the French, with two platinum standards representing the meter and the kilogram on June 22, 1799, in the de la République Archives in Paris.
Scientists supported the idea, the German mathematician Carl Friedrich Gauss was particularly keen. Representatives from 17 countries came together to create the International System of Units by signing the Meter Convention agreement on May 20, 1875.
France, whose belief on the street had fought in the Franco-Prussian war and was not a scientific power it once proposed, offered French chateau beating in Saint-Cloud Forest as an international home to the new system.
The de Breteuil Pavilion has the International Bureau of Poids et Mesures (BIPM), where the International Prototype per kilogram (from then on the Big K) is in two rows and three glass bell jar.
The Big K is a polished block of platinum-iridium used to define the kilogram, each kilogram weight is ultimately measured against. The original has been weighed three times against a number of identical copies.
The British, who had been at the forefront of the negotiations and provided the platinum-iridium kilogram, refused to sign the agreement until 1884.
Even then, the new system was only used by scientists, with everyday life measured in traditional Imperial units such as pounds and ounces, feet and inches.
The US signed the treaty on the day, but then it never put it into practice, hanging on its own version of the British Imperial system, which it's mostly used today. .
Perhaps the United States has ruled that decision in 1999, however, when the Mars Climate Orbiter (MCO) went missing. According to the incident report, “misleading”, which cost US $ 193.1 million in 1999: t
[…] the main cause of the loss of the MCO's spacecraft was the failure to use metric units when coding a ground software file, “Small Forces”, used in trajectory models.
Basically, the spacecraft was lost in the atmosphere of Mars as it entered an orbit below expectation.
New OS definitions
So why change today? The main problems with the previous definitions were that, in the case of the kilogram, they were not fixed and, for the electrical current unit, the ampere, could not be realized.
And weighing against official copies, we think the Big K slowly loses mass.
All the units have now been defined in a common way using what the BIPM calls its “steady constant” formation.
The idea is that we take a general constant – for example, the speed of light in a vacuum – and from now on sets its numerical value at our best value, without uncertainty.
The reality is stable, the number is fixed, and so the units have now been defined.
So we needed to find seven constant and ensure that all measurements were consistent, within measurement uncertainty, and then start counting until Monday. (All technical details are available here.)
Australia had a helping hand to shape the circular macroscopic object on Earth, a silicon field used to measure the Avogadro constant, the number of entities in a fixed substance. This now defines the SI unit, mole, which is mainly used in chemistry.
Quality to artefact
What is the Big K – the standard kilogram? Today it becomes an object of great historical significance that can be weighed and measurement uncertainty will grow.
Now the kilogram is defined using the Planck constant, something that doesn't change from quantum physics.
The challenge now is to explain these new definitions to people – especially non-scientists – so that they understand. Comparing a kilogram with a metal block is easy.
Technically a kilogram (kg) is now defined:
[…] by taking the Planck's fixed numerical value consistently h to be 6.626 070 15 × 10–34 when it's expressed in the J's unit, that's equal to Hoa2 s–1where the meter is defined and second in terms of c a ΔνCs.
Try to explain it to someone!
Update: the phrase “tens of centimeters” was changed to “tens of millimeters” at the request of the author.
David Brynn Hibbert, Emeritus Professor of Analytical Chemistry, UNSW
This article was reissued from The Conversation under a Creative Commons license. Read the original article.