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New definitions of standards (SI) for kilogram (kg), meter (m), second (s), ampere (A), kelvin (K), mol (mol) and candela (cd) . Credit: BIPM, CC BY-ND
We measure things all the time – how long, how much is hot, how hot it is, and so on. – because we have to do it for things like trade, health and knowledge. Ensuring that our measurements compare apples to apples has always been a challenge: how do I know if my weight in kilograms or my length in meters is the same as yours.
Attempts have been made to define units of measurement over time. But today – International Day of Metrology – sees the complete revision of these standards come into play.
You will not notice anything – you will not be heavier or lighter than yesterday – because the transition was made to be transparent.
Only the definitions of the seven basic units of the SI (International System of Units) are now completely different from yesterday.
How we measured
Humans have always been able to count, but as we evolved, we quickly turned to measuring length, weight, and time.
The Egyptian pharaohs built the pyramids on the basis of the length of the royal forearm, known as the Royal Cubit. This was kept and promulgated by engineering priests who maintained the standard on pain of death.
But the cubit was not a fixed unit in time: it was about half a meter, more or less a few tens of millimeters compared to current measurements.
The first suggestion of a universal set of decimal measures was made by John Wilkins, in 1668, then secretary of the Royal Society in London.
International prototype of the kilogram (the Big K). Credit: BIPM
The impetus to do something concrete came with the French Revolution. It was the French who defined the first standards of length and mass, with two platinum stallions representing the meter and the kilogram on June 22, 1799 at the Archives de la République in Paris.
Agreed standards
Scientists have supported this idea, including the German mathematician Carl Friedrich Gauss. Representatives of 17 nations came together to create the International System of Units by signing the Treaty of the Meter Convention on May 20, 1875.
France, whose street credit had been mistreated in the Franco-Prussian War and was no longer the scientific power that it once was, offered a defeated castle in the forest of St. Cloud as a international focus of the new system.
The Breteuil pavilion still houses the International Bureau of Weights and Measures (BIPM), home to the international prototype of the kilogram (now the Big K) in two safes and three glass ringtones.
The Big K is a polished platinum iridium block used to define the kilogram, at which all weights are ultimately measured. (The original was only weighed three times against several nearly identical copies.)
The British, who had played a leading role in the talks and provided the kilogram of platinum iridium, refused to sign the treaty until 1884.
Even then, the new system was only used by scientists, with daily life measured in traditional imperial units such as pounds and ounces, feet and inches.
The United States signed the treaty that day, but never implemented it, clinging to its own version of the British imperial system, which they still use mainly today.
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Lost on Mars: An investigation revealed that the Orbiter for the climate of Mars had probably burned in the atmosphere of the red planet due to a shock of the parameters. Credit: NASA / JPL
The United States may have made this decision in 1999, however, when Mars Climate Orbiter (MCO) went missing. The report on the incident, strangely called an "incident" (which cost US $ 193.1 million in 1999), said: "[…] The fundamental cause of the loss of the MCO space shuttle is the inability to use metric units in the coding of a ground software file, "Small Forces", used in the trajectory models. "
Essentially, the spacecraft was lost in the atmosphere of Mars while it was entering orbit lower than expected.
The new IS definitions
So why change today? The main problems of the previous definitions were that, in the case of the kilogram, they were not stable and that for the unit of electrical current, the ampere, could not be realized.
And from the weighings against the official copies, we think that Big K was losing mass.
All units are now commonly defined using what the BIPM calls the "explicit constant" formulation.
The idea is that we take a universal constant – for example, the speed of light in a vacuum – and now set its numerical value to our best-measured value, without uncertainty.
The reality is fixed, the number is fixed and the units are now defined.
So we had to find seven constants and make sure that all the measurements are consistent, within the limits of measurement uncertainty, and then start the countdown today. (All technical details are available here.)
The seven units are now defined by universal constants such as the speed of light c for the counter. BIPM, CC BY-ND
Australia has contributed to the creation of the world's rounder macroscopic object, a silicon sphere used to measure the Avogadro constant, the number of entities in a fixed amount of substance. This now defines the SI unit, the mole, used mainly in chemistry.
From standard to artifact
What about the Big K: the standard kilogram? Today, it becomes an object of great historical importance that can be weighed and its mass will have an uncertainty of measurement.
As of today, the kilogram is defined using Planck 's constant, which does not change with quantum physics.
However, the challenge now is to explain these new definitions to people – especially non-scientists – so that they understand. Comparing a kilogram to a block of metal is easy.
Technically, a kilogram (kg) is now defined: "[…] taking the fixed numeric value of the Planck constant h to be 6,626 070 15 × 10-34 when it is expressed in the unit J s, which is equal to kg m2 s-1, where the counter and the second are defined in terms of c and Δνcs. "
Try to explain this to someone!
The kilogram is being redefined – says a physicist
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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The way we define kilograms, meters and seconds is changing today (May 20, 2019)
recovered on May 21, 2019
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