For nearly 150 years, the official weight of a kilogram is determined by a platinum cylinder locked in a safe in France.
The kilogram, like the meter and the second, is one of the basic measurement units, also known as the International System of Units. These units were formalized in 1975. It was then concluded that the best way to determine the kilogram was to make a piece of metal and call it "Le Grand K". And for more than a century, all scientific scales are calibrated according to that physical point of reference.
But even hard objects are changing over time. When this piece of metal was measured in the 1980s, several micrograms were easier, which means that all highly accurate scientific scales must be calibrated again.
Fortunately, scientists have already worked on the case and have demanded a universal constant that will give it a constant value of a kilogram and will be true even now, and millions of years later.
This number was found for a second that was defined in 1967. The second was defined as 1 / 86,400th of a second, in a bit puzzling but constant. The same was done with the meter. Instead of being defined as the length of a piece of metal done in 1889, the meter received a new definition in 1983 as the distance that passes the light at 1 / 299,792,458 a second of a second into a vacuum.
Only in 2017 scientists around the world finally agreed on a universal constant for the kilogram. This feat sought to solve one of the most difficult problems in physics from the past century - to determine the numerical value of the Planck constant.
Physicist Max Planck, in 1900, proved that matter emits energy in small pieces called quantums. Its equation for measuring these pieces of energy contains a constant h known as Planck's constant. Thanks to Einstein, we know that energy and mass are mathematically connected, so physicists have realized that Planck's constant can give the most accurate measure of mass.
Calculating the exact value of the Planck constant has lasted for decades and required serious technological innovation, but today it is known that its value is 6.626070150 x 10-34 kg x m2 / s.
In mid-November, at the annual meeting of the International Bureau of Weights and Measures in Versailles, France, representatives from over 60 countries voted to approve the new definition of a kilogram. There is no longer a need for the metal piece, now the kilogram is bound with Planck's constant. New definitions for both the amperes (electric current), the Kelvin (temperature) and the hammer (the number of molecules or atoms in the element) were announced. These definitions will come into force on May 20th, 2019.
The original prototype kilogram of platinum will remain in the French safe, while numerous generations of scientists will make their discoveries using the kilogram 2.0.