The kilogram has been measured the same way for the past 130 years, based on a cylinder constructed from a platinum-iridium alloy (proportionately 90 prevent platinum and 10 percent iridium). This internationally agreed measure has been stored carefully in a suburb of Paris called Saint Cloud, at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). The cylinder is officially known as the International Prototype Kilogram (or often ‘Le Grand K’ or IPK). The cylinder has served as the official definition of a kilogram, which is the internationally accepted basic unit of mass.
Before a consensus was reached around the cylinder, the kilogram was defined as the mass of a liter (cubic decimeter) of water. Around the same time, the gram, 1/1000 of a kilogram, was defined as the mass of one cubic centimeter of water at the melting point of ice. As these were near impossible to replicate, in 1799 a platinum artifact was fashioned to define the kilogram. This has been updated a few times, with the current cylinder created in 1889.
Very few people have seen the kilogram, since the the IPK is stored in a vault nearly all the time and there is no general public access to the vault. For general calibration assessments, working copies of the IPK tend to be used. The image below is a computer simulation of what the IPK is thought to look like:
Mass vs weight
While the terms “mass” and “weight” tend to be used interchangeably in everyday conversation, they mean different things. Mass is the amount of matter in a material while weight is a measure of how the force of gravity acts upon that mass.
As an example, an object with a mass of 1.0 kilogram weighs approximately 9.81 newtons on the surface of the Earth. However, the same object weight would be less on Mars, as gravity is weaker; however, on Saturn, the object would weigh more than it does on Earth. This is all due to different gravitational effects.
New way of assessing the kilogram
From May 20, 2019 (World Metrology Day) there is a new approach for assessing mass. From this date, all of the basic units of measurement will be defined in terms of atomic properties and fundamental physics constants, instead of human-made objects like the Parisian platinum-iridium cylinder. The new definition was approved by the General Conference on Weights and Measures (CGPM) on 16 November 2018 in Versailles.
One reason driving the change is a desire for even greater accuracy when it comes to assessing mass, which is a product of innovations with drug development, nanotechnology and precision engineering. According to MIT News, over its lifetime, that official kilogram has lost about 50 micrograms.
New definition – natural?
With the new assessment, this is no longer a defined metal object held in one part of the world but instead a fixed numerical value of a fundamental constant of nature called the Planck constant. The constant relates the energy of a photon to its frequency, and is coded by the letter h (a photon’s energy is equal to its frequency multiplied by the Planck constant and it is the smallest action an electron can take). This is defined as 6.62607015 times 10^34 kilograms times square meters per second. In other words, the new definition of a kilogram corresponds to the mass of an exact number of particles. As to why this has been used, like the speed of light, the value of Planck’s constant cannot fluctuate. The value is built with exquisite precision into the very fabric of the universe.
According to physicist Wolfgang Ketterle: “Conceptually, the explanation is that 1 kg is now the mass of a defined number of photons, 1.4755214*10^40, at the frequency of the cesium atomic clock.”
This shift in assessment now defines the kilogram in terms of the second and the meter, which means the kilogram is now also defined only in terms of fundamental physical constants. This forms the basis of the International System of Units.
Ever other metric weight has been assessed in similar terms, meaning that the kilogram is the last base unit to be re-defined by a fundamental physical property to replace a physical artifact.
Essential Science
This article is part of Digital Journal’s regular Essential Science columns. Each week Tim Sandle explores a topical and important scientific issue. Last week, we considered how big data analytics is being used to assess and to make predictions about human health.
He week before, we saw how new research presented the first clinical results with CAL02 in patients suffering from severe pneumonia, the first cause of infectious mortality in the world.
