This article was originally published in issue 28 of The Print – The official newspaper of the Queen Mary Students’ Union – and online at theprintnews.co.uk in April 2019.
On the 20th of May 2019, the definition of the kilogram will change. Since 1800 the kilogram has been defined as the mass of a particular object, the latest of these being the International Prototype Kilogram (also known as ‘Le Grand K’) which was manufactured in 1889. This lump of platinum-iridium alloy (Pt-10Ir) resides in a sealed, environmentally controlled chamber, alongside 6 sister copies, basement in the outskirts of Paris.
Approximately every 40 years these masses are compared to one another, which reveals that the mass of Le Grand K is shrinking over time. There are many factors contributing to this change, but the entire process behind it is not fully understood. There is also a bigger change in mass between Le Grand K and the numerous other prototypes that have been distributed to governments around the world, believed to be largely due to the prototypes leaving a few atoms behind every time they are placed on the scales.
The fact that Le Grand K is shrinking therefore meant that the entire definition of a kilogram was changing as well – the mass of Le Grand K is that definition after all. This was enough of a problem that the International Bureau of Weights and Measures, the organization in charge of the International System of Units (SI, from French) decided that we needed a different solution.
As of May 2019, the kilogram will instead be defined from three invariant constants of nature: the speed of light in a vacuum, the natural microwave radiation emitted by a caesium atom and Planck’s constant, a value that relates the frequency of a photon to the energy it carries. By defining the kilogram with respect to these values, our definition will only become more accurate alongside our measurements of these values.
These constants weren’t just picked out of a hat. The inverse of the speed of light in a vacuum (1/299,792,458 m/s) is used as the SI definition of a metre, a definition implemented in 1983, whilst the natural microwave radiation emitted by a caesium-133 atom has been used to define the second since 1967. Planck’s constant is defined as 6.62607015 x10^-34 kg m2 s-2 which means that given the definition of the metre and the second, we can easily obtain the definition of a kilogram from this measurement.
Three other SI units are also being redefined alongside the kilogram: the Ampere with relation to the elementary charge (the charge of an electron), the Kelvin with respect to Boltzmann’s constant, a value which relates the temperature of a gas with the average kinetic energy of its particles, and the Mole with respect to the Avogadro constant, the amount of a substance of exactly 6.02214076×1023 entities, usually atoms or molecules. These definitions were agreed upon on the 16th of November 2018, however for legal reasons are only being adopted in May.
These four definitions, alongside the definitions of the second, the metre, and the candela, a measurement of relative intensity defined using the luminous efficacy constant; make up the seven SI base units, which are used to describe measurements in all areas of science today.
By redefining these units, scientists hope to develop new technologies that utilise these definitions to make more accurate measurements, as well as to reduce the cost of calibrating previous equipment.
What does this mean for Le Grand K? Since scientists are still unsure as to exactly why Le Grand K is shrinking, they plan on continuing to measure them to see how they fare against the test of time.