The kilogram (kg) is the basic unit of mass of the International System of Units (IS) and is defined as being equal to the mass of the International Prototype Kilogram, IPK, called “Le Grand K”, almost equal to a liter of water. This prototype is composed of iridium and platinum and is in the custody of the International Bureau of Weights and Measures (BIPM) in Sèvres, France since 1889, when it was sanctioned by the General Conference of Weights and Measures (GFCM).

Prototype of Le Grand K

Before adopting the IPK, the kilogram was defined as the mass of one liter of demineralized water at fifteen degrees Celsius. This method was abandoned, however, after it was realized that the mass of the same amount of water varied according to its purity.

The Americans had known for some time that there were some problems with France’s “Le Grand K”: it’s getting lighter. The kilogram model is kept under protection, but every 40 years it reappears: eighty replicates of it are created, which are compared to the kilo model present in laboratories around the world. The last time he appeared, he was 0.05mg less – a problem, since his mass simply can not vary.

And why is it getting lighter?

Experts are not sure where these milligrams were, but some theorize that the replicas are manipulated more often, which can change mass to them subtly. Others postulate that Le Grand K’s metal alloy is “losing gas”, meaning air is gradually escaping from the metal.

The stability of the international prototype of the kilogram, the IPK, is of paramount importance, since the kilogram unit sustains a large part of the IS derivatives. For example, Newton is defined as the force required to cause acceleration at one kilogram to one meter per second squared. If the mass of the IPK changes, it will also change the newton and all other units that derive from the kilogram. The pascal, IS pressure unit, is derived from Newton, which in turn is derived from the kilogram; Thus, the pascal is defined indirectly by the kilogram. This chain of dependencies follows by many other units. For example, the joule, unit of work, is defined by the energy expended by the action of a force along a displacement. The equation below shows how the unit joule is dependent on the unit kilogram:

That is why scientists are eager to redefine the kilogram according to a fundamental constant of nature. Unlike a physical artifact, whose mass changes slightly over time, a fundamental constant is just that: constant and unchanging throughout the universe.

Proposed changes to the standard

Planck’s constant (h) is at the heart of quantum mechanics. Atoms can only supply energy in specific quantities, known as “quanta” – just as money is subdivided into specific values ​​of money. This amount of energy is proportional to Planck’s constant.

It is a very small number and, therefore, extremely difficult to measure accurately. Using a high-tech version of a two-plate scale, NIST scientists reached a value of 6.62606983 x 10-34 kg∙m²/s, with precision of 34 parts per billion. Once results from several countries are obtained, they will be entered into a computer that will analyze the data to arrive at the best overall value of the Planck constant. Then, it will become the new base for the kilogram from 2018.

Most people will probably not notice when this happens, but scientists will be much more confident in making sensitive measurements in the future – and they will have to thank a number of scholars who have worked for years to ensure that the kilogram remains truly constant forever.