Just how much is a kilogram? Over the centuries, it has been defined and redefined, with a standard in place since 1889. Called Le Grand K, a cylinder of platinum-iridium is locked up in a jar at the International Bureau of Weights and Measures (BIPM) in Sèvres, near Paris. For nearly 130 years, the mass of this cylinder has been the international standard for the kilogram.

Not anymore. On Friday, representatives from 60 countries voted in Versailles, France, to redefine SI, or the International System of Units. The kilogram, the SI unit for mass, is one of four fundamental units that are being redefined, the others being the ampere (current), the Kelvin (temperature) and the mole (amount of substance). The kilogram’s definition will now be based on a concept of physics called the Planck constant. The changes come into effect on May 20, 2019. Why redefine the fundamental units? Because scientists want to create a measurement system that is based entirely on unchanging fundamental properties of nature. Le Grand K, the “international prototype kilogram”, is the last physical object used to define an SI unit. It is far from unchanging — it gets dusty and is affected by the atmosphere, and when cleaned, it is vulnerable to change, however minute that change is.

The Planck constant, on the other hand, is just that, a constant, if a complex one — it is a quantity that relates a light particle’s energy to its frequency. It is described in a unit that has the kilogram built into it. There are seven fundamental units. Every other unit of measurement can be derived from one or more of these seven units: the unit for speed, for instance, factors in the units for distance and time. While four of the fundamental units, including the kilogram, are being redefined, the other three are already based on unchanging properties of nature. These are the second (time), the metre (distance), and the candela (luminous intensity, a measure for light’s brightness).

**Measurement standards **

Humankind embraced the science of measurement millennia ago, with various civilisations deriving their own units. The Indian measurement of time, for instance, is widely recognised as the oldest in the world. It was only in 1875, with the creation of BIPM, that measurement began to be standardised internationally. A treaty called Metre Convention was signed among 60 countries, leading to international standards. The BIPM reports to the General Conference on Weights and Measures (CGPM), to which

India became a signatory in 1957. The SI system was adopted in 1960.The original definitions for the most basic units were uncomplicated. Until 1875, the metre was defined as 1/10 million of the distance between the North Pole and the Equator. The Metre Convention adopted a standard artifact (later discarded) — a platinum bar kept in Paris, the

“international prototype metre”. From the metre derives the centimetre, from the centimetre, in turn, derives the litre. Before Le Grand K, the kilogram used to be defined as the mass of one litre of water at freezing point.

The second was initially based on the length of a day of 24 hours; in 1956, the standard was set to a fraction of the solar year.

It was only in the middle of the 20th century that the more complex definitions began to be adopted.

**The new definitions **

Since 1967, the second has been defined as the time it takes for a certain amount of energy to be released as radiation from atoms of Caesium-133. This became the basis of all measures of time, and is used in atomic clocks.

Once the second was defined, the metre fell into place. This was based on another universal constant: the speed of light. Today, the metre is defined as the the distance travelled by light in vacuum in 1/299,792,458 of a second (which is already defined). The kilogram comes next. The Planck constant, which it is based on, is usually measured in joule seconds, but this can also be expressed as kilogram square metres per second, physicist Kevin Pimbblet explains in an article in The Conversation. “We know what a second and a metre is from the other definitions. So by adding these measurements, along with an exact knowledge of Planck’s constant, we can get a new, very precise definition of the kilogram,” Pimbblet writes.

Does all this redefining really help science? Indeed, the new definition of the second helped ease communication across the world via technologies like GPS and the Internet. In the same way, a Reuters report notes, “… experts say the change in the kilogram will be better for technology, retail and health — though it probably won’t change the price of fish much.”

**(This report, originally uploaded on Friday, was updated after voting took place) **