How we measure the world underwent a quantum leap as scientists adopted new hyper-accurate definitions for the units of weight, electricity and temperature derived from the universal laws of nature.

The change, on the occasion of World Metrology Day, sees one of science’s most influential objects - a metal cylinder used to define what a kilogram is - retired in favour of an infinitely more precise measurement taken from a quantum ratio.

India adopted a global resolution to redefine four of the seven base units - kilogram, kelvin, mole and ampere, a move expected to have far-reaching effects, including changes in textbooks.

How was the kilogram measured earlier?

Representatives of 17 nations came together to create the International System of Units by signing the Metre Convention treaty on May 20, 1875. Scientists created a cylindrical hunk of mass from platinum and iridium. Since 1889, the kilogram has been defined as the mass of this cylinder, called Le Grand K, or International Prototype Kilogram (IPK).

All other prototypes that served as national reference standards, including the one at New Delhi’s CSIR-National Physical Laboratory (NPL), were calibrated to it.

The BIPM (International Bureau of Weights and Measures) is an international organisation established by the Metre Convention, through which member states act together on matters related to measurement science and measurement standards. India became a member of BIPM in 1957.

When was the method changed?

Scientists have long stressed that the fundamental units should be defined in terms of natural constants. On November 16, 2018, following a vote at BIPM, representatives of 60 countries agreed that the kilogram should be defined in terms of the Planck constant.

What is the Planck constant?

Planck constant is the ratio of a frequency of light, on the one hand, to the quantum energy of that frequency, on the other. Put another way, it is 6.626 x 10-34 joule seconds.

Energy is intrinsically linked to mass, as Einstein demonstrated with his equation E = mc squared.

The Planck constant, combined with two quantum phenomena that allow for the creation of electrical power, can be used to calculate mass based on the equivalent mechanical power needed to displace it.

Proponents of this approach say it will be at least 1 million times more stable than physical artefacts and will have a range of practical applications, from pharmaceutical measurements to scientific research.

Using a machine called a Kibble balance, in which the weight of a test mass is offset by an electromagnetic force, the value of the Planck constant was fixed, the kilogram was redefined, and the date for the new definition was fixed for May 20.

The NPL, which is India’s official reference keeper of units of measurements, on May 20, released a set of recommendations requiring that school textbooks, engineering education books and course curriculum update the definition of the kilogram.

How will it affect our weighing mechanisms?

There won’t be a major change in the measurement of kilogram in day-to-day life. The quantity of rice or vegetables that was bought earlier remains the same. It’s the definition that undergoes a change. Experts say it will guarantee their stability and universality in the long run and help in coming out with accurate data in all sectors.

What are the other units that have changed?

At the General Conference on Weights and Measures (CGPM) in November, representatives from 60 countries also voted to redefine the ampere, the kelvin and the mole.

* The ampere will be defined by the elementary electrical charge (e)

* The kelvin will be defined by the Boltzmann constant (k)

* The mole will be defined by the Avogadro constant (NA)

The other three International System of Units (SI) are second, metre and candela, which will remain unchanged.