The world’s standard definition of four units – the kilogram, the ampere, the kelvin and the mole has been changed after 130 years, as decided by votes of representatives from 60 countries in the International System of Units (SI). So the definition of kilogram is changing from May next year all over the world.

In the latest conference of the International System of Units (SI) on November 16th, scientists and diplomats from 60 countries up-voted to re-define the metric system for four units – the kilogram, the ampere, the kelvin and the mole. The decision to change the definition of these units was made as the current system is 130 years old, which was formalised in the 1875 Treaty of the Metre. The existing unit of weight is determined by a shiny cylinder of platinum called “Le Grand K.” locked away in a French vault, and all existing scales were calibrated to the “physical reference”.

But, even solid matter change with time, and turn out to be unstable and the “Le Grand K.” had actually gotten a few milligrams lighter – compromising accuracy of the standard units. That is what we can call “a real pain in the mass” is. So that the reason of why the SI decided to ditch this old method of using physical objects to measure four quantities – mass (kilogram), current (ampere), temperature (Kelvin) and concentration (mole), and adapt to a new unit system that is based on constants in physics that tend to remain as unchanging constants, for long, long time. And, using constants of physics is no new idea as units for measuring distance (metre) and light-intensity (candela) do use them already.

The new kilogram will be defined by the Planck constant – the fundamental constant of quantum physics, rather than the physical bulk of platinum. The Planck constant is actually measured in joule-seconds but this can also be expressed as kilogram square meters per second. Since we already have unit of time (second) and unit of distance (metre), just adding the measurements with an exact knowledge of Planck’s constant – the extremely precise new definition of kilogram can be derived, anytime, anywhere and will remain stable for centuries to come. For this, an accurate weighing machines called ‘Kibble balance’ will be used to achieve a perfectly precise kilogram, as per new standards.

The change in definition of kilogram, and all three other units will be adopted globally (including India) on 20 May, 2019 – commemorating World Metrology Day. Now for most people, life will continue to be normal after redefinitions – a bag of sugar will have the same amount of sugar it has ever had. But the government will have extra loads of work, redefining the existing weighting system. For India, the Kibble balances will be used all over the country to replace existing kilograms with new constant ones – which will take some three to four years from May 2019 and will cost about Rs 60 crore.

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