Why We Want to Redefine Kg???

What we call a kg might soon change in future.? Yes you heard right, a substantial change to the International System of Units (SI) is currently under discusstion and might be a reality by 2019. The General Conference on Weights and Measures (CGPM) is scheduled to have its 26th meeting in 2018 and will likely vote on a proposal put forward by the International Committee for Weights and Measures (CIPM) to revise the SI. According to this proposal, the structure of the SI will change fundamentally. The present SI is built upon seven base units: the metre, the second, the kilogram, the ampere, the kelvin, the mole, and the candela. The goal of the new SI is to define all of these units completely in terms of fundamental constants with exact values. Some constants, such as the speed of light, are currently defined in this way, as exact quantities.

At present, the kilogram is the SI unit of mass and is equal to the mass of the international prototype of the kilogram which is an artefact, not a constant of nature. The kilogram is the only SI unit still based on a physical object. The kilogram will be defined in terms of a quantum-mechanical quantity known as the Planck constant (h) which will be assigned an exact fixed value based on the best measurements obtained worldwide.

The International Prototype Kilogram(IPK) or “Le Grand K” or “Big K” is the palm-sized platinum and iridium cylinder that for 126 years has defined the kilogram from a high-security vault outside Paris where it was protected from dust, moisture, fingerprints, and other corruptions of the outside world and was hailed as the “perfect” kilogram”. Since hydrocarbons on fingertips or moisture in the air could contaminate its pristine surface, it goes untouched for decades, under triple lock and key at the International Bureau of Weights and Measures. Every 40 years, the weight is ushered from its chamber, washed with alcohol, polished, and weighed against 80 official replicas hand-delivered from laboratories around the world.

It is seen that world’s most perfect weight isn’t so perfect anymore. In its most recent weigh-in in 1988, it was found to be 0.05 milligrams,about the weight of a grain of sand, lighter than its underling replicas. But Le Grand K’s guardians are not sure if it is Le Grand K that has slimmed, or whether its comparators have gained weight. Whatever is right ,its not good for a standard to change. You may think why should we care whether a kilogram in a vault is “perfect” or not? We must, as a small discrepancy can become a gargantuan one if you’re dealing with a large quantity, for eg. a full tanker of Rice. Also the kilogram is used as a building block in other measurements. As in the case of the joule( the amount of energy required to move a one-kilogram weight one meter) and candela, a measure of the brightness of light, is measured in joules per second. Therefore if the kilogram is flawed, so are the joule and candela, which could eventually cause problems in an array of industries, particularly in technology.

The slighter instability of big K is unacceptable in precise fields like medicine or engineering, where tiny differences can cause immense problems. Therfore a more reliable standard has to be defined for measuring kg. For instance, the meter, which was originally represented by a metal bar, was redefined in 1983 as the distance light travels in a vacuum during 1/299,792,458 of a second. However standardizing the kilogram is little trickier as the standard could not be changed physically due to environmental changes, accidental damage, or even sabotage. To overcome such risks and also to improve our system of units, we need a definition that depends on nothing other than the value of a fundamental constant, e.g. the Planck constant h, a fixed quantity linked to E=MC2 and quantum theory that specifies the amount of energy carried by a single particle of light, or photon.

To be continued....