Why Definition of Kilogram has been Changed?

Till two days back, kilogram—the unit to measure mass was defined by a hunk of metal in France. This hunk of metal, also known as the International Prototype Kilogram or Big K, is a platinum-iridium alloy having a mass of 1 kilogram housed at the Bureau of Weights and Measures in France since 1889. The IPK has many copies around the world and are used to calibrate scales to make sure that the whole world follows a standard system of measurement.

But the definition of the Kilogram will no longer be the same. On the International Metrology Day this year, the way a Kilogram has been measured for more than a century has been changed completely. Now, the kilogram would be defined using the Planck constant, something that does not change. Technically a kilogram (kg) is now defined as, "[…] by taking the fixed numerical value of the Planck constant h to be 6.62607015×10-34 when expressed in the unit J s, which is equal to kg m2 s-1, where the meter and the second are defined in terms of c and ΔνCs. "

Even though the definition might look too complicated, which it of course is, but one need not worry much. The change will not be noticeable—nothing is going to be heavier or lighter than it was yesterday. The shift to the new norm has been made seamless.

Why the Change?

It is the Big K that makes sure that a kilogram is a kilogram, no matter where it is measured. But the Big K had a problem, that it is a man-made object and therefore, it can be subjected to changes over time. The Big K has lost around 50 micrograms of its mass since it was created. Even with this tiny amount of loss, the Big K changes and everything else everywhere has to be changed. This in turn can be quite chaotic.

Won’t it be better if the measurement is based on some natural constants, like the speed of light or constant of gravity etc.? That was the impetus for the development of the new definition and when scientists met at the General Conference on Weights and Measures in Versailles in November last year, they voted for the change. With this, the world has moved a step forward in realising the founding dream of the metric system. The metric system, which evolved into the International System of Units or the SI—was primarily designed to be ‘for all times, for all people’.

The Planck constant is a quantum mechanical concept. Precisely, the Planck constant describes how the electrons release energy in discrete quanta or chunks and it never changes. Every unit of the Planck constant is described by an unchanging force of nature.

By defining kilogram with the Planck constant, the inconsistencies related to measuring things with some man made object would go away. But changing the definition would not change the amount of mass a kilogram used to have previously. The new kilogram will have the exact mass as the old kilogram used to have. This is because scientists used the old kilogram to measure the value of the Planck constant.

Also, now on, new definitions for electric current, temperature and amount of substance, time would be in place. But unlike the kilogram, these were not based on a physical object earlier as well. All of these are based on fundamental constants in nature.