Explained: How the kilogram has changed, why your body mass has not

Kabir Firaque
In India, schools and technical institutes have been advised to incorporate the change in their syllabi.

The kilogram is no longer what it used to be. It still means the same amount of mass as before, but the way it is defined changed across the world on Monday, World Metrology Day.

In India, schools and technical institutes have been advised to incorporate the change in their syllabi. The National Physical Laboratory (NPL), custodian of the fundamental units of measurement, has sent recommendations to the NCERT, the All India Council for Technical Education, the IITs, the NITs, and other institutions.

How has the definition changed? Will it affect the way we measure body mass or weigh our groceries?

Why the change

The global standards for measurement are set by the International Bureau of Weights and Measures (BIPM), of which India became a member in 1957. At BIPM in S vres, near Paris, stands a cylinder of platinum-iridium locked in a jar. Since 1889, the kilogram has been defined as the mass of this cylinder, called Le Grand K, or International Prototype Kilogram (IPK). In India, NPL maintains the National Prototype Kilogram (NPK-57), which is calibrated with IPK.

The IPK was the last physical artifact used to define any of the fundamental units. What guarantee is there that that the IPK kept at BIPM has not changed? NPL Director Dinesh K Aswal told The Indian Express. IPK would put on a little extra mass when tiny dust particles settled on it; when cleaned, it would shed some of its original mass.

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, Aswal said. The Planck constant is a quantity that relates a light particle s energy to its frequency.

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, 2019, Aswal said.

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What does not change

What was 1 kg earlier is still 1 kg today. A person hoping to lose weight would still need to shed the same number of kilos she had targeted earlier, and a shopper would not be paying any more or less for their groceries.

All that has changed is the definition, for the sake of accuracy. As Aswal explained, a mass measured as 1 kg earlier would have meant 1 kg, plus or minus 15-20 micrograms. Using the new definition, a mass measured as 1 kg will mean 1 kg, plus or minus 1 or 2 nanograms .

Measure for measure

The new definition for kilogram fits in with the modern definitions for the units of time (second) and distance (metre). Today, the second is defined as the time it takes for a certain amount of energy to be released as radiation from atoms of Caesium-133. Once the second was defined, the metre fell into place. By its modern definition, a metre is the distance travelled by light in vacuum in 1/299,792,458 of a second (which is already defined).

This is where the Planck constant comes in. It has been measured precisely at 6.626069… 10^(-34) kilograms per second per square metre. With the second and the metre already defined, a very precise definition for the kilogram follows.

Along with the units of time and distance, the unit of luminous intensity (candela) is already defined in terms of a natural constant. On Monday, along with the kilogram, the units of current (ampere), temperature(kelvin ), and amount of substance (mole) too took on new definitions. That covers all seven fundamental units.

The modern definition of the second has already helped ease communication across the world via technologies like GPS and the Internet. Scientists have often been quoted as saying the change in the kilogram s definition will be better for technology, retail and health.