Change in the definition of ‘Kilogram’

Why in news?

The Definition of the Kilogram is about to change by redefining the International system of units(SI).

How does the measurement of kilogram evolve?

• There are seven fundamental units and every other unit of measurement can be derived from one or more of these seven units.

• Three of the seven fundamental units are already based on unchanging properties of nature.
• These are the second (time), the metre (distance), and the candela (luminous intensity, a measure for light’s brightness).
• Hence, scientists want to create a measurement system that is based entirely on unchanging fundamental properties of nature.
• The first kilogram (originally called a grave) was defined in 1793 by a commission of the French Academy of Sciences, who wanted a better standard than the fixed amounts of grain that had traditionally been used.
• The commission decided that the new measure would be the mass of one cubic decimetre of distilled water at 4 degree celcius  (the temperature at which water has its highest density under standard conditions).
• This had the advantage in that most properly equipped labs would be able to reproduce this standard.
• Subsequently, a prototype of this mass was cast in brass.
• Unfortunately, this definition/calculation of mass depended upon another variable measurement, the metre.
• At this point, the metre was only provisionally defined as part of the distance from the North Pole to the equator.
• However, once the value of the metre and the temperature of water at its densest were more accurately defined, a new prototype was cast in platinum to represent this mass(kilogram).
• These variable measurements were finally replaced with the international prototype kilogram (IKP), used today, which is a metal cast from a mixture of platinum and iridium to make it very hard and prevent it reacting with oxygen.
• Since 1889, countries who are members of the General Conference on Weights and Measures have agreed to use this standard block of metal kept near Paris to define the kilogram.
• This made the kilogram to be the only base unit in the SI still defined by a physical object.
• Six Copies of the IKP are transported across the world to ensure all participating countries use the same standard.
• But although this metal is stored in a highly controlled environment, its weight can change by tiny amounts as wear and tear causes it to lose mass and dirt causes it to increase.
• Hence, even the modern IPK to measure the kilogram can gradually change in mass.

What is the proposed measure?

• To address this problem, scientists around the world have spent nearly two decades discussing how the kilogram could instead be defined in relation to constant measurements of nature. 
• So they decided that instead of measuring the kilogram against a block stored in a vault, it should be based on precise values of constants of nature.

• Thus the kilogram’s definition is set to change and the new definition of the kilogram uses a measurement from another fixed value from nature, Planck’s constant (h).
• Planck’s constant will be defined as 6.62607015×10−34 joule seconds and can be found by dividing the electromagnetic frequency of a particle of light or “photon” by the amount of energy it carries.
• The constant is usually measured in joule seconds but this can also be expressed as kilogram square metres per second.
• Since 1967, the second has been defined as the time it takes for a certain amount of energy to be released as radiation from atoms of Caesium-133.
• This became the basis of all measures of time, and is used in atomic clocks.
• The SI unit of the metre is also based on another universal constant, namely the speed of light.
• The metre is defined as the distance travelled by light in vacuum in 1/299,792,458 of a second (which is already defined).
• Thus, since definition of a second and a metre have already adjusted to universal constants, by adding these measurements, along with an exact knowledge of Planck’s constant, a very precise definition of the kilogram can be reached easily.

Does this redefining really help science?

• The change in definition of the second, previously, has helped ease communication across the world via technologies like GPS and the Internet.
• In the same way, the change in the kilogram will be better for technology, retail and health.
• For most people, everyday life will carry on as normal despite the redefinitions.
• One standard bag of sugar will contain as much sugar as it ever did.
• But some of these changes will mean practical advantages for scientists making very precise measurements.
• Thus, to answer the question how much is a kilogram, we will no longer have to compare blocks of platinum or worry about scratching them.

Source: The Indian Express, The Wire