How much is a kilogram? Here comes a new way to measure it

The News

• The world’s measurement system will undergo a significant transformation as representatives of the 2018 General Conference on Weights and Measures will redefine some fundamental units including ‘kilogram’ based entirely on fundamental properties of nature.

Key Highlights

• The 2018 General Conference on Weights and Measures, being held in Versailles, France, is historic because the representatives will redefine 4 out of the 7 base units for the International System of Units (SI) used for measurement.
• The 4 fundamental units being redefined are:
  • Kilogram: SI unit for mass
  • Ampere: SI unit for current
  • Kelvin: SI unit for temperature
  • Mole: SI unit for amount of substance
• This is significant because from now on the world’s measurement system will not depend on physical object but be based entirely on unchanging fundamental properties of nature.

Background

Metre Convention

• In accordance with the ‘Metre Convention’, Bureau of Weights and Measures was created in 1875.
• The ‘Metre Convention’ set the first international standardization of measurements.
• India became a signatory to Bureau of Weights and Measures in 1957.
• The SI system was adopted in 1960 in accordance with ‘Meter Convention’.

About SI units

• The International System of Units or SI unit has seven “base” units.
• The base units define measurements of time, distance, mass, electric current, temperature, the amount of a substance and luminous intensity.
• They are called base units because using them which all other measurement units can be derived.
• While some of these base units such as the second and the meter are based on fundamental constants of nature, others like the units for mass and temperature are based on some physical objects or some artificial setup on Earth.

Redefining ‘base units’

Kilogram: Current Definition

• ‘Kilogram’ is the SI unit based on which everything that has mass is measured.
• Currently ‘one kilogram’ is defined as the mass of a cylinder made of platinum-iridium kept in Bureau of Weights and Measures (BIPM) in Sèvres, near Paris.
• The International Prototype Kilogram (IPK) cylinder is called as ‘Le Grand K’.

Redefining Kilogram

• The new definition of kilogram will be based on a constant called the Planck’s constant.

About Planck’s constant

• Light as we know is made up of packets of energy called photons.
• The size of these energy packets that make up light is defined as Planck’s constant.
• In the equation E = hf,
  • E is the energy of each packet of light, measured in Joules = kilogram meter square per second square;
  • f is the frequency of light, measured in hertz or one cycle per second.
  • h is the Planck's constant with the unit kilogram metres square per second.
• This constant therefore describes how to take the frequency of light and use it to determine the size of the packets of energy (or photons) it contains.

Why is the definition being changed?

• The IPK International Prototype Kilogram is a physical object prone to wear and tear and thus susceptible to gaining or losing mass at any time.
• The SI unit of Planck’s constant as seen above is kilogram metres square per second.
• As Planck’s constant is a fundamental “constant” of nature based on light it is unvarying over all and space.
• Accordingly ‘kilogram’ measured using Planck’s constant will also be unvarying over all and space.

Redefining temperature, ampere and mole

• Similarly the definitions of temperature, ampere and mole are being redefined on the basis on fundamental constants of nature that are not subject to variation in time and space.
• The constant used for temperature is ‘Boltzmann’s constant’, that for mole is ‘Avogadro’s constant’ and that for ampere is based on a constant based on ‘charge of an electron’.