New Definition of Kilogram

The 26th General Conference on Weights & Measures (GCWM), which is comprised of 60 member countries, voted for the redefinition of Kilogram. It will be adopted on May 20, 2019 which is World Metrology Day.

  • Now it will be affixed to the Planck constant, a fundamental concept in quantum mechanics that can never, ever change — here on Earth or in the deep reaches of the universe.
  • The existing definition of the kg is over 130 years old and defined as the mass of a hunk of platinum-iridium alloy that’s been housed at the International Bureau of Weights and Measures in Sevres, France since 1889. It’s called the International Prototype Kilogram (a.k.a. Big K, or Le Grand K), and it has many copies around the world.
  • One important reason for the change is that Big K is not constant. It has lost around 50 micrograms (about the mass of an eyelash) since it was created. But, frustratingly, when Big K loses mass, it’s still exactly one kilogram, per the current definition.

Planck’s Constant

Planck’s constant is a number that describes the size of the energy packets (or ‘quanta’) that are contained within light. These packets of energy are called photons. Planck’s constant is given the symbol h in physics and numerically is equal to 6.63 x 10^-34 Joule seconds.

The equation that defines Planck’s constant is called the Planck-Einstein relation and represented as “E = hf”, where E is the energy of each packet (or ‘quanta’) of light, measured in Joules; f is the frequency of light, measured in hertz; and h is Planck’s constant.

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