The Kilogram Is Dead: Long Live the New Kilogram

The kilogram got a new physics-based definition.
Jessica Miley

The kilogram does not mean exactly what it meant once. In a big change for the science world, the definition of the kilogram has been updated - to a measure far more accurate than ever before.

The decision to overhaul the way we previously understood the kilogram was made at 2018's General Conference on Weights and Measures in Versailles and put into practice in 2019.

Accurate measures that are necessary for good science

Metrology - the science of measurement - is critical to all other branches of science. It dictates how we understand and observe our world and its critical to have a precise and constant measurement system so that the scientist across the world can work in synch and share knowledge.

The new changes affect not only the kilogram but the ampere (current), kelvin (temperature), and mole (amount of substance). Each of these and the kilogram got a new method of measurement that uses the laws of physics.

Stable and constant is key

By doing so, the metrologists believe that a more constant and stable measurement can be achieved around the globe. Currently, distance is used in physics to determine its exact length. An official meter is the distance light travels in a vacuum in 1/299792458 of a second. A second is determined by the time it takes for a caesium atom to oscillate 9,192,631,770 times.

Until the big update, a kilogram was well, a kilogram. Literally, a chunk of material that all other kilograms were determined by. The official kilogram was made from 90 percent platinum and 10 percent iridium and was formed in 1889. 

The Kilogram Is Dead: Long Live the New Kilogram
Source: NIST

Old-school kilogram measure is on display

It is housed under super strict conditions in a special vault in the BIPM headquarters. There are copies of the official kilogram in various parts of the world to help scientists locally get an accurate measure. But this pretty archaic system has been changed.

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The new kilogram is based on the Planck constant, the ratio of energy to frequency of a photon, measured to its most precise value. So, under the new definition, the magnitude of a kilogram is "set by fixing the numerical value of the Planck constant to be equal to exactly 6.626 069… × 10–34 when it is expressed in the SI unit s–1 m2 kg, which is equal to J s."

But don’t stress if that all seems a little complicated. The change in kilogram can't affect your Jenny Craig status. In fact, it can't affect your daily life at all. But for the future of precise and constant scientific experimentation - this was a very massive step forward indeed. The change officially came into force on World Metrology Day which fell on May 20, 2019.

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