« What’s heavier a kilogram of steal or a kilogram of feathers? »

I think we’ve all been confronted with this humorous riddle, that made us turn back and figure out what’s a kilogram?

Unfortunately, this riddle will quickly vanish starting from 20 May 2019 when Planck’s constant will officially come into force and replace our King of Kilograms.

Yes! The world officially announced on 16 November 2018 at a meeting on Versailles, France that the reign of the king of kilograms has come to an end, after serving for 140 years as a base unit for weight… So what happened?

The kilogram is one of the seven units of measurement in the SI “ Le système international des unités”: Meter, Second, Mole, Ampere, Kelvin, Candela, Kilogram. These units assure the stability of the industry, the trade, and the scientific world. However, the kilogram was the only unit defined by a physical artifact: a platinum-iridium cylinder known as Le Grand K.

Since 1898, it has been carefully sealed in a vault in Paris so that all mass measurements on the planet can be traced back to it. But as mass measurements became ever more nuanced, scientists started to worry about the cylinder’s mass changing. Even minute mass drifts – due to deposition of particles, for example – change the unit as a whole. This may not be troublesome for non-scientific practices but in medicine and astronomy, this can lead to dangerous outcomes.

As a result, in 1999, metrologists agreed to link the kilogram to the Planck constant; in other words, one kilogram as the amount of mass needed to balance a scale being pulled by an electromagnet using a specific amount of electricity. As a quantum mechanical number derived from nature the constant will – or so it is hoped – be stable for the duration of the universe. Thanks to Einstein’s equation that links mass with energy this transition would be feasible, find the explanation below.

Defined by the Planck constant h (6.626 070 15×10−34kgm2/s), a quantum mechanical quantity that relates mass to energy via E = mc2 with h connected to mass via E = hf

Basically, this change will impact the world of industry and the production of medicine and nanotechnology, which means that for daily practices the kilogram will still hold his reign!

```Sources:

Quartz (2018) : The weight formerly known as the kilogram is dead