# Does a Kilogram Really Weigh a Kilogram?

Image courtesy of BIMP

No one knows what a kilogram really should feel as a weight, you buy a kilogram of apples and that’s it.

A few people know about the “Big K”, and fewer about how it came to be or its relation to the kilogram as an international unit of mass.

The Big K, or “Le Grand K”, served as the International Prototype of the Kilogram (IPK) since 1889 when it was first sanctioned. It’s a cylinder about 39 mm in both diameter and height, made of an alloy of 90 percent platinum and 10 percent iridium.

The IPK is locked under three glass bells in an air-conditioned vault at the International Bureau of Weights and Measures BIMP) headquarters in Paris, France.

The U.S. National Institutes of Standards and Technology (NIST) stores two copies of the Big K: the first is called K20, which was assigned by BIPM to the U.S back in 1889, and the other, K4, is used to check the constancy of K20’s mass.

There are also many other official copies of the Big K in other countries to serve as national standards, and sometimes they get checked against the original prototype in France.

Now, we might start referring to the Big K as the former International Prototype of the Kilogram, because a revised definition has just come into effect.

## Adieu, Le Grand K: The Kilogram Redefined

In the International System of Units — abbreviated as SI, from French — the kilogram is the only unit that’s still determined by a physical object.

All other base units are defined by universal constants.

Take the meter for example, the SI unit of length. A meter is the distance a photon of light travels in a vacuum in 1/299792458 of a second. In its turn, a second is determined by a physical constant, and that’s the time for a cesium atom to oscillate 9,192,631,770 times.

Since May 20, 2019, we can say that we have a system of units that’s entirely defined by universal constants, as the kilogram has finally gone down that road.

The kilogram will now be defined based mainly on the Plank Constant, a universal constant of quantum physics that’s the same here on Earth and across the Universe.

Terry Quinn, Emeritus Director of BIPM explains:

“It is only now that we can define the kilogram in terms of a constant of physics – the Planck constant, the speed of light and the resonant frequency of the caesium atom. Why all three? This is because the units of the Planck constant are kgm2s-1, so we need first to have defined the metre (in terms of the speed of light) and the second (in terms of the caesium atom in the atomic clock).”

From now on, the SI unit kilogram will be “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.”

The three other redefined SI base units are the Kelvin (temperature) the Ampere (electrical current), the Mole (amount of substance):

• The kilogram will be defined by the Planck constant (h)
• The ampere will be defined by the elementary electrical charge (e)
• The kelvin will be defined by the Boltzmann constant (k)
• The mole will be defined by the Avogadro constant (NA)

## Why the Change, Will it Affect us?

Last year, metrology scientists from 60 countries met at the General Conference on Weights and Measures held in France, where they unanimously voted for the definitions of the kilogram, ampere, kelvin and mole to be changed, in a “landmark decision”.

Metrologists had a bit of a problem with the kilogram, or Le Grand K to be exact because it’s been losing mass.

They noticed that over 130 years of existence, Big K has lost around 50 micrograms at last check.

The change of definition of these four base units is now finally official.

“The idea is that by having all the units based on the constants of physics, they are by definition stable and unaltering in the future, and universally accessible everywhere,”  said Quinn.

When it comes to our shopping trips to grocery stores, these changes are of least importance for ordinary people. You’ll still get your kilogram-worth of anything like usual and you won’t notice any perceivable difference.

For metrology and industry, as we’re more and more into the infinitely small world of quantum mechanics, they’d better define their base units with pinpoint accuracy.

### Read More: World First: Scientists use Sound to Entangle Two Quantum Bits

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#### Zayan Guedim

Trilingual poet, investigative journalist, and novelist. Zed loves tackling the big existential questions and all-things quantum.