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Scientists are redefining how much a kilo weighs

Julian Stratenschulte / Picture AllianceWeight is important. My awkward waist after vacation gives me enough proof of it, but it's…

 Research on the new prototype quota
Julian Stratenschulte / Picture Alliance

Weight is important. My awkward waist after vacation gives me enough proof of it, but it’s not enough for science.

On November 1

6, the General Conference on Weights and Actions – which, seeing, I will admit does not sound like the most nut event – begins in France. It is expected that at the conference, researchers will vote to change the definition of one kilo and apply it to one of the universe’s unchangeable phenomena: Planck Constant.

This sounds a little confusing, so let’s take a few steps back.

The most widely used measurement form in the world is based on the metric system and is officially known as the International System of Units (SI). Seven “base units” make these up, including amps, the other and the mill. Some of these measurements were once defined by physical phenomena, such as the other based on the rotation of the earth. Now the second is defined by radiation times in a cesium 133 atom.

The kilometer is the last base unit attached to a physical object.

The physical object is a metal piece that was constantly underground at the International Bureau of Weights and Measures, the International Bureau of Weights and Measures, in Sèvres, France. Platinum iridium alloy, also formally called International Prototype Kilogram and informal as Le Grand K, is like the ring in the Lord of the Rings – it’s the only way to control them all. Each weight is calibrated against Le Grand K, which standardizes the measurement of one kilogram worldwide.

But Le Grand K, erected 1889, has lost 50 micrograms in the past 129 years. The kilometer has become “999.99995-gram”. In addition, despite losing the small part of its mass, the kilogram is still defined by Le Grand K, and changes over time – and researchers do not like it. They want it to be constant, forever.

So one kilogram can be defined by an unchangeable universal phenomenon known as the Planck Constant, rather than a fool of metal in an underground vault. But how does it work?

The new definition of one kilo, if the vote passes, is admittedly scary to read, because it makes me feel that I understand almost nothing about the universe. Here it is:

It is defined by the fact that the fixed numerical value of Planck Constant h is 6,626 070 15 x 10-34 when expressed in unit J⋅s, which is equal to kgm2s -1, where the meter and the other are defined in terms of c and ΔνCs.


Planck Constant is a phenomenon in quantum mechanics that allows mass to be related to electromagnetic energy. It is one of the smallest actions in the physical world and finding it – measuring it – was a big challenge. The trip spanned decades and involved a machine called a Kibble balance that tries to compare no mass without energies.

“We are on a landmark of our trip,” said Dr Barry Inglis, President of the National Measurement Instutionary Australia.

“After years of research, it is now possible to motivate a major revision of SI. This decision, if taken, will constitute a significant scientific achievement.”

The most important of kilograms is the Planck constant only that: constant. Unlike Le Grand K, it will not be 50 micrograms different in 129 years. This means that the kilogram can always be bound to it – and whatever we go over the cosmos, the Planck Constant is expected to remain the same. Thus, the kilogram also comes.

And if all this still confuses you, it’s too long, read it all, quite simple: After decades of hard work and hypotheses, researchers – these genres men and women – have found a way to define the kilot based on a universal truth.

From Friday, the kilogram will always weigh one kilo.


The conference will also see voices taken to change the definition of ampere, mole and Kelvin. If you are interested, you can watch the worldwide event live on YouTube.

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