Kilogram, redefined: Why is the new global definition of mass so great a realization

Monday, May 20 – World Metrology Day – Humanity will have a new definition of the standard kilogram, the standard mass unit used around the world.

The change will occur quieter than when a leap second has been added to a year. It will not disturb anything. The number on your bathroom scale will not change, for better or for worse.

But it still represents an impressive feat: a victory of humanity against the chaos that reigns in the universe.

When scientists met at the General Conference of Weights and Measures at Versailles in November and voted for change, they realized the founding dream of the metric system. The metric system – which has evolved to become the International System of Units (SI) – was designed to be "forever, for everyone".

So far, the kilogram has not been used forever. It's imperfect. But it will be from Monday, May 20th.

Until now, the definition of the kilogram was a piece of metal in Paris

For more than a century, the kilogram had a very simple definition: it was the mass of a piece of platinum-iridium alloy hosted at the International Bureau of Weights and Measures of Sèvres, France, since 1889.

It's called the International Prototype Kilogram (a.k.a Big K or The Big K), and There are many copies around the world, including many at NIST in Gaithersburg, Maryland, used to calibrate scales and ensure that the world uses a single measurement system.

These copies ensure that one kilogram is one kilogram – measured in the aircraft manufacturer's factory or on the digital scale at your grocer's checkout. And for those who think that the kilogram does not matter in the United States, which use imperial units such as pounds, feet and gallons, our measurements are calculated from SI units. Officially, in the United States, 1 pound is set at 0.45359237 kilogram.

The problem is that Big K is a man-made object and therefore imperfect. If Big K changes, everything else needs to be adjusted. And it happened. Big K is not constant. He lost about 50 micrograms (about the weight of an eyelash) since its inception. But, frustrating, when Big K loses mass, he remains exactly one kilogram, according to the old definition.

This is not good. So what's better?

The new definition anchors the value of the kilogram in a constant nature, which can never, never change

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As of Monday, the kilogram will be defined by the Planck constant.

Planck's constant is a concept of quantum mechanics (that is, the study of the operation of the tiniest components of the universe), which describes how the tiniest pieces of matter release energy in stages. or in pieces (called quanta). Basically, you can consider Planck's constant as the smallest action that an electron can take.

The important thing in Planck's constant is that it can never, never change. And this makes it a valid concept for anchoring the definition of the kilogram.

But first of all, to understand why the Planck constant can define the kilogram, it is useful to see how the counter – the standard length unit of the world – has been redefined in terms of the speed of light and is an example of the need to do it.

The counter was originally defined as the length of a bar at the International Bureau of Weights and Measures in France. (It was then redefined to match a certain wavelength of radiation.) Again, the problem with this definition was its vagueness. This was not based on the immutable properties of the universe.

The speed of light, on the other hand, is immutable. In 1983, physicists had become very good at measuring the speed of light. They have therefore used it to fix the counter's length forever, to make it permanent.

Here is how: They have redefined the meter according to the distance traveled by the light in 1/299 792 458 of a second. Essentially, the definition of the meter is now incorporated into the definition of the speed of light.

So back to Planck's constant.

To understand, let's examine it. Written, Planck's constant is 6.62607015 × 10^-34 m² kg / s.

You can treat units of this number – square meters per kilogram divided by seconds – as an equation. The kilograms are in the units and that's important. Once you know the value of Planck's constant and correct it forever, the value of the kilogram does not change either.

Each unit of Planck's constant is defined by an immutable force of nature. The meter is defined by the speed of light. The second is defined by the frequency of atoms in the cesium element. And once the value of the Planck constant has been measured and agreed upon, it means that the kilogram can also be adjusted.

And we know the value of Planck's constant, thanks to years of work to measure it accurately (to learn more, click here). It was an extremely difficult task: to measure, by and large, the smallest thing to measure. And it took decades, involving a beautiful and complicated machine called Kibble Balance.

Again, do not worry: the new kilogram has the same mass as the old one. This is because scientists used the old kilogram to measure the value of Planck's constant.

In doing so, they essentially anchored the old kilogram to the Planck constant. Just as the length of the meter was anchored to the speed of light. (Will also begin Monday with new definitions of electric current, temperature and quantity of substance.These are also linked to natural constants.But contrary to the kilogram, their definitions were not based on a physical object, Big K was the last one.)

If you ignore all of this, here's what all this change is: we will not need a government – the United States, France, anybody – or an international governing body to tell us what is a kilogram. This will be a fundamental truth of the universe, accessible to anyone with the proper equipment to carry it out.