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Kilogram, redefined: Why the world's new definition of mass is such a great achievement



On Monday, May 20 – World Day of Measurement – Humanity will have a new definition of the kilogram, the standard mass unit used worldwide.

The change will appear quieter than when a leap second was added to a year. It will interfere with nothing. The number of your bathroom scale is not to be changed, for better or worse.

But still, it represents an impressive achievement: a victory for humanity against the chaos that permeates the universe.

When researchers met at the General Conference on Weights and Measures in Versailles in November and voted for the change, they were aware of the basic dream of the metric system. The metric system ̵

1; which evolved into the international system of entities or SI – was designed to be "for all time for all people."

Until now, the kilometer has not been forever. It has been incomplete. But from Monday May 20 it will be.

Until now, the definition of a kilo has been a metal knot in Paris

For more than a century, the kilometer had a very simple definition: It was the mass of a platinum-iridium alloy hunk built into the International Bureau for Libra and Measures in Sevres, France since 1889. It is called International Prototype Kilogram (aka Big K or Le Grand K) and it has many copies around the world – including several at NIST in Gaithersburg, Maryland – used to calibrate scales and make sure the whole world is on a measurement system.

These copies ensure that a kilogram is a kilogram, is a kilogram – whether it is measured on the factory floor of an aircraft manufacturer or on the digital scale of your merchant's cash counter. And for those who think that the kilogram doesn't matter in the US using imperial units like pounds, feet and gallons, our measurements are derived from SI units. Officially, in the US, 1 pound is defined as 0.45359237 kilos.

The problem is that Big K is a man-made object, and therefore it is incomplete. If Big K changes, everything else needs to be adjusted. And that has happened. Big K is not constant. It has lost about 50 micrograms (about an eyelash mass) since it was created. But frustrating when Big K loses mass, it is still exactly one kilogram per. The old definition.

It's not good. So what's better?

The new definition anchors the value of kilos to a constant character that can never change.


This ancient early baby's mass was defined by a metal knot in France. Future baby's mass will be defined by a fundamental force in nature.
Fox Photos / Getty Images

From Monday, the kilogram will be defined by the Planck constant.

The Planck Constant is a concept in quantum mechanics (ie the study of how the smallest components of the universe work), describing how the smallest bits of material release energy into discrete steps or clumps (called quanta). Basically, you can think of the Planck constant as the smallest action an electron can take.

What is important with the Planck constant is that it can never change. And it makes it a worthy concept to anchor the definition of the kilogram.

But first, to understand why the Planck constant can define the kilogram, it is useful to look at how the meter – the world's standard length unit – was redefined for the speed of light as an example of why this was necessary.

The meter was originally defined as the length of a bar by the International Bureau of Weights and Measures in France. (It was then redefined to be equal to a certain wavelength of radiation.) Again, the problem with this definition was its imprecision. It was not based on unchanged properties in the universe.

By contrast, light speed is unchanged. In 1983, physicists were really good at measuring the speed of light. So they used it to fix the length of the meter forever, to make it permanent.

Here's how: The redefined gauge to be equal to the distance light moves in a vacuum in 1 / 299,792,458 of a second. Basically, the definition of the meter is now baked into the definition of light speed.

So let's get back to the Planck constant.

To understand, let's take a look at it. Printed out is the Planck constant 6,662607015 × 10 -34 m 2 kg / s. Kg divided by seconds – as an equation. Kilograms are in the units and that is important. Because once the value of the Planck knows the constant and fixes it forever, the value of the kilogram does not change either.

Each unit in the Planck constant is defined by an unchanged force in nature. The meter is defined by the speed of light. The second is defined by the frequency of atoms in the cesium element. And when the value of the Planck constant was measured and agreed, it means that the kilogram can also be set.

And we know the value of the Planck constant because of many years of work to measure it to a precise degree (read more about this here). This was a tremendously difficult task: measuring, basically the smallest thing to measure. And it took decades with a beautiful and complicated machine called the Kibble balance.

Again, don't worry: The new kilo has the same mass as the old kilogram. This is because researchers used the old kilogram to measure the value of the Planck constant.

By 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. (Also debut Monday, new definitions of electric current, temperature and amount of substance. These are also now bound to constants in nature. But unlike kilograms, their definitions were not based on a physical object. Big K was the last one.) [19659031] If you glowed all over, that's what all this change boils down to: We no longer need a government – the US, France, anyone – or an international governance to tell us what a kilo is. It will be a fundamental truth in the universe that is accessible to anyone with the right equipment to realize it.


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