Finally! It was time! Only he was missing. Even the kilogram, from the 20 May, has an acceptable definition. Let me be clear: you don't have to throw away your scales. A kilo remains a kilo, but now it's more fancy.

Until a few days ago, what was a kilogram? It was The Mass (mass, not the weight, remember, They are different) of a particular Platinum-Iridium cylinder deposited at the International Bureau of Weights and Measures in France. The matter that surrounds us, however, has the bad habit of changing: some molecules can detach, it can simply oxidize, change shape thanks to some cosmic ray. Small variations of which, usually, nobody cares. However, when you want to define a unit of measurement, you must be extremely precise, even very small variations cannot be afforded. The solution? Use multiple samples, kept in the best possible isolation conditions, and use them all together to decrease statistical variations.

So, however, the problem is not solved, and, for precision measurements, the kilogram must be defined in an immutable way. The solution is to fix it on the basis of extremely precise processes, or some physical constant, that, of nature, it is constant. So it was done for the second, defining it thanks to the transition from one energy level to the other of the Cesium atom, or with the meter, now the space traveled by light in the void in a very short time.

in conclusion, the goal was to take the kilogram and connect it in some way to a physical constant. The solution involves a complicated system of quantum mirrors and levers, a Watt balance and the Planck's constant. Let's try to understand how the ingredients are mixed.

Planck's constant indicates the minimum amount of energy that can be transported by an electromagnetic radiation per unit of frequency. in conclusion, an electromagnetic wave is made up of small packets (the famous how many) of energy, each of which carries hν energy, where ν is the frequency of the wave and h, precisely, Planck's constant. A fundamental constant of the universe seems an excellent starting point.

The next step is to try to link it to the kilogram. Planck's constant appears in purely quantum contexts, therefore it is necessary to enter this world, very tangled, per dare un senso al tutto. The way forward is that of the Watt balance. This object (also known as Kibble's scales) it works more or less like grandma's sling bars, those with arms, in which you put what you want to weigh on one plate and a series of samples on the other, until balance is found.

The sample needed to find balance, however, this time it's not a piece of iron, but a current-carrying coil is used, immersed in a magnetic field. Thanks to the interaction between the magnetic field and current, a force develops on the coil, which strictly depends on their values. Varying the current until equilibrium is reached, we are doing the equivalent of adding or subtracting samples from grandma's scales. In the end with this instrument it is possible to pass from a mass to an electric current. Perfect, but h still hasn't appeared anywhere.

This is because we have not yet entered the quantum realm, necessary to bring out the Planck constant. And here comes the quantum Hall effect, insieme all’effetto Josephson. By combining these two effects, which involve superconductors at very low temperatures, to measure the potential difference induced by the current in the loop, we are finally able to jump into the infinitely small, having all the ingredients.

Eventually with this complex mechanism of quantum mirrors and levers it was used to weigh one kilogram, see “how many Planck's constants was” and change the definition, Finally.

Watt balance

As you may have noticed, now our kilogram is heavily more complex. However, when you get into the same quantum mechanics that a cat both alive and dead has, what can you expect? The benefits, however, they are huge. There is no longer an object to rely on, a laboratory equipped with the right facilities can do its math using the perfect definition of kilogram, without having to go to France to take one of the cylinders stored in a controlled atmosphere. In addition, we no longer have to worry about this value changing over time: Planck's constant that is and that will remain.

To make matters worse, now we have a much more elegant kilogram. We can imagine it with a tuxedo, who finally goes to important events together with his brothers from the International System.