Perfect silicon spheres were developed to redefine the kilogram because the platinum kilogram was changing in weight even in extremely strictly controlled physical conditions. If dust can ruin the platinum kilogram, why are people allowed to touch the silicon version?



How is it that the sphere doesn’t become ‘contaminated’ with extra atoms from skin, clothes or even breath? Or is it just that these contaminants are very easy to clean off afterwards?

In: Physics

Not sure about silicon spheres, but the kilogram is now defined in terms of fundamental constants rather than a lump of something.

The definition is in terms of the Planck constant, the speed of light and the atomic transition frequency.

A lump of something is simpler, but as you note, is prone to changing over time.

Because the sphere themselves were not the new kilogram they just have 1k of mass for measuring purposes, the real definition of the kilogram is calculated with physical constants. so if the spheres are lost or destroyed it doesn’t matter.

As I understand it, because the silicon version has done it’s job. The kilogram was the last SI unit that relied on a physical object as a reference point.

The platinum kilogram was just that. It was a kilogram.

The silicon spheres were a tool to help define Avogadro’s number (a constant that links the mass number of a chemical to it’s weight in grams) to the required accuracy.

By doing this, the Kg could then be redefined as a mathematical formula based on universal constants, rather than “that block of platinum in the vacuum chamber in France”.

Veritasium has great videos on both [the Silicon sphere]( and the [new Kilogram definition](

They designed the sphere because they didn’t want the kilogram to be based on a physical they made the sphere so that they could calculate the number of atoms of silicon in it, it is made up entirely of silicon 28.

Kilogram is now defined by planks constant.

You could watch an amazing video by veritasium – worlds roundest object.

It’s important to note that they didn’t want to create just any sphere, they wanted to create the closest thing to a perfect sphere as physically possible. If you can create a perfect(or sufficiently close to perfect) sphere, and the material you make it out of is solid and pure, then using the density of the material, it’s volume, and knowing the molecular mass of the material (in this case silicon) you can get a sufficiently accurate measurement of the number of atoms in that object, and therefore how many moles are in that object and then you can call that object’s mass the kilogram, and then you can define the kilogram in terms of Avogadro’s number.

The sphere was an alternative approach to redefining the kilogram. It was an attempt to define Avogadro’s constant, which relates the number of atoms or molecules in a substance to its mass. Because the sphere was made of one particular isotope of silicon, they were able to achieve an extremely precise measurement of its mass and thus a very accurate count of the atoms.

Ultimately, that was not the approach taken. The kilogram was instead defined with Planck’s constant which, as another commenter has correctly pointed out, defines it in terms of the meter and the second. Beyond that the discussion leaves the realm of ELI5.

Source: am metrologist.

The silicone version is no longer the definition of a kilogram. It’s more like an example of a kilogram. If it gets some dust, we can accurately measure how un-kilogramatic it is because we have a non-physical way of describing a kilogram