Until now all fundamental units were defined in terms that depended on the constants of nature.
For instance:
- the metre as the length of the path travelled by light in a vacuum in 1/299 792 458 seconds.
- the second as: the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom
And so on EXCEPT for the kg.
The kg was the only odd one out. There was this object stored somewhere and when people wanted to measure very well the kg they had to go there and try to measure the mass of that object as accurately as possible... which is a rather odd thing to do when aiming for accuracy.
Now we finally have the means to express the kg with a physical experience in universal terms.