Somewhere, lost in the clouded annals of history, lies a place that few have seen — a mysterious place called the Unknown, where long forgotten stories are revealed to those who travel through the wood.
So basically the cylinder that science has used as THE kilogram since 1889 has been losing microscopic weight, like a few billionths of a kilogram. What scientists plan to do is instead of having a physical object set the standard for how much a kilogram weighs, they’re going to express it in terms of Planck’s Constant, a fundamental constant in quantum physics as unchanging as the speed of light in a vacuum. By dividing Planck’s Constant by the Meter and the Second (both already defined by fundamental constants), you get an insanely small weight. Multiply that by a big enough number and you get one kilogram!
So instead of measuring all weights against an object that can change, the kilogram is defined by unchanging physical constants and pure math.
The Knottiness of Existence 