I know that everyone lays awake at night worrying about this, but the international prototype kilogram (IPK) the world's reference kilogram is losing weight. It has lost less weight than a single grain of dust, but it a world where so many things are reference against mass having the kilogram get lighter is a dangerous thing.
I think the kilogram should be referenced off the electron Volt (eV). But wait you say the eV has Joule as unit and the Joule is based on the kilogram, so you can't do that!
However, I would counter that even if you don't know what the resting mass of an electron or proton is, as long as all the particles of interest have the same resting mass and we can accurately measure speed we can accurately derive mass. There are even artifacts of the speed of a particle related the color spectrum of the photons produced, etc so instruments can be double checked.
The next complaint is that to accelerate a particle to speeds where the mass would be a significant fraction of a kilogram would require a particle accelerator and every particle accelerator would be slightly different. However, there are lots of natural processes that produce beta and alpha particles of fairly tight spectrum's that could be used as a natural source of non-resting mass particles. There are also a variety or fission or fusion events that could be a source of data for meaureing the nature of the kilogram.
Final critism of my eV plan is that it is an abitrary measure, however a meter is the distance light travels in 1/299792458 of a second. If we can measure time and speed accurately enough to base a unit of measurement on a time scale that small it should be a simple matter to determine the speed of a subatomic particle moving at less than 99% the speed of light.
That is just my thought...
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