In case you've been feeling lighter lately, you're not alone. Apparently the metal weight used to define the kilogram in France has been too:
What intrigues me here is the attempt to define a kilogram of mass in "a non-physical way." Now...wait a minute! Mass is, of course, one of the fundamental concepts of physics. Perhaps what we're really dealing with is the mystery of the missing mass itself, and the whole notion that the constants of physics are really constant. But are they? Even Einstein used the implicit assumption of "constant constants" in his relativity theories. But his contemporaries - one thinks of Pascual Jordan, not to mention Dr. Ott Christian Hilgenberg - weren't so convinced that constants were indeed constant.
So this little story may be hinting at a much deeper mystery: why is any mass missing to begin with? What process or theory will be advanced to account for it? Jordan and Hilgenberg advanced their theories long ago, and neither of them proved to be entirely satisfactory, except that both men realized there was something amiss with the idea of the "constant constants".
There is an even deeper part of this story as well, one having ancient roots, as I outline in my forthcoming book Genes, Giants, Monsters and Men, and that is, that both the English imperial and the continental metric system both appear to have analogues in very ancient systems of measure (though in my opinion, the evidence is overwhelmingly in favor of the imperial system). If there is one thing that emerged from the ancient views of cosmology, it is change; no matter how minute, how insiginificant, change was in everything. Perhaps they had some understanding, then, that even the idea of a constant measure of mass also varied over time, for whatever reason.
As for the modern story, while the missing mass in the kilo may be insignificant, it may be a significant clue into a deep physics, one for which every theoretical model has an explanation, but none of them seem entirely adequate. Hilgenberg of course posited a vorticular cosmology, and a perpetually changing mass and expansion of the earth; Jordan's ideas eventually became part of the Brans-Dicke-Jordan hypothesis, in which variable constants were a principal conceptual component. Burkhardt Heim entertained similar notions of a temporal cellular structure to space-time, as did Kozyrev with his mighty, mystical utterance "time is not a scalar."
One wonders, if the kilo is missing the mass of a grain of sand, have yardsticks lengthened or contracted? Granted, it's a small story, but the concept behind the story has exercised the greatest minds in physics in the 20th century, so it may well be a story worth following.