I have posted a couple of notes here about the kilogram’s weight-loss problem, and about some of the efforts to develop a new definition of this basic unit of mass. The kilogram is the only fundamental unit of the SI [Le Système International d’Unités] system of units that is defined by a physical object: the mass of a particular cylinder of platinum/iridium alloy, stored in a vault at the Bureau International des Poids et Mesures [BIPM] at Sèvres, outside of Paris. The other six primary SI units (ampere, meter, second, mole, kelvin, and candela) are defined in terms of fundamental physical processes. For example, the kelvin is defined as 1/276.13 of the thermodynamic temperature (above absolute zero) of the triple point of water. The meter is defined as the distance that light travels in 1/299,792,458 second. So far, however, no one has come up with a similar sort of definition for the kilogram.
Wired magazine has an excellent article on the two main efforts that are underway to develop a new definition of the kilogram. One, dubbed Team Avogadro by the author, Jonathon Keats, and based in Brunswick, Germany, at the Physikalisch-Technische Bundesanstalt, is attempting to develop a definition using a silicon-28 [28Si] sphere containing a known number of silicon atoms, and relating its mass to Avogadro’s Constant, the number of particles in a mole (approximately 6.02 × 1023). The other, Team Planck, based in Gaithersburg, Maryland at the National Institute of Standards and Technology [NIST], is working on using a watt balance to define the kilogram in terms of Planck’s constant, which relates the energy of a photon to the frequency of its associated electromagnetic wave:
where E is the energy in joules, ν is the frequency in Hertz, and h is Planck’s constant, whose value is approximately 6.62 × 10−34 joule-seconds.
The folks who are responsible, at places like the NIST and the BIPM, are appropriately conservative when it comes to changing the definition of fundamental units, and it is by no means clear what any new definition of the kilogram might be. The article gives a good overview of the process, along with some of the history of measuring systems; it’s an interesting read.