The proposal has precedence -- the other six of the seven basic units of the international measurement system are defined by natural phenomena. The second, for instance, is defined as 9,192,631,770 cesium atom oscillations, while the meter is defined as the distance light travels in a vacuum in a 299,792,458th of a second. In contrast to these unchanging definitions that can theoretically be measured anywhere, the kilogram cylinder, a physical object available only a single physical location, is subject to slight mass changes due to contamination or surface loss.
In other precision-measurement news, physicists, engineers and statisticians at the National Institute of Standards and Technology are nearing the end of a four-year effort to provide standard rulers for the extremely small. The rulers use the spacing of atoms within silicon crystals to measure the dimensions of structures as tiny as the individual logic gates of microprocessors. The new rulers are accurate to within 2 nanometers -- about the span of 20 hydrogen atoms.
[Via Smalley's Research Watch]
