Naked and dripping, I tipped the gym scales the other day at an undesirable but far from tragic 86 kilograms. Then, without my breathing or moving, the LCD gave me the benefit of the doubt at 85.9. Then flickered back up to 86, then back down again to 85.9.

I concluded there was something inherently unstable about either my body weight or the electronic scales. Then I remembered the Zen parable in which the novice asks the master whether it is the fluttering flag that is moving or the wind, to which the master replies, It’s your mind that’s moving, dude.

When I returned home I read in the the science pages of the newspaper of an incipient rebellion in the metrological community, an ever-louder call for a fundamental revision of the definition of the kilogram. I pictured indignant metrologists being restrained by riot police with tear gas in their noble bid to set things straight. The kilogram, I read, is a total anachronism insofar as it is the only one of the Standard International (SI) base units still defined in relation to an artifact rather than to fundamental physical properties. It is equal to the mass of the international prototype of the kilogram (le grand kilo), which is made from an alloy of platinum and iridium of 39 mm height and diameter, and kept along with six spiffy official copies in a vault at the Bureau International des Poids et Mesures (BIPM) in Sèvres.

As for the other SI base units of measure–the metre, the second, the ampere, the kelvin, the candela, and the mole–they are all defined according to physical equations. (I always thought a mole was a small burrowing animal with grey velvety fur, very small eyes, and very short strong forelimbs adapted for digging, but apparently it moonlights as the amount of substance of a system which contains as many elementary entities as there are unbound atoms in 0.012 kilogram of carbon 12.) For example, a metre is the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458th of a second, while a second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

What’s more, le grand kilo is slowly but surely wasting away. Meagre solace for the dieters of this world, mind you, given that it has lost only 50 micrograms over the last 100 years. (A microgram is equal to one thousandth (10^-3) of a milligram or one millionth (10^-6) of a gram or one billionth (10^-9, one thousand millionth) of a kilogram.) And this imprecision, slight though it may be, has a potential flow-on effect on electrical units, given that the definition of the ampere is ultimately related to the kilogram.

In fact, the kilogram seems to be a permanent thorn in the side of metrologists everywhere, not least because it is the only SI base unit whose name is morphologically composite. As the BIPM website stoically concludes, ‘In any case, we are now stuck with the infelicity of a base unit whose name has a “prefix”.’

Following recommendations in Resolution 7 of the 21st Conférence Genérale des Poids et Mesures, current electro-mechanical experiments linking the kilogram to the Planck constant h are most likely to result in a new definition, provided of course that this definition displays an uncertainty of the order of 1 part in 108. So, a quantum-based redefinition of the kilogram is in the works, which is definitely a weight off my mind.