I can’t remember when I last did any grocery shopping. But now I am looking forward to walking into a store and saying, "Could I please have sugar weighing the fixed numerical value of the Planck constant h (6.62607015×10−34 ) expressed in the unit J/s, which is equal to kg/m2/s−1, where the metre and the second are defined in terms of c and ΔνCs.?" That, for the uninitiated, is one kilogram of sugar.
This is the new world, and in the new world, the kilogram is defined thus. Don’t you just love it? No confusion, no wondering if the scales are being fiddled with. A kilo will be a kilo will be a kilo as Gertrude Stein nearly said.
Remember when a kilogram was equal to a cylinder of platinum and platinum-iridium alloy, kept at the International Bureau of Weights and Measures near Paris? You don’t? No matter, read on.
In the bad old days, the platinum-iridium alloy sometimes gained weight (happens to the best of us), throwing calculations into a spin. Sometimes the reverse happened. It lost weight, and as often happens when you lost weight, the kilogram was not easily recognised then, and your friendly neighbourhood grocery store’s weights and measures turned inaccurate.
Recently the cylinder lost about 50 micrograms, or the weight of an eyelash, throwing everybody into a tizzy. What if the platinum-iridium alloy got stolen, someone asked innocently, causing scientists to pull out more than 50 micrograms of hair.
Now you can’t go wrong.
Except that around 7,705,643,990 people, or roughly the population of our planet will have no idea what you are talking about when you ask for a kilo of anything. The definition involves the speed of light, and the distance it travels in a vacuum in 1/299,792,458th of a second (which is the official definition of a metre, by the way).
As scientists and judges often do, we have chosen precision over comprehensibility. A High Court judge recently gave a verdict which in essence meant, "You are guilty and I sentence you to five years in prison." But he took over 1,200 pages to say it in, and at the end the accused was not sure whether to celebrate or break down in tears.
Poets deal in precision too. Although to be fair, when you read the first two lines of Ode to a Nightingale — My heart aches, and a drowsy numbness pains / My sense, as though of hemlock I had drunk — you can’t be sure whether Keats is talking about a bird or preparing for a visit from his mother-in-law.
Let’s welcome the kilogram then. Like diamonds, kilograms are forever now, thanks to it being married to a universal constant.