Nature Boy shuffled into the kitchen the other morning with a long face. “I’d hoped to avoid it, but I’m paying for all that whipped cream in full-fat eggnog and the extra helpings of pie with ice cream through the holidays,” he muttered.
“Why wouldn’t you pay for that excess?” I finally managed to ask.
“When they agreed to change the new kilogram standard in November, I thought I could catch a break.”
Yes, folks, there’s a new kilogram — or will be once it becomes the official kilogram in May, which means Nature Boy is getting ahead of himself again. The old kilogram standard, based on a golf ball-sized cylinder of platinum and iridium kept under several glass, vacuum domes and behind locked doors in Paris, is being retired. Since 1879, it has served as the International Prototype of the Kilogram, the standard against which all other kilograms are measured.
However, it was noticed about 70 years ago that it was not that, well, reliable. The protoype has, in fact, been losing weight. During the official weigh-ins that take place every few decades, when its 40 or so reference copies are flown in from around the world and compared to the First Standard of Massy Standards, the prototype was found to have lost about 50 micrograms. That’s about equal to an eyelash.
This is bad news. We buy goods by the kilogram. We ship materials by the tonne (based on the kilogram). We measure out prescription drugs by milligram (based on the kilogram). Our sense of self-health and sometimes worth is tied to the kilogram (or pound, which is calculated from the kilogram).
Some applications require much higher precision and greater constancy. Scientists rely on a steadfast kilogram in their experiments and measurements. Engineers measuring stress loads on bridges and dams, and those running nuclear reactors do, too.
The keepers of standards blame the kilogram’s variability on storage, handling and age. Particles of dust settling on the prime kilogram, oil from fingers that once handled it, interactions with the surfaces it has rested on and gases in the air on the few occasions it has been removed from its vacuum chamber have taken their toll. And here we are.
Nature Boy poured himself some coffee, then hesitated by the carton of half-and-half before turning away and sitting at the table.
“I’m afraid it’s cheese parings and half-rations for me,” he mourned.
“But the kilogram has always been a kilogram and always will be,” I said. “Even if the prototype kilogram loses a nanogram or two, it still defines a kilogram, and everything else has to adjust. You, for instance, would just weigh more.”
“That’s the problem,” he said. “While the kilogram could potentially gain weight, I could hope that I might still wake up lighter one morning after eating an entire pie and half a tub of ice cream.”
To eliminate the changeability of the standard, the new kilogram will no longer be a “thing.” Instead, it will be based on a universal constant — a mathematical or physical quantity thought to be unchanging throughout the universe. In this case, the Planck constant will have the constant measure of the new kilogram standard.
The constant, named for 19th-century German theoretical physicist and Nobel Prize-winner Max Planck, links the amount of energy carried by a photon — a particle/wave of light — with its electromagnetic properties. Researchers in Canada and elsewhere have recently measured the Planck constant’s value to an unprecedented precision — to within one-billionth. The constant also draws on other metric standards — metres and seconds — also based on universal constants. In the 1960s, the metre was redefined by the speed of light and the second was tied to the movement of cesium atoms. In fact, the kilogram was the last of the seven basic metric measurements to be related to a universal quantity.
However, even with the change in how a kilogram is defined, we won’t notice it in our daily lives. Nature Boy, for example, would have to pig out more than even he is physically capable of for it to show up on the bathroom scale. This means that any increase in those morning numbers he was bemoaning is due entirely to the amount he ate.
When it comes to bathroom scales, it’s pie, not pi, that matters.