Le Grand K is dead! Long live the New K!
In a temperature and humidity-controlled vault outside of Paris, there is a small cylinder made of iridium and platinum. Its official name is International Prototype Kilogram (IPK) but it is known colloquially as Le Grand K. All kilos in the world are derived from this modest little chunk of metal. It has been sitting in that vault since 1875. The weight of the IPK was derived from the original 1795 definition of a gram as the weight of one cubic centimeter of water at 4 degrees Celsius.
The reign of Le Grand K has come to an end. At a meeting last November in Versailles, France, at a meeting of the General Conference of Weights and Measures, delegates from 60 nations, in a unanimous vote, killed the king of kilos. They followed that heinous act with a champagne toast. They were in France, after all.
Henceforth, the kilo will be defined by…um…Planck’s Constant.
Planck’s constant defines the relationship between a quantum of energy (photon) and the frequency of that energy. It is a measured value, not a mathematically-derived constant. But the accuracy of any measurement is limited by the device used to make the measurement, and therefore it has some of the same limitations as Le Grand K.
Any measurement is inherently imperfect. Let’s say you go to the local butcher shop and ask for a pound of ground beef. If the butcher does not have a furtive finger on the scale, you will probably get pretty close to a pound. You will not get exactly a pound, of course. Even if the butcher is a fanatic perfectionist, and uses a magnifying glass to read the scale, and adds tiny bits of meat to the scale with tweezers to get a perfect pound. If he does that, I suspect someone would make a call to the local mental hospital, and the butcher would be escorted out to a padded van by men in white coats, for a ride to his new residence, with a diagnosis of obsessive-compulsive behavior.
Even if the poor fellow were incredibly lucky…defying odds like those for winning the lottery…and he happened to put on the scale, the last molecule of meat that would keep the total under a pound, he would still not have a perfect pound. It would be off by some fraction of the mass of a proton.
If you are really interested in the new method of defining a kilo, here is a long and detailed description of the agonizing process that has been going on for over a century.
Most of us are probably content to accept the butcher’s scale, whilst keeping our eye on his fingers. Such matters concern only philosophers and lunatics like our fanatical perfectionist butcher. Come to think of it, there may be a fair bit of overlap there.