Here is something I did not know for sure and I'll bet most tracker readers didn't either, not that one is expected to stop often and wonder about such things. Of all the practical units for everyday life (dimension, time, mass mainly), only one's definition depends on some physical object or commonly observed phenomenon such as how long it takes for the Earth to turn on its axis. That latter one of course, was too sloppy. Only mass - specifically, the kilogram - remains dependent on such an easy-to-see example. More surprising is how hard it is to pin it to some fundamental, reproducible physics. A meter, for instance, no longer needs to be checked against a standard metal rod of composition that doesn't shrink or expand much with temperature. A meter was re-defined as a specific number of wavelengths of a narrow, krypton-86 atomic emission line, which in turn was later updated to how far light in a vacuum travels in a teensy, precisely declared time interval.
But the kilogram is that thing in the photo. The cylinder of iridium-platinum alloy inside three nested bell jars is in a vault in Paris, taken out only once in a long while to get polished and checked against copies kept in few other centers of science and industry. One problem is that it is not constant, what with atoms of metallic alloys prone to wander off no matter how careful one is.
All this in a feature article in a more commonly technical journal.
- IEEE Spectrum - Rachel Courtland: The Kilogram, Reinvented ; Where one learns a concerted effort is underway to get a definition that one could, say, radiogram to alpha centauri's ETI's and they could make their kilogram exactly like ours without visiting Paris. Never heard of a Watt balance before? Courtland assures us that it "links mass to quantum-electrical measurement" and that it might be the basis for a new definition. I didn't quite get it, but did learn that Canada has a good one. Others propose to exactly count the atoms in hunks of, say, silicon of high istopic purity and decree exactly how many make a kilogram (or more handy small exact fraction of one).
Nifty yarn. The process to redefine the kilogram, it says here, is deeply underway. That's news that'd be of keen enough interest to enough readers out there to merit a circulation in lay media.
- Charlie Petit