Metrologists are revamping units using fundamental constants of nature
epa european pressphoto agency b.v./Alamy Stock Photo
If scientists had sacred objects, this would be one of them: a single, closely guarded 137-year-old cylinder of metal, housed in a vault outside of Paris. It is a prototype that precisely defines a kilogram of mass everywhere in the universe.
A kilogram of ground beef at the grocery store has the same mass as this one special hunk of metal, an alloy of platinum and iridium. A 60-kilogram woman has a mass 60 times as much. Even far-flung astronomical objects such as comets are measured relative to this all-important cylinder: Comet 67P/Churyumov–Gerasimenko, which was recently visited by the European Space Agency’s Rosetta spacecraft (SN: 2/21/15, p. 6), has a mass of about 10 trillion such cylinders.
But there’s nothing special about that piece of metal, and its mass isn’t even perfectly constant — scratches or gunk collecting on its surface could change its size