The meter is fixed to the speed of light and a second to the radiation of cesium, but the mass of one kilogram is still not defined by a universal constant. Instead, it’s still pegged to an old-fashioned cylinder of platinum iridium alloy kept under lock and key in Sèvres, France.
The method isn’t just old-fashioned, it’s imprecise, which has literal ramifications across the world when the point is to set the kilogram standard. The cylinder is weighed every few decades against official copies that had the same mass when they were all cast in 1899. When they were last weighed in 1988, however, their masses had drifted 70 micrograms apart.
Whip out that red pen and make just a few…little…tweaks…
The physical world should feel a little more comfy now: Gravity is a little bit less than it was last Thursday. And the electromagnetic force? A smidge stronger.
Every four years, the National Institute of Standards and Technology posts internationally determined adjustments to the official values of such natural constants to reflect more accurate measurements made possible by advancing technology. This week, in the latest update, the radius of a proton, the speed of light, the Planck constant, and many, many others have received facelifts that will decrease uncertainty in physics measurements. But this update will also affect units much closer to home: In October, the General Conference on Weights and Measures will vote on a measure to base the definition of a kilogram on the values of such natural constants, instead of the 130-year-old slug of platinum and iridium that currently holds the title.
For the time being, the current upgrade will likely trickle down to we armchair physicists once Google Calculator, the search giant’s handy-dandy constant provider, starts using the new numbers. Judging from its current value for the Planck constant, it’s still working from the 2006 data.
Image credit: Mohr,Talbott/NIST