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.
Last October, the International Bureau of Weights and Measures met to determine a new strategy of defining the kilogram, this time using universal constants. IEEE Spectrum has a riveting feature on how this might happen. The kilogram is way more complicated than a supermarket scale would have you think:
Delegates from the bureau’s then 55 member countries unanimously agreed on a tentative plan to base the kilogram on a fundamental constant of quantum mechanics…This coup is largely the result, after decades of work, of steady strides in two challenging strategies for measuring mass. One approach attempts to pin down the exact electromagnetic force needed to balance the gravitational tug on an object. The other harnesses Cold War–era uranium enrichment technology and a host of experimental techniques to count the number of atoms in extremely round balls of ultrapristine silicon.
Results of initial experiments for both approaches have since come in, and…dun dun dun…they don’t agree. Scientists hope to reconcile their results by the next General Conference on Weights and Measures in 2014. The full feature over at IEEE Spectrum is a wonderful behind-the-scenes look at how science actually works, often dogged by ambiguity and disagreement. If you ever doubted that science can be hard and messy, just read about the battle of the kilogram.
Kilogram image via Shutterstock / aniad