New strategy to define the kilogram
The metre 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.
- Discover magazine
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