Artifact-Free Metrology!

With today’s vote in Versailles, we will soon have a consistent measurement system that isn’t based on a chunk of metal kept in a vault in Paris. Because modern experiments have made it possible to determine the value of universal constants like Planck’s Constant to extremely high precision, the members of the General Conference on Weights and Measures were able to confidently vote to switch us to a new definition of the kilogram — one based on setting Planck’s Constant to an arbitrary value based on careful measurement, and redefining the kilogram based on that.

The International Prototype Kilogram (IPK), or “Le Grand K.” (Image credit: BIPM)

Measuring h to this accuracy involved two different types of experiments, to not only allow for replication but independent confirmation of results using a different method. When two such dissimilar experiments agree with high precision on anything, you’re on the right track. In this case, the results of sensitive Watt-balance measurements were compared with the results of measuring the mass of nearly perfect spheres of a single isotope of silicon to high accuracy. These spheres are probably the most geometrically perfect things anyone has ever made.

With this new definition, set to officially take effect on May 20, 2019, the accepted standard values for several SI values will change very slightly. For most practical purposes, though, this change will be so small as to be lost in the noise. That’s the reason for taking such pains to measure h as closely as possible — we want the new definition to match the old, artifact-based one as closely as possible (since that’s what everyone’s devices are already calibrated to.)

What does this mean? If you love science and like to see it done right, this is an amazing step in the right direction. Going forward, all of the SI units will be defined in terms of natural physical constants that anyone can measure.

Veritasium, not surprisingly, has a great explanation video.

If you’re worried about the measurements changing, don’t be — unless you work with precision measured in parts per billion. It’s a good time for Le Grand K to retire. It will no doubt find a place of honor in a museum — as a 1.0 kilogram mass that used to be the 1.0 kilogram mass.

Because having to explain that your definition of the kilogram is losing weight is more than a little embarrassing.

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