Deep below Paris, in a vault that can only be opened by the simultaneous turning of three keys held by three different people, is the international prototype kilogram — the kilo against which all other kilos are based. The kilo is the last SI unit that is still defined by a physical artifact, rather than a constant of nature, but four of the seven SI units are fundamentally underpinned by the kilogram, making the international prototype extremely important… And yet worryingly, the international prototype is losing weight, changing science as we know it, and no one knows why.

The IPK was specifically fashioned from an alloy of 90% platinum and 10% iridium for its virtual immunity to oxidization, and because it’s extremely hard-wearing. The international prototype is a cylinder of 90% platinum and 10% iridium, and due to the massive density of platinum it’s just 39.17 millimeters (1.54 inches) wide and tall. Dozens of IPK copies were made and given to other nations, so that their scientists have easy access to the standard weight, and every 40 years these copies are brought to Paris and compared against the original IPK, to ensure that everyone’s still working from the same kilo. The IPK and 39 copies were made in 1889, and for some unknown reason the weight of the IPK has been drifting away from the copies ever since.

Weirdly, it’s not even known if the IPK is getting lighter, or if the national prototypes are getting heavier — but either way, something is causing these kilos to change weight, by around 50 micrograms every 100 years. This is a problem, because the weight of the IPK is the kilo. If the IPK changes, as does the value of four other SI units, affecting a whole bunch of science. For example, the newton is defined as the force to accelerate one kilogram at one meter per second squared — and the newton, in turn, is used to define the pascal, joule, and ampere. If the kilo changes in weight, so does the value of these units, and many others.

There are some proposed reasons for the weight discrepancy, but in short we still don’t really know why. It is likely due to microscopic surface effects, such as absorption of hydrogen, or perhaps mercury from the proximity of mercury-based instruments in scientific labs when the IPK and its copies are weighed. Some wackier theories might be changes in the gravitational constant or global warming. Because the weight of the IPK has such far-reaching consequences, the 24th General Conference on Weights and Measures agreed that the physical definition of the kilo should be retired in favor of a constant of nature, or natural unit — in specific, Planck’s constant.

The Planck constant describes the relationship between the energy of a particle/wave (photon), and the frequency of EM radiation that it emits. By using the meter and second — two other SI units that are defined by constants of nature — the kilo can be defined by Planck’s constant. The only problem is that we don’t actually know Planck’s constant to a sufficient certainty to use it as the basis for an SI unit, and thus the General Conference of Weights and Measures decided to hold off on the redefinition of the kilo until we know Planck’s constant with enough certainty.

Now, Patrick Abbott at the National Institute of Standards and Technology thinks he’s come up with a way of ascertaining Planck’s constant. Using a very strong vacuum, so that there’s no contamination or outside influences on the laser interferometry, and an instrument called the watt balance, Abbott says he and fellow researchers should be able to define the kilo in terms of the Planck constant by 2018. Only then, with the final physical artifact retired from that Parisian vault, will we be able to rest assured that SI units, and thus science, isn’t creepily changing under our noses.

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Research link: “The Important Role of Vacuum Technology in the Redefinition of the Kilogram” on October 28 at the AVS 60th International Symposium and Exhibition

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