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Big Test Coming Up For Kilogram Redefinition (ieee.org)
szotz writes: Electromechanical balances have got to be better than an aged lump of platinum and iridium right? Teams are working to get kilograms measured and shipped to Paris in time for a test to see whether the technology (along with another that uses ultrapure silicon spheres) is now ready to redefine the kilogram. Why is this redefinition interesting? Because it's about using physics to overcome one problem with weight standards based on tightly held exemplars in standards bodies' inner sanctums: the mass of those exemplars can change, however subtly, introducing uncertainty and confusion. From the article: The world's metrologists aim to change this state of affairs in 2018 by fixing the kilogram to the Planck constant, a fundamental physical constant. That shift would, at least in principle, allow any laboratory to "realize" the kilogram from scratch with a series of experiments and specialized equipment. But for that scheme to work, the kilogram derived by one laboratory must be the same as those derived by others.
Though the above is true, it is *not* the reason why we can't base the kilo on some arbitrary multiple of Pi. The point is that we want to be able to actually reproduce the reference kilo in any lab. Take for example the definition of the meter as the distance travelled by light in 1/(299,792,458) of a second. A lab can actually measure the length light travels in that amount of time and thus reproduce the canonical meter. If we just defined the meter as 1/Pi, there would be no way to convert this number to an actual length.
When the article says that they define the kilo in terms of Planck's constant, they mean that you take the ratio of all sorts of measured quantities in the lab and the laws of physics say that the result should be the mass of what you are measuring times Planck's constant. The true emphasis is that the measurement is proportional to the mass of what you measured, not that the constant of proportionality is Planck's constant (except of course for the fact that we assume that the constant of proportionality, Planck's constant, being part of the fundamental laws of physics, is independent of where and when we do the measurement (at least in the time and distance scales that physics has managed to probe).