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Sci Tech
A better definition for the kilogram
In a move to create a standard that would eliminate the need for a physical artefact to define what a kilogram is Ronald F. Fox, a Regents’ Professor Emeritus in the School of Physics at the Georgia Institute of Technology and Theodore P. Hill — a Professor Emeritus in the Georgia Tech School of Mathematics – hit upon something that is logically very simple to understand.
Their proposal is that the gram — 1/1000th of a kilogram — would henceforth be defined as the mass of exactly 18 x 140744813 carbon-12 atoms, according to a Georgia Institute of Technology press release. They first assign a specific value to Avogadro’s constant. The constant represents the number of atoms or molecules in one mole of a pure material — for instance, the number of carbon-12 atoms in 12 grams of the element. However, Avogadro’s constant is not a specific number.
The official standard
Their proposal for Avogadro’s constant is 844468863. The move results from the fact that it turns out that nobody can say for sure, how much is a kilogram, at least not in a way that won’t change ever so slightly over time. The official kilogram — a cylinder cast 118 years ago from platinum and iridium — has been losing mass, about 50 micrograms at last check. That’s not so good for a standard the world depends on to define mass.
The kilogram is the last major standard defined by a physical artefact rather than a fundamental physical property.
In 1983, for instance, the distance represented by a metre was redefined by how far light travels in 1/299,792,458 seconds – replacing a metal stick with two marks on it.
Two other proposals
At least two other proposals for redefining the kilogram are under discussion. They include replacing the platinum-iridium cylinder with a sphere of pure silicon atoms, and using a device known as the “watt balance” to define the kilogram using electromagnetic energy. Both would offer an improvement over the existing standard – but not be as simple as what Fox and Hill have proposed, nor be exact, they say.
While the new definition for kilogram would do away with the need for a physical representation of mass, Fox says people who want a physical artefact could still have one — though carbon cannot actually form a perfect cube with the right number of atoms.
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