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The kilogram doesn't weigh a kilogram any more. This sad news was announced during a seminar at CERN on Thursday, 26 October by Professor Klaus von Klitzing, who was awarded the 1985 Nobel Prize in Physics for the discovery of the quantised Hall effect. "We are about to witness a revolutionary change in the way the kilogram is defined," he declared.
Together with six other units – metre, second, ampere, kelvin, mole, and candela – the kilogram, a unit of mass, is part of the International System of Units (SI) that is used as a basis to express every measurable object or phenomenon in nature in numbers. This unit's current definition is based on a small platinum and iridium cylinder, known as "le grand K", whose mass is exactly one kilogram. The cylinder was crafted in 1889 and, since then, has been kept safe under three glass bell jars in a high-security vault on the outskirts of Paris. There is one problem: the current standard kilogram is losing weight. About 50 micrograms, at the latest check. Enough to be different from its once-identical copies stored in laboratories around the world.
To solve this weight(y) problem, scientists have been looking for a new definition of the kilogram.
Dang. That throws the easily memorable conversion of 1kg=2.2lbs right out the window.
(Score: 2) by bob_super on Tuesday November 07 2017, @05:58PM
> Why do they want to define kilogram in terms of not perfectly known value such as Planck's constant? Why not something permanent
> such as a certain number of neutrons at rest? Does mass of a neutron fluctuates by any chance?
The point of defining the kilogram differently than "this piece of metal in Paris" is reproductibility. The variations of the current kilogram are so tiny they only really matter for some (very important) physics research. The fact that you can't calibrate your instruments easily is the bigger issue.
It's like redefining the metre: now you can create it in your own facility. It's expensive, but it's actually doable.
So, like we fixed c a while back, we fix Planck, and the better we get at creating tools expressing Planck, the better our gram gets (at least until we realize that some weird quantum astrophysics phenomenon is causing Planck to change by a tiny amount every million years)