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For decades, metrologists have strived to retire ‘Le Grand K’ — the platinum and iridium cylinder that for 126 years has defined the kilogram from a high-security vault outside Paris. Now it looks as if they at last have the data needed to replace the cylinder with a definition based on mathematical constants.
The breakthrough comes in time for the kilogram to be included in a broader redefinition of units — including the ampere, mole and kelvin — scheduled for 2018. And this week, the International Committee for Weights and Measures (CIPM) will meet in Paris to thrash out the next steps.
“It is an exciting time,” says David Newell, a physicist at the US National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. “It is the culmination of intense, prolonged efforts worldwide.”
[...] In 2011, the CIPM formally agreed to express the kilogram in terms of Planck’s constant, which relates a particle’s energy to its frequency, and, through E = mc2, to its mass. This means first setting the Planck value using experiments based on the current reference kilogram, and then using that value to define the kilogram. The CIPM’s committee on mass recommends that three independent measurements of Planck’s constant agree, and that two of them use different methods.
(Score: 0) by Anonymous Coward on Friday October 16 2015, @05:17AM
Now, if we communicate with aliens by sending laser beams across the galaxy we don't have to be embarrassed when they ask us what a kilogram is.
It's the mass unit that makes hbar (Planck's constant) exactly equal to 6.62606x10^−34.
It's pretty neat that all units will be exactly defined, and the only limit to how precisely we can know a physical quantity is how well we can measure it. Previously, the units themselves were imprecise. Any value that included the kilogram, such as Planck's constant which is truly fundamental, was limited to the precision of the kilogram artifact.
Actually, the original definition of the kilogram (from the year 1795!) as 1000 cm^3 of water would have been acceptable, too. But I guess those French rascals wanted to have something special to keep in Paris, so they invented the platinum kilogram artifact.
(Score: 2) by maxwell demon on Friday October 16 2015, @06:46PM
No, the definition was not acceptable, as the density of water changes with temperature. Now I guess "mass of 1000 cm^3 of water at a temperature of 4°C" would have been acceptable but impractical, as back then you couldn't produce water at a temperature of 4°C whenever you want, as the refrigerator had not yet been invented.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by joshuajon on Friday October 16 2015, @07:34PM
Except that definition would be dependent on the density of water at a particular temperature, which of course would then be based on the definition of a degree celsius, which then adds a further layer of imprecision because it changes with altitude.