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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: 3, Informative) by Thexalon on Thursday October 15 2015, @10:57PM
Kind of. The original idea of metric was this:
1. Metres were 1 / 10,000,000 of the distance from the Equator to the North Pole. Sure, that's arbitrary-ish, but it was defined by a measurable natural feature and the power of 10 that corresponded best to a reasonably convenient size.
2. Grams were the mass of 1 cm^3 of water. So already it's related to another unit.
The problem was that those definitions were not precise enough, which is why metres and kilograms were switched to physical standard objects, and later the second was defined in terms of cesium atom fluctuations at absolute 0 and the meter defined based on the distance light travels in a vacuum during that second. But from the beginning they were at least thinking along the lines of properties that anybody could measure, rather than, say, the extremely arbitrary length known as a "foot".
The only thing that stops a bad guy with a compiler is a good guy with a compiler.
(Score: 2) by Alfred on Friday October 16 2015, @03:39PM
I would think a natural definition would pick a nice round number with few significant figures. Like exactly 1E6 wavelengths of something. I would also be cool with a nice round number like 2^28 (which gives you 9 sig figs in base 10). That would also be arbitrary though, just like picking that orange-red line.
To me this is worse because it now depends on another fundamental (and arbitrary) unit which depends on oscillations of Cesium. Kind of like your second point about the gram. And again, 9 significant figures.
9 significant figures again but this time it is independent from the other base units. To be fair the significant figures are used just to try and match what was grandfathered in.
In reality there are no perfectly natural measurement units. I really shouldn't pick on the decisions of guys that have been dead for 100 years. If the meter was some other arbitrary length I would still use it the same way. Since none of the increased accuracy from this activity will ever find use in my home maybe I shouldn't even care. But still, all units are arbitrary even if thoughtful.
*quotes from https://en.wikipedia.org/wiki/Meter [wikipedia.org] and https://en.wikipedia.org/wiki/Second [wikipedia.org]
(Score: 2) by Thexalon on Friday October 16 2015, @04:33PM
My point was that the original idea was a nice round number: 1e-7 * 1/4 C where C is the circumference of the Earth. The problem was one of precision, not meaning.
If you want a completely not-arbitrary system of measurement, try Planck units [wikipedia.org], which are all based around known fundamental constants in the universe like the speed of light in a vacuum. I mean, it's not exactly convenient to represent the distance from where you are to, say, Times Square, in Planck Lengths, but it's at least possible.
The only thing that stops a bad guy with a compiler is a good guy with a compiler.
(Score: 2) by maxwell demon on Friday October 16 2015, @06:49PM
Well, if the units will be defined by the values of the fundamental constants, then it essentially is the same as defining the meter as a certain number of Planck lengths, etc.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by Alfred on Friday October 16 2015, @07:07PM