Monday, November 07, 2011

The Kilogram To Be Tied to Planck Constant

I've mentioned before of the issue related to the last of the 7 SI units that still is not tied to a fundamental constant - the kilogram. This is compounded by the fact that one of the standard mass that is being kept outside of Paris is changing mass. They think it is losing weight, but they're not sure now if it is a weight loss or a weight gain (really?!).

Still, at the end of the article in that last link, there was something that I didn't know about.

Mr Picard said the kilogram had gained (or lost) the equivalent of a small grain of sand in weight, but that was enough to throw out calculations in everything from precision engineering to trade.

It will be replaced by the Planck Constant, named after Max Planck, which is the smallest packet of energy (or quanta) that two particles can exchange.

Last month the General Conference on Weights and Measures (CGPM) agreed to use the constant to calculate the value of the kilo - but not before 2014.
Whoa! I didn't know they decided on that already! So I went and did a search, and found it immediately. This is a press release from BIPM.

A redefinition of the kilogram first requires highly accurate measurements of a fundamental constant of nature in terms of the mass of the international prototype of the kilogram, currently exactly equal to 1 kilogram. The numerical value of the fundamental constant will then be fixed and the same experiment will later be used to measure the mass of objects including the international prototype. Several facilities throughout the world capable of carrying out such measurements will be needed after the redefinition in order to make practical use of the new kilogram definition.

The target uncertainty for the most accurate of such measurements is 20 microgram per kilogram, which is the same as 20 parts in one thousand million. It is remarkable that at least two experimental approaches are very close to achieving this goal. One approach uses a special electronic balance – a “watt balance” – in order to measure the kilogram in terms of the Planck constant, which is the fundamental constant of quantum mechanics. A second technique compares one kilogram to the mass of a single atom of the chemical element silicon. Physics tells us that the results of these two seemingly different approaches can be accurately compared with each other and, of course, they should agree. The present situation has been examined by the CODATA Task Group on Fundamental Constants based on work published through the end of 2010. They conclude that the present uncertainty of the Planck constant from all relevant experimental approaches is the equivalent of 44 microgram per kilogram.

The CGPM will not adopt the proposed new definitions until present difficulties are resolved. However, on Friday 21 October 2011, the General Conference took a historic step towards the revision by adopting Resolution 1 and thereby outlining the proposed New SI as well as the steps required for the final completion of this project. The text of Resolution 1 is that of Draft Resolution A, which had been publicly available for some months on the BIPM “New SI” website, with only minor changes made during the Conference. One of these asks the International Committee for Weights and Measures (CIPM) to continue its work to render the language of the New SI as far as possible understandable for users in general, while maintaining scientific rigour and clarity and without altering the basic content and structure of the New SI as set forth in Resolution 1.
In the resolution of the most recent meeting, there is a bit more description on how the kilogram will be tied to the Planck constant.

the kilogram will continue to be the unit of mass, but its magnitude will be set by fixing the numerical value of the Planck constant to be equal to exactly 6.626 06X ×10–34 when it is expressed in the SI unit m2 kg s–1, which is equal to J s
the mass of the international prototype of the kilogram m(K) will be 1 kg but with a relative uncertainty equal to that of the recommended value of h just before redefinition and that subsequently its value will be determined experimentally,
Well, there ya go. Looks like some time in 2014, that standard mass will be nothing more than a historical, museum piece, just like the 1 meter rod.


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