* Astronomy

The fundamental mass

At a time when we are about to shed light on the fundamental question of the creation of mass after the Big Bang, we are also close to solving another basic mass-related problem. The kilogram is the only base unit of the International System of Units (SI) whose official definition is still based on a material artefact rather than on invariant quantities. If you are now thinking that this concerns you less than the glamorous Higgs boson, think again: your scales could give you a different value when you use them tomorrow.
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The kilogram is losing weight and many international scientists, including some at Sandia National Laboratories, agree that its time to redefine it.
Scientists are hoping to redefine the kilogram by basing it on standards of universal constants rather than on an artefact standard.

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A better definition for the kilogram? Scientists propose a precise number of carbon atoms

How much is a kilogram?
It turns out that nobody can say for sure, at least not in a way that wont change ever so slightly over time. The official kilogram a cylinder cast 118 years ago from platinum and iridium and known as the International Prototype Kilogram or Le Gran K has been losing mass, about 50 micrograms at last check. The change is occurring despite careful storage at a facility near Paris.
Thats not so good for a standard the world depends on to define mass.
Now, two U.S. professors a physicist and mathematician say its time to define the kilogram in a new and more elegant way that will be the same today, tomorrow and 118 years from now. Theyve launched a campaign aimed at redefining the kilogram as the mass of a very large but precisely-specified number of carbon-12 atoms.

Our standard would eliminate the need for a physical artefact to define what a kilogram is. We want something that is logically very simple to understand - Ronald F. Fox, a Regents Professor Emeritus in the School of Physics at the Georgia Institute of Technology.

Their proposal is that the gram 1/1000th of a kilogram would henceforth be defined as the mass of exactly 18 x 14074481 (cubed) carbon-12 atoms.
The proposal, made by Fox and Theodore P. Hill a Professor Emeritus in the Georgia Tech School of Mathematics first assigns a specific value to Avogadros constant. Proposed in the 1800s by Italian scientist Amedeo Avogadro, 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, Avogadros constant isnt a specific number; its a range of values that can be determined experimentally, but not with enough precision to be a single number.
Spurred by Hills half-serious question about whether Avogadros constant was an even or odd number, in the fall of 2006 Fox and Hill submitted a paper to Physics Archives in which they proposed assigning a specific number to the constant one of about 10 possible values within the experimental range. The authors pointed out that a precise Avogadros constant could also precisely redefine the measure of mass, the kilogram.
Their proposal drew attention from the editors of American Scientist, who asked for a longer article published in March 2007. The proposal has so far drawn five letters, including one from Paul J. Karol, chair of the Committee on Nomenclature, Terminology and Symbols of the American Chemical Society. Karol added his endorsement to the proposal and suggested making the number divisible by 12 which Fox and Hill did in an addendum by changing their numbers final digit from 8 to 6. So the new proposal for Avogadros constant became 84446886 (cubed), still within the range of accepted values.
Fast-forward to September 2007, when Fox read an Associated Press article on the CNN.com Web site about the mass disappearing from the International Prototype Kilogram. While the AP said the missing mass amounted to no more than the weight of a fingerprint, Fox argues that the amount could be significant in a world that is measuring time in ultra-sub-nanoseconds and length in ultra-sub-nanometers.
So Fox and Hill fired off another article to Physics Archive, this one proposing to redefine the gram as 1/12th the mass of a mole of carbon 12 a mole long being defined as Avogrados number of atoms. They now hope to generate more interest in their idea for what may turn out to be a competition of standards proposals leading up to a 2011 meeting of the International Committee for Weights and Measures.
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.

Using a perfect numerical cube to define these constants yields the same level of significance eight or nine digits as in those integers that define the second and the speed of light. A purely mathematical definition of the kilogram is experimentally neutral researchers may then use any laboratory method they want to approximate exact masses - Theodore P. Hill.

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 meter was redefined by how far light travels in 1/299,792,458 seconds replacing a metal stick with two marks on it.

We suspect that there will be some public debate about this issue. We want scientists and science teachers and others to think about this problem because we think they can have an impact. Public discussion may play an important role in determining how one of the worlds basic physical constants is defined - Ronald F. Fox.

How important is this issue to the worlds future technological development

When you make physical and chemical measurements, its important to have as high a precision as possible, and these standards really define the limits of precision. The lack of an accurate standard leaves some inconsistency in how you state results. Having a unique standard could eliminate that - Ronald F. Fox.

While the new definition 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 cant actually form a perfect cube with the right number of atoms. And building one might take some time.

You could imagine having a lump of matter that actually had exactly the right number of atoms in it. If you could build it by some kind of self-assembly process as opposed to building it atom-by-atom, which would take a few billion years you could have new kilogram artefact made of carbon. But theres really no need for that. Even if you built a perfect kilogram, it would immediately be inaccurate as soon as a single atom was sloughed off or absorbed - Ronald F. Fox.

Georgia Institute of Technology

Perfect silicon sphere to redefine the kilogram
Securely tucked away inside a French vault is a lump of metal known as the International Prototype. A mixture of platinum and iridium, it was made in the 1880s to define the mass of a kilogram.
But work by a team of Australians could help pave the way for the retirement of this century- old prototype, as weight and measurement experts across the globe work towards a more scientific definition of the kilogram.

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