�Our standard would eliminate the need for a physical artifact to
define what a kilogram is,� said Ronald F. Fox, a Regents� Professor
Emeritus in the School of Physics at the Georgia Institute of
Technology. �We want something that is logically very simple to
understand.�
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 Avogadro�s 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, Avogadro�s
constant isn�t a specific number; it�s a range of values that can be
determined experimentally, but not with enough precision to be a
single number.
Spurred by Hill�s half-serious question about whether Avogadro�s
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
Avogadro�s 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 number�s final digit from 8 to 6.
So the new proposal for Avogadro�s 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 Avogrado�s 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,� Hill said. �A
purely mathematical definition of the kilogram is experimentally
neutral � researchers may then use any laboratory method they want to
approximate exact masses.�
The kilogram is the last major standard defined by a physical artifact
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,�
Fox said. �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
world�s basic physical constants is defined.�
How important is this issue to the world�s future technological
development"?
�When you make physical and chemical measurements, it�s important to
have as high a precision as possible, and these standards really
define the limits of precision,� Fox said. �The lack of an accurate
standard leaves some inconsistency in how you state results. Having a
unique standard could eliminate that.�
While the new definition would do away with the need for a physical
representation of mass, Fox says people who want a physical artifact
could still have one � though carbon can�t 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,� Fox noted. �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 artifact made of carbon. But there�s 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.�
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