Wednesday, October 20, 2010

Redefining The Kilogram

Is this the last of our standard metrology that isn't tied to a fundamental constant? It might be.

There is a push to redefine the kilogram into something beyond a mass kept in a lab somewhere. While standard length of 1 meter and standard time of 1 second are tied to various fundamental constant, the standard mass of 1 kilogram is still tied to some fiducial object. This latest attempt at a precise measurement of Avogadro's number using silicon-28 strengthened the case to redefine the kilogram.

The latest result from a team led by Peter Becker of the Federal Institute of Physical and Technical Affairs (PTB) in Braunschweig, Germany, published on arXiv (P. Andreas et al. Preprint at; 2010), comes closer than ever to ending the cylinder's reign. The team has measured the number of atoms in a sphere of silicon-28 to calculate Avogadro's constant to nine significant figures: 6.02214084(18) × 1023 mol−1. The constant refers to the number of atoms in a sample whose bulk mass in grams equals the relative atomic mass of the element. This general relationship makes Avogadro's constant a fixed point from which to define mass.



Tometheus said...

.. but but but .. Avagadro's constant is defined in terms of the kg! (well, technically, 0.012 kg) I can define my unit of measurement to be Twillipogs (which is 1.2 kg) then redefine Avagadro's constant to be related to how many molecules of Carbon are in 12 Twillipogs. There's nothing magical about Avagadro's constant that would /define/ a kg, since a kg was already used to define .... Aw, nevermind, the circular logic makes my ead hurt.

Blaise Pascal said...


It's not really circular; it's a reversing of an existing relationship.

Currently, Avogadro's Number a measured quantity, based on the measured mass of C12 atoms. It's constancy is assumed, but it's actual value has an uncertainty based on our ability to count atoms.

The proposed redefinition would fix a numerical value to Avogadro's Number, and as a result the uncertainty would shift to the size of the kg. If the uncertainty in measuring the current size of the kg is larger than the uncertainty from the new definition, then the new definition is better.

This is very similar to what happened with the meter. Previously, the speed of light in a vacuum was a measured quantity, known by theory to be constant, but who's actual value had an uncertainty due to the limits of experiment. When the experimental techniques improved enough, a decision was made to fix the speed of light and redefine the meter in terms of it.

In reality, any fundamental constant who's value can be measured accurately enough that depends on the kg could be used. If, for instance, Planck's Constant was fixed, a watt balance could be used to measure masses without reference to a physical kilogram standard. At least three labs are working on watt balances to do exactly that, and provide yet another alternative definition of the kg.