Getting Down to Basics

I have posted a few notes here about the effort to find a new definition for the kilogram, the only fundamental unit of the SI [Le Système International d’Unités] system of units that is still defined by a physical artifact: the mass of a particular cylinder of platinum/iridium alloy, stored in a vault at the Bureau International des Poids et Mesures [BIPM] at Sèvres, outside of Paris.  Other fundamental SI units are defined in terms of fundamental physical processes; the meter, for example, is defined as the distance traveled by light in a vacuum in 1/299,792,458 second.   The advantages of this kind of definition are that it does not depend on a integrity of a physical object, and that it can be replicated anywhere that suitable conditions and apparatus are available.

According to a report at New Scientist, this year’s meeting of the quadrennial Conférence Générale des Poids et Mesures [CGPM] has unanimously adopted a proposal to change the SI to redefine the kilogram, as well as three related units, the mole, the ampere, and the Kelvin [degree].

The General Conference on Weights and Measures (CGPM) in Paris, France, has unanimously agreed on a proposal that would lead to reform of the mole, kilogram, kelvin and ampere, according to the international system of units (SI).

The change will need to be confirmed at the next meeting of the CGPM in four years’ time; if it is confirmed, it will be the most significant change in SI for a century.

The new definitions will tie the SI units to fundamental physical constants; to put it another way, the units will be defined on the basis that the constants are really constant, and have known values.  The proposed new definitions are:

• Ampere (unit of electric current) will be defined such that the elementary charge (the charge on one proton or electron) is exactly 1.60217653 × 10^-19 coulombs.
• Kelvin (unit of absolute temperature) will be defined such  that the Boltzmann constant is 1.3806505 × 10^-23 joules/Kelvin
• Mole (amount of s substance) will be defined such that Avogadro’s constant is 6.0221415 × 10²³ mole^-1
• Kilogram (unit of mass) will be defined such that Planck’s constant is 6.6260693 × 10^-34 joule-second.

The New Scientist also has a handy chart showing the old and new definitions of the fundamental units, including those that will remain unchanged: the second, the meter, and the candela.

The BIPM Web site has the press release [PDF], as well as the (fairly technical) text of Resolution 1 [PDF].

Making a change like this is a slow process, because the people responsible, from the various national metrology labs, tend to be conservative, for good reasons; one of their objectives is that definition changes will not cause dislocations in everyday life and commerce.  Watching the process can seem almost as exciting as watching paint dry.  It’s probably useful to remember that this problem exists only because we have learned to measure the physical world to a degree of accuracy undreamed of when the metric system was first formulated in the 18th century.