From Artifacts to Physical Constants, continued

In 1983, the length definition changed again. The International Meter is now the distance that light travels in a vacuum in 1/299 792 458 of a second. Iodine stabilized lasers are the preferred method of realizing the current definition.

The unit of mass, alone among the fundamental units, is still defined by an artifact: the International Prototype Kilogram (IPK). However, several possible directions are currently being investigated whereby the mass definition may be linked to an invariant of nature. There are two major approaches taken in this worldwide effort: one is based on electrical measurements while the other is based on counting atoms.

In the electrical measurement approach, an electrical force is produced to balance the gravitational force on a kilogram mass. The electrical force is measured from the current and voltage needed to produce it. Since the electrical force is related to fundamental constants through quantum standards, the kilogram is therefore related to fundamental constants as well.

In the atom counting approach, the goal is to tie the macroscopic and microscopic units of mass --the kilogram and the atomic mass unit-- via Avogadro's constant. This approach requires a perfect 1-kg silicon crystal. If the mass and volume of the crystal are known along with the silicon molar mass, lattice spacing, and isotopic abundance, the number of atoms in the crystal can be calculated. The kilogram is therefore defined as being the mass of a specific number of silicon atoms.

A change of definition is viewed as necessary because the mass of the IPK is changing relative to the mass of the national prototypes by about 20 billionths of a gram per year or about the mass of a grain of pepper every decade. It is unknown whether the national prototypes are growing or the IPK is diminishing or whether the IPK is growing at a slower rate than the national prototypes.

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