The atomic units, the kilogram and the other proposed changes to the SI

The close relationship between the SI units incorporating the proposed changes of Mills et al and the atomic system of units is discussed. It is shown that when the definition of the second is taken into account, a definition of the kilogram in terms of the mass of a photon of specified frequency results in a kilogram that implicitly depends on α4me and other dimensionless constants, where α is the fine structure constant and me is the electron rest mass. Thus the choice of definitions proposed by Mills et al implicitly yields a kilogram that depends either essentially on α4me or on the unified atomic mass unit, mu. Consequently, the kilogram could be regarded as implicitly retaining its present status as an independently defined SI base unit with either definition. The α-dependence of the kilogram and many other SI quantities would, of course, be affected by future changes in the definition of the second.