Measurements for the Future—A Complete SI From Physical Constants

Basing the SI units entirely on the values of fundamental physical constants involves extremely difficult metrology. It is desirable that the reproducibilities of a newly defined kilogram and of a thermodynamic temperature scale are at least equal to those of the present artifact kilogram and the water triple-point temperature. This paper discusses the benefits of an updated SI, the principal difficulties in attaining it, and why the effort expended is justified.