David Newell’s article (Physics Today, July 2014, page 35) on proposed changes to the International System of Units (SI) was an enjoyable read. The author was able to condense the key points of a rather complicated subject into a few pages. As he pointed out, the SI has undergone continuous improvement over its long history, so the focus on an improved standard for mass seems well justified. However, I am not convinced that the changes in the base quantities, as illustrated in tables 1 and 2 of Newell’s article, are a wise choice.
In modern society, the quantities one is most likely to use are time, temperature, mass, and length. Those quantities and their associated SI base units are defined as part of the present SI, as shown in table 1. However, in the new SI, table 2, the base quantities become frequency, heat capacity, action, and velocity. That choice, although it may be logical to the purist, seems to remove the SI from everyday use. Newell comments on the possible difficulties of teaching the new SI and uses the watt balance as an example. I believe many electronic balances work by offsetting the gravitational force of the unknown mass with the force generated by an electromagnet, so the basic concept of the watt balance is not particularly foreign. However, explaining why action is a base quantity may prove more challenging. Who uses an action meter?
Because of practical considerations, the SI can never be logically perfect. For example, it is not clear why the candela should be a base quantity, nor that Avogadro’s number should be a fundamental constant on the same footing as the speed of light. Keeping the same base quantities will not impede the improvements that may be expected from the emphasis on invariants of nature. However, it will provide continuity with the past and leave the basic logic of the SI accessible to a wider public.