Susan Sterrett (Wichita State University)
In mid-November 2018, an international body, the General Conference on Weights and Measures (CGPM) will meet to vote on a proposal to revise the International System of Units ("the SI"). The proposed revision is considered revolutionary, although the motivation for it is to achieve an ideal that has long existed yet remains unfulfilled to date: to provide a system of measurement based entirely on natural measures.
There are two features of the proposed system of units (the "New SI" or "Revised SI") that are bound to arouse interest among those concerned with foundational questions in philosophy of science: (i) the proposed system of units can be defined without drawing on a distinction between base units and derived units; and (ii) the proposed system of units does not restrict (or even specify) the means by which the value of the quantities of the units are to be established. Instead, the system of units is defined by fixing the value of seven "well-recognized fundamental constants of nature." The change is akin to the approach that has already been taken for defining the unit of length (i.e., the meter) in terms of the velocity of light, a well-recognized "constant of nature." To define the entire system of units by fixing the values of seven constants of nature (one of which is the velocity of light) is a far more radical proposal.
The proposal that is expected to be accepted presents two distinct alternative formulations of the definition of SI units. In the first of these, the definition includes not only the units that were previously designated as the seven base units of the SI, but it includes five additional SI units, and, in an unprecedented move, draws no distinction between base units and derived units. Yet, to minimize disruptive consequences of the change, a second formulation of the definition is provided as well, in which each of the base units is given a definition and, as a matter of convenience, the terminology of base units and derived units is retained. Thus, the question of the appropriate role of base units -- and, even, of whether the concept of base unit plays an essential role in the definition of a system of units at all -- arises.
In this poster, I will first briefly present and explain the proposed reform of the SI. I will then highlight what will change and what will remain the same if the proposal is accepted by the international body in mid-November as expected.
Finally, I will address the question of the role of base units in light of the new SI. I formulate and clarify the question: "Do the 'base units' of the SI play any essential role anymore, if they are neither at the bottom of a hierarchy of definitions themselves, nor the only units that figure in the statements for fixing the numerical values of the 'defining constants'?" I present an answer to this question.
