Naturalness and convention in the International System of Units

In 2018 the units of the International System are expected to be redefined by fixing the numerical values of seven constants. In addition to specifying units, these definitions will implicitly specify a set of metric conventions - namely, criteria for determining equality among quantity intervals - for many kinds of quantity. For these kinds of quantity, the determination of differences and ratios will become implicitly tied to the mathematical forms of accepted fundamental laws. This overlooked consequence of the redefinition generates a conceptual tension between the need for long-lasting unit definitions and the need for testable fundamental laws. While fundamental laws will remain testable through the same methods that have been employed for their testing prior to the redefinition, certain kinds of evidence that support challenges to fundamental theory will become unavailable under the 'New SI'.