A Paradigm Shift in the International System of Units (SI)

The international system of units (SI) serves as cornerstone of science, technology, industry and trade. In 2019, it experienced its most comprehensive renewal ever since it was established in 1875 by the Meter Convention. Until recently, seven base units served as anchor points of this system, meaning that all SI units can be derived by multiplying or dividing these base units. The discoveries in quantum physics have enabled a deeper and more coherent understanding of underlying physics and supported a paradigm shift in the choice of the SI’s anchor points, suggested already more than 100 years ago by James C. Maxwell and Max Planck. Instead of defining units by means of physical artefacts, idealized measurement procedures or material properties the new paradigm involves using physical constants as anchor points by fixing their values and thereby implicitly defining their units. Todays ‘new SI’ offers entirely new options for traceability back to its definitions. On the one hand, certain quantities can now be measured more directly in relation to the defining anchor points, rather than necessarily via the base units. On the other hand, self-referencing sensors become possible, which relate directly to the defining base constants. In my talk, I will shed light on several aspects of this paradigm shift, discuss how old anchors were released and new ones were set, and consider implications for future metrology.