ABSTRACT
T he science of measurement, called metrology, is indispensable to the science of physics because the accuracy of measurement limits the ac-
curacy of understanding. In fact, the construction, inter-comparison, and maintenance of a system of units is really an art, often dependent on the latest advances in the art of physics-e.g. quantum Hall effect, laser cooling and trapping, trapped-ion frequency standards, etc. As a result, metrological precision typically marches forward a good fraction of an order of magnitude per decade. Importantly, measurements of the same quantity (e.g. the fine structure constant α) in different fields of physics (e.g. atomic structure, QED , and solid state) provide one of the few cross-disciplinary checks available in a world of increasing specialization. Precise null experiments frequently rule out alternative theories, or set limits on present ones. Examples include tests of local Lorentz invariance and the equivalence principle, searches for atomic lines forbidden by the exclusion principle, and searches for electric dipole moments in fundamental particles which indicate violation of time-reversal symmetry.
