Time and Metrology

The recourse to Quantum Mechanics for the definition of the International System of Units has been the trigger of significant progress. In the first part of the paper, we briefly recall the definitions of units now in use for the basic quantities. We summarize the levels of precision available today within the framework of these definitions. Time, whether we are aware of it or not, is a very special physical quantity. We therefore expose the extended use that could be made of the expression of physical quantities by means of time, like what is already practiced for the masses, expressed by means of energies. We detail a little more the case of the electrical quantities voltage and resistance which, without being basic quantities, benefit from new approaches thanks to two quantum phenomena: the Josephson effect, and the Quantum Hall Effect. But time is not an absolute. The next part of the paper exposes the corresponding teachings brought by Special Relativity on the one hand, and General Relativity on the other hand. Finally, the perspectives of the field are approached under the aspect of the metrological repercussions. Indeed, the technologies applicable to the measurement of time lead to performances that can be considered extraordinary in absolute terms, and in any case superior in relative to anything that can be achieved for other physical quantities. In conclusion, we examine the challenge that may represent the achievement of ever-increasing metrological performance for all physical quantities, and for time in particular, and the perspectives opened up for research in the domain by fields of knowledge not yet covered.