Measuring the Elusive: Quantum Physics Rewrites the Metric System

The International System of Units (the SI), the modern metric system, recently underwent its most revolutionary change since its origins during the French Revolution. The nature of this revolution is that all of the base units of the SI are now defined by fixing values of natural constants, including the “most quantum” one, Planck’s constant. Our measurement system is now, both philosophically and practically, decidedly quantum. This talk will recall some of the history of how units have been defined in the past, describe why the quantum reform was needed, and how it is done.