The International System of Units (the SI), the modern metric system, has recently undergone 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. Our measurement system is now, both philosophically and practically, strongly quantum. Nobel Prize recipient William Phillips, Ph.D., a Distinguished University and College Park Professor of Physics at the University of Maryland, talks about why this reform was needed and how it is done. Series: "UC Berkeley Graduate Lectures" [Science] [Show ID: 37728]