Measuring Mass in Seconds

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Science  01 Feb 2013:
Vol. 339, Issue 6119, pp. 532-533
DOI: 10.1126/science.1232923

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For centuries, humans have measured time by counting oscillations of highly regular periodic motion—the Sun, a pendulum, or a quartz crystal, for example. During the past 50 years, we have chosen to use the electromagnetic oscillations, which drive absorption in an atom—a highly stable and universal frequency reference. Such atomic clocks define the SI second via an atomic resonance in cesium (1). The second is the most precisely defined physical unit. Although it may seem obvious now, making the leap from performing precise spectroscopy on the atomic structure of cesium to using its atomic structure as a precise reference to stabilize other oscillators was profound. On page 554 of this issue, Lan et al. (2) make an analogous distinction between performing momentum-spectroscopy on a recoiling atom, and using that spectroscopy to stabilize an oscillator, effectively locking a clock to the mass of a particle. This result has important implications for fundamental physics and precision measurement, and could play a role in a new definition of the kilogram.