Inside a laboratory nestled in the foothills of the Rocky Mountains, amid a labyrinth of lenses, mirrors, and other optical machinery bolted to a vibration-resistant table, an apparatus resembling a chimney pipe rises toward the ceiling. On a recent visit, the silvery pipe held a cloud of thousands of supercooled cesium atoms launched upward by lasers and then left to float back down. With each cycle, a maser—like a laser that produces microwaves—hit the atoms to send their outer electrons jumping to a different energy state.
The machine, called a cesium fountain clock, was in the middle of a two-week measurement run at a National Institute of Standards and Technology (NIST) research facility in Boulder, Colo., repeatedly fountaining atoms. Detectors inside measured photons released by the atoms as they settled back down to their original states. Atoms make such transitions by absorbing a specific amount of energy and then emitting it in the form of a specific frequency of light, meaning the light’s waves always reach their peak amplitude at a regular, dependable cadence. This cadence provides a natural temporal reference that scientists can pinpoint with extraordinary precision.
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