In a discovery announced on January 10th, however, University of California, Berkeley physicist Holger Müller has given us a new and potentially more fundamental way to define time. His invention is the simplest clock ever constructed—based on a single atom of the soft metal cesium—that also stands to revolutionize (of all things) how we define a kilogram.
To understand how an atom can tell time, first consider the way that all clocks tell time: by counting. A grandfather clock counts swings of a pendulum. A digital watch counts oscillations in an electrical frequency. An atomic clock—the most accurate timepiece we have—counts microwave signals emitted by electrons as they transition between energy levels.
Müller’s clock counts oscillations of what’s called the “matter wave”- in this case, of a cesium atom. In his 1924 doctoral thesis French physicist Louis de Broglie followed on the work of Albert Einstein and Ernst Planck to propose that all matter can also act as a wave. The frequency of this “matter" wave is mindbogglingly fast, on the order of 3x10^25 cycles per second for cesium. Physicists have long thought that this frequency—known as the Compton frequency—is too fast to be measured, thus making matter waves unusable for marking time.
But as a post on Nature.com details, Müller discovered a way to measure cesium matter waves—albeit indirectly. He split a cloud of cesium atoms in half and shot photons from a laser into one half of the cloud, which slowed the frequency of those atoms. Then he rejoined the two groups of atoms, and as they recombined, he was able to measure the difference between their two frequencies—something on the order of 100,000 oscillations, a far more manageable number than the frequency of the matter waves themselves.
If the method behind Müller’s clock is mind-bending, the implications of his work are cool and fairly straightforward. In a press release issued by Berkeley and posted on Eureka Alert, Müller explained, “Every other clock involves a reference using at least two particles that interact. This Compton clock is the first to be based entirely on a single particle’s mass.”
Scientists prefer to base units of measurement on fundamental physical phenomena, because doing so makes science more exact and consistent. And because Müller’s clock is based on a single particle, it could provide the most elemental definition of a second ever constructed. "Our clock is accurate to within 7 parts per billion," he said in the press release. "That's like measuring one second out of eight years, about as good as the very first cesium atomic clock about 60 years ago. Maybe we can develop it further and one day define the second as so many oscillations of the Compton frequency for a certain particle."
At the same time, Müller’s work may also provide us with a more exact definition of a completely different unit of measurement: the kilogram. As the Globe reported in 2011, the current standard for a kilogram is a cylinder of platinum-iridium kept in a vault at the International Bureau of Weights and Measures outside Paris. It is a blunt and also unsustainable way of defining such an important unit of measurement: The reference cylinder is continually gaining mass as it absorbs contamination from the atmosphere. Müller’s technique for measuring matter waves raises the possibility of a more perfectly objective definition of the kilogram. Physicists can measure matter waves and, from those measurements, infer the weight of the underlying particles—which may make it possible in the future to define a kilogram as an exact number of atoms of a specific element.
Hardly bad work, for a clock.
Image of grandfather clock courtesy of Elliott Brown
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