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We May Have A New Definition Of The Second By 2030

 It's time to change how we measure time. Or at least that's what scientists have proposed in a draft resolution for the upcoming 27th General Conference on Weights and Measures held later this year. Thanks to ultraprecise atomic clocks, we could have a new definition of the most fundamental unit of time: the second.

The second was defined back in 1967 as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atoms.” This was refined in 2018 but was still based on the hyperfine transition of the cesium-133 atom.

This definition has served humanity well for many decades, but breakthroughs in optical atomic clocks have gone beyond the precision that can be obtained with the more classical atomic clocks. In this brave new world of ultra-precise timekeeping, cesium is becoming a grandfather clock.


“As a general principle, the definitions for the SI units should be based on the measurement methods that lead to the lowest uncertainty for all secondary measurements of quantities based on those units," Dr Liz Donley, chief of the National Institute of Standards and Technology Time & Frequency Division, told IFLScience.


"Optical frequency standards have reached the level where they can perform frequency measurements that are 100x more accurate than the measurements that are performed using Cesium as the standard.”


But the definition of the second affects the definition of five out of the other six International System of Units (SI), so these uncertainties have impacts on mass measurements, distances, etc. A more precise second has an impact across almost all measurements we perform.

The 27th CGPM is a meeting of the International Bureau of Weights and Measures, which regulates and standardizes the system of SI units. Scientists and representatives from standards organizations around the world will gather to make decisions relating to SI, including redefining base units, as happened in 2018.

The draft resolution for the meeting held in November proposes amongst other things that the second be redefined by 2030, either by a new highly accurate atomic transition or by the weighted average of a group of highly accurate atomic transitions. First, a list of criteria that must be met for this new definition will need to be agreed upon.

There is currently a long list of criteria that needs to be met for optical clocks to become the standard and the cesium to serve as a secondary representation of the second. Work is underway to better clarify those requirements, which could be announced as early as June this year, reports the New York Times. But even with well-defined requirements, institutes across the world will have to go above and beyond to demonstrate a reliable new definition of the second.


“The metrology institutes working on meeting the criteria still have to achieve some of the key milestones and then we have to decide which atom (or atoms) the new definition will be based on. Progress is being made at a rapid pace, but there is not yet a consensus on this important issue,” Dr Donley told IFLScience.


Highly precise measurements are not just needed in labs. They affect technologies we use on a day-to-day basis, as well as new technologies that optical atomic clocks might deliver as they can precisely measure gravitational changes and more.



New York Times

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