Chennai: Modern civilization, it is said, would be impossible without measurement. And measurement would be pointless if we weren’t all using the same units. So, for nearly 150 years, the world’s metrologists have agreed on strict definitions for units of measurement through the International Bureau of Weights and Measures, known by its French acronym, BIPM, and based outside Paris.
Nowadays the bureau regulates the seven base units that govern time, length, mass, electrical current, temperature, the intensity of light and the amount of a substance. Together, these units are the language of science, technology and commerce. Scientists are constantly refining these standards. In 2018, they approved new definitions for the kilogram (mass), ampere (current), kelvin (temperature) and mole (amount of substance). Now, with the exception of the mole, all of the standards are subservient to one: time. The meter, for example, is defined as the distance light travels in a vacuum during one-299,792,458th of a second. Likewise, the new definition of the kilogram rests on the second, in a manner too complicated to explain in fewer than several paragraphs.
“All the units now are not autonomous units, but they are all depending on the second,” said Noël C. Dimarcq, a physicist and the president of the BIPM’s consultative committee for time and frequency. That means that conceptually, if clumsily, you could express other units, such as weight or length, in seconds. “You go to the grocery and say, ‘I would like not one kilogram of potatoes, but an amount of seconds of potatoes,’” Dr. Dimarcq said. Yet now, for the first time in more than a half-century, scientists are in the throes of changing the definition of the second, because a new generation of clocks is capable of measuring it more precisely.
In June, metrologists with the BIPM will have a final list of criteria that must be met to set the new definition. Dr. Dimarcq said he expected that most would be fulfilled by 2026, and that formal approval would happen by 2030. It must be done carefully. The architecture of global measurement depends on the second, so when the unit’s definition changes, its duration must not.
“It’s like a once-in-every-50-year thing,” said Elizabeth A. Donley, chief of the time and frequency division of the National Institute of Standards and Technology, or NIST, in Boulder, Colo. She is on the BIPM’s international consultative committee with Dr. Dimarcq. “And so it’s a big deal that we want to get right, and so there’s a lot of discussion. It’s exciting to work on, for sure.” Once, humans told time by looking at the heavens. But since 1967, metrologists have defined time instead by measuring what’s going on inside an atom — clocking, as it were, the eternal heartbeat of the universe.
But time still has its roots and even its nomenclature in astronomical time keeping. Originally, it was based on the path of Earth in its daily spin, day to night and back again. Eventually, ancient Egyptian astronomers who used the duodecimal counting system, based on 12, divided the day and night into 12 hours each, giving us 24 hours in the day.
Those hours varied in length, depending on where Earth was in its orbit around the sun. A little more than 2,000 years ago, Greek astronomers, who needed fixed hours to calculate things like the movements of the moon, developed the revolutionary idea that a single day ought to be divided into 24 hours of the same length.