Metrologists Launched The New Definition Of A “Kilogram” On World Metrology Day

Metrologists have launched the new definition of a "kilogram" based on the Planck Constant on World Metrology Day.Happy Metrology Day! Photo: Chie | Flickr | CC BY 2.0

It’s official; the kilogram as we know it is no longer the same. As of today, the 130-year definition of the unit of measurement “kilogram” has already been retired and has been redefined to follow a more constant and precise definition.

The change was voted upon in the General Conference on Weights and Measures in Versailles last year, and the organization is implementing the more accurate definition of a “kilogram” today, May 20, 2019 – the World Metrology Day.

Metrologists – the scientists who study measurement systems – argue that the existing definition of a kilogram is no longer accurate and they need a new standard to follow because the precision of measurements is essential in almost all industries.

Of the seven base units of the International System of Units (SI), four are not currently based on the constants of physics: the ampere (current), Kelvin (temperature), mole (amount of substance) and kilogram (mass).

“The idea,” explained Emeritus Director of the International Bureau of Weights and Measures (BIPM) Terry Quinn to ScienceAlert, “is that by having all the units based on the constants of physics, they are by definition stable and unaltering in the future, and universally accessible everywhere.”

For example, a meter is determined by the distance light travels in a vacuum in 1/299792458 of a second. A second is determined by the time it takes for a cesium atom to oscillate 9,192,631,770 times.

Interestingly, before the new updated definition, a kilogram is defined by a … kilogram. It’s a kilogram weight called the International Prototype of the Kilogram (IPK), made in 1889 from 90 percent platinum and 10 percent iridium, and kept in a special vault in the BIPM headquarters.

One of the IPK copies in its double bell jars. Photo: NIST

This physical definition of an abstract concept is something that metrologists argue to be continually changing and fails to become accurate as time goes by. The kilogram is the only the SI unit that remains to be defined by a physical object.

Replicas of the kilogram prototypes are used in different parts of the world to serve as national standards. Now and then, these replicas are sent to France to be compared against the actual kilogram prototype.

As expected, these replicas yielded considerable inaccuracies and variance from the SI kilogram prototype. It has been observed to be drifting away from that of the IPK locked away in the vault. It’s unclear whether the copies were losing mass or the IPK was gaining mass, but neither scenario is ideal for scientific precision, even if we’re dealing with mere micrograms.

The new definition of a kilogram is now based on Planck Constant, the ratio of energy to frequency of a photon, measured to its most precise value yet only last year.

“It is only now that we can define the kilogram in terms of a constant of physics – the Planck constant, the speed of light and the resonant frequency of the cesium atom,” Quinn explained.

“Why all three? This is because the units of the Planck constant are kgm2s-1, so we need first to have defined the meter (in terms of the speed of light) and the second (in terms of the cesium atom in the atomic clock).”

So under the new definition, the magnitude of a kilogram would be “set by fixing the numerical value of the Planck constant to be equal to exactly 6.626 069… × 10–34 when it is expressed in the SI unit s–1 m2 kg, which is equal to J s.”

While the changes would not make any perceivable difference if someone buys a kilo of meat from the supermarket, the difference is an essential factor for scientists to consider in their studies moving forward. Because, as noted, base unit standards can rely on other base units. The candela, the ampere, and the mole will be redefined to greater accuracy based on the kilogram. And, as for scientists,

“[The new definition] will considerably improve the understanding and elegance of teaching about units,” Quinn said. “It will open up the way to unlimited improvements in the accuracy of measurements; it will improve the accuracy greatly and extend the possibilities of making accurate measurements at very small and very large quantities.”

Quinn also noted that, while it may look complicated, the new system can be easily understood by anyone. He built a simple balance out of Lego in his basement that can measure directly against the Planck constant, within 5 percent.

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