The Long-lasting Definition of Kilogram has Changed

The Long-lasting Definition of Kilogram has Changed

The Long-lasting Definition of Kilogram has Changed
Image Credits: Wired

Scientists have replaced the platinum-iridium cylinder with the Planck Constant to give birth to a new standard of kilogram.

The world’s standard definition of the kilogram has finally been altered, after representatives from 60 countries voted in favor of redefining the International System of Units (SI) for mass. Since 1889, a platinum-iridium alloy cylinder, nicknamed as the Le Grand K, has been used to define kilogram. This ultimate standard is located in a triple-locked vault underneath the Pavillon de Breteuil, a building near Paris. Le Grand K lost its special status as the international prototype kilogram (IPK) on 20th May 2019 and it will no longer represent the base unit of mass to the world.

The decision was taken to replace the standard of this crucial unit on the basis of physical and atomic properties, instead of a man-made object. Barry Inglis, the President of the International Committee for Weights and Measures, referred to the ideology that brought this change in the following words:  

“The Metric System was envisioned to be ‘for all people for all times. From its outset it sought to ensure long-term stability by defining the units in terms of an internationally agreed ‘constants of nature’ instead of an arbitrary reference.”

The New Kilogram

The platinum-iridium cylinder has been replaced by the Planck Constant, a fundamental quantity related to the energy of photons. Defined as 6.626 x 10−34 joule-seconds, the constant fixes the kilogram to the speed of light and a temporal unit of measurement—the second. To be extremely precise, the kilogram is now equal to 1.4755214 x1040 photons of frequencies similar to that of a cesium atomic clock. This concept might not be familiar to many but is extremely accurate.

Although, Le Grand K is one of the most carefully protected objects on the planet, it is not immune from physical interactions that can alter its weight. Contact with human handlers—or even just air particles—can add or subtract tiny amounts of mass from the cylinder. Consequently, the functional value of the kilogram changes everywhere. Interestingly, recent measurements of Le Grand K suggest that the prototype has become slightly lighter than its sister copies.

Who will this Affect?

The new values represent a momentous shift towards a universal metrology, but they won’t have a major impact on the average person buying carrots at the grocery store or checking their weight on a scale. Most of the people who will notice the difference between Le Grand K and the new kilo will be scientists who deal with hyper-precise measurements, such as pharmacologists who must prescribe exact dosage of medicines. The influential physicist Max Planck, for whom the Planck constant is named, also predicted that this shift could help humans communicate with aliens, if the opportunity ever arises. He said,

“With the help of fundamental constants, we have the possibility of establishing units of length, time, mass, and temperature that necessarily retain their significance for all cultures, even unearthly and human ones.”

Advantage of the New Kilogram

The new system does have a certain advantage over the old standard. Every few decades, all the replica kilograms in the world had to be checked against Le Grand K. The new system, now that it’s been adopted, will allow anyone with a Kibble balance to check their weights anytime and anywhere. It is believed that a stable standard for kilogram is essential for it to be stable in the foreseeable future. Inglis emphasized on the advantages of the new standard by saying,

“Today marks the culmination of decades of work by measurement scientists around the world, the significance of which is immense. We will now no longer be bound by the limitations of objects in our measurement of the world but have universality accessible units that can pave the way to even greater accuracy, and even accelerate scientific advancement.”

Historic Artifact

In these circumstances, a lot of people were curious about what will happen to the former kilogram. According to the International Bureau of Weights and Measures, the old kilogram is a historic artifact that has been under study for more than 140 years and may retain metrological interest even though it is not a standard anymore. The changes, which will come into force on 20th may 2020, will officially end the use of physical objects to define measurement units.

Get some interesting information about all the units of the Metric System here

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