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Kilogram has got a new definition. Here's why

PW Bureau

It is arguably the most significant redefinition of an SI unit since the second was recalculated in 1967, a decision that helped ease communication across the world via technologies like GPS and the internet New Delhi: On Friday, after a week-long meeting at the Palace of Versailles, scientists have voted to redefine the value of kilogram. It may not affect how people buy fruits but it will boost the accuracy of scientific measurements. Since 1889, a shiny lump of platinum-iridium kept in a special glass case known as the International Prototype of the Kilogram has been defining the kilogram. It is housed at the headquarters of the International Bureau of Weights and Measures (whose French acronym is BIPM), just outside Paris.

Some 60 nations who are the members of BPIM agreed to redefine the kilogram in terms of a tiny but unchanging value called the “Planck Constant.”

It is arguably the most significant redefinition of an SI unit since the second was recalculated in 1967, a decision that helped ease communication across the world via technologies like GPS and the internet. Some 60 nations who are the members of BPIM agreed to redefine the kilogram in terms of a tiny but unchanging value called the “Planck Constant.” It was also agreed upon by the participating scientists that the definitions for the ampere (electrical current), the kelvin (thermodynamic temperature) and the mole (amount of a substance) would be updated. A kilogram is derived from immutable physics and for decades, scientists have been trying to define a constant value for it as they have found for other standard units (SI units) overseen by the BPIM. The problem is the prototype doesn’t always weigh the same. Even inside its three glass bell jars it picks up microparticles of dirt and is affected by the atmosphere. Sometimes it needs cleaning, which can affect its mass. That can have profound implications. If the prototype were to lose mass, atoms would, in theory, weigh more since the base kilogram must by definition always weigh a kilogram. For instance, a meter isn’t 100 centimetres, it’s actually “the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.” The “Planck constant”, which derives from quantum physics, can be used along with a Kibble balance, an exquisitely accurate weighing machine, to calculate the mass of an object using a precisely measured electromagnetic force. “The SI redefinition is a landmark moment in scientific progress,” said Martin Milton, director of BIPM. The new definitions agreed by the BIPM will come into force on May 20, 2019.