Redefining the kilogram: A turning point for humanity

The kilogram is shrinking.

Kilogram No. 20, in the U.S., is one of several “working standards.” Credit: Science Source
Kilogram No. 20, in the U.S., is one of several “working standards.” Credit: Science Source

The kilogram is the base unit of mass in the International System of Units (SI), and is defined as being equal to the mass of the International Prototype of the Kilogram (IPK). But now the Kilogram is shrinking.

The official object that defines the mass of a kilogram may be a little, 139-year-old cylinder of noble metal and atomic number 77 that resides in an exceedingly triple-locked vault close to Paris. As a result of it’s therefore vital, scientists nearly never take it out; instead,d they use copies referred to as operating standards.

However, the last time they did examine the important kilogram, they found it’s roughly 5 elements in a hundred million heavier than all the operating standards, that are relinquishing some atoms of metal on every occasion they’re placed on scales.

This is one among the explanations the metric weight unit might shortly be redefined not by an object however through calculations supported elementary constants.

Peter Mohr, a physicist at the National Institute of Standards and Technology (NIST), who serves on the committee that oversees the International System of Units (SI) said, “This [shrinking] is the kind of thing that happens when you have an object that needs to be conserved in order to have a standard. Fundamental constants, on the other hand, are not going to change over time.”

A consequence of this change is that the new definition of the kilogram is dependent on the definitions of the second and the meter.


The definition of the kg is a part of a planned larger overhaul to create SI units totally obsessed on constants of nature. Representatives from fifty-seven countries can vote on the projected amendment this month at a conference in Versailles, France, and also the new rules are expected to pass. Together with the kg, the ampere (the unit of electrical current), Kelvin (temperature) and mole (amount of a substance) can get new definitions.

All four will be based on Planck’s constant, the elementary charge, the Boltzmann constant and the Avogadro constant, respectively. Every one of these constants is controlled by research facility estimations, which have some inherent uncertainty. Be that as it may, if the vote is successful, nations utilizing SI will concede to a settled an incentive for every steady dependent on the best information accessible and utilize them to infer the units.

The definition of the kilogram is due to change fundamentally – the current definition defines the kilogram as being the mass of the international prototype kilogram, which is an artifact and not a constant of nature, whereas the new definition relates it to the equivalent mass of the energy of a photon given its frequency, via the Planck constant.

  • Current definition: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
  • Proposed definition: The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the meter and the second are defined in terms of c and ΔνCs.

A consequence of this change is that the new definition of the kilogram is dependent on the definitions of the second and the meter.

Physicist Stephan Schlamminger of NIST said, “What will happen to the old kilogram artifacts after the redefinition? Rather than packing them off to museums, scientists plan to keep studying how they fare over time. There is so much measurement history on these. It would be irresponsible to not continue to measure them.”

Ian Robinson, head of the National Physical Laboratory’s engineering measurement division said, “One key reason for doing this work is to provide international security. If the Pavillon de Breteuil” — where the IPK is stored — “burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world’s metric weights system. There would be chaos. The current definition of the kilogram is the weight of that cylinder in Paris, after all. And that’s just not good enough for international science.”