A kilogram no longer weighs a kilogram

A CGI version of the kilogram defining standard cylinder
A kilogram

How is it even possible for a kilogram to not weight a kilogram?

The answer is both simple and convoluted and depends on what is a kilogram, or rather how much does a kilogram weigh?

Is it 5 oranges? or 4 apples? What if their sizes are different? How would you ensure that a kilogram in a shop in USA (the joke is on them, they don’t even use kilogram) is same as that of in the UK and also in India?

Inconsistent (literal) measures

The earliest scientist used empirical methods to define a common standard. In 1795, scientists defined a kilogram as 1 litre of water at 0°C. Of course this was theoretically semi-sound but practically producing 1 litre of water that is completely pure, and and ensuring it remained that way was impossible.

So finally in 1799, a prototype made of iron was produced which had the exact (almost) weight as 1 litre of water at 0°C.

A photo showing IPK
International Prototype Kilogram

Today’s definition and the measure of 1 kilogram made of platinum alloy and  is derived based on that 1799 prototype, which was later replaced by what is now known as “International Prototype Kilogram” (IPK) in 1875.

The 1875 version of kilogram also remeasured the kilogram, and redefined is as 1 litre of water measured at 4°C under normal atmospheric pressure (i.e. 1 atm).

While this was more accurate the challenges facing the scientists remained. if the prototype gathered any dust, the weight will change. Or if the prototype face any wear and tear the weight would reduce.

The fact remained that a kilogram was the only SI unit that was not measured by a fundamental physical property and instead used an object, which meant, if two scientist wanted to produce 1 KG in two different parts of the worlds, their measures will be slightly different owing to several factors like impurity, pressure, gravity, etc.

Need for redefining the kilogram

The fact that the scientists around the world were not able to reproduce exact kilogram weight was and is a major issue as almost 20 units in physics is derived from kilogram.

NIST Kibble balance
Kibble balance, capable of measurring kilogram based on new definition

So eventually, after almost a decade of the issue gaining widespread support the at the 26th General Conference on Weights and Measured (GCPM), the new definition of kilogram which is based on Planck’s Constant, was approved.

Meaning of the new Kilogram

The new Kilogam is defined in terms of Planck’s constant (h), which is a fundamental constant of nature, just like Pi, it’s value is immutable.

And the new Kilogram was defined as a function of Planck’s constant, Second and Meter. The redefinition states that

Planck’s Contant = 6.62607015×10−34 kg⋅m2⋅s−1

Since both second and meter are also defined very accurately by fundamental properties of physics and Placnk’s constant is also fixed, whcih means the kilogram is also fixed and defined in terms of meters, seconds and Planck’s constant

Does this solve the problem?

Well, yes and no ! Depend on the problem being talked about.

While certainly there is a sense of predictablity in using the new definition for kilogram, it also means some other base measures will have to break their link to kilogram, e.g. “a mole”.

But our day to day life remains unchanged, we will continue to get only 5 oranges or 4 apples for a kilogram !

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