Instead of being tied to a physical object in France, the international standard for the kilogram is now based on one of the fundamental constants of nature.
And to achieve this, scientists have agreed to change the very definition of one of the constants of physics.
You might be asking why I am talking about scientists debating the definition of standards.
It’s because not only is this story and its history strange, but it is going to be vital for our future scientific innovations on this planet, and beyond.
Check out the video above to find out why this is so important.
On November 16th 2018, scientists from over 60 countries unilaterally agreed on a change in the definition of the kilogram at the General Conference on Weights and Measures (CGPM).
Up until now, the definition was that 1kg equaled exactly the weight of a physical metal cylinder stored in France, made of a composite of Platinum and Iridium, called the International Prototype Kilogram, or colloquially known as “Le Grand K” (shown below).
Le Grand K: the last official physical kilogram
Every object around the world used to measure weight was first calibrated against a standard weight, which was calibrated against a copy of the IPK.
The problem has been that the mass of this IPK has actually not remained completely stable over the past few decades, and scientists cannot understand why.
This means that every time you weigh it, it might give a slightly different value. This is also true of the weights which were calibrated against it.
Now, in most everyday scenarios where we are using a scale to weigh a bag of flour, oranges or even elephants, the differences aren’t going to result in life or death situations.
But when calibrating machinery to measure on the smallest or the largest scales, the differences can multiply and become more important.
Not only that, the challenge with using a physical object as a reference point is that should anything ever change, or happen to, that physical object, it would by definition change the mass of everything else in the world.
But the biggest issue with measuring against a physical object is that you need access to the object itself to perform calibration.
This is why the world has been moving away from using physical objects for the past few decades, instead moving toward things which are based on fundamental constants in physics which will be universal, whether you measure them in Montreal or on Mars.
For example time (measured in seconds) now is defined as the movement of electrons in a type of Caesium atom, and distance (in meters) is based on how far light travels in a vacuum in a specific amount of time). Previously, both of these units were measured against physical objects (like a pendulum or a metal rod).
The issue has been that some of the physical constants which could be used to define these measures were themselves not completely standardised yet.
For instance, Planck’s Constant, used to define some of the smallest scales in the universe, was not actually defined as a specific constant value, and so has occasionally changed as it was measured.
This Planck Constant would be needed to define the kilogram.
So scientists thought about the problem from a different perspective and decided that they would spend the next few years running experiments across the world to measure Planck’s Constant, and then agree on a single figure which would from then onwards be the official figure set in stone.
This would also have implications on several other units of measure, which you can read about here.
The device they used to measure it is called a Kibble Balance, and you can see how it works here:
When you place a mass on the Kibble balance, the machine produces an electric current proportional to Planck’s constant. With Planck’s constant set, the kilogram will correspond to a specific amount of current in the Kibble balance.
The promise in this design is that even if the balance breaks, they can just fix it—something that you can’t do if you dent a platinum-iridium cylinder.
And based on the experiments, the new definition of The Planck constant is exactly 6.62607015×10−34 joule-second (J⋅s).
And this is where the true importance of this seemingly insignificant change comes in.
By agreeing on all of the fundamental units of measure to be related to physical constants, we can now use these units anywhere and recreate them at any time.
This includes on other planets, or in space.
If we are ever to move off Earth and colonise other planets or the Galaxy, then we will need to be able to perform scientific experiments in order to innovate.
And to perform these experiments, we will need to use the units of measure outlined above.
This is why this boring, boring news is actually so very, very important.
Nick Skillicorn
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