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Explore measurement with the IOP

14 May 2019

When we measure something, it is in units like metres, kilograms or seconds and these units are fixed. At least they are since a 2018 vote to define the last remaining unstable units – the kilogram, kelvin, ampere and mole – by fundamental constants of nature.

Imagine if instead of measuring things, we guessed how big or heavy or fast they were. Built from stone blocks of varying sizes, Egypt’s pyramids would have crashed to the ground long ago. We would never have created complicated and precise gadgets like phones or laptops that are made out of components engineered down to the nanoscale. And society as a whole would be in a permanent state of chaos – without a way to measure weight or size, people would constantly argue over the worth of the goods they are buying or selling.

Measuring the world around us in units (like seconds, metres, kilograms, etc) that everybody agrees upon is therefore the bedrock of modern society. But in recent decades it has become apparent that the way we define some of these units no longer cuts the mustard. These units are defined by an object, experiment or phenomenon that is fundamentally unstable, i.e. it can be different depending on where or when it is measured in the universe. And this is a big problem. If the thing that defines the unit you are measuring changes, how can you tell it is any different? The answer is that you can’t – by definition, it is still that unit. 

This is why in 2018 representatives of over 40 countries met at the General Conference on Weights and Measures to pass a vote to make the most sweeping changes to the International System of Units (SI) in its history, with four brand new definitions of the kilogram, kelvin, ampere and mole. The changes come into effect on 20 May 2019 - the anniversary of the signing of the Metre Convention in 1875, a treaty for international cooperation in metrology, and also World Metrology Day. The new definitions bring these four units into line with the rest of the SI units, like the second which is already defined by a fundamental constant of nature: the speed of light. Now that we are using constants of nature that consistently appear in the basic theoretical equations of physics upon which our understanding of the universe rests, the kilogram, kelvin, ampere and mole will never again vary. 

For example, the kilogram was previously defined by the International Prototype Kilogram, a cylindrical platinum alloy weight kept in a vault on the outskirts of Paris. Just touching this weight and leaving skin particles on its surface would change the definition of the kilogram. To avoid this absurd situation, its definition now comes from the Planck constant. The Planck constant h relates the energy E of one quantum of electromagnetic radiation to its frequency ν by E = hν. How this links with mass is through Einstein’s famous formula E = mc2.

The change is nothing to worry about for the public or even most scientists. For instance, a bag of sugar doesn’t suddenly weigh 50 kilograms, because the actual kilogram unit has not changed at all – only its definition. 

The only major change is in the philosophy of how we measure. The redefinition completes a journey from basing our system of units on things we have made to one built on constants of nature. As a result, all the SI units now come from fundamental truths of the universe that will never change. And they can be realised by anyone with the proper equipment, at any time and anywhere in the world or wider universe.

Interested in learning more about measurement and the new SI? Physics World and IOP’s partners have great content that will satisfy your curiosity: