Measurement is an important aspect of daily life. We measure everything from the amount of salt to be added in food, to ourselves when we stand on the weighing machine. When we do this, we get certain values and we know these values have meaning because they are based on a standard. This standard was, until very recently, a metal kilogram made of platinum and iridium that resides under lock and key in an underground vault in Paris. It’s called the International Prototype Kilogram, or IPK, and since its creation in 1889 it has been the standard by which the world’s weights are defined.
Copies of the IPK are distributed around the world, with countries then creating their own reference weights, as close to the original as possible. These, in turn, are used to calibrate scales and weights throughout every section of society, from labs and factories to supermarkets and bakeries. The IPK served as the world’s standard for 129 years, but now the definition of the kilogram is being changed to that of a universal constant in nature and this change will go into effect on May 20th 2019. The reason for the change is because of the lack of consistency. The IPK has lost around 50 micrograms (the mass of an eyelash) since it was created. But even when it loses mass, it still indicates that it is exactly one kilogram as per the previous the definition.
The need to define the units of measurement started with the creation of the metric system during the French Revolution. The metric system was created with the intention of liberating the common man and woman from the confusing and inconsistent weights. It was a utopian project, with its creators imagining that a shared system of weights and measures would unite the world, allowing for the free exchange of goods and information. In order to ensure that these units of measures were accessible to all, there had to be new definitions of measurement based on experiments that any individual could repeat. Previously, the French people used the pied du Roi, or king’s foot as a unit of length. Unfortunately, this unit was not consistent as at end of the day, the size of person foot can change with age. And thus developed the need for constant units.
The first definition of a kilogram was originally defined as the mass of a cubic decimetre of water (a decimetre being a tenth of a meter), while the meter itself was calculated as a fraction of the distance between the North Pole and the Equator. The problem with this definition was highlighted by the physicist James Clerk Maxwell who said that “the Earth might contract by cooling, or it might be enlarged by a layer of meteorites falling on it,” changing its shape and with it the length of the meter. Maxwell’s anxiety reflected the fundamental importance of metrology. Consistent units of measurement are the foundation of the scientific method. Without them it is not possible to reliably repeat experiments, and if the results of experiments are not reliable, then neither is the understanding of the natural world.
The solution to this problem was to use constants of nature instead of just Earth. These constants are fundamental to the study of physics like the gravitational constant, the speed of light, Planck’s Constant and so on. In 1960, the meter was measured using the wavelength of light, and then in 1983, it was given its current definition as “the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.” Over the past few decades, six of the seven units of the metric system — the meter, the second, the ampere, the Kelvin, the mole, and the candela — have undergone the same transformation. Now, only one artefact-based unit remains, the kilogram, and this definition will now be changed too.
After the redefinition our most basic units, weight and length, will be derived from the two foundational theories of physics: quantum mechanics and special relativity. The former gave us Planck’s constant, which will be used to define the kilogram; and the latter gave us the speed of light, which already defines the meter. However, the work of measurement does not end here. We live in a global society and the needs of industry are always changing along with scientists who are always expanding the boundaries of human knowledge. Therefore, the discipline of measurement will never become obsolete. This is good because measurement is a fundamental aspect of life and a constant study is required so that the ways through which we measure increase in accuracy.
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