As you stand in the produce aisle of the local grocery store weighing bananas, few have wondered exactly how we come by the pounds, kilograms, ounces, and grams that are used to measure the weight of something around the world. As it turns out, deciding on a standard way to know that a kilogram is a kilogram is very difficult. Today knowing that a kilogram is a kilogram is done with a set of international prototype kilogram weights made from very hard metals like platinum, but even that isn’t entirely accurate.
The weight changes depending on wear and tear or dirt on the metal block. Scientists wanted a new method of defining a kilogram, and as it turns out coming to that new definition was incredibly tricky. The new definition of a kilogram uses a fixed value from nature, known as Planck’s Constant defined as 6.62607015×10−34 joule seconds.
Planck’s Constant is derived from dividing the electromagnetic frequency of a particle of light by the amount of energy it carries. This number can also be expressed in kilogram square meters per second, though it is usually measured in joule seconds. The new definition means that the weight of a kilogram no longer has to be compared to a block of platinum.
Scientists had to create very precise devices to measure Planck’s constant, which is why the new definition took so long to publish. After a symbolic vote, the new definition is in effect. Along with redefining a kilogram, other base SI units were also redefined.
Other SI units with new definitions include the mole, kelvin, ampere, and candela. The new definition of a kelvin promises advantages for scientists needing to make very precise measurements.