The question seems simple, but it is not. A group of scientists has met this week in London, convened by the prestigious Royal Society , to try to redefine this universal unit of measurement .
The kilogram is the only one of the seven units included in the international system (known as SI for its acronym in English) that is defined based on an object, a cylinder composed of 90% platinum and 10% iridium , manufactured in London in 1879 and kept under a bell jar in the Office of Weights and Measures in Sèvres, near Paris.
However, measurements made over a hundred years ago show that the kilogram has lost weight. Its mass has changed the equivalent of a 0.4 mm diameter grain of sand. Long enough that scientists around the world have decided that the time has come to find a definition that dispenses with the physical object , just as they did with the meter, now defined by the speed of light.
For the kilogram, the scientists suggest using Planck's constant, a value named after the father of quantum physics, Max Planck . To define the relationship between the kilo of life and the Planck constant, experiments are being carried out all over the world. Michael Stock of the Royal Society of London estimates that the change could be approved "reasonably" at the next conference on weights and measures, to be held in Paris in 2015.
