Establishing a New Definition for a Kilogram

When commercially-available scales point at the weight equivalent of a kilogram, they do so with a certain degree of inaccuracy. Even scientific instruments register these errors, but they are considered to be too small to be of relevance. However, what some consider irrelevant is other people's bread and butter. Precision physicists have been trying to define the kilogram in a manner that is accessible and easy to verify for many years, but thus far they have failed to produce an easy method of establishing just how heavy a kilogram is, Technology Review reports.

They hit the same issue when trying to establish the length of a meter. At the 17th General Conference on Weights and Measures, which was held in 1983, experts decided to equal a meter to the distance light travels in 1/ 299 792 458th of a second, while in vacuum. For more than a hundred years before the new definition, the length of a meter had been the distance between two points on a bar of platinum and iridium, measured at the melting point of ice. But that was very difficult to test even in the most advanced labs. With the new definition, anyone with a stop watch, a laser pointer, and a few other devices could establish the length of a meter.

Now, the same type of effort is ongoing for the kilogram. At this point, its weight is that of a cylinder of platinum and iridium called the International Prototype Kilogram, which is held in Sevres, France, inside a vault. But the problem with it is that it loses atoms each time it's handled, a process that makes it lighter. Additionally, dust setting on its hundred-year-old surface makes it heavier all the time. Therefore, this reference point is imprecise in the eye of precision physicists. A proposal would have had the kilogram defined in terms of a current and voltage. But that would have required a tool known as a watt-balance, which requires at least 5 physicists to operate, costs $1.5 million to set up, and is about two stories high.

Now, Georgia Institute of Technology (Georgia Tech) experts led by scientist Ronald Fox propose a new method of defining a kilogram, in relation to a certain number of carbon-12 atoms. They say that precisely 2250×28148963^3 of them make up the perfect kilogram. This translates in practical terms into a carbon cube with a side precisely 8.11 centimeters long. In this manner, the group argues, anyone with a knife and some carbon could produce their own, precise kilogram, and thus verify the International System.