### It Is Time To Re-Consider The Definition Of Kilogram

One of the biggest goals of science and technology is to deliver the most accurate description of reality as promising, so it is always cool telling people how the unit of mass is defined. The kilogram, for instance, is described as the mass of a platinum-iridium cylinder in Sèvres, France.

Yes, each mass and scale in the world has to be matched with the French block, which is properly called the International prototype kilogram (IPK). When the kilogram was defined in 1889, 40 authorized physical copies were completed and sent around the world. The particular measurement of the kilogram was taken to be equivalent to one thousand cubic cm of water. Clearly, having a physical object to quantify a unit is not impossible. The actual mass of the International prototype kilogram (IPK) and the associated copies have not remained the same. Measurements taken in 1948 and 1989 have shown that the masses are all diverging, with some losing mass and some gaining it, for a variety of causes including simply getting difficult.

This matter has been troubling people working in metrology for quite some time. If things must be measured accurately, the description of the kilogram needs to be more accurate. Dr Stuart Davidson from the National Physical Laboratory(NPL) told IFLScience, “The degree of instability is satisfactory, but scientifically is a bit of an anomaly. While we can live with a few microgram changes over a few years, what we are looking for is something that is going to fundamentally be constant forever."

The meter is described as the distance light travels in 1/299,792,458 seconds, and the second is defined as 9,192,631,770 oscillations of a definite frequency of radiation from the cesium atom. With the accurate apparatus, everyone in the world could measure distances and times according to the international system (IS). To attain this universality for the unit of mass, researchers are confident to re-define the kilogram in terms of Planck’sconstant, an important constant that connects the frequency of a particle to its energy, discovered in many physical formulas.

Replica of the fundamental kilogram at Cité des Sciences et de l'Industrie.
Jasp 88 via Wikimedia Common CC BY-SA 3.0

Mass, at the moment, is earthbound. Scientists have been searching for methods to free the mass definition from a physical object, with the Committee for Weights and Measures (CIPM) deciding in 2011, that a new definition was required. Since the improbability is in the land of micrograms, the significance of stability is in fact seen when we try to make very accurate measurements of either small or large quantities. For instance, the pharmaceutical companies measure very small quantities of active drugs and since the differences in the fundamental kilogram are around the size of the quantities measured, this could produce difficulties and problems.

In a similar method, large masses are affected. If your aim is to accurately measure the mass of a jet, a 0.01% uncertainty could have a huge effect on both cost and fuel efficiency. And it is in this situation where we can witness another advantage of the re-definition. If you solve the mass to a fundamental constant, you can explain any mass you like (you will not have to start with a kilogram and partition). The re-definition would develop the complete mass scale. If you believe that the re-definition is honest and easy method, though, we have got bad update for you. To accept the original measurements, the CIPM demanded that three independent measurements be obtained with at least two independent ways. And this is where stuffs get a bit complex. The international unit of electric current, the ampere, has been defined relative to the kilogram; however the electrical community has found methods to define the ampere in terms of certain quantum effects connected to the Planck’s constant.

What researchers believed of doing was to turn the definition of the ampere on its head and to describe the kilogram through electric units in terms of the Planck’s constant. The instrument to carry out this measurement, known as the watt balance, was designed by Dr Bryan Kibble in 1975. Dr Bryan Kibble died this year, and the instrument has been given another name the Kibble balance in his honor. The Kibble balance is basically a very complex scale. Researchers are now working on the highest precision measurement of the Planck constant. Once that is gained, the Kibble balance could be used to measure the mass of every object.

Simplified by Dr Stuart Davidson, "Theoretically, everyone can design their own Kibble balance, do the experiment, and then generate a ‘kilogram’ without having to wait."

Section of the Kibble balance. National Physical Laboratory.

Another way to measure the kilogram is through Avogadro’sconstant, which signifies the number of atoms in a certain mass of a certain substance. That is obtained by making a perfect kilogram sphere made of a specific type of silicon and measuring its diameter 500,000 times in faintly different positions. Knowing the volume and the properties of silicon, scientists can simply evaluate the number of atoms. That allows for accurate approximation of Avogadro’s number. The Avogadro system has two benefits: It requires one to only count the number of atoms and it's defined in terms of the Planck’s constant. Though the measurement is still interconnected with an artifact, it is not dependent on a particular one. It is how the artifact designed that matters and not about the object itself. Still, the silicon solution has its disadvantages.

The artifact is sphere-shaped and so it is not easy to handle. In addition, it has a huge volume, so you have to weigh it in a vacuum to conclude its mass (the mass of the air would affect). It also has a big surface area, so it will get 10 times as difficult as the fundamental kilogram, making the artifact incorrect very quickly. The two experiments are getting closer and closer to the compulsory precision the CIPM ask for, although, so confidently the kilogram along with the kelvin, the ampere, and the mole (whose definitions are also up for review).They  will be re-defined at the 26th General Conference on Weights and Measures in 2018.

This silicon sphere weighs an exact kilogram. Julian Stratenschulte/dpa