The New Kilogram: Change Brings Permanence
Units of measurement are the foundation of all quantified data. They give data value by providing a scale and purpose. Over the past 200 years, much of the world adopted the International System of Units, more commonly known as the metric system. This base ten unit system was popularized in France during the 1790s revolution as part of an effort to create a unified system of measurement for the country. This included the introduction of the kilogram, the standard unit of mass. A quantity of mass determined to represent a kilogram known as the International Prototype Kilogram has been held in France since 1889.
In some ways, the units of measurement that form the system may seem rather arbitrary. The original meter was intended to be a portion of the distance between the North Pole and the equator along the line connecting Dunkirk and Barcelona. The distance was incorrectly calculated, but the incorrect measurement remains. It also formed the basis for many of the other units. Formal standard versions of each measurement were created in the late 1790s. The United States Customary Units (USCS) feature similar odd values. The US system is based on the old British Imperial system, and it utilizes old definitions from it. This includes basing the original definition of a bushel on a specific type of bushel basket available in Britain during the 1800s. Even the U.S. gallon is intended to replicate the British Queen Anne wine gallon. Today, the USCS units have formal definitions based on the metric system.
While the French government forced its country to accept the metric system, the United States government at the time was less insistent. The Metric Act of 1866 validated the system, but did not mandate its use. By the time the system was spreading around the world, American machinery and tools had already been made for inch units. Additionally, as a country trying to make its way as a new nation, the United States was hesitant to use a measurement that was determined using French land. Today, the United States uses both systems depending on the circumstances. Distances use UCSC units while food products and medicines commonly use metric units. This difference in units has led to failed missions in the past. The Mars Climate Orbiter was lost due to one team calculating in metric units while the other team calculated in USCS units. Units can have a greater effect than we may initially think.
Since units are so important, it makes sense that the value of the prototype kilogram must be carefully controlled. As it is a metal block, the prototype kilogram’s mass could change over time. It actually has already decreased slightly over the centuries. Therefore, it was formally agreed that the kilogram should be redefined based on Planck’s constant (6.626*10^-34 J-s or kg-m2/s2). While this has been a goal for many years, a final strategy was only recently implemented. Previously, two approaches featured in a study of this process were counting atoms and relating mechanical and electrical power (Schwitz et al).
Basing the definition on Planck's constant is possible because of the relationships between Planck’s constant and mass. This can be seen in the classic expression E=mc2. Here, E is energy, m is mass, and c is the speed of light. Planck’s expression is E=hv, where E is energy, h is Planck’s constant, and v is frequency. Through these two equations, we can see how Planck’s constant has a direct relationship with mass. As Planck’s constant is a proven and accepted constant, its value is fixed. This allows the value of a kilogram to be fixed.
It is important that over time the kilogram can remain a precise fixed value. This does not change the actual value of a kilogram as far as we are concerned. It does eliminate the flexibility that was inherently built into the previous definition. The difference would not be noticeable on a normal scale, but for scientific calculations it is important that the greatest possible precision is used. The Mars Climate Orbiter incident is a dramatic example of what can happen when mismatched units are used for the same experiment. However, a lack of data precision can have equally negative effects, depending on the scale of the experiment and the error.
While this may seem like a small change, it is a positive effort to lend some additional nuance to the metric system. Improving the accuracy and consistency of our systems can only advance our scientific efforts. It can increase our confidence in the longevity of the kilogram because the ultimate standard is no longer capable of diminishing with time. Units may be difficult to keep track of and even more difficult to convert for those working between metric and USCS units. However, units play a significant role in science, as well as in everyday life. From now on, the kilogram finally has its own fixed definition, giving it a greater sense of permanence in an impermanent world.
W. Schwitz et al., C. R. Physique 5 (2004).