In (very!) recent news: the concept of the kilogram as we know it could drastically change.
Scientists from around the world gathered together on November 16, 2018 in Versailles to vote on whether the current kilogram system should continue or be rejected and modernized. I started writing this post before the date and so I will provide both sides to the argument before revealing the vote’s outcome.
Currently, the kilogram is based on an actual physical object. Hidden under lock and key outside of Paris, a platinum-iridium cylinder called the International Prototype of the Kilogram is the fundamental unit of mass. This cylinder has been the definition since 1879 and is nicknamed Le Grand K. It is the last base system international (SI) unit to be based on a physical object and poses a couple of problems.
A replica of of Le Grande K, held by The National Institute of Standards and Technology (NIST) in the US. Photo credit to NIST
The mass of Le Grand K is divergent, as it can pick up particles of dust, decay with time and has the possibility of being dropped or damaged. As a result, it seems very inefficient to base all of our scientific research and measurements over something this arbitrary. Le Grande K is also very inaccessible to scientists over the world.
The proposed solution? Set Planck’s constant as a fixed value. It is famously known that energy is correlated to Planck’s constant (h) and frequency (f). Energy is also known to equal mass (m) times the speed of light (c2). By rearranging, (as seen in Figure 1) mass is dependent on (hf)/c2. By setting h to a fixed value, we standardize the value of mass and therefore the kilogram.
Figure 1. Rearrangement of formulas to show mass’ (m) dependency on Planck’s constant (h). Made by Elizabeth Porter.
However, this is not as easy as it sounds. Scientists have been reminded that Planck’s constant comes with a degree of uncertainty. The last few decimal places of the number have been differently suggested by many. What isn’t uncertain? The mass of Le Grand K, locked away in France. Because of this, there may be merit in keeping the system as is. Additionally, the standardization of Planck’s constant will affect not only the kilogram, but other SI units such as the mole, the Kelvin and the ampere. Is deviating from the status-quo worthwhile?
Both sides of this topic have validity, although a conclusion has been made. Over the years, scientists have worked tirelessly to set Planck’s constant to a certain value. Many methods were cross referenced to one another to report that h equals 6.62607015 x 10-34 Js. The kilogram can now be based off of this scientific discovery, rather than an arbitrary object. The mole, the Kelvin and the ampere are also now certainly known.
As for the vote? Scientists unanimously approved the overhaul of Le Grand K with joy and celebration. Nobel Prize winner William Phillips exclaimed that this change is “the greatest revolution in measurement since the French revolution.” Measurements are the foundation of science as they allow us to make observations, and I think it is important to keep modern in our referencing.
This is a photo of people celebrating after Superbowl XLVII, but I like to think the reactions in Paris were similar. Photo credit to David Robert Crews.
