Standard measurement units like the meter and the kilogram used to be defined by physical objects, such as a prototype bar and a platinum cylinder. As of May 2019, all measurement units are now defined by the cesium atom and the fundamental constants of the universe.
In an article for Physics Today, MIT professor Wolfgang Ketterle and Waterloo professor Alan Jamison in Physics and Astronomy describe how we transitioned from a hunk of platinum to Planck’s constant, and how atomic physics is central to the most radical redefinition, that of the kilogram.
“The new kilogram exemplifies how fundamental, curiosity driven research can have unforeseen applications down the road,” said Jamison. “Measuring Planck’s constant to 7 decimal places may not sound ‘useful,’ but the Kibble balance that grew out of that research gives us a way to define masses in pharmaceutical settings far more accurately than was possible before.”
There are many different standards that could be used to define a kilogram, and Ketterle and Jamison highlight some interesting but impractical options like pumping a microwave cavity with one-kilogram of photons—the output of a medium-sized nuclear power plant over a year.
The new definition is much more practical and also has the benefits of being reproducible and precise. Labs all around the world can measure a true kilogram for themselves. There’s no need to compare with a platinum cylinder in Paris. And the precision at which Planck’s constant and cesium transitions can be measured allow for unparalleled accuracy—there’s almost no wiggle room with the new definition of the kilogram.
While the astonishing specificity of the new kilogram might not mean much when you step on the bathroom scale, Jamison notes that the redefinition demonstrates the sometimes unexpected power of scientific research.
“Beautiful ideas from atomic, optical, and condensed matter physics all converge into the redefinition, which also has tremendous practical implications for industry,” said Jamison.