As agreed upon by scientists and representatives from around 60 countries last year, the world’s measuring systems are shifting away from using physical standards. Among the most prominent of these is the international prototype kilogram known as Le Grand K, a small metal cylinder protected underneath Paris that has defined weights and measurements since 1889.
The change is meant to remove the risk of relying on physical items. Even microscopic changes, from air pressure to the metal that has defined a kilogram, could have large implications.
“Unlike a physical object, a fundamental constant doesn’t change,” says Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md, in a press statement. “Now a kilogram will have the same mass whether you are on Earth, on Mars, or in the Andromeda galaxy.”
What’s a Kilogram, Anyway?
It’s the standard unit of measurement used around the world, except for a few countries like England and the United States. Initially created and named in the 1790s, the kilogram was part of what was then known as the metric system, a concept developed during the French Revolution by a group of scientists hoping to end the myriad chaotic measuring systems across the country.
The researchers’ efforts eventually spread across the globe, and in 1875, 17 nations created the International Bureau of Weights and Measures, commonly known by its French acronym, BIPM. Today, BIPM has expanded to 59 countries across the world.
Even in a contentious political climate, representatives from countries including Iran, Saudi Arabia, Israel, China, the United States, Russia, Brazil, South Korea and others come together every four years to reassess the state of what is now known as the International System of Units, or SI.
The kilogram wasn’t the only unit altered on May 20, also known as World Metrology Day. Four out of the seven base units within the SI will be changed in total, including the ampre, the kelvin, and the mole.
What Are Those?
Ampres are used to measure electrical currents. Kelvin measures temperature. Moles, meanwhile, are used to measure the number of molecules. The changes made to these three will most likely be unnoticed by all but scientists requiring the most precise measurements achievable. In the case of kelvin, BIPM says in a statement, the change will not even take effect today, but rather, “lays the foundation for future improvements.”
What Changed With Kilograms?
Kilograms are no longer measured by physical standards like Le Grande K, but rather by what’s known as the Planck constant. Developed in full in the year 1900 by German theorist Max Planck, it’s a cornerstone of quantum physics that measures the energy of particles and waves on the atomic scale. Five years later, Albert Einstein was able to prove that energy and mass are in fact equivalent.
Among Einstein’s most famous discoveries from that year, known as his Annus Mirabilis, was his equation E = mc2, which explains this equivalence. Plank had an equation of his own, E = hv, in which E is the energy of a particle of light. The equal sign shows that energy is equivalent to v, which stands for frequency, by a constant factor—a constant known within the equation as h, and as Planck’s Constant to a larger community.
“Taking the two equations together yields a counterintuitive but hugely valuable insight: Mass—even on the scale of everyday objects—is inherently related to h, which Planck first used to describe the vanishingly small energy content of individual photons emitted by the atoms in hot objects,” explains NIST.
Considering that Planck’s Constant has been proven to be naturally steady, scientists determined it’s reliable enough to be a baseline for universal measurement.
“If we make contact with aliens, what are we going to talk to them about? Physics. There is nothing else,” Schlamminger says. “But if you tell aliens that our units of measurement are based on a hunk of metal, you will be the laughingstock of the galaxy.”
While they’re altering historical precedent, scientists are confident that they’re focusing on more accuracy, which has always been the goal.
“Lord Kelvin, one of the leaders in the field of metrology said, ‘To measure is to know,'” director of NIST Walter Copan says in the statement. “As we are able to measure with increasing precision, we are able to learn more about the fundamentals of our universe and the fundamentals of life.”