The death of the kilogram, the international unit for measuring mass, was trumpeted by major news outlets around the world on November 16, 2018: according to The New York Times "The Kilogram is Dead, Long Live the Kilogram;" from The Washington Post we read both "The massive change: Nations redefine the kilogram" and "The inevitable, tragic--and ultimately necessary--death of the kilogram."
The death of the kilogram? One of the seven S.I. Units! The S.I. Units that form the subject matter of the first chapter in every high school chemistry, physics, and biology text, changed?
What is going on and what does it mean for STEM?
First, a little history.
We owe the modern system of weights and measures, the International Standards of Units (S.I.)* to a committee of thinkers and scientists convened by the French government in 1790 and given the task to recommend an entirely new system of weights and measures.
In 1790, the emergence of the modern world was being delayed by the hodgepodge of arbitrary and incoherent standards in use in France (as well as in every other country in the world): the length of a man's forearm (the Egyptian cubit); the length of Charlemagne's foot (the French pied de roi), the distance traveled by Roman soldier in 1,000 paces (the mile). Without common measures the task of sending goods from one country to another was a nightmare. Science and technology suffered as well because scientists and inventors could not share observations or experimental results without laborious attempts to equate a "gill" with a "grain" or even be certain what any given unit of measurement actually meant.
The committee's recommendation: that the system be decimal and that the weights and measures each be "based entirely on unchanging fundamental properties of nature" and thus ensuring a truly international system, one that would be "for all people, for all time" (pour tous, pour toutes les temps). (NIST, 2018)
The recommendations were accepted and what is now the metric system began with two units: length and mass. The measure of length was to be the meter whose length was to be one ten-millionth of the distance between the Equator and the North Pole as measured through the longitude of Paris.
The measure of mass was based on the mass of one cubic centimeter of water at 4° Celsius. But because this was such a small quantity, the standard was to be 1,000 times that mass, thus, the kilogram.
The standard for each measurement is what scientists specializing in measurement (metrologists) call an artifact, an exact physical representation of the measurement: for the Meter, a platinum-iridium meter stick; for the Kilogram, a platinum-iridium cylinder, known by its French guardians as “Le Grand K.”
Within fifty years the meter and kilogram were used all across Europe and with an 1875 treaty (Treaty of the Meter) its adoption became official in seventeen nations.
From 1875 to the present the international system of weights and measures has continued to evolve. The coming of electrical generation added a measure of current, the Ampere, in 1881. As railroads began to connect cities and countries, better measurements of time became necessary and the unit of a second appeared as 1/86,00th of the 24-hour rotation of the Earth. Luminosity was measured by 60 candles made of spermaceti (Sperm Whale) wax. Measurement of thermodynamic temperature was in centigrade degrees with 0° at the freezing point of water and 100° at its boiling point.
By 1960 the accumulation of units was formalized into seven basic units and twenty-two more units that could be derived from them and called Le Système International d’Unités or S.I. or the “metric system.”
The weakness of the system lay in the fact that the standards were embodied in physical representations of the measurements like the platinum-iridium meter or kilogram. Even though each was carefully protected and only handled rarely, even a gloved hand could wipe away parts of the kilogram’s mass or the meter bar be altered by temperature.
However, during the years between 1889 when the kilogram and the meter artifacts had been placed in their safes in the suburban Paris cellars, science had begun to uncover the laws that govern matter. Mass and energy were related to one another by a constant, the speed of light. The discoveries of the quantum nature of the atom revealed the Planck constant.
In 1967 the first step to the achievement of the 1790 vision for a system of measurements that were truly for all people for all times was taken when the second was defined by the vibrations of the caesium-133 atom.
In 1983 laser technology had made it possible to accurately measure the speed of light in a vacuum. The meter was redefined as the distance that light travels in a vacuum in 1/299 792 458 second. The platinum-iridium meter bar was retired in favor “a ruler made of light.”
So the big news from November 16 is that beginning on May 20, 2019, all the SI units will be defined by numerical constants so that each can “be transformed from definition to practical reality without the need for any type of artifact such as [the kilogram].” The headline editors who claimed that the news was of the “death” of the kilogram should really have called it the rebirth of the kilogram because it can now be calculated by using a very accurately defined numerical constant. (NIST, 2018)
This change will mean that scientists can make more accurate and precise measurements anywhere—Earth, Moon, Mars, beyond Pluto—without needing to check their work against an artifact like the meter bar or le grand K in the suburban Paris suburb.
Meet the constants:
The exact values for each of these constants were proposed in 2017. They were formally adopted on November 16, 2018.”
1. c: the speed of light in vacuum equal to 299 792 458 meters per second. This constand will help define the meter, kilogram and kelvin.
2. h: the Planck constant, one of the fundamental constants of quantum mechanics. h= 6.626 070 15 x 10-34 Joule seconds. Used to define the kilogram, kelvin and candela.
3. e: elementary charge, the amount of charge in an electron and e will be equal to 1.602 176 634 x 10-19 coulombs. It will help define the Ampere.
4. ∆Vcs the hyperfine transition frequency of cesium-133. ∆Vcs is equal to 9,192,631,770 hertz. It will help define the second, the meter, kilogram and ampere.
5. k the Boltzmann constant which relates an object’s energy to its temperature. K will equal 1.380649 x 10-23 joules/kelvin and will be used to help define the kelvin.
6. NA: the Avogadro constant defines the number of particles in a mole, the SI unit that expresses the amount of substance. In the revised SI, NA will equal 6.02214076 x 1023 particles per mole.
7. KCD: the luminous efficacy of monochromatic radiation of frequency 540 x 1012 hertz. Monochromatic radiation of this frequency is the green light that the human eye can pick up most sensitively in a well-lit room. “Luminous efficacy” is the total amount of visible light that a source produces using a certain amount of power. KCD is equal to 683 lumens per watt and help define the candela.
Note: *Why S.I. and not I.S.? The French are the custodians (and actually the inventors) of the Le Système International d’Unités, thus, S.I. or SI. The English name International System of Units is a translation of the French, not the other way ‘round. The international authority that ensures the distribution of the SI is the Conférence Générale des Poids et Mesures (CGPM).
NIST. (2018). A Turning Point for Humanity: Redefining the World’s Measurement System. Retrieved from https://www.nist.gov/si-redefinition/turning-point-humanity-redefining-worlds-measurement-system
Images: (captions added by author)
Woodcut, c. 1800 Usage des Nouvelles Mesures (Using the new measures) https://www.nist.gov/si-redefinition/kilogram-past
Le Grand K https://www.atlasobscura.com/places/le-grand-k
Dr. John Holton
Dr. John Holton joined the S²TEM Centers SC in July of 2013, as a research associate with an emphasis on the STEM literature including state and local STEM plans from around the nation.