From: veritasium
Early Definitions and the Birth of the Kilogram
The journey to define the kilogram began with the “grave,” the original name for the base unit of mass in the metric system, later known as the Systeme International d’Unites (SI units) [01:38:00]. In 1793, a commission that included the notable scientist and aristocrat Antoine Lavoisier defined the base unit of mass as the weight of a cubic decimeter of water at the melting temperature of ice, essentially a litre of ice water [01:49:00]. The name “grave” originated from the Latin “gravitas,” meaning weight [02:01:00].
However, the name “grave” was short-lived due to its similarity to the aristocratic title “graf” (equivalent to an earl or count) [02:07:00]. Amidst the French Revolution’s cry for equality, a unit could not sound nobler than others [02:17:00]. Lavoisier himself, not for his scientific contributions but for collecting taxes as a nobleman, was executed during this period [02:23:00].
The new republican government initially deemed the “grave” too large for common measurements and opted for the “gramme,” a thousandth of the “grave” [02:37:00]. Realizing the gramme was too small, they reverted to the original “grave” mass, but renamed it the “kilogram” – a thousand grams [02:47:00]. This is why the kilogram is the only one of the seven base SI units to have a prefix in its name [02:57:00].
In 1799, the kilogram’s definition was refined to be the mass of a litre of water at 4 degrees Celsius, the temperature at which water is densest [03:04:00].
From Water to Physical Artifacts
Recognizing that water was impractical as a mass standard, a pure platinum cylinder was created to match the water definition’s mass [03:16:00]. This artifact was declared the “Kilogram of the Archives” [03:22:00]. Importantly, at this point, the kilogram was no longer tied to the mass of a volume of water; the Kilogram of the Archives was, by definition, THE kilogram [03:29:00].
Ninety years later, in 1889, the kilogram was upgraded to a platinum-iridium alloy cylinder, which was much harder than the original [03:39:00]. This new artifact, officially called the International Prototype Kilogram (IPK) and affectionately known as “Le Grand K” (Big K), became the definitive standard [03:51:00]. Le Grand K is the only object in the universe with a mass of exactly one kilogram because it is the kilogram [04:06:00]. It is also the sole SI unit still defined by a physical object [04:13:00]. It is stored under three bell jars, alongside six “sister kilograms,” in a climate-controlled vault in the basement of the International Bureau of Weights and Measures near Paris [04:20:00].
The Instability Problem
To monitor Le Grand K, 40 identical replicas were created when it was first made, with their slight mass offsets recorded [04:56:00]. These replicas were distributed globally to serve as national standards [05:06:00].
In 1948, during a weigh-in of these kilograms, problems emerged [05:12:00]. Despite being made of the same alloy and stored under similar conditions, their masses had diverged over time [05:20:00]. Le Grand K’s mass itself had changed relative to its six sister cylinders [05:24:00]. Forty years later, the divergence had increased, up to about 50 micrograms – roughly the weight of a fingerprint [05:36:00]. Although the kilograms were carefully washed before weigh-ins, indicating fingerprints were not the cause, a physical process must have altered their mass, the exact mechanism of which remains speculative [05:47:00].
This instability of the platinum-iridium cylinder’s mass over time poses a significant problem, as a fundamental unit’s value should not change [06:04:00]. The repercussions extend beyond just mass measurements, as four of the seven base SI units, along with many derived units (like Newtons, Joules, Volts, and Watts), depend on the kilogram’s mass [06:13:00]. Even the avoirdupois pound in countries not using the metric system is defined as precisely 0.45359237 kilograms, making it dependent on the IPK [06:35:00].
Proposed Redefinitions
To eliminate the kilogram’s dependence on a physical object, two primary approaches are being considered:
The Silicon Sphere and Avogadro’s Constant
A one-kilogram sphere of silicon-28 atoms, containing about 2.15x10^25 atoms, was created [00:22:00]. The raw material for this sphere was worth one million Euros, and the final precisely sculpted sphere is considered priceless [00:47:00]. This sphere is known as the “roundest object in the world”; if it were the Earth, the highest mountain to the lowest valley would be only about 14 meters apart [00:56:00].
The sphere is a perfect crystal of pure silicon-28, a single isotope of silicon, explaining its high cost [07:26:00]. Its spherical shape is simple, allowing its entire dimension to be characterized by knowing its diameter [07:43:00]. The sphere is made by grinding an oversized sphere progressively finer to control its shape at the atomic level [08:02:00]. Its diameter is precisely measured using a laser within a cavity [08:25:00].
By knowing the sphere’s diameter, its volume can be determined [08:39:00]. Since the atom spacing in silicon is known to high precision, the number of atoms in the sphere can be calculated [08:44:00]. This approach aims to redefine Avogadro’s constant. Currently, Avogadro’s constant is defined based on the kilogram (number of atoms in 12 grams of carbon-12) [08:49:00]. The new proposal reverses this: the number of silicon atoms in the sphere would fix Avogadro’s constant, which would then define the kilogram [09:01:00]. This means the kilogram would be defined by a concept (“the mass of 2.15x10^25 silicon-28 atoms”) rather than a physical object, making its definition stable even if the silicon spheres were lost or damaged [09:10:00].
Alternative Approaches
Another approach to redefining the kilogram involves fixing Planck’s constant, using a device called a Watt Balance [09:39:00]. Both the silicon sphere and Watt Balance methods are complementary, providing checks on each other [09:45:00]. If they show good agreement and can reduce uncertainties to about 20 micrograms, the kilogram could be redefined as early as 2014, becoming an unchanging unit no longer tied to a physical object [09:52:00]. The Watt Balance method appears to be the frontrunner for redefining the kilogram [11:14:00].
Historical Accuracy and Future
The original intention for the kilogram was to be the mass of a litre of water at its densest temperature [10:29:00]. A litre of water at nearly four degrees Celsius has a mass of 999.975 grams [10:36:00]. This means the current kilogram is slightly heavier than the original water definition [10:43:00]. However, the fact that scientists 214 years ago were able to create an artifact correct within the margin of error of a grain of rice is considered truly remarkable [10:50:00].
The creation of the silicon sphere was an international collaboration of scientists [11:20:00]. It is noteworthy that Antoine Lavoisier, who was involved in the initial definition of the grave, also conceived of silicon as an element in 1787, linking him to the kilogram’s definition from start to finish [11:24:00].