The Meter Convention, a year ago, promulgated the redefinition of the SI base units in terms of physical constants. This seemingly unnecessary change starts from humankind’s confidence in ultraprecise measurement and deepened understanding of the quantum mechanics. Redefining time, in the form of the second, by tinkering with a cesium atom swiftly overrules our sense of time as inextricably tied to the planetary motions like the Earth’s rotation. The Earth is now a mere runner sprinting around the sun as humans hold the atomic clock measuring how many seconds it takes. We’ve stripped the Earth of its temporal sovereignty, establishing human dominion.
The logic is, however, somewhat befuddling. Although the physical entity implied by a constant is invariant to units, the numerical values are intrinsically a product of the choice of unit system. Just like the speed of your car remains constant but is expressed by different numbers depending on whether kilometer per hour or miles per hour is used. The numerical figures for the constants used in the 2019 redefinition are estimated and expressed within the metric system. This sounds like circular logic. How do we define units with numbers expressed in those units?
The truth is that the physical constants have been measured a myriad of times, they have been empirically proven to be extremely stable. This is the grounds for holding them as a priori known and redefining the rest accordingly. It’s like, in the equation \(y = ax\), we are switching the problem from solving for \(a\) given that \(x\) and \(y\) are known to solving for \(x\) and \(y\) assuming \(a\) is known. We are, in a sense, swapping the knowns and unknowns.
Ian Hacking, a Canadian philosopher, recounted in his book The Taming of Chance that "[t]he idea of an abstract fundamental constant - as opposed to a stable measurable property of a physical object, such as the weight of the earth - was not fully articulated until the nineteenth century." The philosopher goes on to illustrate that a good many cosmologists entertain a hierarchical view of the world portrayed by the “physical laws”. The universe first operates on a set of laws, the constants are then fixed, and there are boundary conditions. According to this innately sequential flavor of natural science, the science was running its business in the wrong direction, and only in 2019 was it rectified.
Pre-2019 SI Units#
The now-obsolete metric system was a product of the French Revolution in the late eighteenth century. Merchants needed an unchanging ‘yardstick’ to see if the amount presented was consistent with what had been promised. The traditional units at the time of the French Revolution were often inconsistent across regions, and were regarded as part of the Ancien Régime. The French revolutionaries as a result sought to institutionalize a scientifically sound unit system, métrique. Subsequently, the newly introduced metric system was disseminated throughout Europe during the Napoleonic Wars but Britain sidestepped the invasion, thereby withstanding the wave of metrication. Britain’s resistance planted the seed of America’s standing aloof with its own unit system.
As the metric system spread further, the French government sought an international initiative which resulted in the signing of the Meter Convention in 1875. Under the Meter Convention, the International Bureau of Weights and Measures (BIPM) manufactured 30 standard meters and 40 standard kilograms and distributed the prototypes among the member states. These prototypes served as ‘constants’ prior to the 2019 redefinition but, since the materials were subject to physical restrictions, the weight of the standard kilograms and the length of the standard meters frequently changed albeit miniscule in its magnitude.
Metric System in the U.S.#
The Mendenhall Order of 1893 officially ‘adopted’ the metric system, except that it did not mandate the conversion which would have entailed switching all the signage, reprinting the nutritional facts, and updating the related laws. It merely redefined the yard and pound in terms of the SI base units. The treaty between the six Anglophone nations – the United States, United Kingdom, Canada, Australia, New Zealand, and the Union of South Africa (currently the Republic of South Africa) – created the international yard and pound where one yard is defined as 0.9144 meters and a pound as 0.45359237 kilograms.
U.S. customary units are the most common unit system in America but this is technically a subsystem derived from the metric units. The frequent loss of standard pounds and yards, and fluctuating weight and length of the standards prodded the related countries into adopting the stable metric system in a way that minimizes, or ideally, circumvents confusion, inconvenience, and financial burden of transitioning. These considerations produced the eccentric linkage of the yard and pound to the standard meters and standard kilograms.
Due to the separation of powers under the United States Constitution, metrication was left for the state governments to decide, even though the Omnibus Foreign Trade and Competitiveness Act of 1988 incentivizes the use of SI units by designating the metric system as ‘the preferred system of weights and measures for U.S. trade and commerce’. The State of Louisiana adopted and officially recognizes the metric system.
The U.S. Army uses metric measurements to make consistent the units of firearms and ammunitions with those of its allies, and to ensure interoperability. NASA-produced documents are by and large metric. The most notable exception is the U.S. Airforce; not only is the metric system hardly ever used for aviation, the standard practices in the field are predominantly U.S.-centric. Very few depart from the U.S. system, including the former Soviet countries and North Korea.