As of Today, the Last Physical Object Used as a Standard of Measurement is No More

As the Universal Kilogram Enters Retirement Cutter Wood Considers the Implications

Today, May 20th, 2019, a cylinder of platinum-iridium alloy will be removed from a vault in the Pavillon de Breteuil in Saint-Cloud, France, just outside Paris, and relocated to a museum, officially “retired” in the terminology of metrologists, becoming the last object in human history to serve as a physical standard of measurement, or as metrologists tellingly describe such an object, an “artifact.”

This cylinder of dense metal, not much larger than a golf ball and colloquially known as “Le Grand K” or “the international prototype kilogram” or only “the IPK,” was created along with a few dozen sister cylinders at the end of the 19th century to act as the definition of the kilogram. This lonely hunk of platinum and iridium has defined mass around the globe for more than a century—from bathroom scales to medical lab balances. Imagine if Boeing couldn’t figure out precisely what an airplane weighs, or if the pharmaceutical industry couldn’t determine the exact mass of a tiny, potentially lethal, dose of medicine. It has been kept under three nested bell jars in an underground vault at the International Bureau of Weights and Measures in Saint-Cloud, accessible only with three separately controlled keys, while six of its sisters have been kept for reference as temoins, or witnesses. The IPK has been removed occasionally and cleaned with a chamois soaked in ethanol and ether before being steamed in distilled water, but otherwise it is merely left to rest. Of those few dozen cylinders originally made, the remaining prototypes were distributed to each of the world’s metrological powers, the signatories of the 1875 Treaty of the Metre—among which the United States numbers itself—and for the past 100 or so years these kilograms, kept and cleaned under similar conditions, have provided the citizens of each nation with their own approximation of the kilogram.

At intervals throughout the 20th century, the cylinders have been returned to France by representatives of each country and brought together for comparison to ensure a uniformity of standards. The specific alloy of platinum and iridium was chosen for the stability of its composition—its density, its peculiar resistance to corrosion and oxidation, its tremendous hardness—to which end it served its purpose admirably, but over the course of 100 years, the masses of these nearly identical kilograms have slowly and minutely and mysteriously begun to diverge from one another—to drift, as the metrologists say. Specifically, though no one has been able to deduce the underlying mechanism, the prototypes as a whole appear to be steadily gaining millionths of a gram of mass in relation to the IPK, or rather more likely, the IPK is losing mass in relation to its fellow cylinders (though no one can say the IPK is losing mass because to do so would be a kind of metrological apostasy; the mass of the IPK cannot change).

Had some enterprising anarchist or clumsy technician succeeded in damaging the IPK, the world order wouldn’t have known what a kilogram was.

As these cylinders have been the standards by which all other masses are measured, and as their continued divergence would cause greater and greater uncertainty in the world of measurement, at a meeting of the General Conference of Weights and Measures a few years ago, it was decided that some alternative method must be devised to measure mass, and so, on May 20th

 of this year, the kilogram will join the second and the meter and the other four base standards of measurement in being defined not by any terrestrial object or astronomical phenomenon but by what is known as a fundamental physical constant, which is to say one of the quantum properties of atoms themselves. In this case, it will be defined by the number known as Planck’s constant, the amount by which a photon’s frequency must be multiplied to equal its energy, or numerically, 6.626010150 × 10-34 kgm2/s, though speaking technically, as one must when discussing these matters in much detail, a kilogram will henceforth be defined not so much by a thing at all as by a strictly agreed upon process of determining Planck’s constant using a sophisticated piece of machinery known as a Kibble balance. 

For metrologists, there are multiple reasons to rejoice at this moment in history, as they did with both cheers and champagne this past November after unanimously voting to officially retire their metal cylinders, and considering the far-reaching impact of that vote, even those of us without Kibble balances should take some note of the bases for this metrological joie de vivre.

The first and most evident cause for elation is that the retirement of Le Grand K eliminates the dubious practice of relying on physical objects, which are subject to all sorts of worldly damage and decay. Not only is it troublesome that the kilograms have been drifting away from one another, but as one metrologist noted, since the IPK is a kilogram by its very definition, if it were dropped, it would still be a kilogram, but every other unit of measure on earth would be incorrect. Had some enterprising anarchist or clumsy technician succeeded in damaging the IPK, the world order wouldn’t have known what a kilogram was. And though it’s easy for a layperson to think a kilogram is a kilogram is a kilogram, to metrologists and to the realms of science and industry that depend on them, the precise definition of the kilogram is paramount to the continued existence of our world as we know it.

The work of the scientists at NIST, the metrological arm of the United States government, for instance, is fundamental to everything from food safety and nutrition (when Ragu creates a new sauce, it sends the jar to NIST to establish the nutritional information) to the shipping industry (if you are wondering how much a cruise ship weighs, NIST calibrates the scales) to pharmaceuticals (if Merck wants to develop a biological drug, they go to NIST) to cellular phones (the time on your phone right now is provided via radio broadcasts from a NIST lab in the mountains of Colorado), among countless others. It is not for no reason that NIST’s headquarters are located in a largely underground campus in suburban Maryland, or that it has stored its own prototype kilogram several stories beneath the surface of the earth.

With the French Revolution, scientists found their opportunity to enact in the realm of science the same sort of radical redefinition that was being enacted in French society.

The second reason metrologists were cheering is that now, for the first time, not only can no one blow up, steal, scratch, or breathe on the kilogram, no one can even own it. As Chris Oates, Division Chief of Time and Frequency at NIST, describes it, “any person anywhere on earth could make use of these quantities now without access to a physical artifact, anyone in the universe, in principle.” If the first reason was one of practicality, the second forms the theoretical or perhaps spiritual basis for their joy, because the original dream of precise standards of measurement, long before anyone could grasp exactly what the fulfillment of this dream would entail, was a specifically democratic dream, and to really begin to understand the peculiar momentousness of the kilogram’s retirement, you have to understand that dream. 

It began with the meter. At the end of the 18th century, different nations and localities each lived by their own units of measure (in France, famously, it was estimated at the time that there were more than 250,000 different measures), and these variations made for acute difficulties when, for instance, trying to replicate an experiment in Berlin that had originally been performed in Amsterdam, or when trying to sell a certain volume of Spanish sherry on the French market. So, deriving from the Enlightenment ideals of order and reason, the idea arose for a unit of length, decimal-based for ease of calculation, and so that it should be truly universal, derived from the very circumference of the earth itself. With the birth of the French Revolution, scientists found their opportunity to enact in the realm of science the same sort of radical redefinition that was being enacted in French society at large, and the Academy of Science decided to finally give the world its own rational and egalitarian measure.

Leaving Paris in 1792 in specially customized carriages, two astronomers, Jean Baptiste Joseph Delambre and Pierre François André Méchain, set off to measure as precisely as humanly possible that arc of the planet’s meridian running from a belfry in Dunkirk, a city at the extreme north of France, to Montjuic Castle in Barcelona. What was originally planned as a two-year mission became a seven-year ordeal, spanning all the turmoil of warring factions in the years after the revolution. The governmental organization that sponsored this journey went through convulsions, the work of Méchain and Delambre was hampered by war and civil unrest, Méchain was driven nearly mad by the imprecision of his calculations (“Who knows what oversensitive is,” as Maxwell says, “considering all there is to be sensitive to”), and when at last they returned to present their findings, they were received not by a democratically-elected representative democracy but by Napoleon Bonaparte.

And yet they succeeded, producing a definition of a standard meter, from which would be derived the unit of volume, the liter, and the unit of mass, the kilogram, and creating standards, as the Marquis de Condorcet said, “for all people, for all time.” Notably, despite Méchain’s misgivings, they were also remarkably precise. Their commission had been to produce a meter that measured 1/10,000,000th of the earth’s meridian, and based on satellite measurements today, we know that they were off by a mere 2,000 meters, meaning that they produced a meter with an error of only a little more than the width of a human hair.

It’s notable here that precision offers us quite different information from accuracy, and that sometimes precision is far more important.

Which all brings us to the final reason the metrologists were cheering, and that’s because the retirement of the kilogram this May sets us once and for all on the path to ever higher degrees of precision. A measurement can hardly be called a standard, after all, if it is not precise.

For those who haven’t recently looked back through their metrology textbooks, it might be helpful here to run through a quick primer on what exactly we mean when speak of precision in the scientific sense, how it relates to accuracy, and how the two concepts essentially differ. The most common analogy when explaining precision is that of a game of darts. Accuracy, as one would expect, is the ability to hit the bull’s eye. Precision, however, is the ability to hit any spot repeatedly. So, three darts that land close to one another at the dart board’s edge are said to be precise but not accurate, while three darts hitting the bull’s are said to be both accurate and precise. Notably, then, we see that precision necessarily involves consistency and repeatability. In terms of measuring a mass, we are precise if we get the same measurement again and again for the same mass, and our degree of precision is defined by our ability to replicate that measurement at smaller and smaller scales. It’s notable, as well, here that precision offers us quite different information from accuracy, and that sometimes precision is far more important.

For scientists generally, and metrologists specifically, precision is the be-all end-all. Precise measurements not only allow us to measure ever smaller quantities, ever fainter signals, ever softer forces, they underlie the entire project of science. If science is about making falsifiable hypotheses, as the philosopher Karl Popper proposed, precise measurements allow for the replication of experiments necessary to corroborate or falsify an idea. Just as importantly, precision is part of what allows a scientist to distinguish a signal from mere noise. Any experiment involves an interaction of forces or quantities, but generally speaking a scientist seeks to control the effect of certain of these variables so that the impact of others can be calculated. The degree to which the variables not being tested influence an experiment is what scientists call noise, and this noise obscures the signal of the variables being tested. So one of the primary goals in an experiment is to fix as many of these variables as rigidly as possible. The more precisely each variable can be measured, the better it can be controlled. Precision, in other words, allows scientists to divide the truth from everything else—precision is to the scientist as the gill is to the fish—and the entire progress of the scientific enterprise can in many ways be seen as a march toward ever greater degrees of precision.

As vital as precision standards are for science, however, they are perhaps even more vital for the world outside the laboratory, despite the nearly complete ignorance of the general populace.

This clock not only measures the passage of time more precisely than any other device on earth, it is so precise that it can actually measure the effect of gravity on a second.

Our ignorance of the degree to which our lives very literally rely on extreme precision—the understandable inability, in other words, of human beings to grasp the minuscule and gargantuan scales probed by contemporary precision devices—is maybe best illustrated by the tendency of those describing the work of these instruments to attempt to familiarize it to lay readers by placing it metaphorically on the human scale. The most ubiquitous and ever-ready analogy here, of course, is to describe the precision of these measurements in terms of the width of a human hair. So we know that the LIGO interferometer in Washington State, designed to measure the actual stretching of the fabric of spacetime, can measure the distance to the nearest star to the width of a human hair, while CERN’s Infinity machine can measure with a precision about 300 times smaller than the width of a human hair, the microshutters on the James Webb Space Telescope are the width of a human hair, etc., etc., etc. However, such hirsute analogs themselves perhaps best illustrate our inability to grasp the sensitivity of these instruments since, while to a human being all hairs seem of relatively similar thickness, to a machine like the LIGO detector, the variation in caliber of human hairs would be monumental.

Although operating largely unseen, NIST and the corollary metrological organizations of other nations, as the arbiters of all precise measurement standards, exist at a sort of mountaintop of precision. To give a single example of the sort of precision regularly achieved at NIST, they have recently created a clock based upon a lattice of ytterbium atoms—symbol Yb, atomic number 70, a soft silvery metal which, along with yttrium, terbium, and erbium, was first discovered in Ytterby, Sweden; this clock not only measures the passage of time more precisely than any other device on earth, it is so precise that it can actually measure the effect of gravity on a second.

The scientists at NIST calibrate scales that calibrate other scales that calibrate other scales and so on, and with each calibration a certain amount of imprecision finds its way into the calculation, until by the time you purchase a countertop scale at Bed, Bath and Beyond, you are vast orders of magnitude below the sorts of precision regularly achieved at NIST. And though the measurement of a mass to the millionth of a gram seems of little relevance to the baker doling out 500 grams of all-purpose flour, the entire sweep of the increasingly interdependent world economy is captured in that cascade of decreasing precision that begins with NIST and ends at a kitchen countertop.

By syncing generators and maintaining the power supply at a steady 60 Hz, these precise measurements make possible the electrical grids that provide energy across the world. They allow for the manufacture of industrial and commercial machinery, from jet turbines to cell phone components, at almost inconceivable levels of detail, and they underpin the fantastically minute measurements of the pharmaceutical industry. Perhaps most salient to today’s inhabitants of planet earth, they make possible the ever-increasing power of computers. They allow for the bundled transmission of information flowing seamlessly over fiber optic cables, ensuring that the ones and zeros in an email to your boss aren’t confused with the ones and zeros in your neighbor’s Fortnite stream. They coordinate the transmissions to stationary satellites far above the surface of the earth that allow your phone to place you on a map of the planet within an area of a few meters. And they make all of this computational power possible, by allowing for the placement of millions of transistors on a single square millimeter of silicon. 

A precision world is a computer world, and these upwelling forces of automation and artificial intelligence have gone hand-in-hand with increasing disparities in wealth.

And so, seen more broadly, the redefinition of the kilogram, like the redefinition of the second and meter before it, is momentous because it promises to pull the entire scientific community further orders of magnitude into territories of precision not yet grasped by humankind, and to drag the rest of society with it.

It seems strange then, though maybe not entirely surprising, that the ultimate realization of this democratic and literally globalist ideal should come at a time when the very fundament of democracy seems to be shifting underneath us. Britain, led by politicians who derided the very idea of scientific expertise, and under some poorly conceived notion of self-determination has decided to abandon the European Union despite the undeniable impact on its society and economy.

The United States has elected an isolationist leader who regularly threatens to leave NATO, who demonizes people seeking the rights of liberty and self-representation in the United States, and who politicizes science itself. France is roiled by the protests of The Yellow Vests, and countless other member countries of that 1875 Treaty of the Metre—Venezuela is one—have a stunningly uncertain political future. It seems that science has realized its greatest unifying dream just as the centrifugal forces of unreason and economic inequality and tribalism are flinging the world apart.    

Possibly, as economists and talk show hosts have noted, it has something to do with the fact that, in a society where precision reigns, the essential nature of labor has changed. Humans no longer make their own cars, they no longer solve their own equations, they no longer select their own mates. A precision world is a computer world—only computers can drill a hole to within a few micrometers, only computers can divide a second into millionths, only computers can tell the difference between 1 kilogram and 1.00000001 kilograms—and these upwelling forces of automation and artificial intelligence have gone hand-in-hand with increasing disparities in wealth.

If the world’s surface were apportioned as its wealth currently is, North America, South America, Europe, Africa, and Asia would belong to 10 percent of the population, while 64 percent of humanity would be confined to an area one third the size of Australia. And, of course, this is only the beginning of these shifts, as was made evident at the most recent summit in Davos, where CEOs spoke avidly of the potential for automation. As Mohit Joshi of Infosys noted, companies are now looking to replace not 10 or 15 percent of their workforces with computers in the coming years but upwards of 90 percent.

Another possibility is that it has something to do with the increasingly globalist world that precision makes possible. Jobs drift where they are cheapest, and there’s a good chance an MRI taken in Des Moines today will be read by a technician in Hyderabad. Certainly the anti-immigrant rhetoric of populists in the United States and England seems to have its basis in a concern that foreign nationals are taking the jobs of native-born citizens.

“I’d like to believe, though I have no grounds to believe, that this rational scientific world can, in the end, provide a route forward to a more stable society.”

Whatever its derivation, the question now, on the eve of the kilogram’s retirement, is whether the dream can survive its fulfillment. And perhaps the retirement of the final physical standard of measure, the final standard that could be held in the hand or kept on a mantle or in any way appreciated without the interposition of computers and machinery and equations, perhaps this event also offers us the opportunity to consider where exactly it is we are going. “I’d like to believe, though I have no grounds to believe, that this rational scientific world can, in the end, provide a route forward to a more stable society,” says Oates.

With that hope for a more rational and stable society in mind, it might be instructive to return to the period following the French revolution when the dream of precise standards of measure was born, and to recall that after seven years of painstaking work by Delambre and Méchain, the general populace of France, finding the new standards of measure of little use to their lives or their local economies, wholeheartedly rejected them in favor of the ad hoc and imprecise systems of measure they’d long used. Perhaps it’s worth recalling the many billions who have not yet found their existences greatly improved by the cause of scientific reason, for whom a hundred dollars is as faraway as the eruption of gold detected by LIGO in the constellation Hydra.

Or perhaps, as we consider the understandable resentments of those left behind by these scientific achievements, it would be helpful to recall September 4, 1792, in the earliest days of Delambre’s mission when his party was detained near Lagny. Night had begun to settle over the countryside, and the astronomer and his assistants, having completed the day’s measurements, were packing equipment into their carriage when a local militia arrived. As factions warred within the country, and the Prussians advanced from the north, anyone seeming to support the aristocracy was seen as suspicious, and these soigne travelers presented an alarming sight. The militia demanded Delambre’s papers and asked about the nature of his instruments and his purposes in looking toward the Prussian front. Delambre tried to explain that he was on an important scientific mission, that his work was for the good of the country and had been commissioned at the highest levels of the government, but the militia was unconvinced. The party was escorted through the driving rain and muddy fields and detained at a local inn. As Delambre wrote later to a friend, there was little he could do. “They were armed, and we had only reason. The parties were not equal.”

Cutter Wood
Cutter Wood
Cutter Wood was born in Central Pennsylvania and received his BA from Brown University, where he was awarded prizes for nonfiction and poetry. Wood completed an MFA in creative nonfiction at the University of Iowa in 2010, during which he was awarded numerous fellowships and had essays published in Harper’s and other magazines. After serving as a visiting scholar in creative nonfiction at UI and the University of Louisville, Wood moved to New York. He currently lives in Brooklyn with his wife and daughter.





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