The following essay is derived from The Making of Measure and the Promise of Sameness.

If you didn’t notice, it’s OK, but Monday marked an important shift in the sciences, one that has taken decades to implement. Last November, scientists at an international conference in Versailles, France, finally agreed that all the measurement standards employed around the world—the meter, the second, the ampere, the kelvin and so on—would acquire new definitions based on physical constants. (A constant, as its name makes clear, is a quantity that physicists take as fixed and usually retrieve in the atomic world, such as the mass of a proton, or the charge of an electron.) That shift happened Monday, May 20. The new era is upon us. But again, you probably missed it—in fact, you were supposed to miss it.

Before getting into the reasons why our collective failure to notice means the transition was a success, let me briefly recap what this whole business is about.

Since the 18th century, many scientists and international traders have been frustrated by the need to convert dimensions every time they crossed a national border. It was not just that every country was using different systems, but many cities within the same country employed different standards, one for each activity (land was not measured with the same rulers employed for cloth, for example). So, in 1875 (on May 20, not a coincidence), a group of 17 countries that included Italy, the Dominican Republic, Russia, and the United States agreed to adopt the same measurement standards as a way to boost commerce and facilitate technological cooperation.* They decided on the metric system, which had been created in France in the late 1790s ( more on that later) and aggressively promoted across the globe.

At the 1875 conference, each adopting country was rewarded with a platinum bar that measured one meter and a cylinder whose mass corresponded to one kilogram. Other countries followed suit (Japan in 1885, Mexico in 1890, Thailand in 1912, I could go on). Members of this metric covenant felt reassured as more and more countries subscribed to it. Yet, over time, scientific advancements also continued to redefine what the meter meant. In 1960, for instance, at the meeting in which Indonesia joined the group, the participants agreed that it was too risky to have the world’s standard of length embodied by only one metal bar. What would have happened if, despite all the precautions, such a bar were damaged or went missing? How to deal with the countless disputes regarding the replication of an object that was, by its very definition, irreplaceable? The bar had to go, and the length of one meter was defined as a multiple of the wavelength of the orange spectral line so that any laboratory could retrieve it.

But soon after, in 1983, scientists made such progress in measuring the frequency of radiations that they had to redefine the speed of light, and the meter with it. The other standards didn’t receive the same treatment, though. Until Monday, for instance, the kilogram was still defined in relation to the 19th-century cylinder that is still preserved in a suburb of Paris. And this is why the representatives of 85 countries agreed that on May 20, 2019, all standards would switch to definitions based on physical constants, constants that could be derived anywhere, rather than replicas of a literal object.

It’s the paradox of the gesture—changing a few things so that everything can stay the same—that I find intriguing.

And yet, while many of the world’s scientists regard this moment as the epochal transformation it rightly is, they are quick to tell everyone that nothing is changing. Press releases celebrate the change but minimize its significance, stressing that it’s only the definitions of the standards that have been corrected—that is, the instructions as to how to create and replicate them—not their dimensions. We can continue measuring our morning jogs or weighing baking ingredients as we have always done: Everything will stay the same.

It’s the paradox of the gesture—changing a few things so that everything can stay the same—that I find intriguing. And after having researched the history of measurements for 15 years, I can also tell you that this paradox is not circumstantial, but an expression of the ambiguity that gives measurements their exceptional power.

You see: Measurements are strange objects. They occupy a unique place in cultural history as a whole. To start, measurements are ambivalent entities that exist between physical objects and ideal ratios. When we talk about measurements, we simultaneously refer to two things: both the plastic ruler that we may hold in our hands, and the dimension that the ruler represents. The physical object is thought to be a mere representation of a ratio, but things are more complicated than that. Before Monday, many measurement standards were in fact like the pre-1960 meter, for which the ratio did not really exist outside of the physical object that was supposed to simply represent it.

Let’s go back to the creation of the meter. When, in the late 18th century, the members of the Academy of Science in Paris worked to make it a reality, they defined it as one ten-millionth of the distance from the North Pole to the equator. They believed that taking the size of the Earth as the starting point would make the new standard eternally retrievable. Even if the object were lost, such a standard could be found again by remeasuring the earth, which, moreover, made the meter appear universal: a tool that all humans would recognize as deduced from their own planet. Yet, when the French went to measure the meridian, they selected the one crossing Paris without realizing (actually, deliberately ignoring) that not all meridians are identical, since the Earth is not the perfect sphere that we see in the solar systems hanging over cribs. Which means that the final metal bar that the French produced was a peculiar object, related to one specific dimension of the earth. (It’s because of this distinctness that the London parliament refused to adopt it.) And yet, when promoting it, the French omitted this peculiarity and described the standard as a universal, utterly constant ratio.

It was not, however, only the French that played this game. Measurements have always existed in suspension between the material and the ideal. In medieval Italy, for instance, standards of length were not hidden in safes protected by guards, as it is the case today. They were instead incised in the façades of cathedrals for everyone to see (and for everyone to guard). The very officers in charge of the incisions, however, were the first ones to say that the standard was not the incised slab itself, which was regularly substituted, but the ratio that it visualized. Critics further objected that if someone went to chip the rut with a chisel, even the abstract dimension that the wall incision supposedly represented would change. But those voices mattered little, in part because measurements are not just technical tools of quantification, but distinct cultural objects that, like all cultural objects, exist amid the discourse that validate them. Measuring requires instructions: Someone needs to tell you where to start and where to end, how to read an edge that falls between two divisions. Discussing measurements is never just technical: They pose questions about frameworks of reference, theories of knowledge, and the stuff that shapes our beliefs.

Measurements derive their efficacy from an idea of incorruptibility that does not belong to the human world.

Measurement standards are as artificial as anything else produced by humans. Over the centuries they have been chipped, weathered, and destroyed by fires (that is what happened to the 15th-century measurements standards of England, engulfed in the blaze that destroyed the London Parliament in 1834, after which: chaos). Yet, measurements are routinely presented as if their material existence is secondary to their abstract reality. This is because measurements derive their efficacy from an idea of incorruptibility that does not belong to the human world. The moment measurement standards are physically constructed, they are immediately said not to matter as objects, but as transfers to something abstract. It is only as abstract forms that they maintain that promise of incorruptibility that lends them validity. I would also add: It is only through ideals that the various incarnations of measurements (such as your ruler not being exactly like mine) can overcome their differences, regardless of how inconspicuous they may be.

This notion explains why scientists changed the definitions of the standards while insisting that nothing changed. Their anxiety is the same that has seized all scientists of the past three centuries: how to improve the tools employed to describe the laws of nature without disrupting everyone else’s life? (Can you imagine what would happen if people started questioning measurements? If at every visit to the supermarket, every shopper would inspect the calibration of each scale?) It’s better to say that there is nothing to see while scientists continue in their pursuit and skew the oscillation of measurements more toward the abstract than the physical. Which is precisely what happened on Monday: an attempt to erase the last traces of the bodies of measurements to finally turn them into utterly incorruptible dimensions. Or this is, at least, what we are told.

I may be cynical, but May 20 seems to be less a step forward than a reiteration of the very paradox of human life in which measurements participate: the effort to produce something absolute, eternally stable, and utterly uncompromisable in an ever-changing universe while having only human-made, imperfect tools at hand. By recurring to physical constants, scientists can claim to have equipped themselves with definitions that allow them to retrieve standards forever. Yet, such confidence evaporates once they leave their laboratories. For who can measure the mass of a proton or the speed of light outside of scientists? Most of us did not notice what happened because May 20 marks the day when measurements became a scientific appanage.

Correction, May 23, 2019: An earlier version of this article misidentified Russia as the Soviet Union.

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