Preface

This book tells the story of a gesture so everyday that we no longer see it: measuring. We perform it dozens of times a day — a glance at the clock, a kitchen scale, a navigator that announces the minutes to arrival — and every time, without thinking, we take for granted a miracle: that the minute of our clock is the minute of every clock, that the kilogram of the market is the kilogram of every market, that the metre is the metre, everywhere and for anyone. It was never obvious. It is, on the contrary, one of the slowest, most quarrelsome, and most ingenious constructions our species has ever carried to completion: this book recounts how we managed it, what it cost, and what exactly we built.

The thesis that runs through every page can be stated in a sentence: a measure is a promise between strangers. When two people who do not know and do not trust each other accept the same standard — an iron arm walled in beside the door of a town hall, a bar of platinum in a Parisian vault, an atom of caesium that oscillates — something happens that precedes technique and founds it: the two declare themselves equal before a third term, and the exchange becomes possible without distrust having to dissolve into friendship. For this reason the history of measure, which presents itself as a history of instruments, is in truth a history of trust: of how it is instituted, of who guarantees it, of how much it costs to maintain and of what happens when it breaks. The historian Witold Kula, to whom this book owes its conceptual framework, showed once and for all that the measures of preindustrial Europe were social institutions in the fullest sense — stakes in conflicts, attributes of sovereignty, matter for revolts [Kula 1986]; the reader will discover that the lesson concerns not only the medieval bushel: it holds, identical, for the Planck constant.

Within this history of trust there runs then a more visible plot, which gives the book its narrative backbone: the long migration of the standard. The unit of measure has dwelt, over the millennia, in ever stranger lodgings: first the human body — the foot, the arm, the day’s labour — then the body of the sovereign, then the body of the Earth, measured to exhaustion by two astronomers in the midst of the French Revolution; then the artefacts of precious metal, kept with relic-like liturgies; then the atoms, identical and incorruptible; and at last, from 2018, the constants of nature themselves, fixed by acclamation in a congress hall. At every move the measure has grown more precise by orders of magnitude, and at the same time more abstract: the reader will see the unit leave the body, geography, and at last matter — and will discover, on reaching the end, that the gesture of trust has not changed by a millimetre.

A few words on what this book is, and on what it deliberately is not. It is a narrative essay: it proceeds by stories, persons, and places — the carpenter who pursued for forty years a prize never formally won, the astronomer who carried to his grave a deviation of three seconds of arc, the potter who measured kilns with clay, the diplomats who signed a treaty for a bar of metal [Alder 2002] — because the ideas of metrology were born within those stories, and to recount them together is the most honest way of understanding them. It is not, however, a manual: the reader will not find in it procedures of calibration, uncertainty budgets, standards and operative formulas — the professional vocabulary of practical metrology, from traceability to coverage factors, belongs to another kind of text, and whoever needs it will find it treated as it should be in the entries of the technical glossary of ingegnerismo.it, to which these pages refer at the right points. Here we recount where the ideas come from; there one explains how they are used.

The reader needs no prerequisites. The writing proceeds by progressive depth: every chapter opens on a concrete scene — a date, a place, a person — and descends by degrees toward historical, conceptual, and, where needed, quantitative detail, until it touches the state of the art of research; whoever wishes to stop sooner can do so without losing the thread, and whoever wishes for everything will find everything. The formulas are almost absent — a single equation appears in solemn form in the whole book, and it is introduced in words before in symbols — while dates and orders of magnitude abound, for they are the salt of this story. The precise sources are indicated in the text and gathered in the bibliography, where a closing essay suggests reading paths for those who wish to continue.

A warning, which is also a declaration of method. The history of measure is infested with beautiful legends: the sailor hanged for telling the truth about the course, the yard derived from a king’s nose, the scientist guillotined because “the Republic has no need of sages,” the lone hero persecuted by the wicked academy. This book recounts them — it would be a pity not to — but it always declares them for what they are, and then sets them right: the reader will discover that the documented truth is regularly more interesting than the myth, because in place of heroes and villains it puts institutions grappling with new problems, and the new problems are the true engine of this history [Crease 2011]. The same holds for the characters: no one, in these pages, is a caricature — not the doctor who burned out the first transatlantic cable, not the Astronomer Royal who served as counterpart to Harrison, not the bureaucrat who asked a genius to dismantle his own masterpiece before a commission. To each has been left his point of view, for it is of conflicting points of view, not of announced triumphs, that real science is made.

The path, in brief and without spoiling anything. We start from the world that measured with the body and discover that it was not chaos but order — local, negotiated, “just” before it was exact. We follow then the birth of the metre in revolutionary France, amid triangulations, arrests, and a secret carried to the grave; then the conquest of time, from the ocean that swallowed fleets to the time zones of the planet. We witness the transformation of measure into diplomacy — a treaty, a neutral pavilion, prototypes drawn from an urn — and its descent into the factories, among gauges and gauge blocks. We learn how one measures what cannot be touched, temperature and electricity, and how error, from a sin that killed the peace of observers, became the most fruitful of sciences. We arrive at last at the leave-taking from objects: the platinum cylinder pensioned off in favour of the constants of nature, and the clocks so fine as to feel thirty-three centimetres of height difference in the fabric of spacetime. In closing, an essential chronology and the reasoned bibliography.

Whoever loves to read across the grain will find, beneath the chronological plot, certain threads that run through the whole book and that it is worth declaring at the start, as one hands over a legend. There is the thread of rite: the standard always lives within a liturgy — the Egyptian temple, the three keys of Sèvres, the washing of the kilogram with ether and chamois — and the liturgy is not folklore but function, because trust needs forms. There is the thread of money, the disguised measure that power handles on its own account, from the debasement of alloys to the gold standard, the conceptual twin of the standard down to the final dematerialization. There is the thread of the average, the slow discovery that the crowd of imperfect measures beats the single stroke of the virtuoso: from the sixteen feet on the church steps to the three shipboard chronometers, from the blind pairs of computers to the atomic clocks fused into a world scale. And there is the thread of reversal, the most secret: at every turn, what measured becomes measured — the observer calibrated like an instrument, the Earth watched over by the clocks it had once served, the Grand K pensioned off and acquiring an uncertainty. When the reader meets one of these motifs for the third or fourth time, let him know it is no repetition: it is the book keeping its promises.

There remains to say why it is worth reading, today, a history of measure. The short answer is that we live within its outcome without seeing it: the network of standards, calibrations, and institutions that holds up every transaction, every drug, every bridge and every telephone is the greatest invisible infrastructure of civilization — one notices it only when it breaks, and it breaks almost never. The long answer is the whole book; but its moral can be anticipated, and it is less technical than the subject might lead one to fear. In an age that argues bitterly about trust — in numbers, in institutions, in science — the history of measure shows the perhaps unique case of a planetary trust built piece by piece, quarrel after quarrel, and maintained for centuries through wars and revolutions: not because men became better, but because they learned to organize mutual verification instead of demanding faith. How they managed it, starting from an iron arm nailed to a door, is — I believe — one of the fine stories of the world. It begins at the market, on an ordinary day in the fourteenth century.