It only takes a few micrograms of LSD to expand our experience of time onto an epic and magical scale.35 “How long is forever?” asks Alice. “Sometimes, just one second,” replies the White Rabbit. There are dreams lasting an instant in which everything seems frozen for an eternity.36 Time is elastic in our personal experience of it. Hours fly by like minutes, and minutes are oppressively slow, as if they were centuries. On the one hand, time is structured by the liturgical calendar: Easter follows Lent, and Lent follows Christmas; Ramadan opens with Hilal and closes with Eid al-Fitr. On the other, every mystical experience, such as the sacred moment in which the host is consecrated, throws the faithful outside of time, putting them in touch with eternity. Before Einstein told us that it wasn’t true, how the devil did we get it into our heads that time passes everywhere at the same speed? It was certainly not our direct experience of the passage of time that gave us the idea that time elapses at the same rate, always and everywhere. So where did we get it from?
For centuries, we have divided time into days. The word “time” derives from an Indo-European root—di or dai—meaning “to divide.” For centuries, we have divided the days into hours.37 But for most of those centuries, however, hours were longer in the summer and shorter in the winter, because the twelve hours divided the time between dawn and sunset: the first hour was dawn, and the twelfth was sunset, regardless of the season, as we read in the parable of the winegrower in the Gospel according to Matthew.38 Since, as we say nowadays, during summer “more time” passes between dawn and sunset than during the winter, in the summer the hours were longer, and the hours were shorter in wintertime.
Sundials, hourglasses, and water clocks already existed in the ancient world, in the Mediterranean region and in China—but they did not play the cruel role that clocks do today in the organization of our lives. It is only in the fourteenth century in Europe that people’s lives start to be regulated by mechanical clocks. Cities and villages build their churches, erect bell towers next to them, and place a clock on the bell tower to mark the rhythm of collective activities. The era of clock-regulated time begins.
Gradually, time slips from the hands of the angels and into those of the mathematicians—as is graphically illustrated by Strasbourg Cathedral, where two sundials are surmounted, respectively, by an angel (one inspired by earlier sundials from 1200) and by a mathematician (on the sundial put there in 1400).
The usefulness of clocks supposedly resides in the fact that they tell the same time. And yet this idea is also more modern than we might imagine. For centuries, as long as travel was on horseback, on foot, or in carriages, there was no reason to synchronize clocks between one place and another. There was good reason for not doing so. Midday is, by definition, when the sun is at its highest. Every city and village had a sundial that registered the moment the sun was at its midpoint, allowing the clock on the bell tower to be regulated with it, for all to see. But the sun does not reach midday at the same moment in Lecce as it does in Venice, or in Florence, or in Turin, because the sun moves from east to west. Midday arrives first in Venice, and significantly later in Turin, and for centuries the clocks in Venice were a good half hour ahead of those in Turin. Every small village had its own peculiar “hour.” A train station in Paris kept its own hour, a little behind the rest of the city, as a kind of courtesy toward travelers running late.39
In the nineteenth century, the telegraph arrives, trains become commonplace and fast, and the problem arises of properly synchronizing clocks between one city and another. It is awkward to organize train timetables if each station marks time differently. It is in the United States that the first attempt is made to standardize time. Initially, it is proposed to fix a universal hour for the entire world. To call, for instance, “twelve o’clock” the moment at which it is midday in London, so that midday would fall at 12 noon in London and around 6 p.m. in New York. The proposal is not well received, because people are attached to local time. In 1883, a compromise is reached with the idea of dividing the world into time zones, thereby standardizing time only within each zone. In this way, the discrepancy between twelve on the clock and local midday is limited to a maximum of about thirty minutes. The proposal is gradually accepted by the rest of the world and clocks begin to be synchronized between different cities.40
It can hardly be pure coincidence that, before gaining a university position, the young Einstein worked in the Swiss patent office, dealing specifically with patents relating to the synchronization of clocks at railway stations. It was probably there that it dawned on him: the problem of synchronizing clocks was, ultimately, an insoluble one.
In other words, only a few years passed between the moment at which we agreed to synchronize clocks and the moment at which Einstein realized that it was impossible to do so exactly.
For millennia before clocks, our only regular way of measuring time had been the alternation of day and night. The rhythm of day followed by night also regulates the lives of plants and animals. Diurnal rhythms are ubiquitous in the natural world. They are essential to life, and it seems to me probable that they played a key role in the very origin of life on Earth, since an oscillation is required to set a mechanism in motion. Living organisms are full of clocks of various kinds—molecular, neuronal, chemical, hormonal—each of them more or less in tune with the others.41 There are chemical mechanisms that keep to a twenty-four-hour rhythm even in the biochemistry of single cells.
The diurnal rhythm is an elementary source of our idea of time: night follows day; day follows night. We count the beats of this great clock: we count the days. In the ancient consciousness of humanity, time is, above all, this counting of days.
As well as the days, we then count the years and the seasons, the cycles of the moon, the swings of a pendulum, the number of times that an hourglass is turned. This is the way in which we have traditionally conceived of time: counting the ways in which things change.
Aristotle is the first we are aware of to have asked himself the question “What is time?,” and he came to the following conclusion: time is the measurement of change. Things change continually. We call “time” the measurement, the counting of this change.
Aristotle’s idea is sound: time is what we refer to when we ask “when?” “After how much time will you return?” means “When will you return?” The answer to the question “when?” refers to something that happens. “I’ll return in three days’ time” means that between departure and return the sun will have completed three circuits in the sky. It’s as simple as that.
So if nothing changes, if nothing moves, does time therefore cease to pass?
Aristotle believed that it did. If nothing changes, time does not pass—because time is our way of situating ourselves in relation to the changing of things: the placing of ourselves in relation to the counting of days. Time is the measure of change:42 if nothing changes, there is no time.
But what is then the time that I hear coursing in the silence? “If it is dark and our bodily experience is nil,” Aristotle writes in his Physics, “but some change is happening within the mind, we immediately suppose that some time has passed as well.”43 In other words, even the time that we perceive flowing within us is the measure of a movement: a movement that is internal. . . . If nothing moves, there is no time, because time is nothing but the registering of movement.