It has long been the convention to indicate this time in equations with the letter t (the word for “time” begins with t in Italian, French, and Spanish, but not in German, Arabic, Russian, or Mandarin). What does this t stand for? It stands for the number measured by a clock. The equations tell us how things change as the time measured by a clock passes.
But if different clocks mark different times, as we have seen above, what does t indicate? When the two friends meet up again after one has lived in the mountains and the other at sea level, the watches on their wrists will show different times. Which of the two is t? In a physics laboratory, a clock on a table and another on the ground run at different speeds. Which of the two tells the time? How do we describe the difference between them? Should we say that the clock on the ground has slowed relative to the real time recorded on the table? Or that the clock on the table runs faster than the real time measured on the ground?
The question is meaningless. We might just as well ask what is most real—the value of sterling in dollars or the value of dollars in sterling. There is no “truer” value; they are two currencies that have value relative to each other. There is no “truer” time; there are two times and they change relative to each other. Neither is truer than the other.
But there are not just two times. Times are legion: a different one for every point in space. There is not one single time; there is a vast multitude of them.
The time indicated by a particular clock measuring a particular phenomenon is called “proper time” in physics. Every clock has its proper time. Every phenomenon that occurs has its proper time, its own rhythm.
Einstein has given us the equations that describe how proper times develop relative to each other. He has shown us how to calculate the difference between two times.7
The single quantity “time” melts into a spiderweb of times. We do not describe how the world evolves in time: we describe how things evolve in local time, and how local times evolve relative to each other. The world is not like a platoon advancing at the pace of a single commander. It’s a network of events affecting each other.
This is how time is depicted in Einstein’s general theory of relativity. His equations do not have a single “time”; they have innumerable times. Between two events, just as between the two clocks that are separated and then brought together again, the duration is not a single one.8 Physics does not describe how things evolve “in time” but how things evolve in their own times, and how “times” evolve relative to each other.*
Time has lost its first aspect or layer: its unity. It has a different rhythm in every different place and passes here differently from there. The things of this world interweave dances made to different rhythms. If the world is upheld by the dancing Shiva, there must be ten thousand such dancing Shivas, like the dancing figures painted by Matisse. . . .
2 LOSS OF DIRECTION
If more gently than Orpheus
who moved even the trees
you were to pluck the zither
the life-blood would not return
to the vain shadow . . .
Harsh fate,
but its burden becomes lighter
to bear, since everything
that attempts to turn back
is impossible. (I, 24)
WHERE DOES THE ETERNAL CURRENT COME FROM?
Clocks may well run at different speeds in the mountains and in the plains, but is this really what concerns us, ultimately, about time? In a river, the water flows more slowly near its banks, faster in the middle—but it is still flowing. . . . Is time not also something that always flows—from the past to the future? Let’s leave aside the precise measurement of how much time passes that we wrestled with in the preceding chapter: the numbers by which time is measured. There’s another, more essential aspect to time: its passage, its flow, the eternal current of the first of Rilke’s Duino Elegies:
The eternal current
Draws all the ages along with it
Through both realms,
Overwhelming them in both.9
Past and future are different from each other. Cause precedes effect. Pain comes after a wound, not before it. The glass shatters into a thousand pieces, and the pieces do not re-form into a glass. We cannot change the past; we can have regrets, remorse, memories. The future instead is uncertainty, desire, anxiety, open space, destiny, perhaps. We can live toward it, shape it, because it does not yet exist. Everything is still possible. . . . Time is not a line with two equal directions: it is an arrow with different extremities.
And it is this, rather than the speed of its passing, that matters most to us about time. This is the fundamental thing about time. The secret of time lies in this slippage that we feel on our pulse, viscerally, in the enigma of memory, in anxiety about the future. This is what it means to think about time. What exactly is this flowing? Where is it nestled in the grammar of the world? What distinguishes the past, its having been, from the future, its not having been yet, in the folds of the mechanism of the world? Why, to us, is the past so different from the future?
Nineteenth- and twentieth-century physics engaged with these questions and ran into something unexpected and disconcerting—much more so than the relatively marginal fact that time passes at different speeds in different places. The difference between past and future, between cause and effect, between memory and hope, between regret and intention . . . in the elementary laws that describe the mechanisms of the world, there is no such difference.
HEAT
It all began with a regicide. On January 16, 1793, the National Convention in Paris sentenced Louis XVI to death. Rebellion is perhaps among the deepest roots of science: the refusal to accept the present order of things.10 Among those who took the fatal decision was a friend of Robespierre called Lazare Carnot. Carnot had a passion for the great Persian poet Saadi Shirazi. Captured and enslaved at Acre by the Crusaders, Shirazi is the author of those luminous verses that now stand at the entrance of the headquarters of the United Nations:
All of the sons of Adam are part of one single body,
They are of the same essence.
When time afflicts us with pain
In one part of that body
All the other parts feel it too.
If you fail to feel the pain of others
You do not deserve the name of man.
Perhaps poetry is another of science’s deepest roots: the capacity to see beyond the visible. Carnot names his first son after Saadi. Sadi Carnot is thus born out of poetry and rebellion.
As a young man, he develops a passion for those steam engines that at the start of the nineteenth century are beginning to transform the world by using fire to make things turn. In 1824, he writes a pamphlet with the alluring title “Reflections on the Motive Power of Fire,” in which he seeks to understand the theoretical basis of the functioning of these machines. The little treatise is packed with mistaken assumptions: he imagines that heat is a concrete entity—a kind of fluid that produces energy by “falling” from hot things to cold, just as the water in a waterfall produces energy by falling from above to below. But it contains a key idea: that steam engines function, in the final analysis, because the heat passes from hot to cold.