The Order of Time

by Carlo Rovelli

The ability to understand something before it’s observed is at the heart of scientific thinking.

Here on the surface of our planet, on the other hand, the movement of things inclines naturally toward where time passes more slowly,

the arrow of time appears only where there is heat.

Why, in one of the two directions of time—the one we call past—were things more ordered?

the phenomena that characterize the flowing of time are reduced to a “particular” state in the world’s past, the “particularity” of which may be attributed to the blurring of our perspective.

For everything that moves, time passes more slowly.

There is no special moment on Proxima b that corresponds to what constitutes the present here and now.

The idea that a well-defined now exists throughout the universe is an illusion, an illegitimate extrapolation of our own experience.

Time is only a way of measuring how things change, as Aristotle would have it—or should we be thinking that an absolute time exists that flows by itself, independently of things?

the place of a thing is what surrounds that thing.

Newton. He calls “absolute, true, and mathematical” space in itself, which exists even where there is nothing.

For Aristotle, this “empty space” is nonsensical, because if two things do not touch it means that there is something else between them,

the time and space Newton had intuited the existence of, beyond tangible matter, do effectively exist. They are real. Time and space are real phenomena. But they are in no way absolute; they are not at all independent from what happens;

it is made of fields.

Spacetime is the gravitational field—and vice versa. It is something that exists by itself, as Newton intuited, even without matter. But it is not an entity that is different from the other things in the world—as Newton believed—it is a field like the others.

The physical substratum that determines duration and physical intervals—the gravitational field—does not only have a dynamic influenced by masses; it is also a quantum entity that does not have determined values until it interacts with something else. When it does, the durations are granular and determinate only for that something with which it interacts; they remain indeterminate for the rest of the universe.

The world is not a collection of things, it is a collection of events.

The difference between things and events is that things persist in time; events have a limited duration.

The hardest stone, in the light of what we have learned from chemistry, from physics, from mineralogy, from geology, from psychology, is in reality a complex vibration of quantum fields, a momentary interaction of forces,

Newton’s mechanics, Maxwell’s equations, quantum mechanics, and so on, tell us how events happen, not how things are

If by “time” we mean nothing more than happening, then everything is time. There is only that which exists in time.

The fact that we cannot arrange the universe like a single orderly sequence of times does not mean that nothing changes. It means that changes are not arranged in a single orderly succession:

There is no need in any of this to choose a privileged variable and call it “time.” What we need, if we want to do science, is a theory that tells us how the variables change with respect to each other.

macroscopic state (which ignores the details) chooses a particular variable that has some of the characteristics of time.

it is not the evolution of time that determines the state, it is the state—the blurring—that determines a time.

the first germ of temporality in elementary quantum transitions lies in the fact that these interactions are naturally (partially) ordered.

a kind of temporal flow is implicitly defined by the noncommutativity of the physical variables.

And it is this thermal and quantum time, I believe,88 that is the variable that we call “time” in our real universe, where a time variable does not exist at the fundamental level.

The entropy of the world does not depend only on the configuration of the world; it also depends on the way in which we are blurring the world, and this depends on what the variables of the world are that we interact with. That is to say, on the variables with which our part of the world interacts.

among the innumerable small systems S that exist in this extraordinarily vast universe where we happen to live, there will be a few special ones for which the fluctuations of the entropy happen to be such that at one of the two ends of the flow of thermal time entropy happens to be low. For these systems S, the fluctuation is not symmetrical: entropy increases. This growth is what we experience as the flowing of time. What is special is not the state of the early universe: it is the small system S to which we belong.

entire history of the universe consists of this halting and leaping cosmic growth of entropy. It is neither rapid nor uniform, because things remain trapped in basins of low entropy (the pile of wood, the cloud of hydrogen . . . ) until something opens a door onto a process that finally allows entropy to increase. The growth of entropy itself happens to open new doors through which entropy can increase further.

In order to leave a trace, it is necessary for something to become arrested, to stop moving, and this can happen only in an irreversible process—that is to say, by degrading energy into heat.

the existence of common causes in the past is nothing but a manifestation of low entropy in the past.

(Even the attribution of the idea to Descartes seems wrong to me: Cogito ergo sum is not the first step in the Cartesian reconstruction, it is the second. The first is Dubito ergo cogito. The starting point of the reconstruction is not a hypothetical a priori that is immediate to the experience of existing as a subject. It’s a rationalistic a posteriori reflection on the first stage of the process in which Descartes had articulated a state of doubt:

The experience of thinking of oneself as a subject is not a primary experience: it is a complex cultural deduction, made on the basis of many other thoughts.

we are the reflection of the idea of ourselves that we receive back from our kind.

To understand ourselves means to reflect on time. But to understand time we need to reflect on ourselves.

This being between past and future events is central to our mental structure. This, for us, is the “flow” of time.

It is within my mind, then, that I measure time. I must not allow my mind to insist that time is something objective. When I measure time, I am measuring something in the present of my mind.

Music can occur only in time, but if we are always in the present moment, how is it possible to hear it? It is possible, Augustine observes, because our consciousness is based on memory and on anticipation.

“time temporalizes itself only to the extent that it is human.”

This space—memory—combined with our continuous process of anticipation, is the source of our sensing time as time, and ourselves as ourselves.

Time, then, is the form in which we beings, whose brains are made up essentially of memory and foresight, interact with the world: it is the source of our identity.

Time is suffering.