The Order of Time

by Carlo Rovelli

The shapes of atoms will be eventually understood only with solutions to Schrödinger’s equations describing how the electrons in atoms move. Events again, not things.

We therefore describe the world as it happens, not as it is.

Because sooner or later, obviously, everything returns to dust.

Thus time revolves, the passing hour that steals the light brings a message: immortality, for us, is impossible.

Warm winds will be followed by cold.

Grammar developed from our limited experience, before we became aware of its imprecision when it came to grasping the rich structure of the world.

We say that an event “is,” or “has been,” or “will be.” We do not have a grammar adapted to say that an event “has been” in relation to me but “is” in relation to you.

We must not allow ourselves to be confused by an inadequate grammar.

the fact that “past” and “future” do not have a universal meaning. Instead, they have a meaning that changes between here and there.

In the world, there is change, there is a temporal structure of relations between events that is anything but illusory. It is not a global happening. It is a local and complex one that is not amenable to being described in terms of a single global order.

Fragile, brief, full of illusions. It’s a phrase that speaks of things that lie deeper than the physical nature of time.

Sooner or later the exact measurement of our time will resume— and we’ll be on the ship that’s bound for the bitterest shore.

The fundamental equations of quantum gravity are effectively formulated like this: they do not have a time variable, and they describe the world by indicating the possible relations between variable quantities.

The theory does not describe how things evolve in time. The theory describes how things change one in respect to the others,72 how things happen in the world in relation to each other. That’s all there is to it.

He spoke to me with the soft voice of an old man: I could not make out much of what he said but did not dare to ask him too frequently to repeat what he was saying.

This is time for us. Memory and nostalgia. The pain of absence.

even the pain caused by absence is, in the end, something good and even beautiful, because it feeds on that which gives meaning to life.

Thoughts and emotions that create bonds of attachment between us have no difficulty in crossing seas and decades, sometimes even centuries, tied to thin sheets of paper or dancing between the microchips of a computer.

The world is like a collection of interrelated points of view.

There is a time to be born and a time to die, a time to weep and a time to dance, a time to kill and a time to heal. A time to destroy and a time to build.

We see the sky turning around us every day, but we are the ones who are turning.

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

The number of possible microscopic configurations for water is its entropy.

The time of physics is, ultimately, the expression of our ignorance of the world. Time is ignorance.

This re-emergence of time seemed to me like an intrusion, a source of mental confusion, anguish, fear and alienation.

a god closes the strip of days that’s to come and laughs at our human trepidation.

The entire difference between past and future may be attributed solely to the fact that the entropy of the world was low in the past.90 Why was entropy low in the past?

Therefore, our vision of the world is blurred because the physical interactions between the part of the world to which we belong and the rest are blind to many variables.

The speed of an object is not a property of the object alone: it is a property of the object in relation to another object.

The entropy of A with regard to B counts the number of configurations of A that the physical interactions between A and B do not distinguish.

we understood that it is we who turn, not the universe.

The rotation of the heavens is a perspective effect due to our particular way of moving on Earth, rather than a mysterious property of the dynamics of the universe.

the dynamic of the universe with which we interact is governed by entropy, which measures the amount of blurring. It measures something that relates to us more than to the cosmos.

At the fundamental level, the world is a collection of events not ordered in time.

The truth is that there is plenty of energy and it is not consumed. It’s not energy that the world needs in order to keep going. What it needs is low entropy.

Without low entropy, energy would dilute into uniform heat and the world would go to sleep in a state of thermal equilibrium—there would no longer be any distinction between past and future, and nothing would happen.

the sun is a continual rich source of low entropy for us.

The 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.

Photosynthesis deposits low entropy from the sun into plants. Animals feed on low entropy by eating.

Life is this network of processes for increasing entropy—processes that act as catalysts to each other.

The entire coming into being of the cosmos is a gradual process of disordering, like the pack of cards that begins in order and then becomes disordered through shuffling.

What causes events to happen in the world, what writes its history, is the irresistible mixing of all things, going from the few ordered configurations to the countless disordered ones.

The entire universe is like a mountain that collapses in slow motion. Like a structure that very gradually crumbles.

From the most minute events to the more complex ones, it is this dance of ever-increasing entropy, nourished by the initial low entropy of the universe, that...

Traces of the past exist, and not traces of the future, only because entropy was low in the past.

the only source of the difference between past and future is the low entropy of the past.

In a world without heat, everything would rebound elastically, leaving no trace.

It is the presence of abundant traces of the past that produces the familiar sensation that the past is determined. The absence of any analogous traces of the future produces the sensation that the future is open.

The laws of elementary physics do not speak of “causes” but only of “regularities,” and these are symmetrical with regard to past and future.