The Order of Time

by Carlo Rovelli

Wonder is the source of our desire for knowledge,1 and the discovery that time is not what we thought it was opens up a thousand questions.

Why do we remember the past and not the future? Do we exist in time, or does time exist in us? What does it really mean to say that time “passes”? What ties time to our nature as persons, to our subjectivity?

What we call “time” is a complex collection of structures,2 of layers.

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

Times are legion: a different one for every point in space. There is not one single time; there is a vast multitude of them.

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.*

In the elementary equations of the world,13 the arrow of time appears only where there is heat.* The link between time and heat is therefore fundamental: every time a difference is manifested between the past and the future, heat is involved. In every sequence of events that becomes absurd if projected backward, there is something that is heating up.

Clausius’s entropy, indicated by the letter S, is a measurable and calculable quantity15 that increases or remains the same but never decreases, in an isolated process. In order to indicate that it never decreases, we write: ΔS ≥ 0

Entropy never decreases which apparently establishes a unidirectional flow of time

The difference between past and future does not lie in the elementary laws of motion; it does not reside in the deep grammar of nature. It is the natural disordering that leads to gradually less particular, less special situations.

Boltzmann

Why, in the past, was entropy lower?

The notion of “particularity” is born only at the moment we begin to see the universe in a blurred and approximate way.

Boltzmann has shown that entropy exists because we describe the world in a blurred fashion. He has demonstrated that entropy is precisely the quantity that counts how many are the different configurations that our blurred vision does not distinguish between.

the disconcerting conclusion that emerges from Boltzmann’s work: the difference between the past and the future refers only to our own blurred vision of the world.

all 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.

the mind-boggling fact that entropy, as Boltzmann fully understood, is nothing other than the number of microscopic states that our blurred vision of the world fails to distinguish.

Nobody had imagined previously that time could be different for a stationary watch and one that was being moved. Einstein had read this within the equations of electromagnetism, by taking them seriously.

For a moving object, time contracts.*

* "In motion" in relation to what? How can we determine which of the two objects moves, if motion is only relative? This is an issue that confuses many. The correct answer (rarely given) is this: in motion relative to the only reference in which the point in space where the two clocks separate is the same point in space where they get back together. There is only a single straight line between two events in spacetime, from A to B: it's the one along which time is maximum, and the speed relative to this line is the one that slows time. If the clocks separate and are not brought together again, there is no point asking which one is fast and which one is slow. If they come together, they can be compared, and the speed of each one becomes a well-defined notion.

“Proper time” depends not only on where you are and your degree of proximity to masses; it depends also on the speed at which you move.

Our “present” does not extend throughout the universe. It is like a bubble around us.

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

there is no such thing as ‘the same moment’ definable in the universe.”

We must do so instead by placing above and below every event the cones of its future and past events:

Any event has a past and future cone of events in the same manner as each individual has a cone of ancestors and descendants. Events outside of these cones are not temporally related as those within in the cones of any specific event.

The whole idea that the universe exists now in a certain configuration and changes together with the passage of time simply doesn’t stack up anymore.

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.

Time is the measure of change:42 if nothing changes, there is no time.

Aristotle

the existence of a time that is uniform, independent of things and of their movement that today seems so natural to us is not an ancient intuition that is natural to humanity itself. It’s an idea of Newton’s.

That which seems intuitive to us now is the result of scientific and philosophical elaborations in the past.

The “quantization” of time implies that almost all values of time t do not exist.

In other words, a minimum interval of time exists. Below this, the notion of time does not exist—even in its most basic meaning.

Indeterminacy is resolved when a quantity interacts with something else.*

* The technical term for interaction used in this context, "measure," is misleading because it seems to imply that in order to create reality, we need an experimental physicist in a white coat.

Let’s enter the world without time.

Let me reprise the long dive into the depths made in the first part of this book. There is no single time: there is a different duration for every trajectory; and time passes at different rhythms according to place and according to speed. It is not directional: the difference between past and future does not exist in the elementary equations of the world; its orientation is merely a contingent aspect that appears when we look at things and neglect the details. In this blurred view, the past of the universe was in a curiously “particular” state. The notion of the “present” does not work: in the vast universe there is nothing that we can reasonably call “present.” The substratum that determines the duration of time is not an independent entity, different from the others that make up the world; it is an aspect of a dynamic field. It jumps, fluctuates, materializes only by interacting, and is not to be found beneath a minimum scale. . . . So, after all this, what is left of time?

The events of the world do not form an orderly queue, like the English. They crowd around chaotically, like Italians.

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.

What confuses us when we seek to make sense of the discovery that no objective universal present exists is only the fact that our grammar is organized around an absolute distinction—“past/present/future”—that is only partially apt, here in our immediate vicinity. The structure of reality is not the one that this grammar presupposes.

Everything in the world becomes blurred when seen close up.

A generic macroscopic state determines a time. I’ll repeat this point, because it is a key one: a macroscopic state (which ignores the details) chooses a particular variable that has some of the characteristics of time.

This is why distinct configurations of the world seem equivalent to us.

Between ourselves and the rest of the world there are physical interactions. Obviously, not all the variables of the world interact with us, or with the segment of the world to which we belong. Only a very minute fraction of these variables does so; most of them do not react with us at all. They do not register us, and we do not register them.

The entropy of a system depends explicitly on blurring. It depends on what I do not register, because it depends on the number of indistinguishable configurations. The same microscopic configuration may be of high entropy with regard to one blurring and of low in relation to another.

The initial low entropy of the universe, and hence the arrow of time, may be more down to us than to the universe itself.

Every glance that we cast toward the world is made from a particular perspective.

the world that we have been given is the world seen from within it, not from without.

Many things that we see in the world can be understood only if we take into account the role played by point of view. They remain unintelligible if we fail to do so.

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

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. Either this is time, or I have no idea what time is.

Augustine

A hymn, a song, is in some way present in our minds in a unified form, held together by something—by that which we take time to be. And hence this is what time is: it is entirely in the present, in our minds, as memory and as anticipation.

the basis of the temporal structure of the world is to be sought in something that closely relates to our way of thinking and perceiving, to our consciousness.

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

There is a present that is near to us, but nothing that is “present” in a far-off galaxy. The present is a localized rather than a global phenomenon.

The closer we are to a mass (chapter 1), or the faster we move (chapter 3), the more time slows down: there is no single duration between two events; there are many possible ones.