The Order of Time

by Carlo Rovelli

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

Heat is the microscopic agitation of molecules.

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

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

“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.

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

Both the sources of blurring—quantum indeterminacy, and the fact that physical systems are composed of zillions of molecules—are at the heart of time.

Temporality is profoundly linked to blurring. The blurring is due to the fact that we are ignorant of the microscopic details of the world.

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

Our blurred and indeterminate image of reality determines a variable, thermal time that turns out to have certain peculiar properties which begin to resemble what we call “time”: it is in the correct relation with equilibrium states.

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.

The entropy of the world in the far past appears very low to us. But this might not reflect the exact state of the world: it might regard the subset of the world’s variables with which we, as physical systems, have interacted. It is with respect to the dramatic blurring produced by our interactions with the world, caused by the small set of macroscopic variables in terms of which we describe the world, that the entropy of the universe was low. This, which is a fact, opens up the possibility that it wasn’t the universe that was in a very particular configuration in the past. Perhaps instead it is us, and our interactions with the universe, that are particular. We are the ones who determine a particular macroscopic description. The initial low entropy of the universe, and hence the arrow of time, may be more down to us than to the universe itself. This is the basic idea.

IT IS ENTROPY, NOT ENERGY, THAT DRIVES THE WORLD

It’s not energy that the world needs in order to keep going. What it needs is low entropy.

Energy (be it mechanical, chemical, electrical, or potential) transforms itself into thermal energy, that is to say, into heat: it goes into cold things, and there is no free way of getting it back from there to reuse it to make a plant grow, or to power a motor. In this process, the energy remains the same but the entropy increases, and it is this which cannot be turned back. The second law of thermodynamics demands it.

What makes the world go round are not sources of energy but sources of 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...

Near to the Earth we have a rich source of low entropy: the sun. The sun sends us hot photons. Then the Earth radiates heat toward the black sky, emitting colder photons. The energy that enters is more or less equal to the energy that exits; consequently, we do not generally gain energy in the exchange. (Gaining energy in the exchange is disastrous for us: it is global warming.) But for every hot photon that arrives, the Earth emits ten cold ones, since a hot photon from the sun has the same energy as ten cold photons emitted by the Earth. The hot photon has less entropy than the ten cold photons, because the number of configurations of a single (hot) photon is lower than the number of configurations of ten (cold) photons. Therefore, the sun is a continual rich source of low entropy ...

Life is this network of processes for increasing entropy—processes that act as catalysts to each other.98 It isn’t true, as is sometimes stated, that life generates structures that are particularly ordered, or that locally diminish entropy: it is simply a process that degrades and consumes the low entropy of food; it is a self-structured disordering, no more and no less than in the rest of the universe.

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 is the real dance of Shiva, the destroyer.

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. My primary experience—if we grant that this means anything—is to see the world around me, not myself. I believe that we each have a concept of “my self” only because at a certain point we learn to project onto ourselves the idea of being human as an additional feature that evolution has led us to develop during the course of millennia in order to engage with other members of our group: we are the reflection of the idea of ourselves that we receive back from our kind.

We are histories of ourselves, narratives. I am not this momentary mass of flesh reclined on the sofa typing the letter a on my laptop; I am my thoughts full of the traces of the phrases that I am writing; I am my mother’s caresses, and the serene kindness with which my father calmly guided me; I am my adolescent travels; I am what my reading has deposited in layers in my mind; I am my loves, my moments of despair, my friendships, what I’ve written, what I’ve heard; the faces engraved on my memory. I am, above all, the one who a minute ago made a cup of tea for himself. The one who a moment ago typed the word “memory” into his computer. The one who just composed the sentence that I am now completing. If all this disappeared, would I still exist? I am this long, ongoing novel. My life consists of it.

And hence this is what time is: it is entirely in the present, in our minds, as memory and as anticipation.

William of Ockham maintained in his Philosophiae Naturalis that man observes both the sky’s movements and the ones within himself and therefore perceives time through his coexistence with the world.

the deliberate obscurity of Heidegger’s language—that “time temporalizes itself only to the extent that it is human.”

But the mind is the working of our brain. What (little) we are beginning to understand of this functioning is that our entire brain operates on the basis of a collection of traces of the past left in the synapses that connect neurons. Synapses are continually formed in their thousands and then erased—especially during sleep, leaving behind a blurry reflection of that which has acted on our nervous system in the past. A blurred image, no doubt—think of how many millions of details our eyes see every moment that do not stay in our memory—but one which contains worlds. Boundless worlds.

We are stories, contained within the twenty complicated centimeters behind our eyes, lines drawn by traces left by the (re)mingling together of things in the world, and oriented toward predicting events in the future, toward the direction of increasing entropy, in a rather particular corner of this immense, chaotic universe.

The Buddha summed this up in a few maxims that millions of human beings have adopted as the foundations of their lives: birth is suffering, decline is suffering, illness is suffering, death is suffering, union with that which we hate is suffering, separation from that which we love is suffering, failure to obtain what we desire is suffering.124 It’s suffering because we must lose what we have and are attached to. Because everything that begins must end. What causes us to suffer is not in the past or the future: it is here, now, in our memory, in our expectations. We long for timelessness, we endure the passing of time: we suffer time. Time is suffering.

A present that is common throughout the whole universe does not exist (chapter 3). Events are not ordered in pasts, presents, and futures; they are only “partially” ordered. 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 difference between past and future does not exist in the elementary equations that govern events in the world (chapter 2). It issues only from the fact that, in the past, the world found itself subject to a state that, with our blurred take on things, appears particular to us.

Locally, time passes at different speeds according to where we are and at what speed we ourselves are moving. The closer we are to a mass (chapter 1), or the faster we move (chapter 3), the more time slows down: there is no sing...

The rhythms at which time flows are determined by the gravitational field, a real entity with its own dynamic that is described in the equations of Einstein. If we overlook quantum effects, time and space are aspe...

But the world is a quantum one, and gelatinous spacetime is also an approximation. In the elementary grammar of the world, there is neither space nor time—only processes that transform physical quantities from one to another, from which it i...

At the most fundamental level that we currently know of, therefore, there is little that resembles time as we experience it. There is no special variable “time,” there is no difference between past and future, there is no spacetime (Part Two). We still know how to write equations that describe the world. In those equations, the variables evolve with respect to each other (chapter 8). It is not a “static” world, or a “block universe” wher...

The return journey has been the attempt to understand how, from this world without time, it is possible for our perception of time to emerge (chapter 9). The surprise has been that, in the emergence of familiar aspects of time, we ourselves have had a role to play. From our perspective—the perspective of creatures who make up a small part of the world—we see that world flowing in time. Our interaction with the world is partial, which is why we see it in a blurred way. To this blurring is added quantum indeterminacy. The ignorance that follows from this determines the existence of a particular variable—thermal time (chapter 9)—and of an entropy that quantifies our uncertainty.

Perhaps we belong to a particular subset of the world that interacts with the rest of it in such a way that this entropy is lower in one direction of our thermal time. The directionality of time is therefore real but perspectival (chapter 10): the entropy of the world in relation to us increases with our thermal time.