The Order of Time

by Carlo Rovelli

Because the mystery of time is ultimately, perhaps, more about ourselves than about the cosmos.

The difference between past and future is deeply linked to this blurring. . . . So if I could take into account all the details of the exact, microscopic state of the world, would the characteristic aspects of the flowing of time disappear? Yes.

Is it really possible that a perception so vivid, basic, existential—my perception of the passage of time—depends on the fact that I cannot apprehend the world in all of its minute detail?

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.

Dear reader, pause for a moment to let this conclusion sink in. In my opinion, it is the most astounding conclusion arrived at in the whole of contemporary physics.

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.29

If the present has no meaning, then what “exists” in the universe? Is not what “exists” precisely what is here “in the present”? 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.

Concreteness occurs only in relation to a physical system: this, I believe, is the most radical discovery made by quantum mechanics.*

The fundamental equations do not include a time variable, but they do include variables that change in relation to each other.

The entire evolution of science would suggest that the best grammar for thinking about the world is that of change, not of permanence. Not of being, but of becoming.

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 physics and astronomy that will work, from Ptolemy to Galileo, from Newton to Schrödinger, will be mathematical descriptions of precisely how things change, not of how they are.

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

“Things” in themselves are only events that for a while are monotonous.

Therefore, the world should not be thought of as a succession of presents.59

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.

Put more simply: the time determined by macroscopic states and the time determined by quantum noncommutativity are aspects of the same phenomenon.

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 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.

The same may be true for the entropy of the universe: perhaps it was in no particular configuration. Perhaps we are the ones who belong to a particular physical system with respect to which its state can be particular.

It is what philosophers call “indexicality”: the characteristic of certain words that have a different meaning every time they are used, a meaning determined by where, how, when, and by whom they are being spoken.

This blurring suggests that 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. These events manifest relations between physical variables that are, a priori, on the same level. Each part of the world interacts with a small part of all the variables, the value of which determines “the state of the world with regard to that particular subsystem.”

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.

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.

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

But memory, causes and effects, flow, the determined nature of the past and the indeterminacy of the future are nothing but names that we give to the consequences of a statistical fact: the improbability of a past state of the universe.

Happy and master of himself is the man who for every day of his life can say: “Today I have lived; tomorrow if God extends for us a horizon of dark clouds or designs a morning of limpid light, he will not change our poor past he will do nothing without the memory of events that the fleeting hour will have assigned to us.” (III, 29)

in the same way as “chariot,” the name “Nāgasena” designates nothing more than a collection of relations and events.

I believe that our notion of self stems from this, not from introspection. When we think of ourselves as persons, I believe we are applying to ourselves the mental circuits that we have developed to engage with our companions.

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.

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

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, 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.

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.

In the end, therefore, instead of many possible times, we can speak only of a single time: the time of our experience—uniform, universal, and ordered. This is the approximation of an approximation of an approximation of a description of the world made from our particular perspective as human beings who are dependent on the growth of entropy, anchored to the flowing of time.

When we cannot formulate a problem with precision, it is often not because the problem is profound: it’s because the problem is false.

Basically, I believe that the fear of death is the result of an accidental and clumsy interference between two distinct evolutionary pressures—the product of bad automatic connections in our brain rather than something that has any use or meaning.

We are not, in the first place, reasoning beings. We may perhaps become so, more or less, in the second.

The philosopher Mauro Dorato has insisted on the necessity to render the elementary conceptual framework of physics coherent with our experience; see Mauro Dorato, Che cos’è il tempo? (Rome: Carocci, 2013).

Einstein frequently maintained that the experiments of Michelson and Morley were of no importance in allowing him to arrive at special relativity. I believe this to be true, and that it illustrates an important factor in the philosophy of science. In order to make advances in our understanding of the world, it is not always necessary to have new data. Copernicus had no more observational data than Ptolemy: he was able to deduce heliocentrism from the data available to Ptolemy by interpreting it better—as Einstein did with regard to Maxwell.

Newton’s fundamental equation is F = m d2x/dt2. Note that time t is squared: this reflects the fact that the equation does not distinguish t from ‒t, that is to say, it is the same backward or forward in time, as I explain in chapter 2.

A contemporary philosopher who has shed light on these aspects of the perspectival nature of the world is Jenann T. Ismael, The Situated Self (New York: Oxford University Press, 2007). Ismael has also written an excellent book on free will: How Physics Makes Us Free (New York: Oxford University Press, 2016).