The Order of Time, by Carlo Rovelli (2017)

Update on 2nd Jan 2024: Wrote more about this book here: Understanding Carlo Rovelli's theory on the emergence of time

This was engaging and challenging to read. I found it difficult to follow in the middle where it switches from established physics to the author’s own theories about how time works.

I have no idea how plausible his theory is, and likely never will, but what I understood of it is intriguing and the book is beautifully written. It reminds me of Julian Jaynes’ theory on the evolution of consciousness in that it’s presented in a way that makes you will it to be true despite any conclusive evidence to confirm it.

It’s a short book full of elegant explanations and new perspectives which have added to my understanding of physics. Two examples that come to mind are; that things gravitate to where time is slowest, and that time is slower where there is mass because there is quite literally “less time” there as a result of the stretching of the fabric of spacetime. Those seem like pretty obvious conclusions in hindsight, but I had never thought of them before.

I haven’t come away feeling particularly confident in my understanding of the author’s theory of the emergence of time in full. I understood the individual concepts pretty well in isolation, but stitching them together to form a mental model of the entire chain that results in our subjective experience of time is proving difficult.

Chapter 13 summarises the theory and its implications well, which does give me an overview to follow so that I can at least understand what he’s proposing. I also found this well-written article by Nathan Hohipuha helpful.

It is a fun puzzle to try to piece together, I hope to understand it in more depth eventually.

Since the book and its theory are based on entropy and the work of Ludwig Boltzmann, I’ll end with this famous introductory paragraph involving both:

Ludwig Boltzmann who spent much of his life studying statistical mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the work, died similarly in 1933. Now it is our turn to study statistical mechanics.

David Goodstein, The States of Matter

Things to remember

  1. Our experience of time is subjective. There is no Universal “now”, and time is fucked up in all sorts of ways that do not fit with how we describe our experience of it.

  2. Since the Universe is made up of events that are not ordered in time, there is nothing theoretically preventing something from moving forwards in time and still arriving back at the event where it originated.

  3. The only law of physics that has any required order in time is the second law of thermodynamics (law of entropy).

  4. In every interaction where entropy increases there is a transfer of heat, so we can say that the arrow of time only exists when something is heating up.

  5. Entropy is relative quantity. Our interaction with a system dictates what states are deemed special, ordered, or particular, and are therefore a state of low entropy.

  6. This perspective, and therefore entropy, is only possible because we see a “blurred” view of the world where we cannot take everything into account. If we could be aware of every detail at a microscopic level, no states would be particular or ordered, and there would be no notion of entropy increasing, and as a result, no concept of time.

  7. Time that is determined this way, by a macroscopic state that neglects to account for the details of a system, is coined as ‘thermal time’.

  8. Order seems to exist at the quantum level, because measuring the position of an electron before its speed alters the state differently compared to measuring the speed before the position. This is known as ‘noncommutativity’; the order matters. This ordering at the quantum level could be the root of all temporal order in the world.

  9. That the Universe had low entropy at the beginning could just be due to the perspective of our particular subsystem of the Universe.

  10. Our experience of time and it’s familiar ‘flow’ comes as the result of our memories recording traces of the effects of increasing entropy.



If our feet adhere to the pavement, it is because our whole body inclines naturally to where time runs more slowly – and time passes more slowly for your feet than it does for your head.


Only where there is heat is there a distinction between past and future.


It [entropy] is the only equation of fundamental physics that knows any difference between past and future.


If I observe the miscroscopic state of things, then the difference between the past and future vanishes.


For now, I will end with 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.


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


But there is no logical contradiction entailed by the existence of closed temporal lines or journeys to the past; we are the ones who complicate things with our confused fantasies about the supposed freedom of the future.


So, in order to exit from a black hole, you would need to move towards the present rather than towards the future!


Diurnal rhythms are ubiquitous in the natural world. They are essential to life, and it seems to me probable that they played a key role in the very origin of life on Earth, since an oscillation is required to set a mechanism in motion.


Remember the clocks in Chapter I that slow down in the vicinity of mass? They slow down because there is, in a precise sense, ‘less’ gravitational field there. There is less time there.


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


Spacetime is a physical object like an electron. It, too, fluctuates. It, too, can be in a ‘superposition’ of different configurations.


Divested of the trappings with which Newtonian theory had draped it, and to which we had become so accustomed, it now shines out with greater clarity: the world is nothing but change.


The hardest stone, in the light of what we have learned from chemisty, from physics, from mineralogy, from geology, from psychology, is in reality a complex vibration of quantum fields, a momentary interaction of forces, a process that for a brief moment manages to keep its shape, to hold itself in equilibrium before disintegrating again into dust…


We understand the world in its becoming, not in its being.


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.


And this [granular interaction at the quantum level] is the happening of the world: it is the minimum elementary form of time that is neither directional nor linear.


In fundamental relativistic physics, where no vairable plays a priori the role of time, we reverse the relation between macroscopic state and evolution of time: it is not the evolution of time that determines the state, it is the state – the blurring – that determines a time.


This is the fascinating idea suggested by [Alain] Connes: the first germ of temporality in elementary quantum transitions lies in the fact that these interactions are naturally (partially) ordered.


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


Time is ignorance.


Entropy is not an arbitrary quantity, or subjective. It is a relative one, like speed.


This [that entropy is initially low due our interactions with it producing a blurred frame of reference], which is a fact, opens up the possibility that it wasn’t the universe that was in a very partiuclar configuration in the past. Perhaps it is us, and our interactions with the universe, that are particular.


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


But among the innumerable small systems that exist in this extraordinarily vast universe where we happen to live, there will be a few special ones for which the fluctuations of entropy happen to be such that at one of the two ends of the flow of thermal time entropy happens to be low.


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 flame is a process that opens a channel through which the wood can pass into a state of higher entropy.


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


I believe that our notion of self stems from this [interaction with other humans], 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.


What we percieve is not the present […] but rather something that happens and extends in time. It is in our brains that an extension in time becomes condensed into a perception of duration.


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.


We long for timelessness, we endure the passing of time: we suffer time. Time is suffering.


This [ordered time we speak of] 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.


It is in order to escape this anxiety [caused by time] that we have imagined the existence of ‘eternity’, a strange world outside of time that we would like to be inhabited by gods, or by a God or by immortal souls.


Our fear of death seems to me to be an error of evolution.


The hypertrophy of our frontal lobes is considerable, and has taken us to the moon, …


… to realize that delirium is a vast theatrical machinery with the capacity to stage the world, and that it is difficult to find arguments to distinguish it from those great collective deliriums of ours that are foundations of our social and spiritual life, …