The Order of Time - Carlo Rovelli 📚

A fantastic book that cleary and fairly susinctly explains important aspects of special and general relativity and the concept of time.

Part 1

Outlines general effects of special and general relativity like: time dillation by speed and gravitation, partial ordering of events, extended present, time cones, spacetime curvature.

The developments from physics are presented as each giving rise to a ‘loss’ of a certain aspect of time:

  1. Loss of Unity: GR shows that time passes slower in a gravitational field; it’s not the same everywhere
  2. Loss of Direction: the basic eqs. of physics don’t change under a temporal inversion, entropy giives rise to the ‘arrow of time’
  3. Loss of the Present: SR demands that time passes differently for different observers
  4. Loss of Independence; Newton’s absolute, external time is integrated in spacetime by GR

He then goes on to describe time quantisation in general terms.

Part 2

Here Rovelli describes what reality looks like now that it has been released from the shackles of time.

The world is composed of events and not things he states. Which is a truly change of vocabulary and definition, but an important one since the concept of the everlasting thing is nonsensical in reality. Due to quantum effects these events in the world form a network of events, processes and events interacting.

He also briefly touches on the relational interpretation of QM some concepts from his own field LQG (Loop Quantum Gravity); space grains, spin networks and spinfoam.

He makes some linguisitical remarks on the inadequacy of our languages to describe reality as they are so tightly coupled to our version of it.

Part 3

Having removed familiar aspects of time from reality in part 1 and rebuilding what remains of reality in part 2, part 3 attempts to rebuild our experience of time in a world without it.

Time is here presented as an emergent phenomena.

Thermal time is introduced as an intersion of the familiar relation: timeenergymacroscopic state into macroscopic stateenergytime. A generic macroscopic state determines a time as Rovelli writes.

The blurring of the macroscopic state gives rise to thermal time (flows. The blurring of QM gives rise to a Connes flow (temporal flow due to the noncommutaticity of quantum observables) These thermal flows are equivalent up to certain internal symmetries and together form the QM Connes flow

The blurring of reality that a macroscopic state basically consists of added to the inherent blurring of QM gives rise to a certain time variable (not a universal one). Rovelli believes

Moving on, Rovelli discusses the origin of the particular blurring that we as humans have of the world. This he does by saying that the variables through which we interact with it give rise to a certain blurring; a way of seeing the world.

In our part of the Universe the past appear to have lower entropy, it might thus be us and our interactions with the Universe that are particular and not the Universe itself.

Why do we live in such a part of the Universe? The question have to be flipped; we live in this part of the world precisely because its properties allow us to and give rise to the second law of thermodynamics.

Why do we live in such a part of the Universe? The question have to be flipped; we live in this part of the world precisely because its properties allow us to.

In our part of the Universe it is a fact that entropy increases and this gives rise to our flow of time, and thus everything associated with it: traces, memories. Entropy keeps increasing in various ways and all processes lead to it eventually. The growth of entropy itself happes to open new doors through which entropy fan increase further.



  1. If I wath a film that shows a ball rolling, I cannot tell if the film is being projected correctly or in reverse. But, if the ball stops, I know that is is being run properly; run backwards, it would show an nimplausible event: a ball starting to move by itself.

    p. 23

  2. This is what Boltzmann understood. 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.

    p. 28

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

    p. 30

  4. How far does the bubble extend? It depends on the precision with which we determine time. If by nanoseconds, the present is defined only over a few metres; ifby milliseconds it is defined over thousands of kilometers. As humans we distinguish tenths of a second only with great difficulty; we can easily consider our planet to be like a single bubble where we can speak of the present as if it were an istant shared by us all. This is as far as we can go.

    On the extended present p. 40

  5. It is only in the fourteenth century in Europe that people’s lives start to be regulated by mechanical clocks.

    p. 53

  6. For millennia before clocks, our only way regular way of measuring time had been the alternation of day and night. The rythm of day followed by night also regulates the lives of plants and animals. Diurnal rythms 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 lie on Earth, since an oscillation is required to set a mechanism in motion. Living organisms are full of clocks of various kinds - molecular, neuronal, chemical, hormonal - each of them more or less in tune with the others. There are chemical mechanisms that keep to a twenty-four-hour rythm even in nthe biochemistry of single cells.

    On biological clocks p. 56

  7. Newton’s time is not the evidence given to us by our senses: it is ann elegant intellectual construction. If, my dear, cusltivated reader, the existence of this Newtonian concept of time which is independent of things seems to you simple and natural, it’s because you encountered it at school. Because it has gradually become the way in which we all think about time. It has filtered through shcool textbooks througout the world and ended up becoming our common way of understanding time.

    p. 60

  8. Before Newton, time for humannity was the way of counting how things changed. Before him, no one had thought it possible that a time independent of things could exist. Don’t take your intuitions and ideas to be ‘natural’: they are often the products of the ideas of audacious thinkers who came before us.

    p. 61

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

    p. 64

  10. A minimum scale exists for all phenomena. For the gravitational field, this is called the ‘Planc scale’. Minimum time is called the ‘Planck time’.

    p. 74

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

    p. 75

  12. Abstract thought can anticipate by centuries hypotheses that find a use - or confirmation - in scientific inquiiy.

    p. 76

  13. The difference between things and events is that things persist in time; events have a limited duration. A stone is a prototypical ‘thing’: we can ask ourselves where it will be tomorrow. Conversely, a kiss is an ‘event’. It makes no sense to ask where the kiss will be tomorrow. The world is made up of networks of kisses, not of stones.

    p. 87

  14. The temporal relations between events are more complex than we previously thought, but they do not cease to exist on account of this. The relations of filiation do not establish a global order, but this does not make them illusory. If we are not in a single file, it does not mean that there are no relations between us.

    p. 97

  15. We do not have a grammar adapted to say that an event ‘has bee’ inn relation to me but ‘is’ in relation to you.

    p. 99

  16. We are struggling to adapt our language and our intuition to a new discovery: the fact that ‘past’ and ‘future’ do not have a universal meaning. I nstead, they have a meaning which changes between here annd there. That’s all there is to it.

    p. 100

  17. Living beings are made up of similarly intertwined processes. Photosynthesis deposits low entropy from the sun into plants. Animals feed on low entropy by eating. (If all we eeded was energy rather than entropy, we would head for the heat of the Sahara rather than towards our next meal.) Inside every living cell, the complex web of chemical processes is a structure that opens and colses gates through which low entrpy can increase. Molecules function as the catalysts that allow the processes to intertwine; or, conversely, they put a brake on them. … It isn’t true as is sometimes stated, that life life generates structures that are particularily ordered, or that locally diminish entrpy: 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 inn nthe rest of the universe.

    p. 143

  18. 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 thouhgts 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 readinng has deposited in layers of 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.

    p. 154

  19. The possibility of predicting something in the future obviously improves our chances of survival and, cosequently, evolution has selected the neural structures that allow it.

    p. 155

  20. what we perceive is not the present, which in any case makes no sense for a system that functions on a scale of finite time, but rather something that happens and extends inn time. It is in our brains that an extension in time becomes condensed into a perception of duration.

    p. 156

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

    p. 175

  22. I love life, but life is also struggle, suffering, pain. I think of death as akin to a well-earned rest. The sister of sleep, Bach calls it, in his marvellous cantata BWV 56. A kindly sister who will come quickly to close my eyes and caress my head.

    p. 176

  23. Reason helps us to clarify ideas, to discover errors. But that same reason also shows us that the motives by which we act are inscribed in our iinntimate structure as mammals, as hunters, as social beings: reason illuminates these connections it does not generate them we are not, in the first place, reasoning beinngs. We may perhaps become so, more or less, in the second.

    p. 178

  24. We are more complex than our mental faculties are capable of grasping. The hypertrophy of our frontal lobes is considerable, and has taken us to the moon, allowed us to discover black holes and to recognize that we are cousins of ladybirds. But it is still not enough to allow us to explain ourselves clearly to ourselves.

    p. 180