The order of time by Carlo Rovelli


The first half of the book was quite mind blowing. It introduced news ideas like: 1. Time and entropy exists only if you look from far away enough, when things blur; 2. It’s not mass that attracts, it’s where time moves more slowly; and 3. It is theoretically possible to have a time travel loop with gravitational waves. Some are new perspectives to our physical world. Others are things that I thought only exists in science fiction. The parts around the lives of well known physicists were quite touching as well. 

I did not like the second half. At first I thought it was just me, but then realized that the first half comes from well known theories and interpretations and the second half of the book is mostly from the author himself. So the second half is not as sound as the ones that has been questioned and pondered by thousands of physicists. 

Nevertheless it is a great book that I’d recommend to people who enjoy physics.


  • The ability to understand something before it’s observed is at the heart of scientific thinking.
  • Here on the surface of our planet, on the other hand, the movement of things inclines naturally toward where time passes more slowly,
  • Between two events, just as between the two clocks that are separated and then brought together again, the duration is not a single one. 8 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.*
  • It follows that the notion of certain configurations being more particular than others (twenty- six red cards followed by twenty- six black, for example) makes sense only if I limit myself to noticing only certain aspects of the cards (in this case, the colors).
  • This is 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. It’s a conclusion that leaves us flabbergasted: 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?
  • the “dear sweet chubby one,” Ludwig Boltzmann, will end his life by hanging himself. He does so at Duino, near Trieste, while his wife and daughter are swimming in the Adriatic. The same Duino where, just a few years later, Rilke will write his Elegy.
  • In fundamental relativistic physics, where no variable plays a priori the role of time, we can 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. Time that is determined in this way by a macroscopic state is called “thermal time.” In what sense may it be said to be a time? From a microscopic point of view, there is nothing special about it— it is a variable like any other. But from a macroscopic one, it has a crucial characteristic: among so many variables all at the same level, thermal time is the one with behavior that most closely resembles the variable we usually call “time,” because its relations with the macroscopic states are exactly those that we know from thermodynamics.
  • When an interaction renders the position of a molecule concrete, the state of the molecule is altered. The same applies for its speed. If what materializes first is the speed and then the position, the state of the molecule changes in a different way than if the order of the two events were reversed. The order matters. If I measure the position of an electron first and then its speed, its state changes differently than if I were to measure its velocity first and then its position. This is called the “noncommutativity” of the quantum variables, because position and speed “do not commute,” that is to say, they cannot exchange order with impunity.
  • To determine a physical variable is not an isolated act; it involves interaction. The effect of such interactions depends on their order, and this order is a primitive form of the temporal order.
  • The intrinsic quantum indeterminacy of things produces a blurring, like Boltzmann’s blurring, which ensures— contrary to what classic physics seemed to indicate— that the unpredictability of the world is maintained even if it were possible to measure everything that is measurable.
  • Entropy is not an arbitrary quantity, nor a subjective one. It is a relative one, like speed.
  • But among the innumerable small systems S that exist in this extraordinarily vast universe where we happen to live, there will be a few special ones for which the fluctuations of the entropy happen to be such that at one of the two ends of the flow of thermal time entropy happens to be low. 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.