Book Review: The Order of Time, by Carlo Rovelli

The book is written by Carlo Rovelli, who is an Italian physicist and specialist in quantum gravity research about time in physics.

I was curious to read this book after a couple of former colleagues at the University of Chicago, specialized in the Quantum Mechanics field, recommended me to go through it. The twenty-seven takeaways that I got out of this boom are presented below:

• This is time, familiar and intimate. We are taken by it. The rush of seconds, hours, years that hurls us toward life then drags us toward nothingness… We inhabit time as fish live in water. Our being is being in time. Its solemn music nurtures us, opens the world to us, troubles us, frightens and lulls us… we sill dont know how time actually works. The nature of time is perhaps the greatest remaining mystery. Curious threads connect it to those other great open mysteries. the nature of mind, the origin of the universe, the fate of black holes, the very functioning of life on Earch. Something essential continues to draw us back to the nature of time.
• Wonder is the source of our desire for knowledge, and the discovery that time is not what we thought it was opens up a thousand questions… Why do we remember the past and not the future? Do we exist in time, or does time exist in us? What does it really mean to say that time “passes”? What ties time to our nature as persons, to our subjectivity? What am I listening to when I listen to the passing of time?… Because the mystery of time is ultimately, perhaps, more about ourselves than about the cosmos. Perhaps, as in the first and greatest of all detective novels, Sophocles’ Oedipus Rex, the culprit turns out to be the detective.
• Einstein has given us the equations that describe how proper times develop relative to each other. He has shown us how to calculate the difference between two times… Clausius‘s entropy, indicated by the letter S, is a measurable and calculable quantity’s that increases of remains the same but never decreases, in an isolated process.
• We see the water in a glass like the astronauts saw the Earth from the moon: calm, gleaming, blue. From the moon, they could see nothing of the exuberant agitation of life on Earth, its plants and animals, desires and de-spairs. Only a veined blue ball. Within the reflections in a glass of water, there is an analogous tumultuous life, made up of the activities of myriads of molecules–many more than there are living beings on Earth… This tumult stirs up everything. If one section of the molecules is still, it becomes stirred up by the frenzy of neighboring ones that set them in motion, too: the agitation spreads, the molecules bump into and shove each other. In this way, cold things are heated in contact with hot ones: their molecules become jostled by hot ones and pushed into ferment. That is, they heat up.
• The difference between past and future is deeply inked to this blurring… So if I could take into account all the details of the exact, microscopic state of the word, would the characteristic aspects of the flowing of time disappear? Yes. If I observe the microscopic state of things; then the difference between past and future vanishes. The future of the world, for instance, is determined by its present state – though neither more nor less than is the past.
• Is it possible that I have as much knowledge of be past or ignorance of it – as I do of the future? Even allowing for the fact that our perceptions of the world are frequently wrong, can the world really be so profoundly different from our perception of it as this?
• For everything that moves, time passes more slowly. For this effect to become perceptible, one must more very quickly. It was first measured in the 1970, using precision watches on airplanes. The watch on board of a plane displays a time behind that displayed by the one on the ground. Today, the slowing down of time can be observed in many physics experiments.
• “Now” means nothing – What is happening “now” in a distant place? Imagine, for example, that your sister has gone to Proxima b, the recently discovered planet that orbits a star at approximately four light-years’ distance from us. What is your sister doing now on Proxima b? The only correct answer is that the question makes no sense. It is like asking “What is here, in Beijing?” when we are in Venice. It makes no sense because if I use the word “here” in Venice, I am referring to a place in Venice, not in Beijing… If your sister is on Proxima b, however, light takes four years to reach you from there. Hence, if you look at her through a telescope, or receive a radio communication from her, you know what she was doing four years ago rather than what she is doing now. “Now” on Proxima b is definitely not what you see through the telescope, or what you can hear from her voice over the radio.
• There is our past: all the events that happened before what we can witness now. There is our future: the events that will happen after the moment from which we can see the here and now. Between this past and this future there is an interval that is neither past nor future and still has a duration: fifteen minutes on Mars; eight years on Proxima b; millions of years in the Andromeda galaxy. It is the expanded present? It is perhaps the greatest and strangest of Einstein’s discoveries… The idea that a well-defined now exists throughout the universe is an illusion, an illegitimate extrapolation of our own experience.
• 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 any more.
• Aristotle is the first we are aware of to have asked himself the question “What is time?” and he came to the following conclusion: time is the measurement of change. Things change continually. We call “time” the measurement, the counting of this change… Success, as ever, is destined to be short-lived-_even great success. Einstein writes the equations of the gravitational field in 1915, and barely a year later it is Einstein himself who observes that this cannot be the last word on the nature of time and space, because of the existence of quantum mechanics. The gravitational field, like all physical things, must necessarily have quantum properties.
• Granularity is the most characteristic feature of quantum mechanics, which takes its name from this: “quanta” are elementary grains. A minimum scale exists for all phe-nomena.” For the gravitational field, this is called the “Planck scale.” Minimum time is called “Planck time.” Its value can be easily estimated by combining the constants that characterize phenomena subject to relativity, gravity, and quantum mechanics. Together, these determine the time to 10^(-44) seconds: a hundred millionch of a trillionth of a trillionth of a trillionth of a second. This is Planck time: at this extremely minuscule level, quantum effects on time become manifest.
• Quantum superposition of times – The second discovery made by quantum mechanics is indeterminacy: it is not possible to predict exactly, for instance, where an electron will appear tomorrow. Between one appearance and another, the electron has no precise position as if it were dispersed in a cloud of probability. In the jargon of physicists, we say that it is in a “superposition” of positions.
• Relations – “Fluctuation” does not mean that what happens is never determined. It means that it is determined only at certain moments, and in an unpredictable way. Indeterminacy is resolved when a quantity interacts with something else. In the interaction, an electron materializes at a certain point. For example, it collides with a screen, is captured by a particle detector, or collides with a photon – thus acquiring a concrete position… But there is a strange aspect to this materialization of the electron: the electron is concrete only in relation to the other physical objects it is interacting with. With regard to all the others, the effect of the interaction is only to spread the contagion of indeterminacy. Concreteness occurs only in relation to a physical system: this, the author believes. is the most radical discovery made by quantum mechanics.
• There is no single time: there is a different duration for every trajectory; and time passes at different rhythms according to place and according to speed. It is not directional: the difference between past and future does not exist in the elementary equations of the world; its orientation is merely a contingent aspect that appears when we look at things and neglect the details. In this blurred view, the past of the universe was in a curiously “particular” state. The notion of the “present’ does not work: in the vast universe there is nothing that we can reasonably call “present.” The substratum that determines the duration of time is not an independent entity, different from the others that make up the world, it is an aspect of a dynamic field.
• The world is nothing but change. None of the pieces that time has lost (singularity, direction, independence, the present, continuity) puts into question the fact that the world is a network of events. On the one hand, there was time, with its many determinations; on the other, the simple fact that nothing is: things happen… The absence of the quantity “time” in the fundamental equations does not imply a world that is frozen and immobile. On the contrary, it implies a world in which change is ubiquitous, without being ordered by Father Time; without innumerable events being necessarily distributed in good order, or along the single Newtonian time line, or according to Einstein’s elegant geometry… The events of the world do not form an orderly queue, like the English. They crowd around chaotically, like Italians.
• We can think of the world as made up of things. Of substances. Of entities. Of something that is. Or we can think of it as made up of events. Of happenings. Of processes. Of something that occurs. Something that does not last, and that undergoes continual transformation, that is not permanent in time. The destruction of the notion of time in fundamental physics is the crumbling of the first of these two perspectives, not of the second. It is the realization of the ubiquity of impermanence, not of stasis in a motionless time… What works instead is thinking about the world as a network of events. Simple events, and more complex events that can be disassembled into combinations of simpler ones. A few examples: A war is not a thing, it’s a sequence of events. A storm is not a thing, it’s a collection of occurrences. A cloud above a mountain is not a thing, it is the condensation of humidity in the air that the wind blows over the mountain. A wave is not a thing, it is a movement of water, and the water that forms it is always different. A family is not a thing, it is a collection of relations, occurrences, feelings. And a human being? Of course it’s not a thing; like the cloud above the mountain, it’s a complex process, where food, information, light, words, and so on enter and exit… A knot of knots in a network of social relations, in a network of chemical processes, in a network of emotions exchanged with its own kind… The world is more like Naples than Singapore.
• Philosophers call “eternalism” the idea that flow and change are illusory: present, past, and future are all equally real and equally existent. Eternalism is the idea that the whole of spacetime exists all together in its entirety without anything changing. Nothing really flows… The fact that we cannot arrange the universe like a single orderly sequence of times does not mean that nothing changes. It means that changes are not arranged in a single orderly succession: the temporal structure of the world is more complex than a simple single linear succession of instants. This does not mean that it is nonexistent or illusory.
• There is no need in any of this to choose a privileged variable and call it “time.” What we need, if we want to do science, is a theory that tells us how the variables change with respect to each other. That is to say, how one changes when others change. The fundamental theory of the world must be constructed in this way; it does not need a time variable: it needs to tell us only how the things that we see in the world vary with respect to each other. That is to say, what the relations may be between these variables… The fundamental equations of quantum gravity are effectively formulated like this: they do not have a time variable, and they describe the world by indicating the possible relations between variable quantities… In 1967, an equation accounting for quantum gravity was written for the first time without any time variable. This equation was discovered by two American physicists – Bryce DeWitt and John Wheeler – and today it’s known as the Wheeler-DeWitt equation.
• The theory does not describe how things evolve in time. The theory describes how things change one in respect to the others, how things happen in the world in relation to each other. That’s all there is to it… As human beings, we live by emotions and thoughts. We exchange them when we are in the same place at the same time, talking to each other, looking into each other’s eyes, brushing against each other’s skin. We are nourished by this network of encounters and exchanges. But, in reality, we do not need to be in the same place and time to have such exchanges. 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. We are part of a network that goes far beyond the few days of our lives and the few square meters that we tread. This book is also a part of that weave.
• Whatever we human beings may be specifically, in detail, we are nevertheless pieces of nature, a part of the great fresco of the cosmos, a small part among many others…Between ourselves and the rest of the world there are physical interactions. Obviously, not all the variables of the world interact with us, or with the segment of the world to which we belong. Only a very minute fraction of these variables does, so most of them do not react with us at all. They do not register us, and we do not register them. This is why distinct configurations of the world seem equivalent to us. The physical interaction between myself and a glass of water – two pieces of the world – is independent of the motion of the single molecules of wa-ter. In the same way, the physical interaction between myself and a distant galaxy two pieces of the world ignores what happens in detail out there. Therefore, our vision of the world is blurred because the physical interactions between the part of the world to which we belong and the rest are blind to many variables… This blurring is at the heart of Boltzmann’s theory. From this blurring, the concepts of heat and entropy are born and these are linked to the phenomena that characterize the flow of time. The entropy of a system depends explicitly on blurring. It depends on what I do not register, because it depends on the number of indistinguishable configurations. The same microscopic configuration may be of high entropy with regard to one blurring and of low in relation to another.
• There is plenty of energy and it is not consumed. 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.
• There are different ingredients that combine to produce our identity. Three of these are important for the argument of this book: The first is that every one of us identifies with a point of view in the world. The world is reflected in each one of us through a rich spectrum of correlations essential for our survival. Each of us is a complex process that reflects the world and elaborates the information we receive in a way that is strictly integrated… The second ingredient on which our identity is based is the same as for the chariot. In the process of reflecting the world, we organize it into entities: we conceive of the world by grouping and segmenting it as best we can in a continuous process that is more or less uniform and stable, the better to interact with it… The third is that we are not a collection of independent processes in successive moments. Every moment of our existence is linked by a peculiar triple thread to our past – the most recent and the most distant – by memory. Our present swarms with traces of our past. We are histories of ourselves, narratives.
• A recent book by Dean Buonomano devoted to research on the functioning of the brain is entitled Your Brain Is a Time Machine. It discusses the many ways in which the brain interacts with the passage of time and establishes bridges between past, present, and future. To a large extent, the brain is a mechanism for collecting memories of the past in order to use them continually to predict the future. This happens across a wide spectrum of time scales, from the very short to the very long… This space – memory – combined with our continuous process of anticipation, is the source of our sensing time as time, and ourselves as ourselves. Think about it: our introspection is easily capable of imagining itself without there being space or matter, but can it imagine itself not existing in time?
• A present that is common throughout the whole universe does not exist. 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 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 aspects of a great jelly in which we are immersed. But the world is a quantum one, and gelatinous space-time 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 is possible to calculate probabilities and relations.
• 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. We for whom, as Ecclesiastes has it, there is a time to be born and a time to die… This is time for us: a multilayered, complex concept with multiple, distinct properties deriving from various different approximations.
• Many discussions of the concept of time are confused because they simply do not recognize its complex and multilayered aspect. They make the mistake of not seeing that the different layers are independent… This is the physical structure of time as the author understands it, after a lifetime of revolving around it… As the author states, many parts of this story are solid, others plausible, others still are guesses hazarded in an attempt at understanding the whole.