"The Order of Time"

[fiefoe]

Carlo Rovelli really lost me in the later chapters, not that the earlier ones were a walk in the park either.

• One after another, the characteristic features of time have proved to be approximations, mistakes determined by our perspective, just like the flatness of the Earth or the revolving of the sun. The growth of our knowledge has led to a slow disintegration of our notion of time. What we call “time” is a complex collection of structures,2 of layers. Under increasing scrutiny, in ever greater depth, time has lost layers one after another, piece by piece. The first part of this book gives an account of this crumbling of time. The second part describes what we have been left with: an empty, windswept landscape almost devoid of all trace of temporality.
• A world stripped to its essence, glittering with an arid and troubling beauty. The physics on which I work—quantum gravity—is an attempt to understand and lend coherent meaning to this extreme and beautiful landscape. To the world without 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.
• will take you to where I believe our knowledge of time has reached: up to the brink of that vast nocturnal and star-studded ocean of all that we still don’t know.
• If things fall, it is due to this slowing down of time. Where time passes uniformly, in interplanetary space, things do not fall. They float, without falling.  <> Here on the surface of our planet, on the other hand, the movement of things inclines naturally toward where time passes more slowly, as when we run down the beach into the sea and the resistance of the water on our legs makes us fall headfirst into the waves. Things fall downward because, down there, time is slowed by the Earth.
• 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.*
• In the elementary equations of the world, the arrow of time appears only where there is heat.* The link between time and heat is therefore fundamental: every time a difference is manifested between the past and the future, heat is involved. In every sequence of events that becomes absurd if projected backward, there is something that is heating up.
• It was he, this Ludwig, who was the luckless hero of time’s directionality.
• 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.
• The notion of “particularity” is born only at the moment we begin to see the universe in a blurred and approximate way. Boltzmann has shown that entropy exists because we describe the world in a blurred fashion. He has demonstrated that entropy is precisely the quantity that counts how many are the different configurations that our blurred vision does not distinguish between.
• There are regularities, represented by what we call physical laws, that link events of different times, but they are symmetric between future and past. In a microscopic description, there can be no sense in which the past is different from the future.*
• 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?
• entropy, as Boltzmann fully understood, is nothing other than the number of microscopic states that our blurred vision of the world fails to distinguish.
• “Proper time” depends not only on where you are and your degree of proximity to masses; it depends also on the speed at which you move.
• There is no special moment on Proxima b that corresponds to what constitutes the present here and now.
• The notion of “the present” refers to things that are close to us, not to anything that is far away. <> Our “present” does not extend throughout the universe. It is like a bubble around us. How far does this bubble extend? It depends on the precision with which we determine time.
• The temporal structure of the universe is very similar to this one. It is also made of cones. The relation of “temporal precedence” is a partial order made of cones.31 Special relativity is the discovery that the temporal structure of the universe is like the one established by filiation: it defines an order between the events of the universe that is partial, not complete. The expanded present is the set of events that are neither past nor future: it exists, just as there are human beings who are neither our descendants nor our forebears.
• This is the structure of spacetime that Einstein understood when he was twenty-five years old. Ten years later, he comes to understand that the speed at which time flows changes from place to place. It follows that spacetime does not really have the order outlined above but can be distorted. It now looks rather more like this:
  When a gravitational wave passes, for example, the small light cones oscillate together from right to left, like ears of wheat blown by the wind. The structure of the cones can even be such that, advancing always toward the future, one can return to the same point in spacetime,
• It can hardly be pure coincidence that, before gaining a university position, the young Einstein worked in the Swiss patent office, dealing specifically with patents relating to the synchronization of clocks at railway stations. It was probably there that it dawned on him: the problem of synchronizing clocks was, ultimately, an insoluble one. In other words, only a few years passed between the moment at which we agreed to synchronize clocks and the moment at which Einstein realized that it was impossible to do so exactly.
• But the existence of a time that is uniform, independent of things and of their movement that today seems so natural to us is not an ancient intuition that is natural to humanity itself. It’s an idea of Newton’s. The majority of philosophers have in fact responded negatively to this idea. In a still celebrated, furious counterblast, Leibniz defended the traditional thesis according to which time is only the order of events, arguing that there is no such thing as an autonomous time. Legend has it that Leibniz, whose name is still occasionally spelled with a “t” (Leibnitz), had deliberately dropped the letter from his name in accordance with his belief in the nonexistence of the absolute Newtonian time t.
• Aristotle was the first to discuss in depth and with acuity the meaning of “space,” or “place,” and to arrive at a precise definition: the place of a thing is what surrounds that thing.
• For Newton, between two things there may also be “empty space.” For Aristotle, it is absurd to speak of “empty” space, because space is only the spatial order of things.
• The synthesis between Aristotle’s time and Newton’s is the most valuable achievement made by Einstein. It is the crowning jewel of his thought. The answer is that the time and space Newton had intuited the existence of, beyond tangible matter, do effectively exist. They are real. Time and space are real phenomena. But they are in no way absolute; they are not at all independent from what happens; they are not as different from the other substances of the world, as Newton had imagined them to be. We can think of a great Newtonian canvas on which the story of the world is drawn. But this canvas is made of the same stuff that everything else in the world is made of, the same substance that constitutes stone, light, and air: it is made of fields.
• But Newton is wrong in assuming that this time is independent from things—and that it passes regularly, imperturbably, separately, from everything else. For his part, Aristotle is right to say that “when” and “where” are always located in relation to something. But this something can also be just the field, the spatiotemporal entity of Einstein.
• 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 phenomena.51 For the gravitational field, this is called the “Planck scale.”
• Continuity is only a mathematical technique for approximating very finely grained things. The world is subtly discrete, not continuous.
• 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.
• In the world, there is change, there is a temporal structure of relations between events that is anything but illusory. It is not a global happening. It is a local and complex one that is not amenable to being described in terms of a single global order.
• 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.
• No, it was a view from afar. Come to think of it, it’s like this with all surfaces. This dense marble table would look like a fog if I were shrunk to a small enough, atomic scale. Everything in the world becomes blurred when seen close up.
• We cut the world into large slices. We think of it in terms of concepts that are meaningful for us, that emerge at a certain scale.
• Is the daily spectacle of a revolving universe “illusory”? No, it is real, but it doesn’t involve the cosmos alone. It involves our relation with the sun and the stars.
• In these examples, something that is real—a cat, a football team, high and low, the surface of clouds, the rotation of the cosmos—emerges from a world that at a much simpler level has no cats, teams, up or down, no surfaces of clouds, no revolving cosmos. . . . Time emerges from a world without time, in a way that has something in common with each of these examples.
• 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, .. Noncommutativity determines an order and, consequently, a germ of temporality in the determination of two physical variables.
• Temporality is profoundly linked to blurring. The blurring is due to the fact that we are ignorant of the microscopic details of the world. The time of physics is, ultimately, the expression of our ignorance of the world. Time is ignorance.
• 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.
• Among these, through the endless game of probabilities and huge numbers, there will surely be some that interact with the rest of the universe precisely with those variables that found themselves having a particular value in the past.
• If a subset of the universe is special in this sense, then for this subset the entropy of the universe is low in the past, the second law of thermodynamics obtains; memories exist, traces are left—and there can be evolution, life, and thought.
• With regard to these systems, entropy is constantly increasing. There, and not elsewhere, there are the typical phenomena associated with the flowing of time: life is possible, together with evolution, thought, and our awareness of time passing. There, the apples grow that produce our cider: time. That sweet juice that contains all the ambrosia and all the gall of life.
• What makes the world go round are not sources of energy but sources of low entropy. Without low entropy, energy would dilute into uniform heat and the world would go to sleep in a state of thermal equilibrium.
• But for every hot photon that arrives, the Earth emits ten cold ones, since a hot photon from the sun has the same energy as ten cold photons emitted by the Earth. The hot photon has less entropy than the ten cold photons, because the number of configurations of a single (hot) photon is lower than the number of configurations of ten (cold) photons. Therefore, the sun is a continual rich source of low entropy for us.
• Animals feed on low entropy by eating. (If all we needed was energy rather than entropy, we would head for the heat of the Sahara rather than toward our next meal.) Inside every living cell, the complex web of chemical processes is a structure that opens and closes gates through which low entropy 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 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 answer is that when the stone hits the Earth, it warms it: its mechanical energy is transformed into heat. And there is no way back from there. If the second law of thermodynamics did not exist, if heat did not exist, if there existed no microscopic swarming, the stone would rebound perpetually; it would never land and be still.
• In order to leave a trace, it is necessary for something to become arrested, to stop moving, and this can happen only in an irreversible process—that is to say, by degrading energy into heat. In this way, computers heat up, the brain heats up, the meteors that fall into the moon heat it; even the goose quill of a medieval scribe in a Benedictine abbey heats a little the page on which he writes. In a world without heat, everything would rebound elastically, leaving no trace.
• And “particular” is a relative term: it is particular in relation to a perspective. It is a blurring. It is determined by the interactions that a physical system has with the rest of the world. Hence causality, memory, traces, the history of the happening of the world itself can only be an effect of perspective: like the turning of the heavens; an effect of our peculiar point of view in the world. . . . Inexorably, then, the study of time does nothing but return us to ourselves.
• My primary experience—if we grant that this means anything—is to see the world around me, not myself. I believe that we each have a concept of “my self” only because at a certain point we learn to project onto ourselves the idea of being human as an additional feature that evolution has led us to develop during the course of millennia in order to engage with other members of our group: we are the reflection of the idea of ourselves that we receive back from our kind.
• 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.
• To understand ourselves means to reflect on time. But to understand time we need to reflect on ourselves.
• The possibility of predicting something in the future obviously improves our chances of survival and, consequently, evolution has selected the neural structures that allow it. We are the result of this selection. This being between past and future events is central to our mental structure. This, for us, is the “flow” of time.
• We are memory. We are nostalgia. We are longing for a future that will not come. The clearing that is opened up in this way, by memory and by anticipation, is time: a source of anguish sometimes, but in the end a tremendous gift.
• The brief arc of our days, O Sestius, prevents us from launching prolonged hopes. (I, 4)
• Job died when he was “full of days.” It’s a wonderful expression.
• The point is that the description in terms of heat, temperature, and the passage of heat from tea to spoon is a blurred vision of what happens, and that it is only in this blurred vision that a startling difference between past and future appears.