The Order of Time

In his previous book, Reality Is Not What It Seems, Rovelli emphasises the need to remove the Newtonian model of a separately existing time from physical theories. In this new similarly slim and equally lucid volume, he delves deeper into what is this time thing, anyway. He carefully picks apart the many different models of time in physics. Newtonian time has been replaced by many different models of time, all of which remove one or more ‘obvious’ properties. Special relativistic time depends on your speed, and asking what is now somewhere else “is like asking ‘What is here, in Peking?’”[p.37]: the present is defined just in a local bubble whose size depends on our precision. General relativistic time depends on the curvature of space, and so is different everywhere, and things fall because “the movement of things inclines to where time passes more slowly” [p.12]. Thermodynamics is the only basic physical theory that has an ‘arrow’ of time, of entropy increase, the existence of which depends on your scale; that “entropy exists because we describe the world in a blurred fashion”, and if we “observe the microscopic state of things the difference between past and future vanishes” [p.30]. Quantum mechanical (space)time is not continuous, but granular, as is everything else, and different times can coexist in superpositions. Rovelli provides an interesting historical perspective on our current everyday intuitions of it being the same time in different places, and time always passing at the same speed: we didn’t always have these ideas. Clocks didn’t start started regulating our hours until around the 14th century. But these clocks were synchronised to local noon, not to each other. Then train timetables in the 19th century required synchronisation across distances. Time zones were invented in 1883, and cities gradually synchronised their clocks with each other. Then, in 1905, Einstein destroyed the idea of universal synchronicity. (I had known Einstein worked in a patent office; I was not aware he dealt specifically with patents related to synchronising clocks!) Another interesting historical perspective: today we are accustomed to the idea the Newton’s view of an independently flowing absolute time, and think that Leibniz was some maverick suggesting relational time, of time being change. But actually, this view of time being dependent on change was the orthodox Aristotelian view, and it was Newton who was the maverick. We are just nowadays more used to the Newtonian view. Einstein synthesised the Aristotelian and Newtonian views: yes, spacetime is something real, yet it is relative, not absolute. Having spent the first part of the book bringing us up to date with current physics, Rovelli moves into more a speculative realm, a different view of time in terms of change. This is heady stuff. We should think of the world as a network of events, and “the simple fact that nothing is: that things happen instead” [p.85]. In this world there is no time as we currently understand it; instead it is a world in which change is ubiquitous. Rovelli explains how some of the problems we have with this new physics is down to grammar: the human languages we use to talk about the world, with their simple past, present and future tenses, do not fit well with our current view of a more complex structure to physical time. But just because natural language, developed before we knew about this complexity, can’t cope, doesn’t mean our physical models are wrong: we just have to work harder. Rovelli concludes his discussion with some thoughts on the origins of time: how it might emerge from the underlying granular, complex structure of spacetime events; from a particular blurring (ignorance) of macroscopic state; from non-commutative quantum operations imposing a natural (partial) order; from the fact that we have a point of view observing the universe while situated within it. This is a beautifully written book, explaining complex concepts with great clarity and style. It is a translation. There is an amusing translation error on p193, which talks of “a degree of liberty”: after a moment of thought, I decided that this should be “a degree of freedom”. Despite the book’s slimness, there is a great deal to think about here; I have merely scraped the surface in my summary above. It is a wonderfully rich concoction of deep ideas and lucid explanation. Recommended.

As always, Rovelli is clear and well argued. As always he brings a wealth of knowledge of both contemporary scientific research and the history of science and ideas. My only reservation is the closing section in which he covers, sometimes too poetically, human consciousness of time.

Love all his books. So well written, explains difficult subjects really well and increase my understanding of these subjects

A short , comprehensive and exciting book, allowing the reader to familiarise oneself with the arcana of things nuclear and their impact on our world

This book is something of a curate's egg and a bit of a ramble. It is not of the high standard that his earlier books ("Seven Brief Lessons..." and "Reality...") would have led readers to expect this time (pardon the pun). On page 107 he writes, honestly, "But I have digressed and lost my thread...". He seems to lose his thread in other chapters also. He doesn't do all that efficiently what he sets out to do. Beneath or behind the unnecessary verbiage and divergences, there is a good book. But the reader has to be patient in places with the author to reach the meat of what he wants to say. A couple of chapters are hard going, and the author warns of that, but, after you excise the unnecessary, the rest is fairly easy reading even if it is a bit of a ramble. The author could have said the same in fewer pages.

Carlo Rovelli is a charismatic writer; in his popular science writing he has the ability to present simply, intrinsically arcane concepts in a prose that is lyrical. In the present book he deconstructs the widely held concept of time in a broadly accepted science and from the barren landscape, he proposes a plausible reconstruction which, however, far from being accepted is, in fact, hotly debated. Our familiar image of time is something that flows uniformly and equally throughout the universe, in the course of which all things happen. With the idea that there exists throughout the cosmos a present that constitutes reality. The past for everyone is fixed having already happened. The future is open, yet to be determined. Reality flows from the past through the present towards the future and the evolution of things between past and future is intrinsically asymmetrical. This was the prevailing thinking regarding the basic structure of the world. The familiar picture has fallen apart, has shown itself to be only an approximation of a much more complex reality. 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. Locally, time passes at different speeds according to where we are and at what speed, we, ourselves are moving. The closer we are to a mass, or the faster we move, the more time slows down: there is no single duration between two events; there are many possible ones. 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 feasible to calculate possibilities and relations. At the most fundamental level that we currently know of, there is no 'time' variable, there is no difference between past and future, there is no space - time. We know how to write equations that describe the world. In these equations, the variables evolve with respect to each other. It is not a 'static' world where all change is illusory: on the contrary, ours, is a world of events rather than of things. The preceding was the deconstruction of time, a universe without time. The reconstruction of time is an attempt to understand how, from this world without time, it is possible for our perception of time to emerge. The surprise has been that, in the emergence of familiar aspects of time, we, ourselves have had a role to play. From our perspective - the perspective of creatures who make up a small part of the world - we see that world flowing in time. Our interaction with the world is partial, which is why we see it in a blurred way. The ignorance that follows from this determines the existence of a particular variable - thermal time - and an entropy that quantifies our uncertainty. It is possible that we belong to a particular subset of the world that interacts with a small number of variables in a small subset of the universe in such a way that the entropy is lower in one direction of our thermal time. The directionality of time is, therefore, real but perspectival: the entropy of the world in relation to us increases with our thermal time. We see the occurrence of things ordered in this variable, which we simply call 'time', and the growth of entropy distinguishes the past from the future for us and leads us to the unfolding of the cosmos. It determines the existence of traces, residues and memories of the past. We, human beings, are an effect of this great history of the increase of entropy held together by the memory that is enabled by these traces. Each one of us is a unified being because we reflect the world, because we have formed an image of a unified entity by interacting with our kind, and because it is a perspective on the world unified by memory. From this comes what we call the 'flowing' of time. This is what we are listening to when we listen to the passing of time.

The chapter “The sister of sleep” is a truly remarkable and beautiful summary of my feelings at this moment in my life at 83. I would love to think that this will happen to me. Thank you Carlo.

As a layman I've read many books about modern physics theory. All have tended to be hard going. But at last, here is a book that opened the door and enabled me to grasp time and existence. Chapter 6 was the epiphany for me. This is the very best book on the subject I have ever read. I cannot recommend it highly enough.

Carlo has a way with words. Amazing book: Interesting and easy to follow.

Narrativa fluida e claríssima. Rovelli aborda temas sofisticados com maestria e simplicidade. Deixa claro o que é visão pessoal e consenso acadêmico. Leitura de qualidade para leigos e especialistas.

20世紀の大きな思想変革として「相対化」があげられる。神や絶対正義が理性によって否定され、その理性の創り出す世界観も相対性理論や量子理論、不完全性定理によって相対化された。そのなかで依然として確固とした地位を保っているかに見えた「時間」はどうなのか？スリリングな知的刺激と視覚の広がりを与えてくれる。平易な英語で読みやすい。

Una explicación muy clara de lo que es el tiempo. Se lee fácil, quizás porque soy físico y entiendo algo del tema

Very good in condition and delivery.